![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
Flow
angle
at
diffuser
inlet
(deg)
Flow angle of each stage of HELANG
Critical flow angle, rotating stall to occur
Stable
Unstable
HPC
Senoo criteria
Flow
angle
at
diffuser
inlet
[deg]
LPC
It indicates the root cause is not a rotating stall at diffuser inlet.
Impeller exit blade height / Impeller diameter b2/r2](https://image.slidesharecdn.com/cs2013041684644170-240609054708-4a7eea80/85/Root-cause-failure-analyses-of-sub-synchroneous-vibration-in-the-centrifugal-compressor-pdf-7-320.jpg)
Root cause failure analyses of sub synchroneous vibration in the centrifugal compressor. pdf
- 1. Root Cause Investigation of Sub-Synchronous Vibration in a Multi-stage Centrifugal Compressor Jun Inai, Project & Development Engineer Kawasaki Heavy Industries, Japan
- 2. Outline • Introduction • Findings • Root Cause Analysis • Modification • Validation of modification • Site confirmation test • Conclusion • Lesson learnt
- 3. 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.
- 4. 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
- 5. 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) Pressure Ratio Pressure 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
- 6. 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
- 7. 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 Flow angle at diffuser inlet (deg) Flow angle of each stage of HELANG Critical flow angle, rotating stall to occur Stable Unstable HPC Senoo criteria Flow angle at diffuser inlet [deg] LPC It indicates the root cause is not a rotating stall at diffuser inlet. Impeller exit blade height / Impeller diameter b2/r2
- 8. 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.
- 9. Root Cause Analysis (4) Static pressure fluctuation at spacer vane inlet Hz mT f Hz T f T T m r 23 10 145 . 44 1 1 1 23 10 145 . 44 1 1 1 1 . 1 10 145 . 44 10 248 . 10 75 360 360 3 - 3 - 0 3 - -3 2 rev. 0 00 00 0d de eg g 0 06 60 0d de eg g 1 18 80 0d de eg g 3 36 60 0d de eg g 1 12 20 0d de eg g 2 24 40 0d de eg g 3 30 00 0d de eg g ΔT=10.248ms ΔT=10.248ms Δ Δθ θ= =7 75 5° ° 1N ΔT=10.248ms T=44.145ms TIME LOCATION Hz mT f Hz T f T T m r 20 10 3065 . 6 8 1 1 159 10 6.3065 1 1 8 8 . 7 10 3065 . 6 10 248 . 10 75 360 360 3 - 3 - 0 3 - -3 Hz mT f Hz T f T T m r 21 10 766 . 15 3 1 1 63 10 766 . 5 1 1 1 3 1 . 3 10 766 . 15 10 248 . 10 75 360 360 3 - 3 - 0 3 - -3 Time & space distribution of static pressure at spacer vane inlet Station VD5 000deg 090deg 270deg 180deg Rotation Circumferential position 20Hz 21Hz 23Hz Close frequency as observed at site test could be simulated.
- 10. 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.
- 11. N=17.5TH REVOLUTION N=18.0TH REVOLUTION N=18.5TH REVOLUTION N=19.0TH REVOLUTION N=19.5TH REVOLUTION N=20.0TH 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.
- 12. 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.
- 13. 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
- 14. 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.
- 15. N=17.5TH REVOLUTION N=18.0TH REVOLUTION N=18.5TH REVOLUTION N=19.0TH REVOLUTION N=19.5TH REVOLUTION N=20.0TH REVOLUTION Validation of modification (2) Flow stability across the stationary region Static pressure distribution Pressure fluctuation at the stationary region completely disappear.
- 16. Validation of modification (3) Rotor excitation force Pre-modified Modified 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
- 17. 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 Pressure ratio 105% speed 100% speed 80% speed 90% speed Suction volume flow 97.9% speed 93.4% speed Former SSV onset line Surge control line
- 18. Site performance test (2) SSV presence Pre-modified Modified 16μm 1N =164Hz 28Hz 20μm SSV Negligible small (<1μm) Confirmed no dominant SSV presence for all operable range. 5μm 1N =160Hz
- 19. 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
- 20. 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.