S PECIAL FEATURE

Pump troubleshooting through vibration analysis

The goa l o f predic t ive main tenance is to e l i m i n a t e to ta l m a c h i n e r y b r e a k d o w ~ This concept is based on the observat ion that machinery gives advanced warn ing that it is in the process o f failing. These w a r w ings could be high vibration, high temperature, corrosion, debris in oil etc~ This article includes two case studies based on opera t iona l exper~ ences a t the H a z i r a Gas Processing Complex where trouble~ shooting was made possible through the use o f predic t ive maintenance.

Case 1: Tongue rectification of cooling tower pumps

i Introduction

The Phase III cooling tower is suppl ied by four p u m p s with the following specification:

TAG : 22 - P -204 AIBICID RPM : 996 F L O W : 3000 m31hr H E A D : 55 m P O W E R INPUT : 500 kW N O OF IMPELLER V A N E S : 5

By R.S Rajesh, EE (M), ONGC, Hazira, India

These are new p u m p s bought to accomodate the p lan t ' s Phase III expansion. They are single stage, double suct ion p u m p s with a horizontal ly split casing. The t ra in layout is shown in Fig. 1.1.

i Problem

On 7.8.96, high v ibra t ion and a b n o r m a l sound was repor ted by the opera to r on p u m p 22-P-204 D. The Condi t ion Moni tor ing Cell was informed of the problem. The sound level was 101 dB and the v ibra t ion was 19 mm/sec . The sound was as if sand or pebbles were be ing moved up and down in a meta l l ic conta iner . Subsequen t ly spec t rum analysis was done with CSI 2115 FFT analyzer.

i Analysis

The spec t rum of Pump Inboard Horizontal (PIH) is shown in Fig. 1.2. The d o m i n a n t peak is at the vane pass frequency (VPF) and its harmonics.

V P F -- N o of i m p e l l e r v a n e s X R P M of t h e m a c h i n e

= 5 X 9 9 6 = 4 9 8 0 C P M .

M O T O R ! 2

A /F B R G 6 3 1 9

S U C T I O N

1 P U M P

1 - M O T O R O U T B O A R D 2 - M O T O R I N B O A R D 3 - P U M P I N B O A R D 4 - P U M P O U T B O A R D D I S C H A R G E

F ig 1 .1 .

A/F BRG 6319

The VPF was presen t on all the four pumps. This frequency is i nhe ren t in vaned pumps,

compressors and blowers. Large ampl i tudes of VPF and its harmonics are generated when

• the impeller is loose on the shaft • the vanes are incorrectly designed • the impeller is not discharging into the volute

at the correct angle • there is an obs t ruc t ion to flow due to ab rup t

bends • the rotor is posi t ioned eccentrically

Due to high vibrat ion at VPF and abnormal sound it was decided to open the p u m p and inspect the in te rna ls for any mechanica l defects.

WORLD PUMPS OCTOBER 1997 Copyright © 1997, Elsevier Science Ltd. All rights reserved. 0262 1762/96/$17.00

S P E C I A L FEATURE

I Observation

On opening the top half cover, no mechan ica l

defects were observed, thus e l imina t ing two

po ten t ia l causes. However two observat ions were

made:

• Pit t ing on the lower edges of the vane where it

mee t s the hub.

• A small ha i r l ine crack measur ing about 8 m m

on the t ip of the vane.

On 23.8.96 the p u m p D was s ta r ted after

in ternal inspection. The checks were carr ied out

as follows

• Vibrat ion on p u m p D with p u m p s B and C

running.

• Vibrat ion on p u m p D with p u m p B running.

• Vibration on p u m p D when only D is running.

This was done to assess the response of p u m p

D when o ther p u m p s are running. The ampl i tude

at VPF was high on all the p u m p s and was a

cause for concern. Readings on p u m p D are

t abu la ted in Table 1.1.

The kinet ic energy impa r t ed by the impe l le r is

conver ted to pressure energy in the volute or the

diffuser. The profi le of the diffuser is very cri t ical

for a smooth flow and efficient convers ion of

veloci ty energy to pressure energy. Any flaw here

will lead to flow d e t a c h m e n t and tu rbu lence

giving rise to VPF and its harmonics .

I Recommendation

Since there were no visible i r regular i t ies in the

p u m p and VPF was p resen t on all the pumps, the

p u m p manufac tu re r was asked to check the

• cu t -water c learance

• vane design

• ro tor pos i t ioning

The cut -water c learance is the m i n i m u m gap /

c learance tha t should be ma in t a ined between

the impel le r d i ame te r and the volute lip to

p r eve n t noise, pu l sa t i on and vibra t ion . The

spec t rum of 22 - P - 204 D was sent to the

design d e p a r t m e n t of the p u m p manufac turer .

20/1 - CT P U M P P H i l l 27,-P-204 D-PIH P U M P I N B O A R D H O R I Z O N T A L

14 [ - ' . . . . . . '

I I

I" V P F & H A R M O N I C S

- - - - - T . . . . . . . . . • R s , d e S p , ~ m m 3-AUG-96 15:36

O V R A L L = 13.28 V - I ~ P K = 13.30 L O A D = 100.0 R P M = 994. R P S = 16.57

I

't

i

'i _!'i L I

F re~mmcy Im C IPM

m N O 0 0 Fveq: 4111U.O

O r k t g l S S ~ I L 4 S

Fig. 1.2. %

Cut Water Dia = Impel le r Dia + Clearance

This is i l lus t ra ted in Fig. 1.3.

~J

J

l i

Fig. 1.3.

B, C RUNNING PUMP B RUNNING ONLY D RUNNING

PIH 11.7 13.3 6.52 PIV 18.01 12.2 9.2 PIA 15.52 11.1 13.3 POH 12.28 8.3 7.7 POV 20.51 16.4 12.6 POA 16.88 12.1 11.2

Table 1.1.

WORLD PUMPS OCTOBER 1997

S PECIAL FEATURE

Photograph of the pump assembly showing the tongue projection after cutting to the original specification.

I I Act ion taken

Based on our r ecommenda t ion , the engineers

from the OEM vis i ted ONGC, Hazira and once

again v ibra t ion and noise level were measured.

The values were well above the prescr ibed limits.

On 25.9.96, the top ha l f cover of 22 - P - 204 D

was opened. The ro tor along with the impel le r

was pul led out for i n s p e c t i o n / m e a s u r e m e n t of

shaft, impel le r and in terna l casing. Rotor posi-

t i on ing was checked and found okay. The

projec t ion of the tongue from the center l ine of

the p u m p was measured. This d is tance was 170

mm, it was 50 m m more than the design value.

This is wha t de te rmines the cut wa te r c learance

and is very i m p o r t a n t in p u m p design. The ex t r a

50 m m was creat ing ex t r em e turbulence leading

to wa te r h a m m e r on the project ing par t and

erosion marks on the t ip of the impeller .

The e x t r a 50 m m was cu t a t s i te and

chamfered , t he d i s t ance was m a i n t a i n e d a t

220 mm. This c learance was checked on all the

four p u m p s and rectif ied (Fig. 1.4).

I I Resul t

After the rectification, the overall vibration level,

sound and a m p l i t u d e at VPF were reduced

substant ia l ly on all four pumps. Compar ison

spectra before and after rectification are shown

in Fig. 1.5. Nevertheless VPF does exists but its

damaging effects have been reduced. The ampli-

tude at VPF will have to be monitored when all the

trains of Phase III are commissioned and the

pump is operat ing at its duty point.

Even though the machines were new, this was

a ma jo r p rob lem which was effectively d iagnosed

and rectified at a very early stage.

Case 2: De-minera l i zed water t rans fer pump

I I Introduct ion

Fig. 1.4

B E F O R E R E C T I F I C A T I O N

. . . . . . .

~" ~ - ~ ":" ~ " A F T E R R E t L W I F I C A T I O N

The supply of de -minera l i zed w a t e r to HRSG

and KTI boi le rs is f rom the DM Plant . A

schemat i c d i ag ram is shown in Fig. 2.1 for

~ / 1 _ CT PUMP pH I l l Z2-p-214 D-PR.I PUMP ~ B O A R D HORIZONTAL

C m u l p ~ m Slmcl~mm

L R m 51m¢1111~

22-P-~HM D P m

2NG41 41tO0e Frequency In C p M

Frecl: 4dl~,O 8 ~ O r d ~ 4,915

Sp I: |11.4S

Fig. 1.5.

WORLD PUMPS OCTOBER 1997 I

S P E C I A L F E A T U R E

reference. The two DM transfer p u m p s with tag n u m b e r 22 - P - 205 A / B were m a d e by Khiml ine , India. The p u m p s have been in operat ion for the last ten years. The working parameters are as fol lows :

Flow (O) : 125 ma/hr RPM : 2900 Head (H) : 85 m Liquid : De-mineralized (DM) water No of Vanes : 5

The p u m p s are very critical from an opera- t ional point of v iew as they supply DM water to HRSG #1, HRSG #2, KTI boiler and other places.

1 Observation and analysis

The condit ion of p u m p 'A' started showing s igns of deterioration from 29.08.96 indicat ing high velocity readings and it was decided to monitor closely. The maintenance team was also ap- praised of the situation. The vibration increased further on 5.9.96 and a detai led analysis was carried out with CSI 2115 FFT Analyzer. The spectra on p u m p inboard and outboard bearings are shown in Figs. 2.2 and 2.3. At first instance on seeing PIH, the high ampl i tude at 1 * RPM suggests unbalance. However on looking at PIV, the s ignif icant ampl i tude at 15 KCPM and also a broad h u m p of energy from 13 KCPM to 30 KCPM opposes unbalance as the problem. Never- the less phase analysis was carried out to check for unba lance /bent shaft.

A mach ine which is unbalanced wil l have significant ampl i tude at 1 * RPM and very l itt le at its harmonics. The t imewave form in acceleration uni t s (g) will be s inusoidal . The horizontal

A N N E X - I

ro HI~S¢~ ~l

P1 ~,iP N E T W O R K

Z'.-V .~05 B I

2~.p-2~A

TO K T I B O I L £ R

T O I A E ( BOILERS

Fig. 2.1.

i s , I

14 o

12

10

8

2 2 - 5 - D M W A T E R P U M P Z 2 - P - 2 0 5 A - P I H P U M P I N B O A R D H O R I Z O N T A L

2O 4O 6O 8O 100 120 140

ha kCPM

Route Spectrum Z g - A U C ~ 9 6 ] 4 : 5 2

OWRA.L L = 13 .79 V - D ( ;

P K = 13.86 L O A D = 100 .0 R P M = ~*00 . R P S = 50 .00

S L N O 1

Freq: 3 .$00 160 Ordr: 1 .{~0

$1~c : 12.35

Fig. 2.2.

2 2 - 5 - D M W A T E R P U M P 2 2 - P - 2 0 5 A - P O V P U M P O U T B O A R D V E R T I C A L

3.3

3.0

2 .7

2 .4

2.1

1.8

1.5

1.2

0.9

0.6

0.3

20 40 60 80 100 120 140

F r ~ p ~ b k C P M

160

Route S p e c t e ~ l 2 9 - A U G - 9 6 1 4 : 5 2

O V R A L L = 4 .16 V - D G

P K = 4 ,16 L O A D = 100.0 R P M = 3000 ,

R P ~ = 5Q,N

S L N O 2

Freq: 15.00 Ordr: $ . 0 e 0 Spec: 1 .456

Fig. 2.3

versus vertical phase readings on the same bearing hous ing will be 90 degrees apart. But in our case the phase was never constant, it kept changing from 0 - 360 degrees. This was checked on both inboard and outboard bearings. The phase keeps changing if there is rubbing or looseness . Thus the possibi l i ty of unbalance was complete ly ruled out. Moreover it is not poss ible to b a lan ce a m a c h i n e w h o s e p h a s e k eep s changing.

A carefu l l o o k at PIV and POV s h o w s substant ia l ampl i tude at 15000 CPM (15 KCPM) which did not exis t earlier. In addit ion the broad h u m p of energy from 13 KCPM to 30 KCPM could be turbulence due to flow disturbances. Keeping these points in v iew and with the experience gained thus far it was concluded that it should be Vane Pass Frequency (VPF) with the impel ler having five vanes.

WORLD PUMPS OCTOBER 1997

S PECIAL FEATURE

LOCATION

PIH PIV PIA POH POV POA

BEFORE RECTIFICATION (MM/SEC)

13.92 6.71 8.03 11.5 4.16 7.22

AFTER RECTIFICATION (MM/SEC)

4.68 3.5 4.5 5.28 3.7 5.5

Table 2.1.

22-$ - DM W A T E R P U M P 22-P-205 A-Pil l P U M P INBOARD HORIZONTAL

1.4

1.2

L0

0.$

0.6

12

i0

ComparJsom Spemun

l~raremce Spect rum

- 22-P-205 A-PIH - 18-DEC-96 10:25

Versus 22-P-20~ A-PIH

" 29-AUG-96 14:$2

S L N O 3

20 48 ( 4 N 100 120 140 160 Freq: 3.090 Ordr: l,OOe

Freqmmcy b k C P M Sp I: 12.3S

Fig. 2.4.

• Recommendation

Recommenda t ion was sen t to the co-genera t ion m a i n t e n a n c e to inspec t the impe l l e r and i ts componen t s . Subsequen t ly the p u m p was dis- m a n t l e d and following observa t ions made ,

• The impe l l e r h a d become loose on the shaft as the key had shea red off.

• The sleeve and the wear r ings had rubbed out.

In r e t rospec t the high a m p l i t u d e on PIH a t 1 * RPM could have been due to loss of ma te r i a l a t the keyway. The sleeve had r u b b e d ou t by 4 - 5 m m and also the impe l l e r had become loose on the shaft. The looseness a long wi th rubbing m a d e the p h a s e highly uns t ab l e

The read ings before and af te r rec t i f ica t ion on the p u m p are given in Table 2.1.

The p u m p was t aken u n d e r r epa i r j u s t in t ime and any fu r the r runn ing would have lead to a c a t a s t roph i c f a i lu re / acc iden t comple te ly d a m a - ging the casing and all t he a s soc ia t ed compo- nents . The compar i son spec t rum is in Fig. 2.4.

This case emphas i zes the impor t ance of p h a s e analys is a long wi th cr i t ica l analysis of s p e c t r u m / wave fo rm in ident i fy ing t h e e x a c t p rob l em. Always confi rm unba lance wi th p h a s e analysis. A lo t of t i m e w o u l d have been w a s t e d in ba lanc ing th is mach ine t ha t d id not requi re it. Add i t i ona l t es t s d id he lp us in p in -po in t ing the p rob l e m and improved the credib i l i ty of the d e p a r t m e n t . •

i Hazira Gas Processing Complex

The Hazi ra Gas Processing Complex (HGPC) is loca ted on the wes t coas t of Ind ia in the s t a t e of Guja ra t . Sour gas is s u p p l i e d f rom Sou th Bassein gas f ields in wes te rn off-shore t h rough two sub sea t r u n k p i p e line. Af te r in i t i a l t r e a t m e n t in the off-shore p l a t fo rm the gas is t r a n s p o r t e d by 217 km of 36 inch and 42 inch d i a m e t e r u n d e r s e a p ipe l ine to U m b h a r a t and then by 14 km onshore p ipe l ine to Hazira. Phases I & II a re des igned to p rocess 20 mi l l ion m a / d a y of gas and a s soc ia t ed condensa te . With t h e a d d i t i o n of P h a s e III, t h e p r o c e s s i n g

capac i ty has inc reased to 39 mil l ion m3/day of sour gas. The gas af ter sweetening is p u m p e d t h rough the HBJ p ipe l ine to var ious power p l a n t s , f e r t i l i s e r p l a n t s a n d p e t r o c h e m i c a l p lants . In addi t ion , the p l a n t p roduces LPG, s e pa ra t i ng NGL from the a s soc ia t ed gas re- cieved from Bombay High and shor t ly keros ine and n a p t h a will be a d d e d to the p roduc t line. In o rde r to sa feguard the envi ronment , s ix t r a ins of s u l p h u r recovery p l a n t a re o p e r a t e d to conver t hydrogen su lph ide gas to e lementa l sulphur .

WORLD PUMPS OCTOBER 1997