- Email: [email protected]
Chapter 11 Laying and Protection
181
CHAPTER 1 1
LAY I NG AND PROTECT I ON SELECTING A ROUTE The s e l e c t i o n of a s u i t a b l e r o u t e f o r a p i p e l i n e has an important b e a r i n g on t h e c a p i t a l c o s t and o p e r a t i n g c o s t s .
A p i p e l i n e r o u t e i s se-
l e c t e d from a e r i a l p h o t o s , t o p o g r a p h i c a l and c a d a s t r a l p l a n s , o n - s i t e i n s p e c t i o n s and any o t h e r d a t a a v a i l a b l e on t h e t e r r a i n , o b s t a c l e s and l o c a l I n s e l e c t i n g a r o u t e , t h e c o s t s and p r a c t i c a b i l i t y have t o be
services. considered.
Care should be t a k e n t o e n s u r e t h e ground p r o f i l e i s below t h e
h y d r a u l i c g r a d e l i n e . (Low-flow c o n d i t i o n s s h o u l d be c o n s i d e r e d a s w e l l a s peak r a t e s , as t h e h y d r a u l i c g r a d i e n t i s f l a t t e s t f o r low f l o w s ) .
If there
were a peak above t h e grade l i n e between t h e i n p u t and d i s c h a r g e h e a d s , obv i o u s l y pumps would have t o be d e s i g n e d t o pump o v e r t h i s peak.
Peaks may
a l s o be p o i n t s of p o s s i b l e w a t e r column s e p a r a t i o n which r e s u l t i n w a t e r hammer o v e r p r e s s u r e s .
On t h e o t h e r hand t h e g e n e r a l l e v e l of t h e p i p e l i n e
r o u t e s h o u l d be k e p t a s n e a r t o t h e h y d r a u l i c grade l i n e s a s p o s s i b l e t o minimize p r e s s u r e s and c o n s e q u e n t l y p i p e c o s t s . Once a p r e l i m i n a r y r o u t e i s s e l e c t e d , i t i s pegged o u t by a s u r v e y o r . Pegs a r e p u t i n a l o n g t h e c e n t r e - l i n e a t 10 t o 100 m r e g u l a r i n t e r v a l s and a t changes i n g r a d e and a t h o r i z o n t a l d e f l e c t i o n s .
O f f s e t pegs, 2 t o 5 m
from t h e c e n t r e l i n e pegs are a l s o p u t i n f o r u s e d u r i n g l a y i n g .
The l e v e l s
of pegs a r e observed and t h e p r o f i l e i s t h e n p l o t t e d i n t h e drawing o f f i c e , w i t h ground l e v e l s , bed l e v e l s and d e p t h s i n d i c a t e d a t e a c h peg.
Holes may
be augered a l o n g t h e r o u t e t o i d e n t i f y m a t e r i a l s which w i l l have t o be excavated.
S t r e n g t h t e s t s may be done on s o i l samples t o d e c i d e whether t r e n c h
s h o r i n g w i l l be n e c e s s a r y . Underground and overhead s e r v i c e s should be a c c u r a t e l y l o c a t e d a t t h i s s t a g e o r a t l e a s t b e f o r e e x c a v a t i o n t o a v o i d them.
( D r a i n s and underground
c a b l e s may have t o be s u p p o r t e d on b r i d g e s , o r a heading could be hande x c a v a t e d under them). LAYING AND TRENCHING
The t y p e of p i p e t o be used f o r any j o b w i l l l a r g e l y be a m a t t e r of economics, a l t h o u g h t h e f a c i l i t i e s f o r l a y i n g , l i f e of t h e p i p e m a t e r i a l s and ot.her f a c t o r s a f f e c t t h e d e c i s i o n .
The l e n g t h s of d e l i v e r e d p i p e s w i l l
1 8 2
depend on t h e t y p e of p i p e , t h e w e i g h t , t h e r i g i d i t y , s i z e of t r a n s p o r t v e h i c l e s and method of j o i n t i n g .
Concrete p i p e s a r e o f t e n s u p p l i e d i n 2 t o
4 m l e n g t h s , w h i l e s t e e l p i p e s may be 10 o r even 20 m l o n g , and t h i n w a l l e d p l a s t i c p i p e s may be s u p p l i e d i n r o l l s .
P i p e s s h o u l d be handled c a r e f u l l y
t o avoid damage t o c o a t i n g s o r p i p e o r d i s t o r t i o n . and t r a n s p o r t e d on Qadded c r a d l e s o r sandbags.
They should be s t o r e d
P i p e s may be lowered i n t o
t h e t r e n c h by b e l t s l i n g s from mechanical booms o r t r i p o d s . diameter t h i c k walled
For s m a l l
welded p i p e s , j o i n t i n g and wrapping may be done a t t h e
s i d e of t h e t r e n c h , and t h e p i p e t h e n ' s n a k e d ' i n t o t h e t r e n c h .
The working
w i d t h r e q u i r e d f o r c o n s t r u c t i o n of p i p e l i n e s v a r i e s w i t h t h e method of exc a v a t i o n and l a y i n g .
I f t h e e x c a v a t i o n and l a y i n g i s done by hand, 3 m may
be wide enough f o r s m a l l d i a m e t e r p i p e s .
On t h e o t h e r hand f o r l a r g e mains
where e x c a v a t i o n , l a y i n g and p o s s i b l y even s i t e c o a t i n g i s done m e c h a n i c a l l y , working w i d t h s up t o 50 m may be r e q u i r e d .
Reserve w i d t h s of 20 t o 30 m a r e
common, and a l l o w f o r one o r two a d d i t i o n a l p i p e l i n e s a t a l a t e r s t a g e . Once t h e t r e n c h i s e x c a v a t e d , s i g h t r a i l s may be s e t up a c r o s s t h e t r e n c h w i t h t h e c e n t r e l i n e of t h e p i p e marked t h e r e o n .
The frames a r e s e t
a d e f i n i t e h e i g h t above t h e bed and t h e p i p e l e v e l l e d w i t h t h e a s s i s t a n c e of t h e s e . P i p e s a r e normally l a i d w i t h a minimum c o v e r o f 0.9 m t o 1.2 m t o avoid damage by superimposed l o a d s .
A minimum cover of 1 m i s n e c e s s a r y i n t h e U.K.
t o prevent f r o s t penetration.
The d e p t h may be reduced i n rocky o r s t e e p
ground t o minimize c o s t s i n which c a s e a c o n c r e t e cover may be n e c e s s a r y . The w i d t h o f t r e n c h i s n o r m a l l y 0 . 5 m t o 0.8 m more t h a n t h e p i p e d i a m e t e r , w i t h l o c a l widening and d e e p e n i n g f o r j o i n t s .
(0.8 m i s used f o r p i p e s o v e r
300 mm d i a m e t e r which a r e d i f f i c u l t t o s t r a d d l e w h i l e l a y i n g and j o i n t i n g ) . The s i d e s of t h e open t r e n c h w i l l have t o be s u p p o r t e d i n uncohesive o r wet s o i l s and f o r deep t r e n c h e s , f o r s a f e t y r e a s o n s .
The s h o r i n g s h o u l d
be d e s i g n e d t o r e s i s t t h e a c t i v e l a t e r a l s o i l p r e s s u r e .
Alternatively the
s i d e s of t h e t r e n c h may be b a t t e r e d back t o a s a f e a n g l e . k e p t w e l l away from t h e e x c a v a t i o n t o avoid s l i p s . d e p t h away i s a d v i s a b l e .
S p o i l should be
A t l e a s t one t r e n c h
D r a i n s may be r e q u i r e d i n t h e bottom o f a t r e n c h
i n w e t ground t o keep t h e t r e n c h d r y w h i l e working, t o r e d u c e t h e p o s s i b i l i t y of embankment s l i p s and t o e n a b l e t h e b a c k f i l l t o be compacted p r o p e r l y . D r a i n p i p e s s h o u l d b e open j o i n t e d and bedded i n a s t o n e f i l t e r w i t h f r e q u e n t c r o s s - d r a i n s t o l e a d away w a t e r .
I n v e r y w e t ground c u t o f f d r a i n s be-
s i d e t h e t r e n c h may be n e c e s s a r y . P i p e s may be bedded on t h e f l a t t r e n c h bottom, b u t a p r o p e r bedding
i s p r e f e r a b l e t o minimize d e f l e c t i o n s of f l e x i b l e p i p e s o r t o r e d u c e damage
183
t o r i g i d p i p e s . The bed of t h e t r e n c h may be pre-shaped of t h e p i p e .
t o f i t the profile
I n rocky ground a bed of sandy m a t e r i a l may be b u i l t up and Beds of s a n d , g r a v e l o r crushed s t o n e up t o 20 mm i n s i z e ,
trimmed t o shape.
brought up to t h e haunches of t h e p i p e , a s e o f t e n u s e d , a s t h e y do n o t exh i b i t much s e t t l e m e n t and are e a s y to compact.
Concrete beds and haunches o r
s u r r o u n d s a r e sometimes used f o r low-pressure r i g i d p i p e s and sewers.
Bedding
s h o u l d be c o n t i n u o u s under j o i n t s b u t haunches should s t o p s h o r t o f j o i n t s t o f a c i l i t a t e r e p l a c i n g damaged p i p e s .
Fig. 1 1 . 1
shows t y p i c a l t y p e s of bedding.
( R e f e r t o Chapter 7 f o r bedding f a c t o r s a s s o c i a t e d w i t h d i f f e r e n t t y p e s of bedding). The b e s t method of b a c k f i l l i n g and compaction depends on t h e t y p e of s o i l . S o i l p r o p e r t i e s may be d e s c r i b e d i n terms of t h e p l a s t i c l i m i t ( P I ) , l i q u i d l i m i t (LL) and p l a s t i c i t y i n d e x ( P I
=
LL-PL).
For s o i l s w i t h low PI'S
f i l l may o f t e n be s a t i s f a c t o r i l y compacted merely by i n u n d a t i o n . LL s o i l s , dynamic compaction i s r e q u i r e d i . e .
f i l l densities
For h i g h
stampers o r v i b r a t o r s .
Back-
may be s p e c i f i e d i n terms of t h e P r o c t o r S t a n d a r d D e n s i t y .
For l a r g e p i p e s i t i s p r a c t i c e t o compact t h e bed w i t h a w i d t h of a t l e a s t h a l f t h e p i p e d i a m e t e r under t h e p i p e t o 95 t o 100 % P r o c t o r d e n s i t y .
The s o i l
f i l l around t h e p i p e s h o u l d be compacted i n 100 t o 150 mm l a y e r s t o 90 o r 95% P r o c t o r d e n s i t y up t o haunch l e v e l o r t o 3 / 4 of t h e d e p t h of t h e p i p e .
It may be
n e c e s s a r y to s t r u t f l e x i b l e p i p e i n s i d e t o p r e v e n t d i s t o r t i o n d u r i n g t h i s operation.
The f i l l around t h e t o p h a l f of t h e p i p e , up to 300 mm above t h e
crown s h o u l d be compacted t o 85 to 90% P r o c t o r d e n s i t y . For b a c k f i l l under r o a d s , t h e t o p l a y e r s may have t o be compacted t o 90-95% P r o c t o r d e n s i t y , w h i l e f o r runways 110% may be r e q u i r e d .
85% i s nor-
m a l l y s u f f i c i e n t i n open c o u n t r y and d e n s i t i e s a s low as 80% a r e o b t a i n e d i f c o n t r o l i s bad.
To avoid damage to p i p e o r c o a t i n g s , f i l l around t h e p i p e should n o t have s t o n e s i n i t .
S u r p l u s s p o i l may be s p r e a d o v e r t h e t r e n c h w i d t h t o
compensate f o r s e t t l e m e n t o r c a r t e d away.
R e i n s t a t e m e n t of t o p s o i l and
v e g e t a t i o n , p r o v i s i o n of l a n d d r a i n a g e , and f e n c i n g o f f of working a r e a s should be c o n s i d e r e d . THRUST BORES
To avoid e x c e s s i v e l y deep e x c a v a t i o n s , o r t o avoid d i s r u p t i n g t r a f f i c on a road o v e r a p i p e , t h e p i p e may be j a c k e d t h r o u g h t h e s o i l f o r s h o r t lengths.
A h o l e i s dug a t e a c h end of t h e l e n g t h t o be j a c k e d .
A jack is s e t
up i n one h o l e w i t h i t s b a c k a g a i n s t a t h r u s t b l o c k o r t i m b e r and t h e p i p e
Class A bedding z 0 4 OiM= 3 4
concrete 15Omm mt
CONCRETE ARCH
Class B bedding ME
1.9
Oenleh Cmpaeled Boch?ill
0 6 D
SHAPED SUBGRADE WITH GRANULAR FOUNMTION
Class C bedding M:
I 5
0 125H
- Lioh?ly
~~mpacie
SHAPEDI~ G B G R ~ D E
GRANULAR FOUNDATION
Class D bedding
FLAT SUBGRADE
FIG.
11.1
NOTE For cock or o?her Incompressible malar~als,thalranch should be amrexcovoled 0 minimum of 150mm ond rsfi1l.d w ! h grond~rmaleiirrl
Types o f b e d d i n g s .
set up on r a i l s i n f r o n t o f t h e j a c k .
M-bedding
factor
Small p i p e s ( l e s s t h a n 600 mm diam-
e t e r ) may have a s h a r p head o r s h o e f i t t e d t o t h e f r o n t t o e a s e t h e j a c k i n g l o a d and m a i n t a i n t h e d i r e c t i o n o f t h e p i p e , a s t h e s l i g h t e s t o b s t a c l e w i l l deviate t h e pipe.
Occasionally t h e outside of t h e pipe i s lubricated with
bentonite t o reduce f r i c t i o n . may s t i c k .
Pushing should never s t o p f o r long o r the pipe
With l a r g e r p i p e s t h e s o i l ahead of t h e p i p e i s dug o u t by a u g e r
o r hand and removed t h r o u g h t h e p i p e .
The p i p e i s t h e n pushed i n f u r t h e r ,
185
t h e j a c k r e t r a c t e d and a n o t h e r p i p e i n s e r t e d behind t h e p i p e s i n t h e b o r e and t h e p r o c e s s r e p e a t e d .
The j a c k i n g f o r c e r e q u i r e d t o push a p i p e w i t h a
shoe may be up t o 1 200 t o n s , b u t i f a n a u g e r i s used, a 100 t o n
j a c k may
suffice. PIPE BRIDGES P i p e s f r e q u e n t l y have t o span o v e r r i v e r s o r g o r g e s .
For s h o r t s p a n s ,
r i g i d j o i n t e d p i p e s may have s u f f i c i e n t s t r e n g t h t o s u p p o r t themselves p l u s the fluid.
For l a r g e r s p a n s , i t may be n e c e s s a r y t o s u p p o r t t h e p i p e on
t r u s s e s o r c o n c r e t e b r i d g e s , o r hang t h e p i p e from a n e x i s t i n g t r a f f i c bridge.
I f a t r u s s b r i d g e i s t o be c o n s t r u c t e d f o r t h e p i p e , t h e p i p e could
a c t a s t h e t e n s i o n member a t t h e bottom, o r t h e compression member a t t h e top.
An a t t r a c t i v e form of b r i d g e i s one with t h e p i p e s u p p o r t e d from sus-
pension cables.
Two c a b l e s a r e p r e f e r a b l e t o one, a s t h e y may be spaced
a p a r t and t h e hangers a t t a c h e d a t an a n g l e t o t h e v e r t i c a l p l a n e through the pipe.
This arrangement h e l p s t o s u p p o r t t h e p i p e a g a i n s t wind f o r c e s
and r e d u c e s wind v i b r a t i o n s .
The p i p e could a l s o be d e s i g n e d t o a c t a s an
a r c h , b u t a g a i n l a t e r a l s u p p o r t would be n e c e s s a r y - e i t h e r two p i p e s c o u l d be d e s i g n e d t o a c t t o g e t h e r w i t h c r o s s b r a c i n g , o r c a b l e s t a y s could be erected.
I n a l l c a s e s e x c e p t i f s u p p o r t e d on an independent b r i d g e , t h e p i p e
s h o u l d be r i g i d j o i n t e d ; p r e f e r a b l y welded s t e e l .
This means t h e r e w i l l
have t o be some form of e x p a n s i o n j o i n t a l o n g t h e p i p e t o p r e v e n t t h e r m a l s t r e s s e s developing. UNDERWATER PIPELINES
The l a y i n g of p i p e s underwater i s v e r y e x p e n s i v e , n e v e r t h e l e s s i t i s often inescapable. coming common.
I n a d d i t i o n t o r i v e r c r o s s i n g s , undersea l a y i n g i s be-
Gas and o i l l i n e s from u n d e r s e a beds o r o f f - s h o r e t a n k e r
b e r t h s are f r e q u e n t .
There a r e a l s o many i n d u s t r i e s and towns which d i s -
c h a r g e e f f l u e n t o r s l u d g e i n t o t h e sea t h r o u g h u n d e r s e a p i p e s .
The method
used f o r underwater l a y i n g w i l l depend on c o n d i t i o n s such as c u r r e n t s , wave h e i g h t , t y p e of bed and l e n g t h of under-water p i p e : (1)
F l o a t i n g and S i n k i n g :
I n calm w a t e r t h e p i p e may be j o i n t e d on s h o r e ,
f l o a t e d o u t and sunk.
Lengths up t o 500 m have been l a i d t h u s .
Care
i s needed i n s i n k i n g t h e p i p e and winches on b a r g e s should be p o s i tioned along the l i n e t o assist.
Lengths of p i p e up t o a few k i l o m e t r e s may be assembled
Bottom Towing:
on l a n d i n s t r i n g s , e a c h up t o 1 km l o n g , and towed a l o n g t h e bed of t h e s e a by a winch on a b a r g e anchored a t sea.
S t r i n g s are j o i n t e d to-
The p i p e i s bouyed up by a i r i n t h e p i p e o r by
g e t h e r on t h e s h o r e .
buoys t i e d t o t h e p i p e , t o r e d u c e f r i c t i o n on t h e sea bed.
I f t h e pipe
i s f i l l e d w i t h a i r i t may have t o be weighted down by a c o n c r e t e c o a t i n g to g e t t h e c o r r e c t buoyancy.
Too l i g h t a p i p e would b e s u s c e p t i b l e
t o s i d e c u r r e n t s and may bend out of l i n e .
The s t r e s s e s i n t h e p i p e due
t o t h e t e n s i o n and bending caused by underwater c u r r e n t s o r waves may be c r i t i c a l .
The l a t e r a l d r a g p e r u n i t l e n g t h due t o waves and c u r r e n t s
is CDw ( u + v ) ~D/2g
(11.1)
where CD i s t h e d r a g c o e f f i c i e n t (about 0.9),
w i s t h e s p e c i f i c weight
of t h e w a t e r , u i s t h e wave v e l o c i t y and v i s t h e c u r r e n t v e l o c i t y ( o r t h e i r components p e r p e n d i c u l a r t o t h e p i p e ) , D i s t h e o u t s i d e d i a m e t e r of t h e p i p e and g i s g r a v i t a t i o n a l a c c e l e r a t i o n . Lay Barge:
P i p e s may b e t a k e n t o s e a i n a p i p e b a r g e , j o i n t e d on a
s p e c i a l lay-barge and lowered c o n t i n u o u s l y down an arm ( o r s t r i n g e r ) i n t o place.
The b a r g e s proceed forward a s l a y i n g p r o c e e d s .
Underwater J o i n t i n g :
Underwater j o i n t i n g i s d i f f i c u l t a s e a c h p i p e
has t o be accurately aligned.
A d i v e r w i l l be r e q u i r e d , which l i m i t s
t h e method t o s h a l l o w d e p t h s . I t i s u s u a l l y n e c e s s a r y t o bury t h e p i p e t o avoid damage due t o cur-
Dredging i s d i f f i c u l t as sediment may f a l l o r be washed i n t o t h e
rents.
trench before the pipe i s laid.
The t r e n c h normally h a s t o be overexcavated
A n o v e l way o f l a y i n g t h e p i p e i n s o f t beds i s w i t h a bed f l u i d i z e r (Ref.
11.7).
High-pressure
j e t s of w a t e r f l u i d i z e t h e bed under t h e p i p e s and t h e
p i p e s i n k s i n t o t h e bed.
A number of p a s s e s of t h e f l u i d i z e r a r e r e q u i r e d s o
t h a t t h e pipe s i n k s gradually over a long length.
Too deep a t r e n c h may
c a u s e bending s t r e s s e s i n t h e p i p e a t t h e head of t h e t r e n c h . Most underwater p i p e s are s t e e l w i t h welded j o i n t s .
P i p e s and j o i n t s
s h o u l d b e w e l l c o a t e d and l i n e d and c a t h o d i c p r o t e c t i o n i s d e s i r a b l e under t h e sea. JOINTS AND FLANGES
The t y p e of j o i n t t o u s e on a p i p e l i n e w i l l depend on t h e t y p e of p i p e ,
197
the facilities available on site, cost, watertightness of joint, strength and flexibility requirements. The following types of joints are used:(1)
Butt- welded: The ends of the steel pipe are trued and bevelled approximately 30'
back, leaving a 2 mm root.
The joint is clamped and/
or tacked in places and then welded right around with one or more runs. Large pipes with thick walls may a l s o require a weld pass inside, although it is difficult to make good the internal lining in pipes less than 600 mm bore. (2)
Sleeve welded :
One end of a steel pipe may be flared out to form a
socket and the leading edge fillet welded to the barrel of the other pipe.
If possible the joint should be welded inside too.
11.2).
(see Fig.
The external pipe coating and the internal lining, if possible,
should be made good at the weld after all slag has been wire-brushed off. Two methods of welding are available: and metallic arc welding.
Oxy-acetylene gas welding
Top-class welding is essential to
achieve high weld strength and avoid cavities.
A thorough testing
programme should accompany the welding. Destructive testing should be confined where possible to the factory and to sample welds on short lengths of pipe but occasional field destructive tests may be necessary.
Strips including the weld are cut from the pipe wall and bent
over a predetermined radius.
The weld is ground and etched to reveal
air pockets or slag inclusions. Normally field tests will be nondestructive: either by X-ray, Gamma-ray, magnetic or ultra-sonic methods.
X-rays are preferred for high definition and contrast but
Gamma-ray equipment is more portable and useful for otherwise inaccessible corners. With large-bore pipes, the X-ray equipment may be inserted in the pipe and a film is wrapped around the pipe to receive the image from a single wall.
For smaller pipes, double, over-
lapping exposures are made from outside, thereby exaggerating defects in the near wall. Welding of high tensile steels require better craftmanship than for low-tensile steels. Pre-heating of the pipe ends, and post-heating for stress relief are normally desirable. (3)
Screwed : Threaded and screwed ends and sockets are only used on small steel pipes.
(4)
Spigot and Socket :
Concrete, cast iron and plastic pipes frequently
have one end socketed and the other end plain.
The male end has a
1 8 6
FIG.
11.2
FIG. 11.3
Sleeve-welded
joint
Push-in t y p e j o i n t .
r u b b e r s e a l i n g g a s k e t f i t t e d o v e r i t and t h e s o c k e t i s f o r c e d o v e r the ring.
(see Fig.ll.3).
Various shaped r u b b e r r i n g s are used.
An-
o t h e r way o f s e a l i n g t h e j o i n t i s t o c a u l k it w i t h bitumen compound, cement m o r t a r , l e a d o r r e s i n .
J o i n t s f i t t e d with rubber r i n g s a r e
o f t e n c a u l k e d a s w e l l t o hold t h e r i n g i n p l a c e , improve s e a l i n g and prevent d i r t g e t t i n g i n t o the j o i n t .
S p i g o t and s o c k e t j o i n t s w i t h
r u b b e r r i n g s can normally accommodate one o r two d e g r e e s of d e f l e c t i o n .
(5)
Clamp-on j o i n t s : p l a i n ended p i p e s .
Various p r o p r i e t r y j o i n t s a r e a v a i l a b l e f o r j o i n t i n g They normally i n v o l v e clamping a r u b b e r s e a l between
e a c h p i p e b a r r e l and a c o v e r s l e e v e .
F i g . 11.4 i l l u s t r a t e s one such
t y p e of j o i n t ( a l s o r e f e r r e d t o a s a Viking Johnson o r D r e s s e r coupling).
These j o i n t s accommodate movement e a s i l y , and may be designed
t o t a k e s e v e r a l d e g r e e s of d e f l e c t i o n between p i p e s o r l o n g i t u d i n a l movement.
I f t h e j o i n t i s t o r e s i s t t e n s i o n , t i e b o l t s may be f i x e d
189
FIG. 11.4
Slip-on type coupling.
to the barrel of each pipe by brackets to take the tension.
The
brackets cause local bending and tension stresses in the walls of the pipe and may cause damage unless carefully designed.
Normally four or
even two tie bolts are sufficient. The diameter of the bolts is selected to take the tension and the brackets designed so that the bolts are as close as possible to the barrel to minimize eccentricity. The brackets are normally "U" shaped in plan with the legs welded parallel to the pipe axis.
The length of the legs of the bracket
should be such that longitudinal shear and bending stresses are acceptable.
In fact it is impossible to eliminate all bending stresses on the
pipe barrel with brackets and for this reason rings, like flanges, may be preferable for transferring the tie-bolt load t o the pipe wall. The Victaulic coupling used for cast iron pipes, uses lips on the ends of the pipes to transfer tension to the coupling. (6)
Flanged :
Steel and cast iron pipes are often flanged at the ends,
especially if pipes or fittings are likely to be removed frequently. Faces are machined and bolted together with 3 mm rubber or other insertion gasket between.
Flanges are drilled to standard patterns
according to the diameter and working pressure. F u l l face gaskets, with holes for bolts and the bore, are used
f o r high-pressure joints and cast iron or soft metal flanges.
Joint
rings with an outside diameter slightly less than the inside diameter of the bolt holes are used for thick steel flanges and low-pressure pipes. Some flanges have a raised face inside the bolt circle for use with
joint rings.
Flanges used w i t h j o i n t r i n g s t e n d t o d i s h when t h e
b o l t s a r e t i g h t e n e d though. the pipe v a r i e s .
The method of a t t a c h i n g t h e f l a n g e s t o
The s i m p l e s t method i s t o s l i p t h e f l a n g e over t h e
p i p e b u t t o l e a v e approximately 10 mm e x t e n d i n g beyond t h e end of t h e pipe.
A f i l l e t weld i s t h e n c a r r i e d around t h e f r o n t of t h e p i p e and
a t t h e back of t h e f l a n g e around t h e p i p e b a r r e l .
Alternatively both
o r one of t h e i n s i d e edges of t h e f l a n g e may be b e v e l l e d f o r welding.
A method p r e f e r r e d f o r h i g h p r e s s u r e work i s t o have a l i p on t h e f l a n g e p r o j e c t i n g o v e r t h e end o f t h e p i p e s o t h a t t h e b o r e a t t h e j o i n t w i l l be f l u s h a f t e r welding. J o i n t i n g and t h r u s t f l a n g e s a r e f a i r l y t h i c k , as i n d i c a t e d by t h e s t a n d a r d codes of p r a c t i c e , and must be s e c u r e l y welded t o t h e p i p e o r c a s t integrally.
Two o t h e r t y p e s of f l a n g e s a r e common:-
Puddle f l a n g e s are used on p i p e s which are c a s t i n w a l l s o f w a t e r retaining structures.
The o b j e c t of t h e s e f l a n g e s i s t o p r e v e n t leak-
age of w a t e r and t h e y are n o t n e c e s s a r i l y a s t h i c k a s f l a n g e s which
are d e s i g n e d t o t r a n s f e r l o n g i t u d i n a l p i p e s t r e s s , a l t h o u g h t h e f l a n g e should be s e c u r e l y a t t a c h e d t o t h e p i p e w a l l . Blank f l a n g e s a r e , a s t h e name i m p l i e s , i n s t a l l e d a t b l a n k e n d s . T h e i r t h i c k n e s s s h o u l d be c a l c u l a t e d t o r e s i s t bending moments a t t h e edges and c e n t r e .
For a c i r c u l a r d i s c r i g i d i t y clamped a t t h e edges
t h e maximum r a d i a l bending s t r e s s o c c u r s a t t h e p e r i m e t e r b o l t s , and
i s e q u a l t o 3 p d 2 / 1 6 t z where p i s t h e f l u i d p r e s s u r e , d i s t h e p i t c h c i r c l e d i a m e t e r and t i s t h e f l a n g e t h i c k n e s s .
I f t h e edges were n o t
r i g i d l y clamped t h e maximum s t r e s s would o c c u r a t t h e c e n t r e and would be 50% i n e x c e s s of t h i s amount. COATINGS
Buried s t e e l p i p e s are s u b j e c t t o c o r r o s i o n and damage u n l e s s t h e pipe is coated.
Coatings s h o u l d i d e a l l y b e r e s i s t a n t t o s c r a t c h i n g d u r i n g
t r a n s p o r t and l a y i n g of t h e p i p e , t o m o i s t u r e , chemical and b i o l o g i c a l a t t a c k , e l e c t r i c c u r r e n t s and t e m p e r a t u r e v a r i a t i o n s .
They s h o u l d be hard
enough t o p r e v e n t damage d u r i n g h a n d l i n g and due t o s t o n e s i n t h e t r e n c h , y e t s u f f i c i e n t l y a d h e s i v e t o a d h e r e w e l l t o t h e p i p e w a l l and f l e x i b l e enough t o w i t h s t a n d t h e f l e x i n g of t h e p i p e w a l l . The most common c o a t i n g f o r p i p e s i s a t h i n a d h e s i v e c o a t followed by a c o a t i n g r e i n f o r c e d w i t h f i b r e s and t h e n p o s s i b l y a n o u t e r wrapping.
1 9 1
The p i p e s u r f a c e i s i n i t i a l l y c l e a n e d by w i r e b r u s h i n g , s a n d b l a s t i n g o r a c i d p i c k l i n g , and t h e prime c o a t i s t h e n a p p l i e d by s p r a y , b r u s h o r d i p p i n g Bitumen o r c o a l t a r enamel i s t h e p r e f e r r e d prime c o a t -
the pipe i n a bath. ing.
A f t e r t h e primary c o a t t h e p i p e may be s p i r a l l y wrapped w i t h impregT h i s i s sometimes followed by
n a t e d f e l t o r woven g l a s s f i b r e m a t t i n g .
p a p e r o r a s b e s t o s f e l t impregnated w i t h bitumen o r c o a l t a r .
The p i p e i s
t h e n whitewashed t o a s s i s t i n d e t e c t i n g damage and t o s h i e l d t h e c o a t i n g from t h e s u n .
The c o a t i n g may be a p p l i e d i n t h e f i e l d a f t e r welding t h e
p i p e j o i n t s , o r i n t h e f a c t o r y , i n which c a s e t h e ends a r e l e f t b a r e f o r j o i n t i n g and c o a t e d i n t h e f i e l d .
The t o t a l t h i c k n e s s o f c o a t i n g should be
a t l e a s t 5 mm.
Other t y p e s of c o a t i n g s i n c l u d e an a s b e s t o s f i b r e bitumen m a s t i c 3 t o
6 mm t h i c k , c o a l t a r p i t c h , epoxy p a i n t s , PVC o r p o l y t h e n e t a p e s ( e i t h e r s e l f a d h e s i v e o r bedded on an a d h e s i v e ) , r e s i n s o r p l a s t i c s , cement m o r t a r and z i n c a p p l i e d by g a l v a n i z i n g .
Exposed p i p e s may be primed and p a i n t e d
w i t h bitumen based aluminium o r enamel. Cement m o r t a r c o a t i n g s o f f e r a d d i t i o n a l r e s i s t a n c e to b u c k l i n g i n t h e c a s e of l a r g e b o r e t h i n w a l l e d p i p e s . u s u a l l y 1 2 t o 20 m
(4
Cement m o r t a r c o a t i n g s a r e
t o 3 / 4 inch) thick.
F i n i s h e d c o a t i n g s may be checked f o r f l a w s , p i n h o l e s e t c . by means of a Holiday d e t e c t o r .
An e l e c t r i c a l conductor i n t h e form o f m e t a l b r u s h e s
o r r o l l i n g springs i s run along t h e pipe coating.
An e l e c t r i c a l p o t e n t i a l
i s a p p l i e d a c r o s s t h e c o a t i n g and a c u r r e n t i s observed when f l a w s i n t h e coating are detected. LININGS S t e e l p i p e s a r e l i n e d t o r e s i s t i n t e r n a l c o r r o s i o n and minimize t h e
f r i c t i o n losses. in the fluid.
Unlined s t e e l p i p e may be o x i d i z e d by c o r r o s i v e s u b s t a n c e s
I n t h e c a s e of s o l i d s - c o n v e y i n g systems i n p a r t i c u l a r , t h e
oxide is r a p i d l y scraped o f f leading t o f u r t h e r corrosion.
C o r r o s i o n of
water-conveying p i p e s may be i n h i b i t e d by m a i n t a i n i n g a h i g h pH e . g . by a d d i n g l i m e . L i m e on t h e o t h e r hand could c a u s e c a r b o n a t e s c a l i n g . The.most p o p u l a r l i n i n g s are bitumen ( 3 t o 5 mm t h i c k ) o r c o a l t a r enamel (2 t o 3 mm t h i c k ) .
Bitumen, which i s a by-product of petroleum, i s
t h e cheaper o f t h e two. Before a p p l y i n g t h e l i n i n g t h e p i p e w a l l i s c l e a n e d by s a n d b l a s t i n g o r o t h e r methods and t h e l i n i n g i s t h e n a p p l i e d by b r u s h , s p r a y , d i p p i n g o r s p i n n i n g t o o b t a i n a smooth s u r f a c e .
Spun enamel i n p a r t i c u l a r p r o v i d e s
a smooth f i n i s h .
Coal t a r enamel i s a l s o more r e s i s t a n t t o m o i s t u r e t h a n
bitumen, a l t h o u g h i t i s more b r i t t l e and c o n s e q u e n t l y s u b j e c t t o damage by impact and f l e x i n g of t h e p i p e .
P l a s t i c i s e d c o a l t a r enamels have, however,
now been developed t o overcome t h e problems of b r i t t l e n e s s . Epoxy p a i n t s a r e a l s o used s u c c e s s f u l l y f o r l i n i n g s , a l t h o u g h c a r e f u l a p p l i c a t i o n i s necessary t o ensure successive coats adhere t o each other. The recommended t h i c k n e s s of t h e l i n i n g v a r i e s w i t h t h e t y p e of p a i n t , b u t
i t i s normally of t h e o r d e r of 0 . 3 mm a p p l i e d i n 2 t o 4 l a y e r s .
Coal t a r
epoxy l i n i n g s s h o u l d be avoided f o r p o t a b l e w a t e r p i p e s a s t h e y t a i n t t h e Lead based p r i m e r s s h o u l d a l s o be avoided as t h e y a r e t o x i c .
water.
Cement m o r t a r l i n i n g , a p p l i e d c e n t r i f u g a l l y by s p i n n i n g t h e p i p e , i s a l s o used f o r l a r g e b o r e p i p e s . (1/4 to
4
inch) t h i c k .
pipes i n t h e f i e l d .
The a p p l i e d l i n i n g i s u s u a l l y 6 t o 12 mm
Mortar l i n i n g s have been a p p l i e d s u c c e s s f u l l y t o
The o l d s u r f a c e i s f i r s t t h o r o u g h l y c l e a n e d by wire-
b r u s h i n g o r sand b a l s t i n g , and t h e n c o a t e d c e n t r i f u g a l l y by a machine drawn o r propelled slowly along t h e l i n e . On p i p e s o v e r about 600 mm b o r e , most f a c t o r y l i n i n g s may be made good a t f i e l d j o i n t s manually o r by mechanical a p p l i c a t o r s . CATHODIC PROTECTION
D e s p i t e t h e u s e of p r o t e c t i v e c o a t i n g s , c o r r o s i o n of t h e w a l l s of s t e e l p i p e s o f t e n o c c u r s t h r o u g h f l a w s , pin-holes
o r a t exposed f i t t i n g s .
C o r r o s i o n i s due p r i m a r i l y t o two c a u s e s ; g a l v a n i c c o r r o s i o n and s t r a y current electrolysis. Galvanic Corrosion When two d i s s i m i l a r materials a r e connected t h r o u g h an e l e c t r o l y t e , c u r r e n t may f l o w from one m a t e r i a l t o t h e o t h e r .
The r e s u l t i n g e l e c t r i c cur-
r e n t f l o w s from t h e anode t o t h e c a t h o d e t h r o u g h t h e e l e c t r o l y t e . o r i o n s , l e a v e t h e anode c a u s i n g c o r r o s i o n .
Particles
The c a t h o d e i s n o t a t t a c k e d .
Such a c t i o n may t a k e p l a c e when two d i s s i m i l a r m e t a l s a r e i n c o n t a c t i n conductive s o i l ( f o r instance f i t t i n g s of a d i f f e r e n t m a t e r i a l t o t h e pipe).
Another more f r e q u e n t form o f g a l v a n i c c o r r o s i o n o c c u r s w i t h p i p e s
i n corrosive soils.
The e f f e c t i s p a r t i c u l a r l y marked i n s o i l
with varying
c h a r a c t e r i s t i c s , d i f f e r e n t i a l oxygen c o n c e n t r a t i o n s o r i n w a t e r w i t h h i g h chemical c o n t e n t , e s p e c i a l l y s u l p h u r .
C o r r o s i o n i s a l s o caused by bio-
chemical a c t i o n i n t h e s o i l which i s a t y p e of g a l v a n i c a c t i o n r e s u l t i n g from b a c t e r i a .
A method which has been used s u c c e s s f u l l y t o c o u n t e r a c t
193
s o i l c o r r o s i o n was t o add l i m e t o t h e t r e n c h b a c k f i l l .
Stress c o n c e n t r a t i o n
i n t h e s t e e l i n a n e l e c t r o l y t e may a l s o l e a d t o c o r r o s i o n .
The amount of
c o r r o s i o n due t o t h e l a s t named two i n f l u e n c e s i s u s u a l l y s m a l l . P o t e n t i a l c o r r o s i v e a r e a s may be d e t e c t e d by s o i l r e s i s t a n c e t e s t s , p i p e - s o i l p o t e n t i a l t e s t s o r measurement of t h e c u r r e n t i n t h e p i p e . I f t h e s o i l i s a t a l l s u s p e c t , o r a s a s t a n d a r d p r a c t i c e f o r major p i p e l i n e , a s o i l r e s i s t i v i t y s u r v e y should be conducted. S o i l
i s measured i n ohm - c e n t i m e t r e s .
resistivity
Readings a r e normally t a k e n i n - s i t u u s i n g
a b r i d g e c i r c u i t o r by measuring a c u r r e n t and t h e a s s o c i a t e d v o l t drop. S t a n d a r d p r o b e s are a v a i l a b l e f o r t h e s e measurements, b u t any major s u r v e y s s h o u l d be done by a n e x p e r i e n c e d c o r r o s i o n e n g i n e e r . A h i g h l y c o n d u c t i v e s o i l may have a r e s i s t i v i t y of 500 ohm cm, and a
p o o r l y c o n d u c t i v e s o i l , more t h a n 10 000 ohm cm. The p o t e n t i a l d i f f e r e n c e between a b u r i e d p i p e and t h e s o i l i s important i n evaluating corrosive conditions.
The p o t e n t i a l i s measured by
c o n n e c t i n g a v o l t m e t e r between t h e p i p e and a s p e c i a l e l e c t r o d e i n c o n t a c t with the s o i l .
A copper s u l p h a t e h a l f c e l l i s f r e q u e n t l y used a s t h e e l e c -
t r o d e under normal c o n d i t i o n s .
The p o t e n t i a l of a p i p e i s 0.5 t o 0 . 7 v o l t s
below t h a t of t h e s u r r o u n d i n g s o i l .
I f t h e pipe is a t a higher voltage than
0.85 v o l t s below t h a t of t h e s o i l , c u r r e n t s a r e l i k e l y t o f l o w from p i p e t o s o i l , thereby corroding t h e pipe. To p r e v e n t t h i s c o r r o s i o n , a s a c r i f i c i a l anode may be connected by a conductor t o t h e p i p e i n t h e v i c i n i t y o f p o s s i b l e c o r r o s i o n
(Fig. 11.5).
The s a c r i f i c i a l anode i s b u r i e d i n a c o n d u c t i v e s u r r o u n d , p r e f e r a b l y below t h e w a t e r t a b l e and c u r r e n t s w i l l t e n d t o l e a v e from t h e anode i n s t e a d of t h e p i p e , t h e r e b y l i m i t i n g t h e c o r r o s i o n t o t h e anode.
I f t h e anode i s s u f -
f i c i e n t l y d i s s i m i l a r from t h e s t e e l p i p e t o cause g a l v a n i c a c t i o n no e x t e r n a l e l e c t r i c a l p o t e n t i a l need be a p p l i e d .
I n f a c t , a r e s i s t o r may sometimes
be i n s t a l l e d i n t h e c o n n e c t i o n t o l i m i t t h e c u r r e n t .
Common s a c r i f i c i a l
anodes are magnesium, z i n c and aluminium, o r a l l o y s of t h e s e m e t a l s .
Mag-
nesium h a s t h e g r e a t e s t p o t e n t i a l d i f f e r e n c e from i r o n , has a h i g h e l e c t r o chemical e q u i v a l e n t (ampere h o u r s p e r k i l o g r a m of m a t e r i a l ) and i s f a i r l y r e s i s t a n t t o anodic p o l a r i z a t i o n .
( A l a y e r of hydrogen i o n s may r e p l a c e t h e
i o n s l e a v i n g t h e anodes, t h e r e b y a c t i v e l y i n s u l a t i n g t h e m e t a l a g a i n s t further attack.
This i s termed p o l a r i z a t i o n ) .
Magnesium anodes g i v e 200 t o
1 200 ampere h o u r s p e r kg. Magnesium anodes are normally d e s i g n e d f o r about a 10 y e a r l i f e b u t z i n c anodes o f t e n l a s t 20 o r 30 y e a r s .
Junclion
Box W i t h
Reslitor i f Required
J
FIG.
11.5
Insulated Sinpl. Core Cobla
Sacrificial anode installation for cathodic protection.
The spacing and size of anodes will be determined by the current requirements to bring the pipe to a safe potential (at least 0.85 volts below the soil potential along the entire length of the pipe).
The spacing of
sacrificial anodes will vary from 3 m in poorly conductive soil and poorly coated pipe to 30 m in highly conductive soil provided the pipe is well coated.
The most reliable way of estimating the required current is to
actually drain current from the pipe by means of a ground bed and d.c. source and measure the resulting pipe to soil potential along the pipe. A s a rule of thumb, the current required to protect a pipe against
corrosion is i
=
10/r per square metre of bared pipe surface, where i is
the current in amps, r is the soil resistance in ohm cm and a factor of safety of 3 is incorporated.
The area of exposed pipe may be as low as 0.5
per cent for a good coating, rising to 20 per cent for a poor coating. Once the required draw-off current per km of pipe is known, the total anode size may be calculated. I n highly conductive soils (less than 500 ohm cm) large anodes (20 kg) may be used but in soils with lower conductivities (greater than about 1 000 ohm cm) a number of smaller anodes should be used to ensure an adequate current output. If an electricity supply is available, it is usually cheaper to use an impressed current type of protection (Fig. 11.6) instead of a sacrificial anode.
A transformer-rectifier may be installed to provide the necessary dc
current. With impressed current installations, the anode need not be selfcorrosive.
Scrap iron, or graphite rods buried in coke fill, are frequently
195
used.
S t e e l anodes w i l l q u i c k l y c o r r o d e i n h i g h l y c o n d u c t i v e s o i l s and f o r
t h i s r e a s o n h i g h s i l i c o n c a s t i r o n anodes are p r e f e r r e d . The t y p e of p r o t e c t i o n t o u s e w i l l depend on t h e long-term economics, Impressed c u r r e n t i n s t a l l a t i o n s are f r e q u e n t l y t h e c h e a p e s t i n t h e l o n g r u n ,
as m a i n t a i n a n c e c o s t s are low and fewer i n s t a l l a t i o n s a r e r e q u i r e d t h a n f o r s a c r i f i c i a l anodes.
I n t h e c a s e of impressed c u r r e n t i n s t a l l a t i o n s a r e l a t i v e -
l y h i g h v o l t a g e may be a p p l i e d , which i n t u r n p r o t e c t s a l o n g e r l e n g t h of p i p e t h a n a s a c r i f i c i a l anode c o u l d .
The p i p e - s o i l p o t e n t i a l f a l l s o f f r a p i d -
l y away from t h e a p p l i e d v o l t a g e though, c o n s e q u e n t l y a h i g h v o I t a g e may have
t o be applied t o p r o t e c t a long length.
The p i p e - t o - s o i l
potential i n the
immediate v i c i n i t y of t h e p o i n t should n o t , i n g e n e r a l exceed 3 v o l t s . p i p e p o t e n t i a l b e i n g below t h a t of t h e s o i l ) . coatings.
(the
L a r g e r v o l t a g e s may damage
Impressed c u r r e n t i n s t a l l a t i o n s may be a t 1 t o 100 km s p a c i n g de-
pending on t h e q u a l i t y of t h e p i p e c o a t i n g .
Impressed c u r r e n t i n s t a l l a t i o n s
a r e p r e f e r a b l e t o s a c r i f i c i a l anodes i f t h e s o i l r e s i s t i v i t y i s h i g h e r t h a n about 3 000 t o 5 000 ohm cm. C o n s i d e r a b l e economic s a v i n g s a r e o f t e n achieved by p r o t e c t i n g o n l y "hot s p o t s " , o r p o i n t s downs can be t o l e r a t e d
subject
t o a g g r e s s i v e a t t a c k , and i f o c c a s i o n a l s h u t -
t h i s should b e c o n s i d e r e d .
S t r a y Current E l e c t r o l y s i s
The most s e v e r e form of c o r r o s i o n i s o f t e n caused by s t r a y d c c u r r e n t s leaving a pipe.
Railways and o t h e r u s e r s of dc c u r r e n t r e t u r n c u r r e n t s
t h r o u g h r a i l s , b u t i f t h e r e i s a n o t h e r conductor such a s a s t e e l p i p e nearby, a p r o p o r t i o n of t h e c u r r e n t may f l o w t h r o u g h t h e conductor i n s t e a d of the r a i l .
Where t h e c u r r e n t l e a v e s t h e p i p e , s t e e l w i l l be corroded a t a
Eurrd or Submerged Slrvclure Mad. Cathodic
Iron or prophila)
FIG.
11. 6
Impressed c u r r e n t c o r r o s i o n p r o t e c t i o n .
1 9 6
r a t e of 9 kg ( 2 0 l b s ) p e r y e a r p e r ampere of c u r r e n t .
The c u r r e n t may be
d e t e c t e d by a c t u a l c u r r e n t measurements o r from p i p e / s o i l p o t e n t i a l measurements.
Continuous r e c o r d i n g s should be t a k e n o v e r a d a y , a s t h e c u r r e n t s
may f l u c t u a t e w i t h t i m e . The c o r r o s i o n a s s o c i a t e d w i t h s t r a y c u r r e n t s may be p r e v e n t e d by conn e c t i n g t h e p i p e a t t h e p o i n t where t h e c u r r e n t l e a v e s i t , t o t h e d e s t i n a t i o n of t h e c u r r e n t , w i t h a c o n d u c t o r .
The c u r r e n t may l e a v e t h e p i p e a l o n g
some l e n g t h , i n which c a s e a c u r r e n t would have t o be impressed on t h e p i p e t o m a i n t a i n t h e v o l t a g e s u f f i c i e n t l y low t o p r e v e n t c u r r e n t e s c a p i n g .
A
ground bed and t r a n s f o r m e r r e c t i f i e r may b e r e q u i r e d f o r t h i s p r o t e c t i o n . When a p i p e i s c a t h o d i c a l l y p r o t e c t e d , c a r e s h o u l d b e t a k e n t h a t mecha n i c a l j o i n t s a r e e l e c t r i c a l l y bonded, by welding a c a b l e a c r o s s them i f necessary.
Branch p i p e s may have t o be i n s u l a t e d from t h e main p i p e t o con-
t r o l the currents. THERMAL INSULATION
F l u i d i n a p i p e l i n e may o f t e n be h e a t e d o r c o o l e d by t h e s u r r o u n d i n g s . I c e f o r m a t i o n i n a r c t i c c l i m a t e s and h e a t i n g i n t r o p i c a l c l i m a t e s a r e somet i m e s a s e r i o u s problem.
Heat i s t r a n s f e r r e d t o o r from t h e i n t e r i o r of an
exposed p i p e i n a number of ways: by c o n d u c t i o n t h r o u g h t h e p i p e w a l l and wrapping and a boundary l a y e r of f l u i d i n s i d e t h e p i p e w a l l , by r a d i a t i o n from t h e e x t e r n a l f a c e of t h e p i p e and by c o n v e c t i o n and wind c u r r e n t s i n t h e air surrounding t h e pipe. The h e a t l o s s by c o n d u c t i o n t h r o u g h an homogeneous p i p e w a l l i s prop o r t i o n a l t o t h e t e m p e r a t u r e g r a d i e n t a c r o s s t h e w a l l and t h e h e a t t r a n s f e r r e d p e r u n i t a r e a of p i p e w a l l p e r u n i t t i m e i s
3 =& AT t
(1 1.2)
where Q i s t h e amount of h e a t conducted i n k i l o c a l o r i e s o r Btu, A i s t h e a r e a of p i p e s u r f a c e , T i s t h e d u r a t i o n of t i m e , A 0
i s t h e temperature d i f -
f e r e n c e a c r o s s t h e w a l l , t i s t h e w a l l t h i c k n e s s and k i s t h e t h e r m a l cond u c t i v i t y , t a b u l a t e d i n Table 1 1 . 1 f o r v a r i o u s m a t e r i a l s :
197
TABLE 1 1 . 1
THERMAL C O N D U C T I V I T I E S
Thermal c o n d u c t i v i t y k cal Btu i n
Material
m sec
c0
s q f t h r Fo
0.000 14
Water
4
Air
0.000 005
0.15
Steel
0.014
420
Bituminized Wrapping
0.000 01
0.3
Concrete
0.0002
S l a g wool
0.000 01 ~
~
6 0.3
~~
An e q u a t i o n w a s developed by Riddick (Ref. 1 1 .16) t r a n s f e r t h r o u g h t h e w a l l of a p i p e conveying w a t e r .
f o r the t o t a l heat
The e q u a t i o n h a s been
modified t o a g r e e w i t h r e l a t i o n s h i p s i n d i c a t e d by EEUA (Ref. 11.17) and i s converted t o metric u n i t s here: Rate of h e a t l o s s of w a t e r k c a l / k g / s e c -
-
(0; -0
0 ) I 250D t l + t 2 + 1 + 1
- - - _ _ _ k~ k2 Kf Kr+Kc
(1 1.3)
and s i n c e t h e s p e c i f i c h e a t of w a t e r i s u n i t y , t h e r a t e of drop i n temperat u r e i n degrees
Centigrade per sec equals t h e heat l o s s i n kcal/kg/sec,
t e m p e r a t u r e of w a t e r
CO
ambient t e m p e r a t u r e o u t s i d e Co diameter m thicknes.s of p i p e m c o n d u c t i v i t y of p i p e w a l l (kcal/m sec Co) t h i c k n e s s of c o a t i n g m c o n d u c t i v i t y of c o a t i n g kcal/m s e c Co 0.8 0.2 h e a t l o s s t h r o u g h boundary l a y e r = 0 . 3 4 ( 1 + 0 . 0 2 0 ; ) ~ /D kcal/m2 s e c Co water v e l o c i t y m/s h e a t l o s s by r a d i a t i o n = 0.054 E ( kcal/m2 sec Co
00 + 2 7 3 3 )
too
106
E
=
emissivity factor
=
0.9 for black asphalt and concrete 0.7 for cast iron and steel 0.4 for aluminimum paint
KC
=
heat l o s s by convection
V
=
wind velocity km/hr
0.
1 - ‘0 0.25 ( 7 )
0.000 34J1+0.78V
=
kcal/m2 sec Co
The heat generation by fluid friction is usually negligible although it theoretically increases the temperature of the fluid. Example An uncoated 300 mm diameter pipeline 1 000 m long with 5 mm thick
steel walls, exposed to a 10 km/hr wind, conveys 100 k?/s of water at an initial temperature of 10°C.
The air temperature is 3OoC.
Determine
the end temperature o f the water.
E
=
0.7
v
=
0.1/0.785 x 0.32
Kf
=
Kr
=
Kc
=
0.3411 + 0.78 x 10T(30
-
o .005 0.014
-t l _ kl
A0/sec
=
1.42 m/s
0.34 x 1.42°’8/0.30’2 0.054 x 0.7 ( 30
=
=
+
=
273)3
0.67 10-3 =
-
10) 0.25
o.ool ,0-3
o.oo29
=
0.35
(30 - 10)/(250 x 0.3) 0.35 + 110.67 + (0.001 + 0.0029 )
Temperature rise over 1 000 m
=
x 0.00103
=
0.00103~~1sec =
0.7OC
1.42 The insulation of industrial pipework carrying high or low temperature fluids is a subject on its own.
The cost of the heat transfer should
be balanced against the cost of the lagging.
Heat transfer to or from buried pipelines depends on the temperature of the surroundings, which in turn is influenced by the heat transfer.
The
temperature gradient and consequently rate of heat transfer may vary with time and are difficult to evaluate. is
If the temperature of the surroundings
known, Equ. 11.3 may be used, omitting the term l/(Kr + Kc).
190
REFERENCES
11.1
BSCP 2010 Part 2, Design and Construction of Steel Pipelines in Land, BSI, London, 1970
11.2
Concrete Pipe Assn., Bedding and Jointing of Flexibly Jointed Concrete Pipes, Techn.Bu1. No.10, Tonbridge, 1967.
11.3
American Concrete Pipe Assn., Concrete Pipe Design Manual, Arlington, 1970
11.4 H.M.Reitz, Soil mechanics and backfilling practice, J.Am.Water Works Assn., 42(12) 11.5
(Dec., 1950).
G.F.Sowers, Trench excavation and backfilling, J.Am.Water Works Assn.,
48(7)
(July, 1956).
11.6 J.M.Reynolds, Submarine pipelines, Pipes and Pipeline Manual. 3rd
Ed., Scientific Surveys, London, 1970. 11.7
H.I.Schwartz, Hydraulic trenching of submarine pipelines, Proc. Am.Soc.Civi1 Engs., 97(TE4)
(Nov., 1971).
11.8 Engineering Equipment Users Assn., Pipe Jointing Methods, Handbook No. 23, Constable, London, 1968. 11.9
AWWA Standard C203, Coal Tar Enamel Protective Coatings for Steel Water Pipe 30 I n s . and Over, N.Y.,1962.
11.10 AWWA Standard C205, Cement Mortar Protection Lining and Coating for Steel Water Pipe 30 I n s . and Over, N.Y. 1962.
1 1 . 1 1 AWWA Standard C602, Cement Mortar Lining of Water Pipelines in Place, N.Y., 1954. 11.12 E.S.Cole, Design of steel pipe with cement coating and lining, J.Am. Water Works Assn., 48(2)
(Feb., 1956).
11.13 W.H.Cates, Coating for steel water pipe, J.Am.Water Works Assn., 45(2)
(Feb., 1953).
11.14 H.H.Uhlig, The Corrosion Handbook, Wiley, N.Y., 1948. 11.15 W.R.Schneider, Corrosion and cathodic protection of pipelines, J.Am. Water Works Assn., 44 (5) (May., 1952).
11.16 T.M.Riddick, N.L.Lindsay and A.Tomassi, Freezing of water in exposed pipelines, J.Am.Water Works Assn., 42(11) (Nov., 1950). 11.17 Engineering Equipment User's Assn., Thermal Insulation of Pipes and Vessels, Handbook No.12, Constable, London, 1964.
11.18 Pipeline protection review. Pipes and Pipelines International 20 (4) (Aug., 1975).
23 0
L I S T OF SYMBOLS
A
area
AS
area of steel
B
width of trench
cD
drag coefficient
d
inside diameter
D
outside diameter
E
thermal emissivity factor
f
stress
g H
backfill above top o f pipe
i
current in amps
gravitational acceleration
KC
Kf Kr kl
heat l o s s by convection heat l o s s through water film heat l o s s by radiation conductivity of pipe wall conductivity of coating
k2 M
bedding factor
P
pressure
Q
amount of heat
r
resistance in ohms
T
time
tl t2
thickness of pipe wall thickness of coating
U
component of wave velocity perpendicular to pipe
V
mean water velocity, or component of current velocity perpendicular
V
wind velocity
to pipe W
specific weight of water
Y
depth of bedding material below pipe
0
temperature inside pipe
0
temperature outside pipe
i
CHAPTER 1 1
LAY I NG AND PROTECT I ON SELECTING A ROUTE The s e l e c t i o n of a s u i t a b l e r o u t e f o r a p i p e l i n e has an important b e a r i n g on t h e c a p i t a l c o s t and o p e r a t i n g c o s t s .
A p i p e l i n e r o u t e i s se-
l e c t e d from a e r i a l p h o t o s , t o p o g r a p h i c a l and c a d a s t r a l p l a n s , o n - s i t e i n s p e c t i o n s and any o t h e r d a t a a v a i l a b l e on t h e t e r r a i n , o b s t a c l e s and l o c a l I n s e l e c t i n g a r o u t e , t h e c o s t s and p r a c t i c a b i l i t y have t o be
services. considered.
Care should be t a k e n t o e n s u r e t h e ground p r o f i l e i s below t h e
h y d r a u l i c g r a d e l i n e . (Low-flow c o n d i t i o n s s h o u l d be c o n s i d e r e d a s w e l l a s peak r a t e s , as t h e h y d r a u l i c g r a d i e n t i s f l a t t e s t f o r low f l o w s ) .
If there
were a peak above t h e grade l i n e between t h e i n p u t and d i s c h a r g e h e a d s , obv i o u s l y pumps would have t o be d e s i g n e d t o pump o v e r t h i s peak.
Peaks may
a l s o be p o i n t s of p o s s i b l e w a t e r column s e p a r a t i o n which r e s u l t i n w a t e r hammer o v e r p r e s s u r e s .
On t h e o t h e r hand t h e g e n e r a l l e v e l of t h e p i p e l i n e
r o u t e s h o u l d be k e p t a s n e a r t o t h e h y d r a u l i c grade l i n e s a s p o s s i b l e t o minimize p r e s s u r e s and c o n s e q u e n t l y p i p e c o s t s . Once a p r e l i m i n a r y r o u t e i s s e l e c t e d , i t i s pegged o u t by a s u r v e y o r . Pegs a r e p u t i n a l o n g t h e c e n t r e - l i n e a t 10 t o 100 m r e g u l a r i n t e r v a l s and a t changes i n g r a d e and a t h o r i z o n t a l d e f l e c t i o n s .
O f f s e t pegs, 2 t o 5 m
from t h e c e n t r e l i n e pegs are a l s o p u t i n f o r u s e d u r i n g l a y i n g .
The l e v e l s
of pegs a r e observed and t h e p r o f i l e i s t h e n p l o t t e d i n t h e drawing o f f i c e , w i t h ground l e v e l s , bed l e v e l s and d e p t h s i n d i c a t e d a t e a c h peg.
Holes may
be augered a l o n g t h e r o u t e t o i d e n t i f y m a t e r i a l s which w i l l have t o be excavated.
S t r e n g t h t e s t s may be done on s o i l samples t o d e c i d e whether t r e n c h
s h o r i n g w i l l be n e c e s s a r y . Underground and overhead s e r v i c e s should be a c c u r a t e l y l o c a t e d a t t h i s s t a g e o r a t l e a s t b e f o r e e x c a v a t i o n t o a v o i d them.
( D r a i n s and underground
c a b l e s may have t o be s u p p o r t e d on b r i d g e s , o r a heading could be hande x c a v a t e d under them). LAYING AND TRENCHING
The t y p e of p i p e t o be used f o r any j o b w i l l l a r g e l y be a m a t t e r of economics, a l t h o u g h t h e f a c i l i t i e s f o r l a y i n g , l i f e of t h e p i p e m a t e r i a l s and ot.her f a c t o r s a f f e c t t h e d e c i s i o n .
The l e n g t h s of d e l i v e r e d p i p e s w i l l
1 8 2
depend on t h e t y p e of p i p e , t h e w e i g h t , t h e r i g i d i t y , s i z e of t r a n s p o r t v e h i c l e s and method of j o i n t i n g .
Concrete p i p e s a r e o f t e n s u p p l i e d i n 2 t o
4 m l e n g t h s , w h i l e s t e e l p i p e s may be 10 o r even 20 m l o n g , and t h i n w a l l e d p l a s t i c p i p e s may be s u p p l i e d i n r o l l s .
P i p e s s h o u l d be handled c a r e f u l l y
t o avoid damage t o c o a t i n g s o r p i p e o r d i s t o r t i o n . and t r a n s p o r t e d on Qadded c r a d l e s o r sandbags.
They should be s t o r e d
P i p e s may be lowered i n t o
t h e t r e n c h by b e l t s l i n g s from mechanical booms o r t r i p o d s . diameter t h i c k walled
For s m a l l
welded p i p e s , j o i n t i n g and wrapping may be done a t t h e
s i d e of t h e t r e n c h , and t h e p i p e t h e n ' s n a k e d ' i n t o t h e t r e n c h .
The working
w i d t h r e q u i r e d f o r c o n s t r u c t i o n of p i p e l i n e s v a r i e s w i t h t h e method of exc a v a t i o n and l a y i n g .
I f t h e e x c a v a t i o n and l a y i n g i s done by hand, 3 m may
be wide enough f o r s m a l l d i a m e t e r p i p e s .
On t h e o t h e r hand f o r l a r g e mains
where e x c a v a t i o n , l a y i n g and p o s s i b l y even s i t e c o a t i n g i s done m e c h a n i c a l l y , working w i d t h s up t o 50 m may be r e q u i r e d .
Reserve w i d t h s of 20 t o 30 m a r e
common, and a l l o w f o r one o r two a d d i t i o n a l p i p e l i n e s a t a l a t e r s t a g e . Once t h e t r e n c h i s e x c a v a t e d , s i g h t r a i l s may be s e t up a c r o s s t h e t r e n c h w i t h t h e c e n t r e l i n e of t h e p i p e marked t h e r e o n .
The frames a r e s e t
a d e f i n i t e h e i g h t above t h e bed and t h e p i p e l e v e l l e d w i t h t h e a s s i s t a n c e of t h e s e . P i p e s a r e normally l a i d w i t h a minimum c o v e r o f 0.9 m t o 1.2 m t o avoid damage by superimposed l o a d s .
A minimum cover of 1 m i s n e c e s s a r y i n t h e U.K.
t o prevent f r o s t penetration.
The d e p t h may be reduced i n rocky o r s t e e p
ground t o minimize c o s t s i n which c a s e a c o n c r e t e cover may be n e c e s s a r y . The w i d t h o f t r e n c h i s n o r m a l l y 0 . 5 m t o 0.8 m more t h a n t h e p i p e d i a m e t e r , w i t h l o c a l widening and d e e p e n i n g f o r j o i n t s .
(0.8 m i s used f o r p i p e s o v e r
300 mm d i a m e t e r which a r e d i f f i c u l t t o s t r a d d l e w h i l e l a y i n g and j o i n t i n g ) . The s i d e s of t h e open t r e n c h w i l l have t o be s u p p o r t e d i n uncohesive o r wet s o i l s and f o r deep t r e n c h e s , f o r s a f e t y r e a s o n s .
The s h o r i n g s h o u l d
be d e s i g n e d t o r e s i s t t h e a c t i v e l a t e r a l s o i l p r e s s u r e .
Alternatively the
s i d e s of t h e t r e n c h may be b a t t e r e d back t o a s a f e a n g l e . k e p t w e l l away from t h e e x c a v a t i o n t o avoid s l i p s . d e p t h away i s a d v i s a b l e .
S p o i l should be
A t l e a s t one t r e n c h
D r a i n s may be r e q u i r e d i n t h e bottom o f a t r e n c h
i n w e t ground t o keep t h e t r e n c h d r y w h i l e working, t o r e d u c e t h e p o s s i b i l i t y of embankment s l i p s and t o e n a b l e t h e b a c k f i l l t o be compacted p r o p e r l y . D r a i n p i p e s s h o u l d b e open j o i n t e d and bedded i n a s t o n e f i l t e r w i t h f r e q u e n t c r o s s - d r a i n s t o l e a d away w a t e r .
I n v e r y w e t ground c u t o f f d r a i n s be-
s i d e t h e t r e n c h may be n e c e s s a r y . P i p e s may be bedded on t h e f l a t t r e n c h bottom, b u t a p r o p e r bedding
i s p r e f e r a b l e t o minimize d e f l e c t i o n s of f l e x i b l e p i p e s o r t o r e d u c e damage
183
t o r i g i d p i p e s . The bed of t h e t r e n c h may be pre-shaped of t h e p i p e .
t o f i t the profile
I n rocky ground a bed of sandy m a t e r i a l may be b u i l t up and Beds of s a n d , g r a v e l o r crushed s t o n e up t o 20 mm i n s i z e ,
trimmed t o shape.
brought up to t h e haunches of t h e p i p e , a s e o f t e n u s e d , a s t h e y do n o t exh i b i t much s e t t l e m e n t and are e a s y to compact.
Concrete beds and haunches o r
s u r r o u n d s a r e sometimes used f o r low-pressure r i g i d p i p e s and sewers.
Bedding
s h o u l d be c o n t i n u o u s under j o i n t s b u t haunches should s t o p s h o r t o f j o i n t s t o f a c i l i t a t e r e p l a c i n g damaged p i p e s .
Fig. 1 1 . 1
shows t y p i c a l t y p e s of bedding.
( R e f e r t o Chapter 7 f o r bedding f a c t o r s a s s o c i a t e d w i t h d i f f e r e n t t y p e s of bedding). The b e s t method of b a c k f i l l i n g and compaction depends on t h e t y p e of s o i l . S o i l p r o p e r t i e s may be d e s c r i b e d i n terms of t h e p l a s t i c l i m i t ( P I ) , l i q u i d l i m i t (LL) and p l a s t i c i t y i n d e x ( P I
=
LL-PL).
For s o i l s w i t h low PI'S
f i l l may o f t e n be s a t i s f a c t o r i l y compacted merely by i n u n d a t i o n . LL s o i l s , dynamic compaction i s r e q u i r e d i . e .
f i l l densities
For h i g h
stampers o r v i b r a t o r s .
Back-
may be s p e c i f i e d i n terms of t h e P r o c t o r S t a n d a r d D e n s i t y .
For l a r g e p i p e s i t i s p r a c t i c e t o compact t h e bed w i t h a w i d t h of a t l e a s t h a l f t h e p i p e d i a m e t e r under t h e p i p e t o 95 t o 100 % P r o c t o r d e n s i t y .
The s o i l
f i l l around t h e p i p e s h o u l d be compacted i n 100 t o 150 mm l a y e r s t o 90 o r 95% P r o c t o r d e n s i t y up t o haunch l e v e l o r t o 3 / 4 of t h e d e p t h of t h e p i p e .
It may be
n e c e s s a r y to s t r u t f l e x i b l e p i p e i n s i d e t o p r e v e n t d i s t o r t i o n d u r i n g t h i s operation.
The f i l l around t h e t o p h a l f of t h e p i p e , up to 300 mm above t h e
crown s h o u l d be compacted t o 85 to 90% P r o c t o r d e n s i t y . For b a c k f i l l under r o a d s , t h e t o p l a y e r s may have t o be compacted t o 90-95% P r o c t o r d e n s i t y , w h i l e f o r runways 110% may be r e q u i r e d .
85% i s nor-
m a l l y s u f f i c i e n t i n open c o u n t r y and d e n s i t i e s a s low as 80% a r e o b t a i n e d i f c o n t r o l i s bad.
To avoid damage to p i p e o r c o a t i n g s , f i l l around t h e p i p e should n o t have s t o n e s i n i t .
S u r p l u s s p o i l may be s p r e a d o v e r t h e t r e n c h w i d t h t o
compensate f o r s e t t l e m e n t o r c a r t e d away.
R e i n s t a t e m e n t of t o p s o i l and
v e g e t a t i o n , p r o v i s i o n of l a n d d r a i n a g e , and f e n c i n g o f f of working a r e a s should be c o n s i d e r e d . THRUST BORES
To avoid e x c e s s i v e l y deep e x c a v a t i o n s , o r t o avoid d i s r u p t i n g t r a f f i c on a road o v e r a p i p e , t h e p i p e may be j a c k e d t h r o u g h t h e s o i l f o r s h o r t lengths.
A h o l e i s dug a t e a c h end of t h e l e n g t h t o be j a c k e d .
A jack is s e t
up i n one h o l e w i t h i t s b a c k a g a i n s t a t h r u s t b l o c k o r t i m b e r and t h e p i p e
Class A bedding z 0 4 OiM= 3 4
concrete 15Omm mt
CONCRETE ARCH
Class B bedding ME
1.9
Oenleh Cmpaeled Boch?ill
0 6 D
SHAPED SUBGRADE WITH GRANULAR FOUNMTION
Class C bedding M:
I 5
0 125H
- Lioh?ly
~~mpacie
SHAPEDI~ G B G R ~ D E
GRANULAR FOUNDATION
Class D bedding
FLAT SUBGRADE
FIG.
11.1
NOTE For cock or o?her Incompressible malar~als,thalranch should be amrexcovoled 0 minimum of 150mm ond rsfi1l.d w ! h grond~rmaleiirrl
Types o f b e d d i n g s .
set up on r a i l s i n f r o n t o f t h e j a c k .
M-bedding
factor
Small p i p e s ( l e s s t h a n 600 mm diam-
e t e r ) may have a s h a r p head o r s h o e f i t t e d t o t h e f r o n t t o e a s e t h e j a c k i n g l o a d and m a i n t a i n t h e d i r e c t i o n o f t h e p i p e , a s t h e s l i g h t e s t o b s t a c l e w i l l deviate t h e pipe.
Occasionally t h e outside of t h e pipe i s lubricated with
bentonite t o reduce f r i c t i o n . may s t i c k .
Pushing should never s t o p f o r long o r the pipe
With l a r g e r p i p e s t h e s o i l ahead of t h e p i p e i s dug o u t by a u g e r
o r hand and removed t h r o u g h t h e p i p e .
The p i p e i s t h e n pushed i n f u r t h e r ,
185
t h e j a c k r e t r a c t e d and a n o t h e r p i p e i n s e r t e d behind t h e p i p e s i n t h e b o r e and t h e p r o c e s s r e p e a t e d .
The j a c k i n g f o r c e r e q u i r e d t o push a p i p e w i t h a
shoe may be up t o 1 200 t o n s , b u t i f a n a u g e r i s used, a 100 t o n
j a c k may
suffice. PIPE BRIDGES P i p e s f r e q u e n t l y have t o span o v e r r i v e r s o r g o r g e s .
For s h o r t s p a n s ,
r i g i d j o i n t e d p i p e s may have s u f f i c i e n t s t r e n g t h t o s u p p o r t themselves p l u s the fluid.
For l a r g e r s p a n s , i t may be n e c e s s a r y t o s u p p o r t t h e p i p e on
t r u s s e s o r c o n c r e t e b r i d g e s , o r hang t h e p i p e from a n e x i s t i n g t r a f f i c bridge.
I f a t r u s s b r i d g e i s t o be c o n s t r u c t e d f o r t h e p i p e , t h e p i p e could
a c t a s t h e t e n s i o n member a t t h e bottom, o r t h e compression member a t t h e top.
An a t t r a c t i v e form of b r i d g e i s one with t h e p i p e s u p p o r t e d from sus-
pension cables.
Two c a b l e s a r e p r e f e r a b l e t o one, a s t h e y may be spaced
a p a r t and t h e hangers a t t a c h e d a t an a n g l e t o t h e v e r t i c a l p l a n e through the pipe.
This arrangement h e l p s t o s u p p o r t t h e p i p e a g a i n s t wind f o r c e s
and r e d u c e s wind v i b r a t i o n s .
The p i p e could a l s o be d e s i g n e d t o a c t a s an
a r c h , b u t a g a i n l a t e r a l s u p p o r t would be n e c e s s a r y - e i t h e r two p i p e s c o u l d be d e s i g n e d t o a c t t o g e t h e r w i t h c r o s s b r a c i n g , o r c a b l e s t a y s could be erected.
I n a l l c a s e s e x c e p t i f s u p p o r t e d on an independent b r i d g e , t h e p i p e
s h o u l d be r i g i d j o i n t e d ; p r e f e r a b l y welded s t e e l .
This means t h e r e w i l l
have t o be some form of e x p a n s i o n j o i n t a l o n g t h e p i p e t o p r e v e n t t h e r m a l s t r e s s e s developing. UNDERWATER PIPELINES
The l a y i n g of p i p e s underwater i s v e r y e x p e n s i v e , n e v e r t h e l e s s i t i s often inescapable. coming common.
I n a d d i t i o n t o r i v e r c r o s s i n g s , undersea l a y i n g i s be-
Gas and o i l l i n e s from u n d e r s e a beds o r o f f - s h o r e t a n k e r
b e r t h s are f r e q u e n t .
There a r e a l s o many i n d u s t r i e s and towns which d i s -
c h a r g e e f f l u e n t o r s l u d g e i n t o t h e sea t h r o u g h u n d e r s e a p i p e s .
The method
used f o r underwater l a y i n g w i l l depend on c o n d i t i o n s such as c u r r e n t s , wave h e i g h t , t y p e of bed and l e n g t h of under-water p i p e : (1)
F l o a t i n g and S i n k i n g :
I n calm w a t e r t h e p i p e may be j o i n t e d on s h o r e ,
f l o a t e d o u t and sunk.
Lengths up t o 500 m have been l a i d t h u s .
Care
i s needed i n s i n k i n g t h e p i p e and winches on b a r g e s should be p o s i tioned along the l i n e t o assist.
Lengths of p i p e up t o a few k i l o m e t r e s may be assembled
Bottom Towing:
on l a n d i n s t r i n g s , e a c h up t o 1 km l o n g , and towed a l o n g t h e bed of t h e s e a by a winch on a b a r g e anchored a t sea.
S t r i n g s are j o i n t e d to-
The p i p e i s bouyed up by a i r i n t h e p i p e o r by
g e t h e r on t h e s h o r e .
buoys t i e d t o t h e p i p e , t o r e d u c e f r i c t i o n on t h e sea bed.
I f t h e pipe
i s f i l l e d w i t h a i r i t may have t o be weighted down by a c o n c r e t e c o a t i n g to g e t t h e c o r r e c t buoyancy.
Too l i g h t a p i p e would b e s u s c e p t i b l e
t o s i d e c u r r e n t s and may bend out of l i n e .
The s t r e s s e s i n t h e p i p e due
t o t h e t e n s i o n and bending caused by underwater c u r r e n t s o r waves may be c r i t i c a l .
The l a t e r a l d r a g p e r u n i t l e n g t h due t o waves and c u r r e n t s
is CDw ( u + v ) ~D/2g
(11.1)
where CD i s t h e d r a g c o e f f i c i e n t (about 0.9),
w i s t h e s p e c i f i c weight
of t h e w a t e r , u i s t h e wave v e l o c i t y and v i s t h e c u r r e n t v e l o c i t y ( o r t h e i r components p e r p e n d i c u l a r t o t h e p i p e ) , D i s t h e o u t s i d e d i a m e t e r of t h e p i p e and g i s g r a v i t a t i o n a l a c c e l e r a t i o n . Lay Barge:
P i p e s may b e t a k e n t o s e a i n a p i p e b a r g e , j o i n t e d on a
s p e c i a l lay-barge and lowered c o n t i n u o u s l y down an arm ( o r s t r i n g e r ) i n t o place.
The b a r g e s proceed forward a s l a y i n g p r o c e e d s .
Underwater J o i n t i n g :
Underwater j o i n t i n g i s d i f f i c u l t a s e a c h p i p e
has t o be accurately aligned.
A d i v e r w i l l be r e q u i r e d , which l i m i t s
t h e method t o s h a l l o w d e p t h s . I t i s u s u a l l y n e c e s s a r y t o bury t h e p i p e t o avoid damage due t o cur-
Dredging i s d i f f i c u l t as sediment may f a l l o r be washed i n t o t h e
rents.
trench before the pipe i s laid.
The t r e n c h normally h a s t o be overexcavated
A n o v e l way o f l a y i n g t h e p i p e i n s o f t beds i s w i t h a bed f l u i d i z e r (Ref.
11.7).
High-pressure
j e t s of w a t e r f l u i d i z e t h e bed under t h e p i p e s and t h e
p i p e s i n k s i n t o t h e bed.
A number of p a s s e s of t h e f l u i d i z e r a r e r e q u i r e d s o
t h a t t h e pipe s i n k s gradually over a long length.
Too deep a t r e n c h may
c a u s e bending s t r e s s e s i n t h e p i p e a t t h e head of t h e t r e n c h . Most underwater p i p e s are s t e e l w i t h welded j o i n t s .
P i p e s and j o i n t s
s h o u l d b e w e l l c o a t e d and l i n e d and c a t h o d i c p r o t e c t i o n i s d e s i r a b l e under t h e sea. JOINTS AND FLANGES
The t y p e of j o i n t t o u s e on a p i p e l i n e w i l l depend on t h e t y p e of p i p e ,
197
the facilities available on site, cost, watertightness of joint, strength and flexibility requirements. The following types of joints are used:(1)
Butt- welded: The ends of the steel pipe are trued and bevelled approximately 30'
back, leaving a 2 mm root.
The joint is clamped and/
or tacked in places and then welded right around with one or more runs. Large pipes with thick walls may a l s o require a weld pass inside, although it is difficult to make good the internal lining in pipes less than 600 mm bore. (2)
Sleeve welded :
One end of a steel pipe may be flared out to form a
socket and the leading edge fillet welded to the barrel of the other pipe.
If possible the joint should be welded inside too.
11.2).
(see Fig.
The external pipe coating and the internal lining, if possible,
should be made good at the weld after all slag has been wire-brushed off. Two methods of welding are available: and metallic arc welding.
Oxy-acetylene gas welding
Top-class welding is essential to
achieve high weld strength and avoid cavities.
A thorough testing
programme should accompany the welding. Destructive testing should be confined where possible to the factory and to sample welds on short lengths of pipe but occasional field destructive tests may be necessary.
Strips including the weld are cut from the pipe wall and bent
over a predetermined radius.
The weld is ground and etched to reveal
air pockets or slag inclusions. Normally field tests will be nondestructive: either by X-ray, Gamma-ray, magnetic or ultra-sonic methods.
X-rays are preferred for high definition and contrast but
Gamma-ray equipment is more portable and useful for otherwise inaccessible corners. With large-bore pipes, the X-ray equipment may be inserted in the pipe and a film is wrapped around the pipe to receive the image from a single wall.
For smaller pipes, double, over-
lapping exposures are made from outside, thereby exaggerating defects in the near wall. Welding of high tensile steels require better craftmanship than for low-tensile steels. Pre-heating of the pipe ends, and post-heating for stress relief are normally desirable. (3)
Screwed : Threaded and screwed ends and sockets are only used on small steel pipes.
(4)
Spigot and Socket :
Concrete, cast iron and plastic pipes frequently
have one end socketed and the other end plain.
The male end has a
1 8 6
FIG.
11.2
FIG. 11.3
Sleeve-welded
joint
Push-in t y p e j o i n t .
r u b b e r s e a l i n g g a s k e t f i t t e d o v e r i t and t h e s o c k e t i s f o r c e d o v e r the ring.
(see Fig.ll.3).
Various shaped r u b b e r r i n g s are used.
An-
o t h e r way o f s e a l i n g t h e j o i n t i s t o c a u l k it w i t h bitumen compound, cement m o r t a r , l e a d o r r e s i n .
J o i n t s f i t t e d with rubber r i n g s a r e
o f t e n c a u l k e d a s w e l l t o hold t h e r i n g i n p l a c e , improve s e a l i n g and prevent d i r t g e t t i n g i n t o the j o i n t .
S p i g o t and s o c k e t j o i n t s w i t h
r u b b e r r i n g s can normally accommodate one o r two d e g r e e s of d e f l e c t i o n .
(5)
Clamp-on j o i n t s : p l a i n ended p i p e s .
Various p r o p r i e t r y j o i n t s a r e a v a i l a b l e f o r j o i n t i n g They normally i n v o l v e clamping a r u b b e r s e a l between
e a c h p i p e b a r r e l and a c o v e r s l e e v e .
F i g . 11.4 i l l u s t r a t e s one such
t y p e of j o i n t ( a l s o r e f e r r e d t o a s a Viking Johnson o r D r e s s e r coupling).
These j o i n t s accommodate movement e a s i l y , and may be designed
t o t a k e s e v e r a l d e g r e e s of d e f l e c t i o n between p i p e s o r l o n g i t u d i n a l movement.
I f t h e j o i n t i s t o r e s i s t t e n s i o n , t i e b o l t s may be f i x e d
189
FIG. 11.4
Slip-on type coupling.
to the barrel of each pipe by brackets to take the tension.
The
brackets cause local bending and tension stresses in the walls of the pipe and may cause damage unless carefully designed.
Normally four or
even two tie bolts are sufficient. The diameter of the bolts is selected to take the tension and the brackets designed so that the bolts are as close as possible to the barrel to minimize eccentricity. The brackets are normally "U" shaped in plan with the legs welded parallel to the pipe axis.
The length of the legs of the bracket
should be such that longitudinal shear and bending stresses are acceptable.
In fact it is impossible to eliminate all bending stresses on the
pipe barrel with brackets and for this reason rings, like flanges, may be preferable for transferring the tie-bolt load t o the pipe wall. The Victaulic coupling used for cast iron pipes, uses lips on the ends of the pipes to transfer tension to the coupling. (6)
Flanged :
Steel and cast iron pipes are often flanged at the ends,
especially if pipes or fittings are likely to be removed frequently. Faces are machined and bolted together with 3 mm rubber or other insertion gasket between.
Flanges are drilled to standard patterns
according to the diameter and working pressure. F u l l face gaskets, with holes for bolts and the bore, are used
f o r high-pressure joints and cast iron or soft metal flanges.
Joint
rings with an outside diameter slightly less than the inside diameter of the bolt holes are used for thick steel flanges and low-pressure pipes. Some flanges have a raised face inside the bolt circle for use with
joint rings.
Flanges used w i t h j o i n t r i n g s t e n d t o d i s h when t h e
b o l t s a r e t i g h t e n e d though. the pipe v a r i e s .
The method of a t t a c h i n g t h e f l a n g e s t o
The s i m p l e s t method i s t o s l i p t h e f l a n g e over t h e
p i p e b u t t o l e a v e approximately 10 mm e x t e n d i n g beyond t h e end of t h e pipe.
A f i l l e t weld i s t h e n c a r r i e d around t h e f r o n t of t h e p i p e and
a t t h e back of t h e f l a n g e around t h e p i p e b a r r e l .
Alternatively both
o r one of t h e i n s i d e edges of t h e f l a n g e may be b e v e l l e d f o r welding.
A method p r e f e r r e d f o r h i g h p r e s s u r e work i s t o have a l i p on t h e f l a n g e p r o j e c t i n g o v e r t h e end o f t h e p i p e s o t h a t t h e b o r e a t t h e j o i n t w i l l be f l u s h a f t e r welding. J o i n t i n g and t h r u s t f l a n g e s a r e f a i r l y t h i c k , as i n d i c a t e d by t h e s t a n d a r d codes of p r a c t i c e , and must be s e c u r e l y welded t o t h e p i p e o r c a s t integrally.
Two o t h e r t y p e s of f l a n g e s a r e common:-
Puddle f l a n g e s are used on p i p e s which are c a s t i n w a l l s o f w a t e r retaining structures.
The o b j e c t of t h e s e f l a n g e s i s t o p r e v e n t leak-
age of w a t e r and t h e y are n o t n e c e s s a r i l y a s t h i c k a s f l a n g e s which
are d e s i g n e d t o t r a n s f e r l o n g i t u d i n a l p i p e s t r e s s , a l t h o u g h t h e f l a n g e should be s e c u r e l y a t t a c h e d t o t h e p i p e w a l l . Blank f l a n g e s a r e , a s t h e name i m p l i e s , i n s t a l l e d a t b l a n k e n d s . T h e i r t h i c k n e s s s h o u l d be c a l c u l a t e d t o r e s i s t bending moments a t t h e edges and c e n t r e .
For a c i r c u l a r d i s c r i g i d i t y clamped a t t h e edges
t h e maximum r a d i a l bending s t r e s s o c c u r s a t t h e p e r i m e t e r b o l t s , and
i s e q u a l t o 3 p d 2 / 1 6 t z where p i s t h e f l u i d p r e s s u r e , d i s t h e p i t c h c i r c l e d i a m e t e r and t i s t h e f l a n g e t h i c k n e s s .
I f t h e edges were n o t
r i g i d l y clamped t h e maximum s t r e s s would o c c u r a t t h e c e n t r e and would be 50% i n e x c e s s of t h i s amount. COATINGS
Buried s t e e l p i p e s are s u b j e c t t o c o r r o s i o n and damage u n l e s s t h e pipe is coated.
Coatings s h o u l d i d e a l l y b e r e s i s t a n t t o s c r a t c h i n g d u r i n g
t r a n s p o r t and l a y i n g of t h e p i p e , t o m o i s t u r e , chemical and b i o l o g i c a l a t t a c k , e l e c t r i c c u r r e n t s and t e m p e r a t u r e v a r i a t i o n s .
They s h o u l d be hard
enough t o p r e v e n t damage d u r i n g h a n d l i n g and due t o s t o n e s i n t h e t r e n c h , y e t s u f f i c i e n t l y a d h e s i v e t o a d h e r e w e l l t o t h e p i p e w a l l and f l e x i b l e enough t o w i t h s t a n d t h e f l e x i n g of t h e p i p e w a l l . The most common c o a t i n g f o r p i p e s i s a t h i n a d h e s i v e c o a t followed by a c o a t i n g r e i n f o r c e d w i t h f i b r e s and t h e n p o s s i b l y a n o u t e r wrapping.
1 9 1
The p i p e s u r f a c e i s i n i t i a l l y c l e a n e d by w i r e b r u s h i n g , s a n d b l a s t i n g o r a c i d p i c k l i n g , and t h e prime c o a t i s t h e n a p p l i e d by s p r a y , b r u s h o r d i p p i n g Bitumen o r c o a l t a r enamel i s t h e p r e f e r r e d prime c o a t -
the pipe i n a bath. ing.
A f t e r t h e primary c o a t t h e p i p e may be s p i r a l l y wrapped w i t h impregT h i s i s sometimes followed by
n a t e d f e l t o r woven g l a s s f i b r e m a t t i n g .
p a p e r o r a s b e s t o s f e l t impregnated w i t h bitumen o r c o a l t a r .
The p i p e i s
t h e n whitewashed t o a s s i s t i n d e t e c t i n g damage and t o s h i e l d t h e c o a t i n g from t h e s u n .
The c o a t i n g may be a p p l i e d i n t h e f i e l d a f t e r welding t h e
p i p e j o i n t s , o r i n t h e f a c t o r y , i n which c a s e t h e ends a r e l e f t b a r e f o r j o i n t i n g and c o a t e d i n t h e f i e l d .
The t o t a l t h i c k n e s s o f c o a t i n g should be
a t l e a s t 5 mm.
Other t y p e s of c o a t i n g s i n c l u d e an a s b e s t o s f i b r e bitumen m a s t i c 3 t o
6 mm t h i c k , c o a l t a r p i t c h , epoxy p a i n t s , PVC o r p o l y t h e n e t a p e s ( e i t h e r s e l f a d h e s i v e o r bedded on an a d h e s i v e ) , r e s i n s o r p l a s t i c s , cement m o r t a r and z i n c a p p l i e d by g a l v a n i z i n g .
Exposed p i p e s may be primed and p a i n t e d
w i t h bitumen based aluminium o r enamel. Cement m o r t a r c o a t i n g s o f f e r a d d i t i o n a l r e s i s t a n c e to b u c k l i n g i n t h e c a s e of l a r g e b o r e t h i n w a l l e d p i p e s . u s u a l l y 1 2 t o 20 m
(4
Cement m o r t a r c o a t i n g s a r e
t o 3 / 4 inch) thick.
F i n i s h e d c o a t i n g s may be checked f o r f l a w s , p i n h o l e s e t c . by means of a Holiday d e t e c t o r .
An e l e c t r i c a l conductor i n t h e form o f m e t a l b r u s h e s
o r r o l l i n g springs i s run along t h e pipe coating.
An e l e c t r i c a l p o t e n t i a l
i s a p p l i e d a c r o s s t h e c o a t i n g and a c u r r e n t i s observed when f l a w s i n t h e coating are detected. LININGS S t e e l p i p e s a r e l i n e d t o r e s i s t i n t e r n a l c o r r o s i o n and minimize t h e
f r i c t i o n losses. in the fluid.
Unlined s t e e l p i p e may be o x i d i z e d by c o r r o s i v e s u b s t a n c e s
I n t h e c a s e of s o l i d s - c o n v e y i n g systems i n p a r t i c u l a r , t h e
oxide is r a p i d l y scraped o f f leading t o f u r t h e r corrosion.
C o r r o s i o n of
water-conveying p i p e s may be i n h i b i t e d by m a i n t a i n i n g a h i g h pH e . g . by a d d i n g l i m e . L i m e on t h e o t h e r hand could c a u s e c a r b o n a t e s c a l i n g . The.most p o p u l a r l i n i n g s are bitumen ( 3 t o 5 mm t h i c k ) o r c o a l t a r enamel (2 t o 3 mm t h i c k ) .
Bitumen, which i s a by-product of petroleum, i s
t h e cheaper o f t h e two. Before a p p l y i n g t h e l i n i n g t h e p i p e w a l l i s c l e a n e d by s a n d b l a s t i n g o r o t h e r methods and t h e l i n i n g i s t h e n a p p l i e d by b r u s h , s p r a y , d i p p i n g o r s p i n n i n g t o o b t a i n a smooth s u r f a c e .
Spun enamel i n p a r t i c u l a r p r o v i d e s
a smooth f i n i s h .
Coal t a r enamel i s a l s o more r e s i s t a n t t o m o i s t u r e t h a n
bitumen, a l t h o u g h i t i s more b r i t t l e and c o n s e q u e n t l y s u b j e c t t o damage by impact and f l e x i n g of t h e p i p e .
P l a s t i c i s e d c o a l t a r enamels have, however,
now been developed t o overcome t h e problems of b r i t t l e n e s s . Epoxy p a i n t s a r e a l s o used s u c c e s s f u l l y f o r l i n i n g s , a l t h o u g h c a r e f u l a p p l i c a t i o n i s necessary t o ensure successive coats adhere t o each other. The recommended t h i c k n e s s of t h e l i n i n g v a r i e s w i t h t h e t y p e of p a i n t , b u t
i t i s normally of t h e o r d e r of 0 . 3 mm a p p l i e d i n 2 t o 4 l a y e r s .
Coal t a r
epoxy l i n i n g s s h o u l d be avoided f o r p o t a b l e w a t e r p i p e s a s t h e y t a i n t t h e Lead based p r i m e r s s h o u l d a l s o be avoided as t h e y a r e t o x i c .
water.
Cement m o r t a r l i n i n g , a p p l i e d c e n t r i f u g a l l y by s p i n n i n g t h e p i p e , i s a l s o used f o r l a r g e b o r e p i p e s . (1/4 to
4
inch) t h i c k .
pipes i n t h e f i e l d .
The a p p l i e d l i n i n g i s u s u a l l y 6 t o 12 mm
Mortar l i n i n g s have been a p p l i e d s u c c e s s f u l l y t o
The o l d s u r f a c e i s f i r s t t h o r o u g h l y c l e a n e d by wire-
b r u s h i n g o r sand b a l s t i n g , and t h e n c o a t e d c e n t r i f u g a l l y by a machine drawn o r propelled slowly along t h e l i n e . On p i p e s o v e r about 600 mm b o r e , most f a c t o r y l i n i n g s may be made good a t f i e l d j o i n t s manually o r by mechanical a p p l i c a t o r s . CATHODIC PROTECTION
D e s p i t e t h e u s e of p r o t e c t i v e c o a t i n g s , c o r r o s i o n of t h e w a l l s of s t e e l p i p e s o f t e n o c c u r s t h r o u g h f l a w s , pin-holes
o r a t exposed f i t t i n g s .
C o r r o s i o n i s due p r i m a r i l y t o two c a u s e s ; g a l v a n i c c o r r o s i o n and s t r a y current electrolysis. Galvanic Corrosion When two d i s s i m i l a r materials a r e connected t h r o u g h an e l e c t r o l y t e , c u r r e n t may f l o w from one m a t e r i a l t o t h e o t h e r .
The r e s u l t i n g e l e c t r i c cur-
r e n t f l o w s from t h e anode t o t h e c a t h o d e t h r o u g h t h e e l e c t r o l y t e . o r i o n s , l e a v e t h e anode c a u s i n g c o r r o s i o n .
Particles
The c a t h o d e i s n o t a t t a c k e d .
Such a c t i o n may t a k e p l a c e when two d i s s i m i l a r m e t a l s a r e i n c o n t a c t i n conductive s o i l ( f o r instance f i t t i n g s of a d i f f e r e n t m a t e r i a l t o t h e pipe).
Another more f r e q u e n t form o f g a l v a n i c c o r r o s i o n o c c u r s w i t h p i p e s
i n corrosive soils.
The e f f e c t i s p a r t i c u l a r l y marked i n s o i l
with varying
c h a r a c t e r i s t i c s , d i f f e r e n t i a l oxygen c o n c e n t r a t i o n s o r i n w a t e r w i t h h i g h chemical c o n t e n t , e s p e c i a l l y s u l p h u r .
C o r r o s i o n i s a l s o caused by bio-
chemical a c t i o n i n t h e s o i l which i s a t y p e of g a l v a n i c a c t i o n r e s u l t i n g from b a c t e r i a .
A method which has been used s u c c e s s f u l l y t o c o u n t e r a c t
193
s o i l c o r r o s i o n was t o add l i m e t o t h e t r e n c h b a c k f i l l .
Stress c o n c e n t r a t i o n
i n t h e s t e e l i n a n e l e c t r o l y t e may a l s o l e a d t o c o r r o s i o n .
The amount of
c o r r o s i o n due t o t h e l a s t named two i n f l u e n c e s i s u s u a l l y s m a l l . P o t e n t i a l c o r r o s i v e a r e a s may be d e t e c t e d by s o i l r e s i s t a n c e t e s t s , p i p e - s o i l p o t e n t i a l t e s t s o r measurement of t h e c u r r e n t i n t h e p i p e . I f t h e s o i l i s a t a l l s u s p e c t , o r a s a s t a n d a r d p r a c t i c e f o r major p i p e l i n e , a s o i l r e s i s t i v i t y s u r v e y should be conducted. S o i l
i s measured i n ohm - c e n t i m e t r e s .
resistivity
Readings a r e normally t a k e n i n - s i t u u s i n g
a b r i d g e c i r c u i t o r by measuring a c u r r e n t and t h e a s s o c i a t e d v o l t drop. S t a n d a r d p r o b e s are a v a i l a b l e f o r t h e s e measurements, b u t any major s u r v e y s s h o u l d be done by a n e x p e r i e n c e d c o r r o s i o n e n g i n e e r . A h i g h l y c o n d u c t i v e s o i l may have a r e s i s t i v i t y of 500 ohm cm, and a
p o o r l y c o n d u c t i v e s o i l , more t h a n 10 000 ohm cm. The p o t e n t i a l d i f f e r e n c e between a b u r i e d p i p e and t h e s o i l i s important i n evaluating corrosive conditions.
The p o t e n t i a l i s measured by
c o n n e c t i n g a v o l t m e t e r between t h e p i p e and a s p e c i a l e l e c t r o d e i n c o n t a c t with the s o i l .
A copper s u l p h a t e h a l f c e l l i s f r e q u e n t l y used a s t h e e l e c -
t r o d e under normal c o n d i t i o n s .
The p o t e n t i a l of a p i p e i s 0.5 t o 0 . 7 v o l t s
below t h a t of t h e s u r r o u n d i n g s o i l .
I f t h e pipe is a t a higher voltage than
0.85 v o l t s below t h a t of t h e s o i l , c u r r e n t s a r e l i k e l y t o f l o w from p i p e t o s o i l , thereby corroding t h e pipe. To p r e v e n t t h i s c o r r o s i o n , a s a c r i f i c i a l anode may be connected by a conductor t o t h e p i p e i n t h e v i c i n i t y o f p o s s i b l e c o r r o s i o n
(Fig. 11.5).
The s a c r i f i c i a l anode i s b u r i e d i n a c o n d u c t i v e s u r r o u n d , p r e f e r a b l y below t h e w a t e r t a b l e and c u r r e n t s w i l l t e n d t o l e a v e from t h e anode i n s t e a d of t h e p i p e , t h e r e b y l i m i t i n g t h e c o r r o s i o n t o t h e anode.
I f t h e anode i s s u f -
f i c i e n t l y d i s s i m i l a r from t h e s t e e l p i p e t o cause g a l v a n i c a c t i o n no e x t e r n a l e l e c t r i c a l p o t e n t i a l need be a p p l i e d .
I n f a c t , a r e s i s t o r may sometimes
be i n s t a l l e d i n t h e c o n n e c t i o n t o l i m i t t h e c u r r e n t .
Common s a c r i f i c i a l
anodes are magnesium, z i n c and aluminium, o r a l l o y s of t h e s e m e t a l s .
Mag-
nesium h a s t h e g r e a t e s t p o t e n t i a l d i f f e r e n c e from i r o n , has a h i g h e l e c t r o chemical e q u i v a l e n t (ampere h o u r s p e r k i l o g r a m of m a t e r i a l ) and i s f a i r l y r e s i s t a n t t o anodic p o l a r i z a t i o n .
( A l a y e r of hydrogen i o n s may r e p l a c e t h e
i o n s l e a v i n g t h e anodes, t h e r e b y a c t i v e l y i n s u l a t i n g t h e m e t a l a g a i n s t further attack.
This i s termed p o l a r i z a t i o n ) .
Magnesium anodes g i v e 200 t o
1 200 ampere h o u r s p e r kg. Magnesium anodes are normally d e s i g n e d f o r about a 10 y e a r l i f e b u t z i n c anodes o f t e n l a s t 20 o r 30 y e a r s .
Junclion
Box W i t h
Reslitor i f Required
J
FIG.
11.5
Insulated Sinpl. Core Cobla
Sacrificial anode installation for cathodic protection.
The spacing and size of anodes will be determined by the current requirements to bring the pipe to a safe potential (at least 0.85 volts below the soil potential along the entire length of the pipe).
The spacing of
sacrificial anodes will vary from 3 m in poorly conductive soil and poorly coated pipe to 30 m in highly conductive soil provided the pipe is well coated.
The most reliable way of estimating the required current is to
actually drain current from the pipe by means of a ground bed and d.c. source and measure the resulting pipe to soil potential along the pipe. A s a rule of thumb, the current required to protect a pipe against
corrosion is i
=
10/r per square metre of bared pipe surface, where i is
the current in amps, r is the soil resistance in ohm cm and a factor of safety of 3 is incorporated.
The area of exposed pipe may be as low as 0.5
per cent for a good coating, rising to 20 per cent for a poor coating. Once the required draw-off current per km of pipe is known, the total anode size may be calculated. I n highly conductive soils (less than 500 ohm cm) large anodes (20 kg) may be used but in soils with lower conductivities (greater than about 1 000 ohm cm) a number of smaller anodes should be used to ensure an adequate current output. If an electricity supply is available, it is usually cheaper to use an impressed current type of protection (Fig. 11.6) instead of a sacrificial anode.
A transformer-rectifier may be installed to provide the necessary dc
current. With impressed current installations, the anode need not be selfcorrosive.
Scrap iron, or graphite rods buried in coke fill, are frequently
195
used.
S t e e l anodes w i l l q u i c k l y c o r r o d e i n h i g h l y c o n d u c t i v e s o i l s and f o r
t h i s r e a s o n h i g h s i l i c o n c a s t i r o n anodes are p r e f e r r e d . The t y p e of p r o t e c t i o n t o u s e w i l l depend on t h e long-term economics, Impressed c u r r e n t i n s t a l l a t i o n s are f r e q u e n t l y t h e c h e a p e s t i n t h e l o n g r u n ,
as m a i n t a i n a n c e c o s t s are low and fewer i n s t a l l a t i o n s a r e r e q u i r e d t h a n f o r s a c r i f i c i a l anodes.
I n t h e c a s e of impressed c u r r e n t i n s t a l l a t i o n s a r e l a t i v e -
l y h i g h v o l t a g e may be a p p l i e d , which i n t u r n p r o t e c t s a l o n g e r l e n g t h of p i p e t h a n a s a c r i f i c i a l anode c o u l d .
The p i p e - s o i l p o t e n t i a l f a l l s o f f r a p i d -
l y away from t h e a p p l i e d v o l t a g e though, c o n s e q u e n t l y a h i g h v o I t a g e may have
t o be applied t o p r o t e c t a long length.
The p i p e - t o - s o i l
potential i n the
immediate v i c i n i t y of t h e p o i n t should n o t , i n g e n e r a l exceed 3 v o l t s . p i p e p o t e n t i a l b e i n g below t h a t of t h e s o i l ) . coatings.
(the
L a r g e r v o l t a g e s may damage
Impressed c u r r e n t i n s t a l l a t i o n s may be a t 1 t o 100 km s p a c i n g de-
pending on t h e q u a l i t y of t h e p i p e c o a t i n g .
Impressed c u r r e n t i n s t a l l a t i o n s
a r e p r e f e r a b l e t o s a c r i f i c i a l anodes i f t h e s o i l r e s i s t i v i t y i s h i g h e r t h a n about 3 000 t o 5 000 ohm cm. C o n s i d e r a b l e economic s a v i n g s a r e o f t e n achieved by p r o t e c t i n g o n l y "hot s p o t s " , o r p o i n t s downs can be t o l e r a t e d
subject
t o a g g r e s s i v e a t t a c k , and i f o c c a s i o n a l s h u t -
t h i s should b e c o n s i d e r e d .
S t r a y Current E l e c t r o l y s i s
The most s e v e r e form of c o r r o s i o n i s o f t e n caused by s t r a y d c c u r r e n t s leaving a pipe.
Railways and o t h e r u s e r s of dc c u r r e n t r e t u r n c u r r e n t s
t h r o u g h r a i l s , b u t i f t h e r e i s a n o t h e r conductor such a s a s t e e l p i p e nearby, a p r o p o r t i o n of t h e c u r r e n t may f l o w t h r o u g h t h e conductor i n s t e a d of the r a i l .
Where t h e c u r r e n t l e a v e s t h e p i p e , s t e e l w i l l be corroded a t a
Eurrd or Submerged Slrvclure Mad. Cathodic
Iron or prophila)
FIG.
11. 6
Impressed c u r r e n t c o r r o s i o n p r o t e c t i o n .
1 9 6
r a t e of 9 kg ( 2 0 l b s ) p e r y e a r p e r ampere of c u r r e n t .
The c u r r e n t may be
d e t e c t e d by a c t u a l c u r r e n t measurements o r from p i p e / s o i l p o t e n t i a l measurements.
Continuous r e c o r d i n g s should be t a k e n o v e r a d a y , a s t h e c u r r e n t s
may f l u c t u a t e w i t h t i m e . The c o r r o s i o n a s s o c i a t e d w i t h s t r a y c u r r e n t s may be p r e v e n t e d by conn e c t i n g t h e p i p e a t t h e p o i n t where t h e c u r r e n t l e a v e s i t , t o t h e d e s t i n a t i o n of t h e c u r r e n t , w i t h a c o n d u c t o r .
The c u r r e n t may l e a v e t h e p i p e a l o n g
some l e n g t h , i n which c a s e a c u r r e n t would have t o be impressed on t h e p i p e t o m a i n t a i n t h e v o l t a g e s u f f i c i e n t l y low t o p r e v e n t c u r r e n t e s c a p i n g .
A
ground bed and t r a n s f o r m e r r e c t i f i e r may b e r e q u i r e d f o r t h i s p r o t e c t i o n . When a p i p e i s c a t h o d i c a l l y p r o t e c t e d , c a r e s h o u l d b e t a k e n t h a t mecha n i c a l j o i n t s a r e e l e c t r i c a l l y bonded, by welding a c a b l e a c r o s s them i f necessary.
Branch p i p e s may have t o be i n s u l a t e d from t h e main p i p e t o con-
t r o l the currents. THERMAL INSULATION
F l u i d i n a p i p e l i n e may o f t e n be h e a t e d o r c o o l e d by t h e s u r r o u n d i n g s . I c e f o r m a t i o n i n a r c t i c c l i m a t e s and h e a t i n g i n t r o p i c a l c l i m a t e s a r e somet i m e s a s e r i o u s problem.
Heat i s t r a n s f e r r e d t o o r from t h e i n t e r i o r of an
exposed p i p e i n a number of ways: by c o n d u c t i o n t h r o u g h t h e p i p e w a l l and wrapping and a boundary l a y e r of f l u i d i n s i d e t h e p i p e w a l l , by r a d i a t i o n from t h e e x t e r n a l f a c e of t h e p i p e and by c o n v e c t i o n and wind c u r r e n t s i n t h e air surrounding t h e pipe. The h e a t l o s s by c o n d u c t i o n t h r o u g h an homogeneous p i p e w a l l i s prop o r t i o n a l t o t h e t e m p e r a t u r e g r a d i e n t a c r o s s t h e w a l l and t h e h e a t t r a n s f e r r e d p e r u n i t a r e a of p i p e w a l l p e r u n i t t i m e i s
3 =& AT t
(1 1.2)
where Q i s t h e amount of h e a t conducted i n k i l o c a l o r i e s o r Btu, A i s t h e a r e a of p i p e s u r f a c e , T i s t h e d u r a t i o n of t i m e , A 0
i s t h e temperature d i f -
f e r e n c e a c r o s s t h e w a l l , t i s t h e w a l l t h i c k n e s s and k i s t h e t h e r m a l cond u c t i v i t y , t a b u l a t e d i n Table 1 1 . 1 f o r v a r i o u s m a t e r i a l s :
197
TABLE 1 1 . 1
THERMAL C O N D U C T I V I T I E S
Thermal c o n d u c t i v i t y k cal Btu i n
Material
m sec
c0
s q f t h r Fo
0.000 14
Water
4
Air
0.000 005
0.15
Steel
0.014
420
Bituminized Wrapping
0.000 01
0.3
Concrete
0.0002
S l a g wool
0.000 01 ~
~
6 0.3
~~
An e q u a t i o n w a s developed by Riddick (Ref. 1 1 .16) t r a n s f e r t h r o u g h t h e w a l l of a p i p e conveying w a t e r .
f o r the t o t a l heat
The e q u a t i o n h a s been
modified t o a g r e e w i t h r e l a t i o n s h i p s i n d i c a t e d by EEUA (Ref. 11.17) and i s converted t o metric u n i t s here: Rate of h e a t l o s s of w a t e r k c a l / k g / s e c -
-
(0; -0
0 ) I 250D t l + t 2 + 1 + 1
- - - _ _ _ k~ k2 Kf Kr+Kc
(1 1.3)
and s i n c e t h e s p e c i f i c h e a t of w a t e r i s u n i t y , t h e r a t e of drop i n temperat u r e i n degrees
Centigrade per sec equals t h e heat l o s s i n kcal/kg/sec,
t e m p e r a t u r e of w a t e r
CO
ambient t e m p e r a t u r e o u t s i d e Co diameter m thicknes.s of p i p e m c o n d u c t i v i t y of p i p e w a l l (kcal/m sec Co) t h i c k n e s s of c o a t i n g m c o n d u c t i v i t y of c o a t i n g kcal/m s e c Co 0.8 0.2 h e a t l o s s t h r o u g h boundary l a y e r = 0 . 3 4 ( 1 + 0 . 0 2 0 ; ) ~ /D kcal/m2 s e c Co water v e l o c i t y m/s h e a t l o s s by r a d i a t i o n = 0.054 E ( kcal/m2 sec Co
00 + 2 7 3 3 )
too
106
E
=
emissivity factor
=
0.9 for black asphalt and concrete 0.7 for cast iron and steel 0.4 for aluminimum paint
KC
=
heat l o s s by convection
V
=
wind velocity km/hr
0.
1 - ‘0 0.25 ( 7 )
0.000 34J1+0.78V
=
kcal/m2 sec Co
The heat generation by fluid friction is usually negligible although it theoretically increases the temperature of the fluid. Example An uncoated 300 mm diameter pipeline 1 000 m long with 5 mm thick
steel walls, exposed to a 10 km/hr wind, conveys 100 k?/s of water at an initial temperature of 10°C.
The air temperature is 3OoC.
Determine
the end temperature o f the water.
E
=
0.7
v
=
0.1/0.785 x 0.32
Kf
=
Kr
=
Kc
=
0.3411 + 0.78 x 10T(30
-
o .005 0.014
-t l _ kl
A0/sec
=
1.42 m/s
0.34 x 1.42°’8/0.30’2 0.054 x 0.7 ( 30
=
=
+
=
273)3
0.67 10-3 =
-
10) 0.25
o.ool ,0-3
o.oo29
=
0.35
(30 - 10)/(250 x 0.3) 0.35 + 110.67 + (0.001 + 0.0029 )
Temperature rise over 1 000 m
=
x 0.00103
=
0.00103~~1sec =
0.7OC
1.42 The insulation of industrial pipework carrying high or low temperature fluids is a subject on its own.
The cost of the heat transfer should
be balanced against the cost of the lagging.
Heat transfer to or from buried pipelines depends on the temperature of the surroundings, which in turn is influenced by the heat transfer.
The
temperature gradient and consequently rate of heat transfer may vary with time and are difficult to evaluate. is
If the temperature of the surroundings
known, Equ. 11.3 may be used, omitting the term l/(Kr + Kc).
190
REFERENCES
11.1
BSCP 2010 Part 2, Design and Construction of Steel Pipelines in Land, BSI, London, 1970
11.2
Concrete Pipe Assn., Bedding and Jointing of Flexibly Jointed Concrete Pipes, Techn.Bu1. No.10, Tonbridge, 1967.
11.3
American Concrete Pipe Assn., Concrete Pipe Design Manual, Arlington, 1970
11.4 H.M.Reitz, Soil mechanics and backfilling practice, J.Am.Water Works Assn., 42(12) 11.5
(Dec., 1950).
G.F.Sowers, Trench excavation and backfilling, J.Am.Water Works Assn.,
48(7)
(July, 1956).
11.6 J.M.Reynolds, Submarine pipelines, Pipes and Pipeline Manual. 3rd
Ed., Scientific Surveys, London, 1970. 11.7
H.I.Schwartz, Hydraulic trenching of submarine pipelines, Proc. Am.Soc.Civi1 Engs., 97(TE4)
(Nov., 1971).
11.8 Engineering Equipment Users Assn., Pipe Jointing Methods, Handbook No. 23, Constable, London, 1968. 11.9
AWWA Standard C203, Coal Tar Enamel Protective Coatings for Steel Water Pipe 30 I n s . and Over, N.Y.,1962.
11.10 AWWA Standard C205, Cement Mortar Protection Lining and Coating for Steel Water Pipe 30 I n s . and Over, N.Y. 1962.
1 1 . 1 1 AWWA Standard C602, Cement Mortar Lining of Water Pipelines in Place, N.Y., 1954. 11.12 E.S.Cole, Design of steel pipe with cement coating and lining, J.Am. Water Works Assn., 48(2)
(Feb., 1956).
11.13 W.H.Cates, Coating for steel water pipe, J.Am.Water Works Assn., 45(2)
(Feb., 1953).
11.14 H.H.Uhlig, The Corrosion Handbook, Wiley, N.Y., 1948. 11.15 W.R.Schneider, Corrosion and cathodic protection of pipelines, J.Am. Water Works Assn., 44 (5) (May., 1952).
11.16 T.M.Riddick, N.L.Lindsay and A.Tomassi, Freezing of water in exposed pipelines, J.Am.Water Works Assn., 42(11) (Nov., 1950). 11.17 Engineering Equipment User's Assn., Thermal Insulation of Pipes and Vessels, Handbook No.12, Constable, London, 1964.
11.18 Pipeline protection review. Pipes and Pipelines International 20 (4) (Aug., 1975).
23 0
L I S T OF SYMBOLS
A
area
AS
area of steel
B
width of trench
cD
drag coefficient
d
inside diameter
D
outside diameter
E
thermal emissivity factor
f
stress
g H
backfill above top o f pipe
i
current in amps
gravitational acceleration
KC
Kf Kr kl
heat l o s s by convection heat l o s s through water film heat l o s s by radiation conductivity of pipe wall conductivity of coating
k2 M
bedding factor
P
pressure
Q
amount of heat
r
resistance in ohms
T
time
tl t2
thickness of pipe wall thickness of coating
U
component of wave velocity perpendicular to pipe
V
mean water velocity, or component of current velocity perpendicular
V
wind velocity
to pipe W
specific weight of water
Y
depth of bedding material below pipe
0
temperature inside pipe
0
temperature outside pipe
i