Root Cause Investigation of Sub-Synchronous Vibration in a …€¦ · Root Cause Investigation of Sub-Synchronous Vibration in a Multi-stage Centrifugal Compressor Jun Inai, Project

Root Cause Investigation of Sub-Synchronous Vibration in a Multi-stage Centrifugal Compressor

Jun Inai, Project & Development Engineer

Kawasaki Heavy Industries, Japan

Outline

• Introduction

• Findings

• Root Cause Analysis

• Modification

• Validation of modification

• Site confirmation test

• Conclusion

• Lesson learnt

Introduction

During the commercial operation of an eight-stage back to back GT driven centrifugal compressor locates at the end-user’s off-shore platform, high level shaft vibration alarm under specific operating conditions was reported from end user.

According to the site evaluation test with dynamic measurements, sub-synchronous vibration (SSV) was observed under higher load conditions of high pressure compressor for every operating speed.

This case study features the root cause analysis of SSV problem using large scale unsteady CFD and the final result of site confirmation test after improvement.

Findings (1) Site Evaluation Test

GAS

COOLER KNOCK OUT

DRUM

Casing drain

GAS

COOLER

PT TT

FT TT

PT

DPT

KNOCK OUT

DRUM

TT PT

FT TT

PT DPT

LP COMPRESSOR(LPC) HP COMPRESSOR(HPC)

XE XE

ANTI SURGE VALVE ANTI SURGE VALVE

Site evaluation test was conducted to understand the circumstances. Dynamic measurements of rotor vibration and discharge pressure at HPC casing drain and downstream piping were implemented.

GT

DPT XE : Dynamic measurement points

Pressure transducer

Vibration transducer

Findings (2) Operable Range and Shaft Vibration

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

4.0

0 1000 2000 3000 4000 5000 6000 7000 8000

Suction Volume Flow - m3/hr(corr)

Pre

ssu

re R

atio

Pre

ssure

ratio

SSV not present

105% speed

100% speed

80% speed

90% speed

Suction volume flow

97.9% speed

Shaft vibration alarm detected !!

SSV onset line

Operable range was restricted by high level shaft vibration.

SSV onset points correspond to increase of shaft vibration.

HPC operating map

Findings (3) Sub Synchronous Vibration

SSV around 20~30 Hz were dominantly present. They were approximately 1/6~1/7 times the machine rotational speed. Same frequency of discharge pressure fluctuation were also detected at casing drain and down stream piping. Is this a typical vaneless diffuser rotating stall ? At first we suspected it as the most possible root cause.

80% speed 90% speed 100% speed

Observed radial shaft vibration (Y-NDE)

16μm

1N =146Hz

28Hz

8μm

18Hz 20Hz 1N

=130Hz

1N =164Hz

Root Cause Analysis (1) Rotating stall check at vaneless diffuser inlet

Rotating stall at diffuser inlet is checked at the design phase

based on Senoo criteria. And it was re-confirmed that sufficient acceptable

margin were secured.

Flow angle at diffuser inlet at design condition

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

22.0

24.0

26.0

28.0

30.0

32.0

34.0

36.0

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

b2/r2

Flo

w a

ngle

at diffu

ser

inle

t (d

eg)

Flow angle of each stage of HELANG

Critical flow angle, rotating stall to occur

Stable

Unstable

HPC Senoo criteria

Flo

w a

ngle

at diffu

ser

inle

t [d

eg]

LPC

It indicates the root cause is not a rotating stall at diffuser inlet.

Impeller exit blade height / Impeller diameter b2/r2

Root Cause Analysis (2) CFD Analysis of 8th stage

IGV

Impeller (φd=0.0118)

Parallel wall vaneless diffuser

Diffuser outlet with spacer vane

Eye seal

Center seal

Discharge Volute

Shunt holes

Deflector

• Number of vane - Impeller = 22 w/t splitter - IGV = 16 - Spacer vane = 8

• Number of shunt holes = 3 • Rotating speed =9514 rpm • Calculation time = 1day/rotation

Birdview from upstream

from 7R

to 4R

Center seal leakage

Rabbet fit

Large scale unsteady CFD analysis was carried out for the 8th stage.

Root Cause Analysis (4) Static pressure fluctuation at spacer vane inlet

HzmT

f

HzT

f

T

Tm

r 2310145.441

11

2310145.44

11

11.110145.44

10248.10

75

360360

3-

3-0

3-

-3

2 rev.

000000ddeegg

006600ddeegg

118800ddeegg

336600ddeegg

112200ddeegg

224400ddeegg

330000ddeegg

ΔT=10.248ms ΔT=10.248ms

ΔΔθθ==7755°°

1N

ΔT=10.248ms

T=44.145ms

TIME

LO

CA

TIO

N

HzmT

f

HzT

f

T

Tm

r 20103065.68

11

159106.3065

11

88.7103065.6

10248.10

75

360360

3-

3-0

3-

-3

HzmT

f

HzT

f

T

Tm

r 2110766.153

11

6310766.51

11

31.310766.15

10248.10

75

360360

3-

3-0

3-

-3

Time & space distribution of static pressure at spacer vane inlet

Station VD5

000deg

090deg 270deg

180deg

Rotation

Cir

cum

fere

nti

al p

osi

tio

n

20Hz 21Hz 23Hz

Close frequency as observed at site test could be simulated.

Root Cause Analysis (5) Static pressure fluctuation at other stationary region

Static pressure time & space distribution & FFT spectra

Strong pressure fluctuation at spacer vane inlet affects to the other stationary region.

N=17.5TH REVOLUTION N=18.0TH REVOLUTION N=18.5TH REVOLUTION


Root Cause Analysis (6) Unsteady pressure distribution across the stationary region

Static pressure distribution

Periodic pressure fluctuation was shown across the whole stationary region.

Root Cause Analysis Summary

A large scale unsteady CFD analysis achieved to simulate the sub-synchronous phenomena as close frequency as measured SSV at site test and indicates strong flow fluctuation due to large flow separation at the diffuser outlet with spacer vane at the final stage.

It was considered that the root cause is complete stall induced from diffuser outlet due to excess flow passage expansion between diffuser outlet and discharge volute at the final stage.

Therefore, configuration of ‘diffuser outlet with spacer vane’ shall be re-designed.

Modification Improved diaphragm of 8th stage

Cross-sectional configuration of diffuser outlet was changed from expanded shape to parallel wall shape

Spacer vane shape was also changed from cusped to elliptical blunt.

Spacer vane

Pre-modified Modified

Validation of modification (1) Static pressure fluctuation at other stationary region

Static pressure time & space distribution & FFT spectra

Confirmed no presence of noticeable time & space distribution of static pressure at spacer vane inlet and other stationary region.



Validation of modification (2) Flow stability across the stationary region

Static pressure distribution Pressure fluctuation at the stationary region completely disappear.

Validation of modification (3) Rotor excitation force


View from upstream View from upstream

Rotor excitation force at the stage 8th occurs in the direction of discharge nozzle.

Excitation force time averaged/dynamic have both decreased in association with modification.

Dynamic Dynamic

Site confirmation test (1) Operable range & Shaft vibration

Surge line

Confirmed wide operable range is secured as estimated

Overall vibration is less than 25μm for whole operable range

Pre

ssu

re r

atio

105% speed

100% speed

80% speed

90% speed

Suction volume flow

97.9% speed

93.4% speed Former SSV onset line

Surge control line

Site performance test (2) SSV presence


16μm

1N =164Hz

28Hz

20μm

SSV Negligible small (<1μm)

Confirmed no dominant SSV presence for all operable range.

5μm

1N =160Hz

Conclusion

With regard to the natural gas export compressor on the off-shore platform which was restricted its operable range due to SSV as 1/7 times the machine rotational speed, a large scale unsteady CFD analysis was carried out in order to investigate the root cause.

The CFD analysis achieved to simulate those sub-synchronous phenomena. And it was found that the root cause was a typical stall at diffuser outlet due to excess flow passage expansion between diffuser outlet and discharge volute at the final stage.

Modified stationary flow passage was designed and validated its effectiveness by CFD analysis in the same manner as root cause analysis.

Modified diaphragm was already installed to the site machine. The followings were confirmed through the site evaluation test. * No presence of dominant SSV for whole operable range * Operable range is secured as estimated

Lesson & Learnt

Even the stalls in such a stationary flow passage region apart from the rotor can be the excitation force of shaft vibration especially under high pressure condition.

Sufficient consideration and care with a broad view shall be taken during the engineering phase.

Root Cause Investigation of Sub-Synchronous Vibration in a …€¦ · Root Cause Investigation of Sub-Synchronous Vibration in a Multi-stage Centrifugal Compressor Jun Inai, Project

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