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Hydrodynamic sealing of centrifugal pumps — keys to success
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PECIAL FEATURE
Hydrodynamic sealing o f keys centrifilgal p u m p s to s u c c e s s M e c h a n i c a l seals a n d c o m p r e s s i o n p a c k i n g used in centrifugal p u m p s have seen i m p r o v e m e n t in the last 5 to 10 y e a r s . This t y p e o f sealing c a n p e r f o r m well a n d g e n e r a l l y h a n d l e most p u m p applications. However, Michael W D a y , M a r k e t M a n a g e r f o r Pulp & Paper at Goulds Pumps believes the p r o c e s s i n d u s t r i e s a n d , in p a r t i c u l a r , the p u l p & p a p e r i n d u s t r y , m a y h a v e o v e r l o o k e d another s o l u t i o n - the h y d r o d y n a m i c seal Figure 1. Typical end-suction paper stock pump with dynamic seal
Introduction C o n v e n t i o n a l p a c k i n g a n d m e c h a n i c a l seals f u n c t i o n to p r o v i d e a b a r r i e r b e t w e e n t h e p u m p e d liquid a n d the atmosphere. Frequently, seal faces m u s t be flushed with fresh water from a n outside source b e c a u s e the p u m p e d fluid is u n s u i t a b l e as a l u b r i c a n t for shaft seals. Unless a s u i t a b l e e n v i r o n m e n t is provided, c o m p r e s s i o n packing a n d m e c h a n i c a l seal face m a t e r i a l s may have very short service lives. The r e q u i r e m e n t for a fresh w a t e r source can be a p r o b l e m in m a n y installations. In some climates, the w a t e r source m u s t be protected from freezing. Distance can play a p a r t when the p u m p is located remotely from the w a t e r source. In arid regions w a t e r is scarce a n d expensive. Recycle mills located in m e t r o p o l i t a n areas m u s t use costly w a t e r from m u n i c i p a l systems. In a l m o s t any s i t u a t i o n i n s t a l l a t i o n of water lines is costly a n d requires c o n s t a n t m o n i t o r i n g a n d maintenance.
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Priciple of operation
The design of the h y d r o d y n a m i c seal helps to e l i m i n a t e most of these problems. In general, the s t a n d a r d e n d s u c t i o n p u m p (Figure 1) is fitted with a repeller (also k n o w n as expeller) between the stuffing box c h a m b e r a n d the impeller.
© 1995, Elsevier Science Ltd 0262 1762/95/$7.00
The repeller functions like a n impeller u p o n s t a r t u p a n d p u m p s liquid from the stuffing box. This action allows outside air to follow the p u m p fluid t h r o u g h the stuffing box into the repeller c h a m b e r where an air-liquid interface forms at some p o i n t on the repeller. The stuffing box t h u s r e m a i n s free of the p u m p e d liquid. When the p u m p is s h u t down, a d i a p h r a g m secondary seal (Figure 2) prevents liquid from escaping from the stuffing box chamber. The m a j o r a d v a n t a g e of the d y n a m i c seal design is t h a t the repeller does the sealing w h e n the p u m p is operating. The secondary seal does n o t r u n in the p u m p a g e d u r i n g o p e r a t i o n t h u s is n o t subjected to the degrading effects of the fluid. E x t e r n a l flush w a t e r is not r e q u i r e d for most applications. The basic benefits of the h y d r o d y n a m i c s e a l i n g m e c h a n i s m r e s u l t s in e l i m i n a t i o n of leakage, fluid c o n t a m i n a t i o n , a n d r e d u c t i o n of wear on the p u m p a n d sealing c o m p o n e n t s d u r i n g operation. To realize the effective use of the hydrodyn a m i c seal for centrifugal p u m p s , one m a j o r aspect m u s t be the full u n d e r s t a n d i n g of the stuffing box pressure for the p u m p involved. Analytical t e c h n i q u e s can p e r m i t r e a s o n a b l e prediction of stuffing box pressure. This becomes the pressure a g a i n s t which the d y n a m i c
WORLD PUMPS JANUARY 1995
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S
PECIAL FEATURE
Gland
Diaphragm
,o,, ....
Figure 2. Diaphragm secondary seal for dynamic seal seal m u s t o p e r a t e . The b e s t m e t h o d for e s t a b l i s h ing stuffing b o x p r e s s u r e involves a c t u a l l a b o r a tory t e s t i n g u n d e r varying suction c o n d i t i o n s t h r o u g h o u t t h e o p e r a t i n g r a n g e of t h e centrifugal p u m p curve from zero to m a x i m u m flow. Effects of o p e r a t i n g p a r a m e t e r s such as inc r e a s e d i m p e l l e r c l e a r a n c e o r t h e a d d i t i o n of i m p e l l e r b a l a n c e h o l e s m u s t be a c c u r a t e l y d e t e r m i n e d to p e r m i t p r o p e r a p p l i c a t i o n ; b o t h on new e q u i p m e n t a n d t h e retrofit of e x i s t i n g equipment.
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Key influencing factors
Figure 3 p o r t r a y s t h e p r e s s u r e d i s t r i b u t i o n in a h y d r o d y n a m i c a l l y s e a l e d centrifugal p u m p . Beginning at the pump suction, the section p r e s s u r e is r e p r e s e n t e d by Hs. The p r e s s u r e g e n e r a t e d w i t h t h e i m p e l l e r is d e s i g n a t e d Hd. Therefore t h e v o l u t e p r e s s u r e is equal to Hs + Hd. D e p e n d i n g u p o n t h e p u m p design ( i m p e l l e r specific s p e e d a n d / o r casing c o n f i g u r a t i o n ) t h e typical r a n g e for t h e v o l u t e p r e s s u r e will be 75% 90% of t h e d i s c h a r g e p r e s s u r e . The p r e s s u r e d r o p b e h i n d t h e i m p e l l e r will vary d e p e n d i n g u p o n t h e configuration of t h e impeller. In t h e e x a m p l e
C h a r a c t e r i s t i c s of a good h y d r o d y n a m i c sealing design s h o u l d include little o r no d e g r a d a t i o n of suction head sealing capability with axial i m p e l l e r a d j u s t m e n t a s s o c i a t e d with i m p e l l e r wear. Provision s h o u l d be i n c l u d e d for a x i a l m o v e m e n t d u e to i m p e l l e r wear. C a p a b i l i t y to handle published sealing head limitations t h r o u g h o u t t h e p u m p curve; for e x a m p l e , back to m i n i m u m flow conditions, s h o u l d be provided. Clear identification of modifications, such as i m p e l l e r b a l a n c e h o l e s r e q u i r e d to r e d u c e stuffing b o x p r e s s u r e are necessary. The i m p a c t of t h e a d d i t i o n of i m p e l l e r b a l a n c e holes to such c r i t i c a l p u m p p e r f o r m a n c e c r i t e r i a s u c h as NPSHr s h o u l d be clearly identified. The a d d i t i o n a l h o r s e p o w e r r e q u i r e m e n t s cons u m e d by the r e p e l l e r s h o u l d be clearly indicated. The sealing capability and brake h o r s e p o w e r r e q u i r e m e n t t r a d e - o f f m u s t be c o n s i d e r e d in t h e i n i t i a l d e s i g n p h a s e s by e v a l u a t i n g t h e n u m b e r of r e p e l l e r v a n e s a n d t h e w i d t h of t h e r e p e l l e r vanes from b o t h an analytical and empirical test standpoint.
Figure 3. Pressure distribution in a dynamic seal centrifugal pump
Figure 4. During operation the dynamic seal evacuates the stuffing box
Hydrodynamic seal: pressure distribution
|(
shown, p u m p o u t vanes were utilized which will r e d u c e t h e p r e s s u r e from Hs ÷ H d a t t h e i m p e l l e r periphery, to Hs + X at t h e i m p e l l e r hub. F r o m t h e b a s e of t h e repeller, t h e p r e s s u r e d r o p w o u l d increase d u e to s i m p l e d i s k friction on t h e fiat side of t h e r e p e l l e r so t h e p r e s s u r e at t h e p e r i p h e r y of t h e r e p e l l e r is equal to Hs ÷ Y. The p r e s s u r e b r e a k d o w n or, in o t h e r words, t h e p r e s s u r e g e n e r a t i o n of t h e r e p e l l e r vanes m u s t t h e n equal Hs ÷ Y so t h a t at t h e b a s e of t h e repeller vane the total pressure has been r e d u c e d to a t m o s p h e r i c p r e s s u r e (or p e r h a p s a slight v a c u u m ) . This w o u l d t h e n yield a stuffing b o x c h a m b e r ( F i g u r e 4) w h i c h w o u l d n o t c o n t a i n any p u m p a g e d u r i n g o p e r a t i o n of t h e pump.
WORLD PUMPS JANUARY 1995
S PECIAL FEATURE
[m A p p l i c a t i o n c o n s i d e r a t i o n s -- w h e n t o c o n s i d e r ? A p p l i c a t i o n s which should receive serious consideration for h y d r o d y n a m i c seals include the following liquids or services: • • • • • •
Paper stock (up to 6%) Liquids t h a t are poor l u b r i c a n t s Chemical recovery services (i.e. weak black liquor, white liquor) Raw w a t e r services Mining a p p l i c a t i o n s / l i g h t slurries (i.e. coating services) Packing or single seals using e x t e r n a l seal water flush
Each of the above categories p r e s e n t s certain c r i t e r i a w h i c h involve e i t h e r solids in t h e p u m p a g e ( p a p e r stock, black liquor, slurries) w h i c h p r e s e n t s e a l i n g p r o b l e m s for e i t h e r c o n v e n t i o n a l p a c k i n g or m e c h a n i c a l seals. E v a p o r a t o r services are good c a n d i d a t e s when e x t e r n a l seal w a t e r is utilized. Energy m u s t be e x p e n d e d to evaporate off the injected seal water. Raw w a t e r p u m p services are good c a n d i d a t e s to i n c r e a s e service life d u e to p o t e n t i a l s a n d or grit p r e s e n t in the p u m p a g e which reduces packing or mechanical seal life. This is e n h a n c e d by the fact t h a t raw water p u m p s are generally remotely located from the m a i n process area a n d t h u s receive little or no m a i n t e n a n c e a t t e n t i o n . Light slurries such as coating slurries in the p a p e r m a c h i n e area offer excellent o p p o r t u n i t i e s for dynamic seals. Finally, any p u m p which is utilizing e x t e r n a l seal water for either packing or m e c h a n i c a l seals should be viewed as a potential c a n d i d a t e for the i n s t a l l a t i o n of a hydrodynamic seal. The service o p e r a t i n g conditions, suction conditions, a n d m a i n t e n a n c e h i s t o r y will be key factors in evaluating the suitability of the p u m p u n d e r c o n s i d e r a t i o n for retrofit to a hydrodynamic seal.
For example, d y n a m i c seals can be used on v a c u u m services b u t the impeller should n o t utilize b a l a n c e holes to f u r t h e r reduce stuffing box pressure. C a u t i o n should be exercised on liquids at the boiling p o i n t or highly viscous liquids since a d d i t i o n a l heat i n p u t may raise the liquid t e m p e r a t u r e sufficiently to cause flashing or solids to come out of s u s p e n s i o n a n d solidify t h u s creating o p e r a t i n g problems. A controlled e x t e r n a l flush is often a practical solution to this situation. For higher consistency p a p e r stock applications; a seal c h a m b e r flush to p r e v e n t stock from plugging d u r i n g s h u t d o w n c o n d i t i o n s is a excellent c o n s i d e r a t i o n to i n c o r p o r a t e into the i n i t i a l c o n f i g u r a t i o n . In m a n y of t h e s e situations, provision for a very controlled seal water flush a n d repeller c h a m b e r flush is a wise c o n s i d e r a t i o n with the actual utilization of these c o n n e c t i o n s d e p e n d e n t u p o n the final o p e r a t i n g c o n d i t i o n s found in the field.
[[~ W h e n n o t to u s e hydrodynamics seals? Situations which m a n d a t e zero leakage applicat i o n s such as toxic or c a r c i n o g e n i c l i q u i d s c a n n o t use d y n a m i c seals a n d should utilize sealless p u m p designs. Normally severe corrosive liquids are not good c a n d i d a t e s for d y n a m i c seals due to the e x p o s u r e of the p u m p a g e to a t m o s p h e r e when the centrifugal p u m p is s h u t down. In general when a liquid is sensitive to moisture, heat, or air (such as latex), it is p r u d e n t to avoid a d y n a m i c seal for t h o s e applications. One i m p o r t a n t factor w h e n considering the use of h y d r o d y n a m i c seals is t h a t they are n o t an absolute solution for all situations, b u t they are a very viable tool for m a n y a p p l i c a t i o n s a n d can yield significant economic benefits when properly applied.
Summary B
When to use caution?
Centrifugal p u m p a p p l i c a t i o n s which can utilize hydrodynamic seals b u t should be e x a m i n e d with c a u t i o n would include the following: • Vacuum services or suction lift c o n d i t i o n s • Liquids at boiling p o i n t ( c o n d e n s a t e ) • Highly Viscous liquids (i.e. high solids black liquor -- 70%) • Liquids t h a t crystallize when exposed to air Centrifugal p u m p s used in these a p p l i c a t i o n s can effectively h a n d l e these liquids with the p r o p e r c a u t i o n s a n d considerations.
This review has illustrated the basic o p e r a t i o n of t h e h y d r o d y n a m i c seal b e g i n n i n g with t h e u n d e r s t a n d i n g of the p u m p stuffing box pressure u n d e r a variety of conditions. The significant design characteristics which i m p a c t the selection a n d a p p l i c a t i o n of a hydrodynamic seal are identified. A p p l i c a t i o n s which are good candidates for d y n a m i c seals are given as well as s i t u a t i o n s in which c a u t i o n should be exercised. The use of d y n a m i c seals p r e s e n t s a n excellent o p p o r t u n i t y to reduce m a i n t e n a n c e costs, reduce seal w a t e r usage a n d improve h o u s e k e e p i n g when applied properly. •
WORLD PUMPS JANUA[¢Y 1995
:]
PECIAL FEATURE
Hydrodynamic sealing o f keys centrifilgal p u m p s to s u c c e s s M e c h a n i c a l seals a n d c o m p r e s s i o n p a c k i n g used in centrifugal p u m p s have seen i m p r o v e m e n t in the last 5 to 10 y e a r s . This t y p e o f sealing c a n p e r f o r m well a n d g e n e r a l l y h a n d l e most p u m p applications. However, Michael W D a y , M a r k e t M a n a g e r f o r Pulp & Paper at Goulds Pumps believes the p r o c e s s i n d u s t r i e s a n d , in p a r t i c u l a r , the p u l p & p a p e r i n d u s t r y , m a y h a v e o v e r l o o k e d another s o l u t i o n - the h y d r o d y n a m i c seal Figure 1. Typical end-suction paper stock pump with dynamic seal
Introduction C o n v e n t i o n a l p a c k i n g a n d m e c h a n i c a l seals f u n c t i o n to p r o v i d e a b a r r i e r b e t w e e n t h e p u m p e d liquid a n d the atmosphere. Frequently, seal faces m u s t be flushed with fresh water from a n outside source b e c a u s e the p u m p e d fluid is u n s u i t a b l e as a l u b r i c a n t for shaft seals. Unless a s u i t a b l e e n v i r o n m e n t is provided, c o m p r e s s i o n packing a n d m e c h a n i c a l seal face m a t e r i a l s may have very short service lives. The r e q u i r e m e n t for a fresh w a t e r source can be a p r o b l e m in m a n y installations. In some climates, the w a t e r source m u s t be protected from freezing. Distance can play a p a r t when the p u m p is located remotely from the w a t e r source. In arid regions w a t e r is scarce a n d expensive. Recycle mills located in m e t r o p o l i t a n areas m u s t use costly w a t e r from m u n i c i p a l systems. In a l m o s t any s i t u a t i o n i n s t a l l a t i o n of water lines is costly a n d requires c o n s t a n t m o n i t o r i n g a n d maintenance.
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Priciple of operation
The design of the h y d r o d y n a m i c seal helps to e l i m i n a t e most of these problems. In general, the s t a n d a r d e n d s u c t i o n p u m p (Figure 1) is fitted with a repeller (also k n o w n as expeller) between the stuffing box c h a m b e r a n d the impeller.
© 1995, Elsevier Science Ltd 0262 1762/95/$7.00
The repeller functions like a n impeller u p o n s t a r t u p a n d p u m p s liquid from the stuffing box. This action allows outside air to follow the p u m p fluid t h r o u g h the stuffing box into the repeller c h a m b e r where an air-liquid interface forms at some p o i n t on the repeller. The stuffing box t h u s r e m a i n s free of the p u m p e d liquid. When the p u m p is s h u t down, a d i a p h r a g m secondary seal (Figure 2) prevents liquid from escaping from the stuffing box chamber. The m a j o r a d v a n t a g e of the d y n a m i c seal design is t h a t the repeller does the sealing w h e n the p u m p is operating. The secondary seal does n o t r u n in the p u m p a g e d u r i n g o p e r a t i o n t h u s is n o t subjected to the degrading effects of the fluid. E x t e r n a l flush w a t e r is not r e q u i r e d for most applications. The basic benefits of the h y d r o d y n a m i c s e a l i n g m e c h a n i s m r e s u l t s in e l i m i n a t i o n of leakage, fluid c o n t a m i n a t i o n , a n d r e d u c t i o n of wear on the p u m p a n d sealing c o m p o n e n t s d u r i n g operation. To realize the effective use of the hydrodyn a m i c seal for centrifugal p u m p s , one m a j o r aspect m u s t be the full u n d e r s t a n d i n g of the stuffing box pressure for the p u m p involved. Analytical t e c h n i q u e s can p e r m i t r e a s o n a b l e prediction of stuffing box pressure. This becomes the pressure a g a i n s t which the d y n a m i c
WORLD PUMPS JANUARY 1995
:!
S
PECIAL FEATURE
Gland
Diaphragm
,o,, ....
Figure 2. Diaphragm secondary seal for dynamic seal seal m u s t o p e r a t e . The b e s t m e t h o d for e s t a b l i s h ing stuffing b o x p r e s s u r e involves a c t u a l l a b o r a tory t e s t i n g u n d e r varying suction c o n d i t i o n s t h r o u g h o u t t h e o p e r a t i n g r a n g e of t h e centrifugal p u m p curve from zero to m a x i m u m flow. Effects of o p e r a t i n g p a r a m e t e r s such as inc r e a s e d i m p e l l e r c l e a r a n c e o r t h e a d d i t i o n of i m p e l l e r b a l a n c e h o l e s m u s t be a c c u r a t e l y d e t e r m i n e d to p e r m i t p r o p e r a p p l i c a t i o n ; b o t h on new e q u i p m e n t a n d t h e retrofit of e x i s t i n g equipment.
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Key influencing factors
Figure 3 p o r t r a y s t h e p r e s s u r e d i s t r i b u t i o n in a h y d r o d y n a m i c a l l y s e a l e d centrifugal p u m p . Beginning at the pump suction, the section p r e s s u r e is r e p r e s e n t e d by Hs. The p r e s s u r e g e n e r a t e d w i t h t h e i m p e l l e r is d e s i g n a t e d Hd. Therefore t h e v o l u t e p r e s s u r e is equal to Hs + Hd. D e p e n d i n g u p o n t h e p u m p design ( i m p e l l e r specific s p e e d a n d / o r casing c o n f i g u r a t i o n ) t h e typical r a n g e for t h e v o l u t e p r e s s u r e will be 75% 90% of t h e d i s c h a r g e p r e s s u r e . The p r e s s u r e d r o p b e h i n d t h e i m p e l l e r will vary d e p e n d i n g u p o n t h e configuration of t h e impeller. In t h e e x a m p l e
C h a r a c t e r i s t i c s of a good h y d r o d y n a m i c sealing design s h o u l d include little o r no d e g r a d a t i o n of suction head sealing capability with axial i m p e l l e r a d j u s t m e n t a s s o c i a t e d with i m p e l l e r wear. Provision s h o u l d be i n c l u d e d for a x i a l m o v e m e n t d u e to i m p e l l e r wear. C a p a b i l i t y to handle published sealing head limitations t h r o u g h o u t t h e p u m p curve; for e x a m p l e , back to m i n i m u m flow conditions, s h o u l d be provided. Clear identification of modifications, such as i m p e l l e r b a l a n c e h o l e s r e q u i r e d to r e d u c e stuffing b o x p r e s s u r e are necessary. The i m p a c t of t h e a d d i t i o n of i m p e l l e r b a l a n c e holes to such c r i t i c a l p u m p p e r f o r m a n c e c r i t e r i a s u c h as NPSHr s h o u l d be clearly identified. The a d d i t i o n a l h o r s e p o w e r r e q u i r e m e n t s cons u m e d by the r e p e l l e r s h o u l d be clearly indicated. The sealing capability and brake h o r s e p o w e r r e q u i r e m e n t t r a d e - o f f m u s t be c o n s i d e r e d in t h e i n i t i a l d e s i g n p h a s e s by e v a l u a t i n g t h e n u m b e r of r e p e l l e r v a n e s a n d t h e w i d t h of t h e r e p e l l e r vanes from b o t h an analytical and empirical test standpoint.
Figure 3. Pressure distribution in a dynamic seal centrifugal pump
Figure 4. During operation the dynamic seal evacuates the stuffing box
Hydrodynamic seal: pressure distribution
|(
shown, p u m p o u t vanes were utilized which will r e d u c e t h e p r e s s u r e from Hs ÷ H d a t t h e i m p e l l e r periphery, to Hs + X at t h e i m p e l l e r hub. F r o m t h e b a s e of t h e repeller, t h e p r e s s u r e d r o p w o u l d increase d u e to s i m p l e d i s k friction on t h e fiat side of t h e r e p e l l e r so t h e p r e s s u r e at t h e p e r i p h e r y of t h e r e p e l l e r is equal to Hs ÷ Y. The p r e s s u r e b r e a k d o w n or, in o t h e r words, t h e p r e s s u r e g e n e r a t i o n of t h e r e p e l l e r vanes m u s t t h e n equal Hs ÷ Y so t h a t at t h e b a s e of t h e repeller vane the total pressure has been r e d u c e d to a t m o s p h e r i c p r e s s u r e (or p e r h a p s a slight v a c u u m ) . This w o u l d t h e n yield a stuffing b o x c h a m b e r ( F i g u r e 4) w h i c h w o u l d n o t c o n t a i n any p u m p a g e d u r i n g o p e r a t i o n of t h e pump.
WORLD PUMPS JANUARY 1995
S PECIAL FEATURE
[m A p p l i c a t i o n c o n s i d e r a t i o n s -- w h e n t o c o n s i d e r ? A p p l i c a t i o n s which should receive serious consideration for h y d r o d y n a m i c seals include the following liquids or services: • • • • • •
Paper stock (up to 6%) Liquids t h a t are poor l u b r i c a n t s Chemical recovery services (i.e. weak black liquor, white liquor) Raw w a t e r services Mining a p p l i c a t i o n s / l i g h t slurries (i.e. coating services) Packing or single seals using e x t e r n a l seal water flush
Each of the above categories p r e s e n t s certain c r i t e r i a w h i c h involve e i t h e r solids in t h e p u m p a g e ( p a p e r stock, black liquor, slurries) w h i c h p r e s e n t s e a l i n g p r o b l e m s for e i t h e r c o n v e n t i o n a l p a c k i n g or m e c h a n i c a l seals. E v a p o r a t o r services are good c a n d i d a t e s when e x t e r n a l seal w a t e r is utilized. Energy m u s t be e x p e n d e d to evaporate off the injected seal water. Raw w a t e r p u m p services are good c a n d i d a t e s to i n c r e a s e service life d u e to p o t e n t i a l s a n d or grit p r e s e n t in the p u m p a g e which reduces packing or mechanical seal life. This is e n h a n c e d by the fact t h a t raw water p u m p s are generally remotely located from the m a i n process area a n d t h u s receive little or no m a i n t e n a n c e a t t e n t i o n . Light slurries such as coating slurries in the p a p e r m a c h i n e area offer excellent o p p o r t u n i t i e s for dynamic seals. Finally, any p u m p which is utilizing e x t e r n a l seal water for either packing or m e c h a n i c a l seals should be viewed as a potential c a n d i d a t e for the i n s t a l l a t i o n of a hydrodynamic seal. The service o p e r a t i n g conditions, suction conditions, a n d m a i n t e n a n c e h i s t o r y will be key factors in evaluating the suitability of the p u m p u n d e r c o n s i d e r a t i o n for retrofit to a hydrodynamic seal.
For example, d y n a m i c seals can be used on v a c u u m services b u t the impeller should n o t utilize b a l a n c e holes to f u r t h e r reduce stuffing box pressure. C a u t i o n should be exercised on liquids at the boiling p o i n t or highly viscous liquids since a d d i t i o n a l heat i n p u t may raise the liquid t e m p e r a t u r e sufficiently to cause flashing or solids to come out of s u s p e n s i o n a n d solidify t h u s creating o p e r a t i n g problems. A controlled e x t e r n a l flush is often a practical solution to this situation. For higher consistency p a p e r stock applications; a seal c h a m b e r flush to p r e v e n t stock from plugging d u r i n g s h u t d o w n c o n d i t i o n s is a excellent c o n s i d e r a t i o n to i n c o r p o r a t e into the i n i t i a l c o n f i g u r a t i o n . In m a n y of t h e s e situations, provision for a very controlled seal water flush a n d repeller c h a m b e r flush is a wise c o n s i d e r a t i o n with the actual utilization of these c o n n e c t i o n s d e p e n d e n t u p o n the final o p e r a t i n g c o n d i t i o n s found in the field.
[[~ W h e n n o t to u s e hydrodynamics seals? Situations which m a n d a t e zero leakage applicat i o n s such as toxic or c a r c i n o g e n i c l i q u i d s c a n n o t use d y n a m i c seals a n d should utilize sealless p u m p designs. Normally severe corrosive liquids are not good c a n d i d a t e s for d y n a m i c seals due to the e x p o s u r e of the p u m p a g e to a t m o s p h e r e when the centrifugal p u m p is s h u t down. In general when a liquid is sensitive to moisture, heat, or air (such as latex), it is p r u d e n t to avoid a d y n a m i c seal for t h o s e applications. One i m p o r t a n t factor w h e n considering the use of h y d r o d y n a m i c seals is t h a t they are n o t an absolute solution for all situations, b u t they are a very viable tool for m a n y a p p l i c a t i o n s a n d can yield significant economic benefits when properly applied.
Summary B
When to use caution?
Centrifugal p u m p a p p l i c a t i o n s which can utilize hydrodynamic seals b u t should be e x a m i n e d with c a u t i o n would include the following: • Vacuum services or suction lift c o n d i t i o n s • Liquids at boiling p o i n t ( c o n d e n s a t e ) • Highly Viscous liquids (i.e. high solids black liquor -- 70%) • Liquids t h a t crystallize when exposed to air Centrifugal p u m p s used in these a p p l i c a t i o n s can effectively h a n d l e these liquids with the p r o p e r c a u t i o n s a n d considerations.
This review has illustrated the basic o p e r a t i o n of t h e h y d r o d y n a m i c seal b e g i n n i n g with t h e u n d e r s t a n d i n g of the p u m p stuffing box pressure u n d e r a variety of conditions. The significant design characteristics which i m p a c t the selection a n d a p p l i c a t i o n of a hydrodynamic seal are identified. A p p l i c a t i o n s which are good candidates for d y n a m i c seals are given as well as s i t u a t i o n s in which c a u t i o n should be exercised. The use of d y n a m i c seals p r e s e n t s a n excellent o p p o r t u n i t y to reduce m a i n t e n a n c e costs, reduce seal w a t e r usage a n d improve h o u s e k e e p i n g when applied properly. •
WORLD PUMPS JANUA[¢Y 1995
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