ABSTRACT

Analysis of Vibration and Failure Reciprocating Triplex Pumps for

J. C. WACHEL, President Member ASME

Engineering Dynamics Inc.

Problems in Oil Pipelines

F. R. SZENASI, Senior Project Engineer Member ASME

Engineering Dynamics Inc. San Antonio, Texas

S. C. DENISON, Production Engineer Consultant Member AIME/SPE

Tenneco Oil Exploration & Production, Inc. Houston, Texas

An a n a l y s i s was made t o i d e n t i f y t he causes of v i b r a t i o n and f a i l u r e problems wi th t he p ip ing and r e c i p r o c a t i n g pump i n t e r n a l s on an o i l p i p e l i n e pump s t a t i o n . A f i e l d i n v e s t i g a t i o n was made t o o b t a i n v i b r a t i o n s and p u l s a t i o n s over t h e e n t i r e range of p l a n t ope ra t ing cond i t i ons . The d a t a showed t h a t c a v i t a t i o n was p re sen t a t n e a r l y a l l ope ra t ing cond i t i ons due t o t h e h igh p u l s a t i o n s i n t he s u c t i o n system. The d i scha rge system exper ienced h igh v i b r a t i o n s and p ip ing f a i l u r e s due t o t h e i n e f f e c t i v e n e s s of the accumulator. An a c o u s t i c a l a n a l y s i s o f t he s u c t i o n and d i scha rge system was made t o des ign t h e optimum a c o u s t i c a l f i l t e r systems t o a l l e v i a t e t he problems. The a c o u s t i c a l ana lyses were performed wi th a d i g i t a l computer program which p r e d i c t s t h e a c o u s t i c a l resonant f r equenc ie s and the p u l s a t i o n ampl i tudes over t he speed range. Th i s paper d i s c u s s e s t he i n v e s t i g a t i o n s and g ives recommendations f o r p reven t ion of t hese types of problems i n t h e f u t u r e .

INTRODUCTION

Problems were exper ienced wi th fou r t r i p l e x r e c i p r o c a t i n g crude o i l pumps ope ra t ing i n p a r a l l e l a t t he Dina Pumping S t a t i o n loca t ed i n Colombia (F igu re 1 ) . The pumps had a r a t e d speed o f 275 rpm wi th a c a p a c i t y of 388 g a l l o n s per minute. The nominal s u c t i o n p re s su re was 60 ps ig (414 kPa) and the d i scha rge p re s su re was 1800 p s i g (12400 kPa) . The D e l r i n pump v a l v e s had r epea t ed f a t i g u e f a i l u r e s beginning t h r e e months a f t e r s t a r t u p . The d i scha rge valve d i s k s were replaced wi th s t e e l and t h e De l r in d i s k s used on t h e s u c t i o n va lves were r ep l aced eve ry 90 days t o avoid f a t i g u e f a i l u r e s . Valve f a i l u r e s were c o n t r o l l e d a f t e r t he f i r s t 9 months o f s t a t i o n ope ra t ion . For t h e f i r s t f o u r months t h e r e were no p u l l rod f a i l u r e s ; however, t h e r e have been 18 f a i l u r e s i n t h e fo l lowing year and a h a l f . Many of t hese f a i l u r e s r equ i r ed replacement of t h e

crosshead, t h e guideways, and on two occas ions a broken o r bent connect ing rod. The s u c t i o n and d i scha rge p ip ing systems v i b r a t e d exces s ive ly , r e s u l t i n g In s e v e r a l p ip ing f a t i g u e f a i l u r e s . Attempts t o c o n t r o l t he p ip ing v i b r a t i o n s with p ipe clamps and a d d i t i o n a l suppor ts were unsuccessful .

The four pumps had a common s u c t i o n header suppl ied by a charge pump which was capable of supplying p re s su re s up t o 90 p s i (621 kPa) . The d i scha rge of the four pumps fed i n t o a common header which connected t o t he main p ipe l ine . The o r i g i n a l p ip ing des ign inc luded bladder- type accumulators on both the s u c t i o n and d i scha rge .

I t was d i f f i c u l t t o keep the pumps running smoothly s i n c e cons t an t maintenance was needed t o keep the accumulator b ladder p re s su re s charged t o approximate ly 60 t o 70 percent of l i n e p re s su re . The s t a t i c d i scha rge p re s su re could change from 700 ps ig (4826 kPa) t o more than 1600 ps ig (11032 kPa) In a few minutes i f t he down-line boos t e r s t a t i o n went down. When t h i s happened, t he accumulator was i n e f f e c t i v e .

The c o s t o f the p a r t s and l abo r t h a t could be a t t r i b u t e d t o t h i s problem was i n excess of $500,000. Tenneco, t he p ip ing des igne r and the pump manufacturer began a s tudy t o determine t h e cause o r causes of t he v i b r a t i o n s and f a i l u r e s . However, the complex r e l a t i o n s h i p of the system v a r i a b l e s made i t d i f f i c u l t t o develop d e f i n i t e conclus ions .

There were s e v e r a l changes made i n t he p ip ing system dur ing t h i s phase i n an a t tempt t o improve the v i b r a t i o n s and reduce the f a i l u r e s . These inc luded changing the p ip ing ( a t t he recommendation of the accumulator vendor) so t h a t t he flow would be d i r e c t e d a t t he b ladder . Th i s p ip ing mod i f i ca t ion d i d not improve t h e p u l s a t i o n c h a r a c t e r i s t i c s of the sy s tem.

Presented at the Energy-Sources and Technology Conference and Exhibition Dallas, Texas - February 17-21, 1985

Another mod i f i ca t ion which was t r i e d on t h e s u c t i o n s i d e of pumps 1 and 3 was the replacement of t he bladder- type accumulators w i th n i t rogen- charged, flow- through accumulators (F igu re 1 ) . No n o t i c e a b l e imurovements were observed a f t e r t h e s e changes were implemented.

BLADDER PDF\ ACCUMULATOR I I

I PUMP CASE I 101 I (

GAS-CHAROEDJ BOTTLE 1 I FIGURE 1 . Pump Piping Layout Showing P res su re

Measurement Locat ions

The s e v e r i t y of the problems brought the b a s i c des ign of t he system i n t o q u e s t i o n s i n c e t h e s u c t i o n and d i scha rge lead l i n e s from t h e headers t o t h e pump manifold were s h o r t e r than normal f o r most p i p e l i n e s t a t i o n s . The pumps were loca t ed on 16 foo t (4.88 m) c e n t e r s w i th t he s u c t i o n and d i scha rge headers l oca t ed 10 t o 12 f e e t (3.05 t o 3.66 m) away from t h e pump f l anges . The s t a t i o n capac i ty was 39900 b a r r e l s pe r day (264 m3 /h) when the pumps were a t t h e i r r a t e d capac i ty of 388 g a l l o n s per minute (88 m3/h). Th i s r e s u l t s i n a f l u i d v e l o c i t y of 3.3 f t / s ( 1 m/s) i n t he 12 inch schedule 40 s u c t i o n manifold and 6.9 f t / s (2.1 m/s) i n t he 10 inch schedule XS d i scha rge manifold . The flow v e l o c i t i e s i n t he i n d i v i d u a l pump p ip ing were 1.1 f t / s (0.34 m/s) i n t he 12 inch s t anda rd weight s u c t i o n p ipe and 2.7 f t / s (0.82 m/s) i n t h e 8 inch e x t r a heavy d i scha rge p ipe .

Engineering Dynamics Inco rpo ra t ed (EDI) was reques ted t o i n v e s t i g a t e and make recommendations t o a l l e v i a t e t he problems. The f i r s t s t e p i n t he a n a l y s i s was t o model t he a c o u s t i c a l c h a r a c t e r i s t i c s of t he p ip ing systems on a d i g i t a l computer program t o d e f i n e the expected p u l s a t i o n resonances and the o v e r a l l amplitudes i n t he s u c t i o n and d i scha rge p ip ing . A d e t a i l e d f i e l d i n v e s t i g a t i o n was then made t o e v a l u a t e t he p u l s a t i o n and v i b r a t i o n c h a r a c t e r i s t i c s of the pumps. So lu t ions were then developed t o e l i m i n a t e t he problems.

FIELD INVESTIGATION

Ins t rumen ta t ion And T e s t Procedures

The in s t rumen ta t ion and d a t a a c q u i s i t i o n system used t o determine the p u l s a t i o n and v i b r a t i o n c h a r a c t e r i s t i c s a r e shown i n Figure 2. P i e z o e l e c t r i c p re s su re t r ansduce r s and accelerometers were used t o measure t he p re s su re p u l s a t i o n s and the v i b r a t i o n s . A ske t ch of the pump

s u c t i o n and d i scha rge p ip ing i l l u s t r a t i n g some of the p re s su re t e s t p o i n t s a r e shown i n Figure 1. The p u l s a t i o n and v i b r a t i o n s i g n a l s were analyzed f o r frequency con ten t w i th a two channel Hewlett-Packard 3582A FFT analyzer and documented on a HP 74708 d i g i t a l p l o t t e r . The analyzer and ins t ruments were c o n t r o l l e d by an Apple 11+ microcomputer us ing sof tware w r i t t e n s p e c i a l l y f o r ana lyz ing v i b r a t i o n and p u l s a t i o n d a t a . To r s iona l v i b r a t i o n s were measured wi th a HBM t o r s iog raph mounted on the s t u b end o f t h e p in ion gea r s h a f t on pump 1 .

ITEM DESCRIPTION

2 Channel FFT Analyzer ~icro-Computer Floppy Disk Drive Digi ta l Plot ter Tuneable F i l t e r s 2 Channel Oscil loscope Transducer Signal Conditioner and Power Supply 8 Channel FH Tape Recorder Function Generator Strain Gage Amplifier and Frequency Demodulator

FIGURE 2. Data Acqu i s i t i on System

V i b r a t i o n And P u l s a t i o n T e s t i n g

The i n i t i a l v i b r a t i o n surveys r evea l ed h igh v i b r a t i o n amplitudes on the p ip ing , i n d i c a t i n g l a r g e e x c i t a t i o n fo rces p re sen t i n t h e p ip ing systems. Analys is of the p re s su re p u l s a t i o n waveforms revealed seve re c a v i t a t i o n i n t he s u c t i o n p ip ing system. Th i s c a v i t a t i o n was the source of t he h igh energy caus ing the h igh p ip ing v i b r a t i o n s , va lve f a i l u r e s , and pump p a r t f a i l u r e s .

Cav i t a t i on . For l i q u i d r e c i p r o c a t i n g pumps, t he s t a t i c p re s su re i n the s u c t i o n system must be adequate t o compensate f o r f r i c t i o n a l p re s su re drop l o s s e s , the r equ i r ed a c c e l e r a t i o n head, and the p u l s a t i o n s p re sen t i n t he system. Th i s ensu res t h a t t he p re s su re remains above t h e vapor p re s su re . The vapor p re s su re of t h e o i l was l e s s than 2 p s i a (13.8 kPa). When p u l s a t i o n s e x i s t i n a system, they w i l l c o n s i s t of a p o s i t i v e peak of p re s su re which w i l l be added t o t he s t a t i c p re s su re and a nega t ive peak which w i l l be s u b s t r a c t e d from t h e s t a t i c p re s su re . I f t he nega t ive peak of the p u l s a t i o n , when s u b t r a c t e d from the s t a t i c p re s su re , reaches t he vapor p re s su re , t h e f l u i d w i l l c a v i t a t e , r e s u l t i n g i n h igh p re s su re s p i k e s a s t he l i q u i d vapor i ze s and

then c o l l a p s e s a s the p re s su re i n c r e a s e s above the vapor p re s su re .

To i l l u s t r a t e t he e f f e c t s of c a v i t a t i o n , cons ide r t he plunger pressure- t ime wave shown i n Figure 3 which shows t h a t c a v i t a t i o n occurs on the s u c t i o n s t r o k e . Note t h a t when the c a v i t a t i o n p o r t i o n of the waveform i s expanded, t he p r e s s u r e sp ikes a r e approximately 800 p s i (5516 kPa) wi th a t ime per iod of approximate ly 0.00025 seconds. The presence of c a v i t a t i o n can u s u a l l y be observed on the complex wave s i n c e p u l s a t i o n s , which a r e g e n e r a l l y sine- shaped waves, w i l l " square- off" a t t he t rough of t he waves when the vapor p r e s s u r e i s reached. The type of d a t a t o s u b s t a n t i a t e c a v i t a t i o n (F igu re 4 ) i l l u s t r a t e s t h e squaring- off of t he wave, followed by t h e sha rp sp ik ing c h a r a c t e r i s t i c o f severe c a v i t a t i o n . Th i s d a t a , taken on pump u n i t 1 , showed p re s su re s p i k e s of 600 p s i (4137 kPa) .

FIGURE 3 . Pump Plunger Pressure-Time Wave Showing Cav i t a t i on

EMINEERING DYNAMICS INCORPORATED

FIGURE 4 . C a v i t a t i o n Caused By High Pu l sa t ions

The e f f e c t of t he s t a t i c p re s su re on t h e c a v i t a t i o n was i n v e s t i g a t e d by r a i s i n g the s u c t i o n p re s su re t o t he maximum p o s s i b l e (90 psig/621 kPa). The inc rease i n s u c t i o n p re s su re a lone was not s u f f i c i e n t t o e l i m i n a t e the c a v i t a t i o n . Severe

p u l s a t i o n s were found wi th l e v e l s i n excess of 200 p s i peak-to-peak (1379 kPa) . A t a s u c t i o n p re s su re of 76.5 p s i a (527 kPa) , pu l sa t ions of approximately 75 p s i (517 kPa) zero-peak a r e r equ i r ed t o cause c a v i t a t i o n . Th i s va lue i s obta ined by s u b t r a c t i n g the nega t ive ~ u l s a t i o n peak from the s t a t i c p re s su re . S ince p u l s a t i o n s g r e a t e r than 75 p s i (517 kPa) were always p re sen t a t t h e h ighe r speeds , c a v i t a t i o n always occurred .

In t he presence of c a v i t a t i o n , i t i s p r a c t i c a l l y imposs ib le t o e v a l u a t e the i n f luence of v a r i a b l e s , such a s t he e f f e c t of o t h e r u n i t s , speeds and the accumulator des ign. Obviously, a r educ t ion of the p re s su re p u l s a t i o n s was necessary i n o rde r t o ob ta in meaningful t e s t d a t a on the u n i t s .

Acous t i ca l Resonances. The major s u c t i o n p u l s a t i o n components were a t f r equenc ie s near 110 t o 150 Hz with p u l s a t i o n amplitudes of approximately 100 - 150 p s i (689 - 1034 kPa) peak-to-peak, which, when combined wi th the p u l s a t i o n a t t he lower pump harmonics, caused the o v e r a l l s t a t i c p re s su re t o d rop below the vapor pressure . I t was determined t h a t a c o u s t i c a l resonances were caus ing the h igh amplitude p u l s a t i o n s . Acous t i ca l resonances amplify the p u l s a t i o n s whenever one of t h e harmonics of the pump speed passes through the r e sonan t frequency. The a c o u s t i c a l resonance a t 130 Hz was a quarter-wave resonance of t he s u c t i o n p ipe and was a s s o c i a t e d wi th t he 9 foot (2.74 m) l eng th from the end of the s u c t i o n manifold t o t he accumulator.

When an a c o u s t i c a l resonance i s encountered i n a system, t he p re s su re pu l sa t ions can be reduced by e l i m i n a t i n g the resonance o r by a t t e n u a t i n g the ampl i tudes through the a d d i t i o n of a r e s i s t i v e element, such a s an o r i f i c e . Therefore , an o r i f i c e p l a t e was i n s t a l l e d a t t he s u c t i o n f l ange i n an a t tempt t o a t t e n u a t e t he p u l s a t i o n ampl i tudes and poss ib ly move the a c o u s t i c a l n a t u r a l frequency. A d iameter r a t i o ( o r i f i c e d iameter t o i n s i d e d iameter of p ipe) of approximately 0.4 was used t o ensure a s i g n i f i c a n t a c o u s t i c a l e f f e c t . When the o r i f i c e p l a t e was i n s t a l l e d , t he p u l s a t i o n s were reduced; however, t h e r educ t ion was no t s u f f i c i e n t t o completely e l i m i n a t e t he c a v i t a t i o n .

I n t e r a c t i o n With Other Pumps. A l l t h e o t h e r pumps were shu t down and pump 1 was run t o determine i f the c a v i t a t i o n was caused by i n t e r a c t i o n wi th t he o t h e r pumps o r was a func t ion of t h e i n d i v i d u a l p ip ing des ign. These t e s t s i nd i ca t ed t h a t t he pu l sa t ions were caused by the i n d i v i d u a l pumps and t h a t t h e major f a c t o r was the a c o u s t i c a l resonances near 130 Hz. Thi s t e s t a l s o gave evidence t h a t t he l o c a t i o n of the pump i n t he manifold system was not a major f a c t o r i n the c a v i t a t i o n . Th i s i s v e r i f i e d by the f a c t t h a t c a v i t a t i o n occurred on u n i t s 1 and 3 a t t he exac t same speed under the same ope ra t ing cond i t i ons . Uni ts 1 and 3 a r e s epa ra t ed by 32 f e e t (9.75 m) with u n i t 2 midway between them. I f t he l o c a t i o n of the pump i n t he header was a prime f a c t o r , t h e r e would have been d i f f e r e n t p u l s a t i o n and c a v i t a t i o n c h a r a c t e r i s t i c s . To f u r t h e r i n v e s t i g a t e the i n t e r a c t i o n of t he o t h e r pumps, t e s t s were made wi th pump 1 on the verge o f c a v i t a t i o n and the ad j acen t u n i t 2 was swept through the e n t i r e speed range t o determine i f i t a f f e c t e d t h e speed a t which c a v i t a t i o n occurred . This t e s t showed t h a t the ad j acen t u n i t d i d not i n f luence t h e c a v i t a t i o n .

In an a t tempt t o determine whether t h e a c o u s t i c a l resonance was a s s o c i a t e d wi th a p ip ing l eng th from the o t h e r u n i t s , t he s u c t i o n b lock va lve was pinched momentarily t o s ee i f a p re s su re drop taken on the upstream s i d e o f t he accumulator would a f f e c t t he resonances i n t he 130 Hz range. The p r e s s u r e drop of approximately 10 p s i i n t he b lock va lve d i d not have a s i g n i f i c a n t e f f e c t .

F i n a l Tes t ing . Af t e r t he o r i f i c e p l a t e was i n s t a l l e d and t h e nitrogen- charged accumulator b o t t l e on the s u c t i o n system had the maximum gas charge , t he c a v i t a t i o n was e l imina t ed over much of the speed range making it ~ o s s i b l e t o s tudy the e f f e c t of varying system parameters . The normal procedure f o r t he t e s t i n g was t o e s t a b l i s h a s e t of s t eady- s ta t e cond i t i ons , ( such a s s u c t i o n p r e s s u r e , gas volume i n t he b o t t l e , o r charge p r e s s u r e i n t he b ladder accumulator, speeds on the o t h e r pumps, e t c . ) and then change t h e pump speed from 190 rpm t o 290 rpm. During the speed run , t h e p u l s a t i o n s i n the s u c t i o n and d i scha rge p ip ing were t ape recorded f o r l a t e r eva lua t ion . The r e s u l t i n g d a t a p r e s e n t a t i o n f o r t he speed v a r i a t i o n i s g iven i n Figure 5, showing the harmonics of pump speed p u l s a t i o n p re s su re s i n t he s u c t i o n manifold of pump 3 over the speed range. The d a t a shows t h a t the primary cause of t he c a v i t a t i o n was the h i g h l e v e l p u l s a t i o n s a t t h e a c o u s t i c a l n a t u r a l f r equenc ie s i n the system near 130 and 140 Hz which were e x c i t e d by the 21s t through the 30 th harmonics of pump speed.

ENQlNEERlNO OVNUAlC.5 INCORPORATED

FIGURE 5. Speed Raster Of Pump Suct ion P u l s a t i o n s

Speed E f f e c t s . The e f f e c t of speed on - c a v i t a t i o n can be seen i n Figure 6 which g i v e s t he complex p re s su re wave f o r speeds from 220 t o 270 rpm f o r a s u c t i o n p re s su re of 60 ps ig (414 kPa) . P u l s a t i o n s g e n e r a l l y i nc rease wi th speed un le s s t h e r e a r e a c o u s t i c a l resonances . As shown, when t h e speed inc reased above 250 rpm, t h e p u l s a t i o n s i nc reased t o t h e p o i n t t h a t the n e g a t i v e p re s su re

p u l s a t i o n amplitude was near t he vapor p re s su re and the wave became f l a t t e n e d on the trough. As the speed was f u r t h e r i nc reased , t he c a v i t a t i o n became more seve re .

FIGURE 6. Complex Wave Of P re s su re Pu l sa t ion Versus Speed For Suct ion Pressure Of 60 P s i g

S t a t i c P r e s s u r e E f f e c t s . When the s t a t i c s u c t i o n p re s su re was increased t o 90 p s i g , the p u l s a t i o n ampl i tudes were reduced and t h e u n i t could be run a t 280 rpm wi thout c a v i t a t i o n (F igu re 7). The h ighe r s u c t i o n p re s su re seemed t o i n h i b i t t h e amplitude of t he p u l s a t i o n s . The r e s u l t s of t hese t e s t s i nd i ca t ed t h a t t he c a v i t a t i o n could be minimized by i n c r e a s i n g the s u c t i o n p re s su re t o t he maximum p o s s i b l e , i n s t a l l i n g an o r i f i c e p l a t e t o reduce t h e p u l s a t i o n ampl i tudes , and ensu r ing t h a t the accumulator was p rope r ly charged.

FIGURE 7. Complex Wave Of P re s su re Pu l sa t ion Versus Speed For Suct ion P re s su re Of 90 Ps ig

The e f f e c t i v e n e s s of t he gas-charged flow- through accumulator was s t r o n g l y inf luenced by t h e volume of

the n i t r o g e n gas i n t h e accumulator b o t t l e , F igure 8. The inc reased gas charge volume e l imina t ed p u l s a t i o n components of 46 p s i a t 12 Hz and 22 p s i a t 65 Hz.

FIGURE 8. E f f e c t Of Increased Gas Volume In Suct ion Accumulator

Discharge P u l s a t i o n s . The measured f i e l d d a t a showed h igh a m ~ l i t u d e p u l s a t i o n s i n t he - d i scha rge p ip ing wi th l e v e l s exceeding 1000 p s i (6895 kPa) peak-to-peak i n some t e s t s (F igu re 9). An i n v e s t i g a t i o n was made t o determine i f t he d i scha rge p u l s a t i o n s were a f f e c t e d by t h e c a v i t a t i o n on the s u c t i o n s i d e . The complex p re s su re wave a t t he s u c t i o n and d i scha rge va lves were captured s imul taneously du r ing t h e t ime t h a t s eve re c a v i t a t i o n was p r e s e n t and showed t h a t t h e d i scha rge s i d e was i s o l a t e d from the s u c t i o n s i d e . The p u l s a t i o n ampl i tudes were very h igh ; however, they were not caused by the c a v i t a t i o n on t h e suc t ion . The p u l s a t i o n s were a func t ion of t he energy ou tpu t from the p lungers and were s t r o n g l y in f luenced by the va lve r i n g i n g and t h e a c o u s t i c a l resonances a s d i c t a t e d by t h e a c o u s t i c a l p r o p e r t i e s of t he bladder- type accumulator and the p ip ing system. Whenever t he s t a t i o n d i scha rge p re s su re dropped below t h e charge p re s su re i n t h e b l adde r of the d i scha rge accumulator, a n o t i c e a b l e i n c r e a s e i n t h e p u l s a t i o n s occurred .

FIGURE 9 . Complex Wave Of Discharge ~ulsatione

ACOUSTIC SIMULATION

The r a p i d advances of d i g i t a l computers has made i t more p r a c t i c a l t o analyze t h e a c o u s t i c a l c h a r a c t e r i s t i c s ( p u l s a t i o n s ) of p ip ing sys tems d i g i t a l l y . A comprehensive computer program has been w r i t t e n by ED1 t o p r e d i c t the a c o u s t i c a l a t t e n u a t i o n c h a r a c t e r i s t i c s , pass bands, and p u l s a t i o n l e v e l s f o r p ip ing systems wi th l i q u i d pumps o r gas compressors. The program can be used t o des ign p u l s a t i o n f i l t e r s o r t o e v a l u a t e the e f f e c t i v e n e s s of systems wi th l i q u i d f g a s accumulators.

The program i s based on c l a s s i c a l f l u i d mechanics theory ( Navier-Stokes equa t ion , t he c o n t i n u i t y equa t ion , and t h e thermodynamic equa t ion of s t a t e ) . The assumption is made t h a t plane-wave propogat ion w i l l adequate ly s imu la t e t he motion of p re s su re d i s tu rbances and the a c o u s t i c response o f t y p i c a l p ip ing systems found i n most i n d u s t r i a l p l a n t s . The c l a s s i c a l equa t ions of f l u i d mechanics a r e combined wi th t h e p ipe element geometry t o d e f i n e the response of t he p ip ing system t o dynamic v a r i a t i o n s i n t he p re s su re and flow. The e f f e c t s of mean flow on damping ( p r e s s u r e drop, r e s i s t a n c e , e t c ) a r e inc luded i n t he a n a l y s i s . The program i s w r i t t e n i n a g e n e r a l manner s o t h a t any p ip ing system can be s imula ted by combinations of d i s t r i b u t e d o r lumped elements . In l i q u i d pump systems, t he p re s su re pu l se s genera ted by the plunger can be q u i t e complex. Because of t he incompress ib le medium, t h e f low r a t e i s a func t ion of p i s t o n v e l o c i t y , which i s not s i n u s o i d a l because of a f i n i t e cranklrod r a t i o . Geometrically- caused d i s t o r t i o n s produce h ighe r harmonics which must be inc luded i n t he forced p u l s a t i o n a n a l y s i s . The computer program gene ra t e s the plunger pressure- t ime wave and uses i t t o e x c i t e the p ip ing system.

The f i e l d t e s t s i d e n t i f i e d an a c o u s t i c resonance a s the cause of t he problem. Therefore , t he s o l u t i o n was t o move the a c o u s t i c a l resonances away from the s t rong pump harmonics. The ED1 d i g i t a l a c o u s t i c a l program was used t o s imu la t e t h e p ip ing system and t o develop t h e s o l u t i o n . The p ip ing geometry and the l i q u i d thermophysical p r o p e r t i e s d e f i n e the a c o u s t i c n a t u r a l f r equenc ie s of t he s u c t i o n and d i scha rge p ip ing systems. The r e s u l t s o f the computer a n a l y s i s of t he o r i g i n a l s u c t i o n p ip ing system wi th the gas-charged f i l t e r a r e g iven i n Figure 10. The d a t a p r e s e n t s t he p red ic t ed p u l s a t i o n s a t each harmonic of pump speed from minimum t o maximum speed. The harmonic numbers a r e i nd ica t ed ad j acen t t o the app ropr i a t e curve. The i n t e r a c t i o n of t he i n d i v i d u a l harmonics w i th the a c o u s t i c resonant f r equenc ie s a t 130 and 140 Hz can be seen. Th i s d a t a can be compared t o t he measured p u l s a t i o n s g iven i n Figure 5. Genera l ly , t h e r e was good agreement w i th t he a c o u s t i c a l resonances a t 130 and 140 Hz; however, t he c a l c u l a t e d ampl i tudes were lower. I t must be remembered i n t h e assessment of the f i e l d d a t a t h a t c a v i t a t i o n was s t i l l occu r r ing and the amplitudes measured would be expected t o be h ighe r than c a l c u l a t e d f o r t h e s t eady s t a t e ope ra t ing cond i t i ons . Note t h a t t he lower order harmonics, 3X, 6X, 9X, 12X, e t c . a r e c l o s e t o t he c a l c u l a t e d va lues .

FIGURE 10. P u l s a t i o n s P red ic t ed In E x i s t i n g Suct ion P ip ing System

Whlle i n t he f i e l d , an o r i f i c e p l a t e was i n s t a l l e d i n t he s u c t i o n f l ange and was s u c c e s s f u l i n reducing the p u l s a t i o n s and c a v i t a t i o n (F igu re 1 1 ) . The f i e l d d a t a i n d i c a t e s t h a t t he p u l s a t i o n s were reduced and a s l i g h t s h i f t down i n t h e f requency of the major responses i s noted. Th i s was analyzed on the computer and t h e r e s u l t s a r e g iven i n Figure 12. The major response was lowered t o 120 Hz and some r educ t ion i n t h e o v e r a l l peak-peak ampl i tudes was p red ic t ed .

FIGURE 11. Speed Raster Of Pump Suct ion P u l s a t i o n s With O r i f i c e P l a t e And Maximum Gas

Volume In Suct ion Accumulator

FIGURE 12. P u l s a t i o n s P red ic t ed In Suct ion Piping With O r i f i c e P l a t e I n s t a l l e d

I n t h e development of the s o l u t i o n , t he analyses showed t h a t a gas-charged, flow- through l i q u i d accumulator l oca t ed a t the pump f l ange i n s t e a d of two f e e t away, would have r e s u l t e d i n lower p u l s a t i o n s a s p l o t t e d i n Figure 13. The ampl i tudes a t t he resonant f r equenc ie s were reduced a s w e l l a s the lower pump harmonics.

FIGURE 13. P u l s a t i o n s P red ic t ed In Suct ion Piping With Gas-Charged, Flow-Through Accumulator

A t Suct ion Flange

The p u l s a t i o n c h a r a c t e r i s t i c s of the d i scha rge p ip ing system were analyzed and i n d i c a t e d t h a t t he bladder- type accumulator was not e f f e c t i v e du r ing some o f the s t a t i o n ope ra t ing cond i t i ons . For example, t he d i scha rge p re s su re would sometimes f a l l t o 700 p s i (4826 kPa) from the normal 1600 p s i g (11032 kPa). When t h i s happens, the b ladder becomes f u l l y expanded, b locking o f f t he en t r ance of t he accumulator and voiding t h e b e n e f i c i a l e f f e c t s of the gas volume. Also, the h igh l e v e l p u l s a t i o n s caused b ladder f a i l u r e s which then e l imina t ed the gas cushion on the back s i d e of t h e b ladder . The e f f e c t i v e volume of t he accumulator was reduced s i g n i f i c a n t l y . These e f f e c t s were analyzed wi th t he d i g i t a l computer program. Figure 14 compares t he p red ic t ed p u l s a t i o n f o r t he normal cond i t i ons and the ca se w i th low d i scha rge p re s su re . It can be seen t h a t t he p u l s a t i o n ampl i tudes a t t he lower harmonics s i g n i f i c a n t l y i nc reased when the gas cushion was e l imina t ed .

FIGURE 14. P red ic t ed Discharge Pu l sa t ions For Normal Opera t ion (------ ) And Low Discharge P re s su re

(-1 With Bladder Type Accumulator And For Proposed All-Liquid F i l t e r ( . . . . . .)

An a l l - l i q u i d a c o u s t i c f i l t e r des ign was analyzed f o r t h e d i scha rge system which should s i g n i f i c a n t l y lower t he p u l s a t i o n energy a s i nd ica t ed i n Figure 14. An a l l - l i q u i d p u l s a t i o n f i l t e r system c o n s i s t s of a volume-choke-volume o r a volume-choke pip ing arrangement which i s s p e c i a l l y des igned t o a t t e n u a t e the p u l s a t i o n s above s p e c i f i e d f r equenc ie s . The a l l - l i q u i d f i l t e r des ign f o r d i scha rge and s u c t i o n systems is based on t h e a l lowable p re s su re drop i n the choke tube and t h e Helmholtz frequency.

CONCLUSIONS

The r e s u l t s of the f i e l d t e s t s and the a c o u s t i c a l analyses led t o t hese conc lus ions :

Suc t ion System

1. Whenever p u l s a t i o n l e v e l s a r e s u f f i c i e n t l y h igh

t o cause the abso lu t e s t a t i c p re s su re t o drop below the vapor p re s su re , c a v i t a t i o n can occur . This c r e a t e s l a r g e fo rces which can cause such problems a s f a t i g u e of the va lves , c rossheads , rods , and o t h e r r o t a t i n g o r r e c i p r o c a t i n g p a r t s o f pumps.

2. C a v i t a t i o n causes shock type pu l se s t o be t r ansmi t t ed through the s u c t i o n p ip ing. These can e x c i t e a c o u s t i c a l resonances and cause h igh p ip ing v i b r a t i o n a t t he mechanical n a t u r a l f r equenc ie s of the p ip ing spans, p ip ing w a l l ( s h e l l resonances) , and p ip ing appendages, such a s ven t s , d r a i n s , gage l i n e s , e t c . The f o r c e s can be so h igh t h a t normal p ipe clamps and suppor t s may be i n e f f e c t i v e i n c o n t r o l l i n g the v i b r a t i o n s .

3. The genera ted pu l sa t ions i n t h e s u c t i o n p ip ing a r e a s t r o n g func t ion of the type and l o c a t i o n of the p u l s a t i o n f i l t e r . For t h i s p ip ing system, the gas-charged, flow- through accumulator was more e f f e c t i v e than the bladder- type accumulator i n a t t e n u a t i n g the p u l s a t i o n s .

Discharge System

4. The p u l s a t i o n s i n t he d i scha rge p ip ing were exces s ive , p a r t l y due t o t h e changing s t e a d y s t a t e cond i t i ons which made the bladder- type accumulator i n e f f e c t i v e f o r many of the ope ra t ing cond i t i ons . An a l l - l i q u i d a c o u s t i c f i l t e r was des igned f o r a l l ope ra t ing cond i t i ons t o minimize the p u l s a t i o n s and pip ing v i b r a t i o n s i n t he d i scha rge p ip ing.

Liquid Pumps

5. Many v i b r a t i o n and f a i l u r e problems i n r e c i p r o c a t i n g pumps i n o i l p i p e l i n e a p p l i c a t i o n a r e caused by system r e l a t e d a c o u s t i c a l resonances which cause h igh l e v e l p u l s a t i o n s i n the s u c t i o n and d i scha rge p ip ing.

5. The a c o u s t i c a l c h a r a c t e r i s t i c s of a pump i n s t a l l a t i o n a r e a func t ion of t he speed of sound i n t he f l u i d and the a c o u s t i c a l c h a r a c t e r i s t i c s o f the p ipe s i z e s (d iameters and l eng ths ) o f a l l t he p ip ing elements i n t he l i n e from the plunger va lves t o the supply header and beyond.

7 . The a c o u s t i c a l n a t u r a l f r equenc ie s and the p u l s a t i o n amplitudes t h a t w i l l occur i n any given p ip ing system can be p red ic t ed by modeling the e n t i r e system us ing a computer program which inc ludes a l l the important v a r i a b l e s .

8. When the p ip ing systems d i scussed i n t h i s paper were s imula ted us ing the d i g i t a l program, the a c o u s t i c a l resonances and the p red ic t ed p u l s a t i o n amplitudes were i n agreement.

9. Commercially a v a i l a b l e accumulators and p u l s a t i o n f i l t e r s can be q u i t e e f f e c t i v e i n c o n t r o l l i n g p u l s a t i o n s ; however, they should normally be i n s t a l l e d as c l o s e a s p o s s i b l e t o the pump plungers . S p e c i f i c accumulators can be modeled us ing the d i g i t a l computer program and t h e i r performance s t u d i e d under proposed ope ra t ing cond i t i ons . 10. A l l- l iqu id a c o u s t i c f i l t e r systems can be designed f o r p r a c t i c a l l y any s u c t i o n and d i scha rge system t o minimize p u l s a t i o n s . A l l- l i q u i d f i l t e r sy s tems a r e advantageous f o r some i n s t a l l a t ions s ince they r e q u i r e p r a c t i c a l l y no maintenance once

they a r e i n s t a l l e d .

11. The f i n a l recommended s o l u t i o n f o r t h i s o i l pumping s t a t i o n was t o move t h e gas- charged, flow-through accumulator t o the s u c t i o n f l ange and t o i n s t a l l an a l l - l i q u i d f i l t e r i n t h e d i scha rge system a t t he pump f l ange . These recommendations a r e be ing implemented and t h e u n i t s w i l l be t e s t e d a f t e r t he i n s t a l l a t i o n .

F i e l d T e s t i n g

12. The t e s t i n g showed t h a t , f o r t h i s system, t he pu l sa t ions genera ted were p r imar i ly a f u n c t i o n of the i n d i v i d u a l s u c t i o n and d i scha rge p ip ing and the b a s i c pump des ign and no t s t r o n g l y in f luenced by the in t e r connec t ing p ip ing wi th t he o t h e r u n i t s . Other systems t e s t e d have shown t h a t t he p u l s a t i o n s can be inf luenced by the p ip ing from o t h e r pumps and t h e l o c a t i o n of t h e pumps i n t h e system. The e f f e c t s of the i n t e r connec t ing p ip ing can be s t u d i e d i n t he des ign phase by t h e d i g i t a l a c o u s t i c a l s imu la t ion .

13. Tes t ing r evea l ed t h a t an inc reased s t a t i c p re s su re l e v e l lowered the l e v e l o f t h e p u l s a t i o n s i n t he pump manifold and i n h i b i t e d the c a v i t a t i o n . The t e s t i n g a l s o showed t h a t t he p u l s a t i o n l e v e l s i nc reased wi th speed.

14. When p u l s a t i o n and v i b r a t i o n problems occur i n a r e c i p r o c a t i n g pump i n s t a l l a t i o n , f i e l d d a t a can be obta ined t o d e f i n e the b a s i c cause us ing f i e l d measurement techniques a s desc r ibed i n t h i s paper.

REFERENCES

1. Hicks, E. J . and Grant , T. R., "Acoustic F i l t e r Con t ro l s Recip Pump Pu l sa t ion , ' ' The O i l and Gas J o u r n a l , January 15, 1979, pp 67-73.

2. Ludwig, M., "Design of P u l s a t i o n Dampeners f o r High Speed Reciprocat ing Pumps," Div i s ion of T ranspor t a t i on , American Petroleum I n s t i t u t e Vol 36 [V] 1956, pp 47-54.

3. Sparks , C. R. and Wachel, J. C., " Pu l sa t ions i n C e n t r i f u g a l Pumps and Piping Systems," Hydrocarbon P roces s ing J u l y 1977, pp 183-189.

4. Wachel, J. C. and Szenas i , F. R., "Vibra t ion and Noise i n Pumps," Pump Handbook, 1 s t E d i t i o n , McGraw-Hill, 1976, pp 9-87 t o 9-97.

5. M i l l e r , J . E., "Liquid Dynamics of Reciprocat ing Pumps - P a r t s 1 and 2," The O i l and Gas J o u r n a l , A p r i l 18, 1983.

This paper received t h e Eugene W . Jacobson Award on February 26, 1986, f o r t h e most o r i g i n a l , t imely and outstanding technica l paper presented a t t h e Energy-Sources Technology Conference & Exhibi t ion, Dal las , Texas, February, 1985.