A guide to piping design

A guide to piping design FLEXIBLE CONNECTORS

PRESSURE REDUCER C 0.1 m_(~_

lm

AVERAGE PRESSURE IN A B C SAY ABOUT 13 MNIm 2

2m

__

0.9

JOURNAL BEARINGS, OIL FLOW 32 cm3/s EACH, AT 900 kN/m 2 OILWAYS 30 mm LONG, 5rnm DIA.

I

).2m FILTER PRESSURE DROP 300 kNIm 2

A24

-"-" O

0.6m

-rooo.3 m

HYDROSTATIC BEARING OIL FLOW 250 cm3/s PRESSURE DROP ACROSS BEARING 6 MNIm 2

LINEAR RESTRICTOR PRESSURE DROP 6 MNIm 2

RELIEF VALVE K = 12 AT 300 cmSls

2.5m RETURN TRAY

OIL LEVEL 0.3m

2.2m UNLOADING VALVE

~, ' ~ ! ]

E

AVAILABLE FALL 0.55m

F J.

OIL VISCOSITY AT 50~ 70cP ; DENSITY 900kglm 3

PUMP STRAINER AREA RATIO 1:3

Figure 24.1 Typical lubrication s y s t e m Table 24.1 Selection of pipe materials

Approx. relativeprice

Range

Apflications

.Notes

3

Not recommended. Occasionally used for mains in large high-pressure systems where cost important

Thoroughly clean and de-scale by acid pickling. Seal ends after pickling until installation

12

6

Large permanent systems

Best material for high flow rates. Can be untidy on small systems

50(O-1000

4

4

Universal. Especially favoured for low flow rates

Brass resists corrosion better than copper. Neat runs and joints possible

0.125--0. 75

5000-2500

9

4

As final coupling to bearing to assist maintenance. Connection to moving part or where subject to heavy vibration

Nylon or rubber base obtainable

Hard nylon

0.12521.125

700-350

2

2

As cheap form of flexible coupling. Large centralised low-flow lubrication systems for c h e a p nca3s

Deteriorates in acid atmosphere. Heavily pigmented variety should be used in strong light

PV C reinforced

0.125--0.Y5

150

2

1

Low-pressure systems. Gravity returns

Readily flexible. Pigmented variety best in sunlight

PVC unreinforced

0.125-2

20-10

1

1

Gravity returns

Pigmented variety best in sunlight. Can be untidy. Relatively easily damaged

Ma~'i4d

Bore alia.

Pressure* lbf/in 2

on/),

Installed

Mild steel

No limit

No limit

2

Stainless steel

No limit

No limit

Half-hard copper and brass

0.07-2

Flexible reinforced high-pressure hose

in

Material

* The pressures quoted are approximate working pressures corresponding to the minimum and maximum bores, respectively, and are intended as a guide to selection only. Use manufacturers' values for design purposes.

A24.1

A24

A guide to piping design

Table 24.2 Selection of control valves Valve, type,

Operation

Common use,

~fotes

Bypass (unloading)

Bleeds to drain when primary pressure exceeds given value

Control main circuit pressure

Designed to operate in open position without oil heating. Often included in pump unit, otherwise requires return to sump

Relief (safety)

As above

Safety device to protect pipework fittings and pump

If used to control circuit pressure, oil heating may result. Do not undersize or forget return to sump

Non- return

Prevents reverse flow

Prevent back-flow through a bypass

Sometimes incorporated in bypass or relief valve

Pressure-reducing (regulating)

Gives fixed pressure drop or fixed reduced pressure

Cater for different bearing needs from one supply pressure

Requires drain to sump

Flow-regulating and -dividing

Controls or divides flow rate irrespective of supply pressure

Pass fraction of flow to open (non-pressure) system or to filter

Approximate control only. Orifices are cheaper alternative, as effective for many applications

Admits oil to secondary circuit only after primary reaches given pressure

Protection of a particular bearing circuit in complex system

Rather uncommon. May be used in hydraulic power/bearing circuit

(check)

Sequence

I ^A

r-4 '-4\ b

I

Directional

Switches flow on receipt of signal

Activate various sections of circuit as required

Remote or manual operation. For simple systems, stop valves may be better

I II

Volume metering

Meter quantity of lubricant to bearing on receipt of pressure pulse

Intermittent lubrication system only, placed at lubrication points

A24.2

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A24

A guide to piping design Table 24.3 Pipe sizes and pressure-drop calculations Item

Equation* figure

Example

JVotes

See Figure 24.1

Essential for all but the simplest

Determination of pipe bore (a) Draw flow diagram

cases

(b~ Size lines assuming velocities: (i) 3 m/s (10 ft/s) for delivery (ii) 1 m/s (3 ills) for pump suction (iii) 0.3 m/s (1 ft/s) for return (iv) 20 m/s (60 ft/s) for linear restrictors

For ABC in Figure 24.1: d = 4~/n-~'or

Q. = 314 cmS/s From Figure 24.2, d -~ 12 mm or 0.5 in

use Figure 24.2

Keep suction lines short, free of fittings

Pressure drop--delivery and pump suction lines (a) Determine correct viscosity for working conditions of temperature and pressure

= Xxqo for pressure, Figure 24.3

qvl

.32~ or

(b) Evaluate pipe viscous losses

P,

(c) Determine losses in valves, fittings and strainers

P, = k >< ~ pv2, find K from Table 24.4

=

use Figure 24.4 !

Manufacturer's data (d) Determine total loss

Adjust sizes, if necessary, after calculation of pressure drops. Use reducer, if necessary, in pump suction. If air entrainment likely in return line, size to run half full

From Figure 24.3 at 13 MN/m2: X = 1.4, F/-- 1.4 x 70 -- 98 cP

Use supplier's value for viscosity at working temperature (usually `30-50~ above ambient)

From Figure 24.4 for ABC: P~ = 65 kN/mZ/m x 2.1 = 1`37 kN/m 2

Check to ensure flow is laminar (Reynolds number < 2000). Find Re from Fig. 24.4 Use manufacturers' figures for K or Pc, where available. Where the flow changes direction, e.g. in a bend, losses increase at low Reynolds number (2x at Re---- 2 0 0 , 4 x at Re -- 100 and 8 x at Re = 50) The total loss is simply the arithmetical sum of the individual losses

For the 90 ~ bend at B: Pc = 2 x 8 9 = 8.1 kN/m 2 For the reliefvalve: Pc = 1 2 x 8 9 -- 48.6 kNlm 2 For the filter: P, = 300 kN/m 2

P =ZP,+EP,

For ABC : P = 137+48.6+`300+8.1 ~- 494 kN/m 2

As for 2(a) to (d)

For DEF: Pp = 2.8 x 0.8 = 2.24 kN/m 2

Pressure drop--return lines (a) Calculate pressure losses as for delivery lines

K (2 bends, entrance and exit) -2+2+1+1 --6

Keep lines as direct as possible, with minimum fittings. Selfdraining lines are best ( 189~ minimum slope)

Pe -- 6 x 89x 900 x 0.32 -- 0.25 kN/m 2 P = 2.49 kN/m 2 (b) Express Pus hydrostatic head of oil

P h----

Pg (c) Check to ensure positive pressure after each fitting

h =

2490

-- 0.28 m

900 x 9.81

h must be less than vertical drop available, i.e. <0.55 m in the example of Figure 24.1

For the entrance D: K -- 1, Pc -- 40 N/m 2, h -- 45 mm Thus, return tray must be at least 45 mm deep

Increase bore size if this condition cannot be met. The entrance to the return pipe is often a source of difficulty

For G in Figure 24.1: Q. = 250 cm3/s; d = 4 mm, say, when v = 18 m/s (Figure 24.2)

Fine bore tubes make better restrictors than orifices for large pressure drops Base calculations on actual bore sizes not nominal bores If/impractically longor short,

Pressure drop--coiled /~'pes Linear restrictors for hydrostatic bearing circuits are often coiled capilliary tubing. Coiling increases the pressure drop and must be allowed for when calculating the length of the restrictor

132qvl

P,- ~--or use Fig. 4 e, = e , x z ,

Z from Fig. 6

Pp = 3.5 MN/m2/m (Figure 24.4), giving 1 -- 1.71 m f o r 6 MN/m 2

Now Re = 650 (Figure 24.5) and if D (dia. of increase or reduce v, respectively, and repeat coil) = 36 mm, Z - 2 calculation Thus required I -- 1.71/2 = 0.85 m

* When using the equations with Imperial units, these must be self-consistent. For example, ~ i n in3/s; v in in/s; d, k, e in in; ~ in reyns (1 reyn = 69000 poise); p in slugs/in 3 (1 slug/in 3 = 32.2 !b/in 3 ); p in lbf/in 2 ; g - ~36 in/s 2

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A24.4

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A guide to piping design kN/m2/m

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Ibf/in2/ft "---5O0

10000--

d mm in 50.----2

5000"m200

V

m/s

ft/s mlO0

--100 2000-- --

E w

.,-.,._

-.-! cP --1000

20-~

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1000"-, --

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----500 500--

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50--'I_._ 2 B

m

0.5-m

.--.20

---I

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20---B m

--0.1

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0.2-'--"

nl0

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---I

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2---

0.1~

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---0.05

0.05~ ---0.1 ---0.2

2----

nO.1

1---

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0.02 --.

--1

m0.05 m

0.01--

0.5-0,5-

---0.02

TRANSFER LINE

Figure 24.4 Pressure losses per unR length in pipes

(Re < 2 0 0 0 )

A24.5

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A guide to piping design

A24 V

m/s

ft/s --200

50---

10000r-

TRANSFER LINE

~=~

- --100

cS --1000

d

20--

in --10

mrn

m

5000

" --50 200-~-"

--500 10--

--5 100-2000

. -20 5-"--

--200 B

50.- --2 1000

Lloo 2--

---1 500

-~5

20---

B

--50

1--

B

-0.5 10 --2

200

0.5--_

--20 -0.2

5"~

B

100

"-10 0.2"-

--0.1 --0.5

50

--,.

n

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m

--5 0.1--

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m

1--- --0.2

20 0.05--

.-2

-

0.5-- --0.02 --0.1 .-1 0.3"0.01

Figure 24.5 Nomogram for Reynolds No.

Re

pVd

Vd

77

v

-

A24.6

A24

A guide to piping design Table 24.4 Loss coefficients Enlargement

Contraction

..,.j=.m--

_.~

----u

~

'

Restrictions

Im..

_

A ...~.

m

a

-.>.d

Bends

~

...w

~

I

r---

Lid<5

(~)

(A/a) 2 w h e r e A is the area of the pipe and a the open

K= K=

0"5

1

2

area available

FITTINGS

Entrances

+

Exit

+

K= 0

-"

§

0.5

T-junction ........

1

--II-"

1

3.0

Stop

0.5

jVon.return Spool

Gate

(b)

Plug

Needle

Flap

Ball

-.t.-.

VALVES

= =

K =

,

2

20

60

10

50-1 O0

Use manufacturers' figures, where available. Approximate loss coefficient in doubtful cases may be obtained from the formula

(c) Approximate loss coefficient K

=

, .

5-1 O0

Cross-sectional area of approach pipe ~ 2

(Cross-sectional area of pipe~ 2 \- ~ ~ o'fs~ra~n~r /

STRAINERS

(a)

Not Susceptible to calculation--use manufacturers' data. If a relief bypass valve is included the actual pressure drop may be two or three times the nominal setting.

FILTERS

/

/

3 N

3

/

/

O i-u z O . = 2 u uJ I~' '

/

0 u

I I I I I

II

9

10

I

I

I

lit

10 3

10 2

Re x ( - ~ ) ~

Figure 24.6 Correction factor Z for flow through curved capillary tubes of bore diameter d and coil diameter D

A24.7