Reverse Rotation in Centrifugal Compressors

40th Turbomachinery Symposium12-15 September 2011 – Houston,TX

Presented by Amr Gad

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Outline

• Introduction• Process overview• Compressor design & construction • Narrative of reverse rotation events• Cause of reverse rotation• Possible impacts of reverse rotation• Mitigating actions taken • Lessons learned• Conclusion

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Introduction

RasGas RasGas Condensate operation Condensate operation

Plant phase 1Plant phase 1Plant phase 2Plant phase 2

Loading CompressorsLoading Compressors

Shutdown statusShutdown statusphase 1 sitephase 1 site

Compressors reverse rotation Compressors reverse rotation existsexists

Presentation Purpose :Presentation Purpose :11-- Explain the impacts ,cause, and mitigations related to phase 1 Explain the impacts ,cause, and mitigations related to phase 1 compressors operation.compressors operation.22-- Phase 1 compressor operation lessons learned implemented in phaPhase 1 compressor operation lessons learned implemented in phase 2 .se 2 .

Shutdown statusShutdown statusPhase 2 sitePhase 2 site

No compressors reverse No compressors reverse ––rotation rotation

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Process Overview

Phase 1 (since 2005)• Six compressors.• Design pressure ratio for LP ,MP, HP is about 3 for each stage

(Pd / Ps).• Design overall pressure ratio for the machine about 28 (Pd /

Ps).• Volume of suction vessels LP, MP, HP more than 50 meter

cube for each.

Phase 2 (since 2009)• Four compressors in operation. • Similar design pressure ratio & overall pressure ratio as phase

1.• Triple the volume of suction vessels compare to phase 1.

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Process Overview

ITVs

1ST

2N

D

3RD

ASV 100% Close

Cooler

SDV Open

SDV Open

Machines During Normal Operation

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Compressor Design & Construction

Phase 1 • Unit configuration: Fixed speed electric motor &

gear box& compressor.• Compressor rated at: 3.7MW• Motor rated at: 4.2 MW • Gear box speed: 1480 / 10258 RPM• Compressor operating speed: 10258 RPM

• Seal configuration: Unidirectional dry gas seal.• Bearing construction:

• Compressor: Tilt pad self equalizing• Motor: Cylindrical sleeve bearing

• Gear box design: Double helical.• Lube oil: Rundown tank with two AC pumps (Main

& Auxiliary).5

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Reverse Rotation Events

• Reverse rotation happened since commissioning of the machines.

• However, they were only discovered after 18 month of units operations. Late discovery was due to:• No reverse rotation detection system installed.• No prior experience in-house with reverse rotation in this type of

process configuration.

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Cause of Reverse Rotation

RCFA findings:• Reverse hydraulic

flow due to large amount of entrapped hydrocarbon volume in the piping and suction vessels of the MP & HP sections.

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Cause of Reverse Rotation

• Hydraulic flow occurs after 10-15 seconds from a stop command till compressor coast down to zero RPM speed.

• Hydraulic flow during the reverse rotation from zero speed till 5000 RPM and back to zero again within 4 to 6 minutes.

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Cause of Reverse Rotation

Speed trend (Figure A)• Unit stop very fast after stop

command.• Unit speed ramp up again in

reverse rotation to almost 5000 RPM for the compressor.

• After speed reaches 5000 RPM it cost down again to zero speed in about 4-6 minutes.

• This phenomena is repeated each time the machine stops.

Flow rate trend (Figure B)• Showing flow exchange &

equalizing between stages.

Figure A

Figure B

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Possible Impacts of Reverse Rotation

Lube oil system design doesnLube oil system design doesn’’t support reverset support reverserotation with total electric power black out , asrotation with total electric power black out , asauxiliary and emergency pumps is AC driven and auxiliary and emergency pumps is AC driven and the overhead tank is designed for only 3 minutes .the overhead tank is designed for only 3 minutes .

Existing unidirectional dry gas seal are not Existing unidirectional dry gas seal are not guaranteed for such modes of operations as per guaranteed for such modes of operations as per OEM guide lines.OEM guide lines.

No detailed study conducted by OEM to confirmNo detailed study conducted by OEM to confirmimpacts on various machines components impacts on various machines components ( Impeller vanes, diffusers, Labyrinth seals ,( Impeller vanes, diffusers, Labyrinth seals ,……etc ). etc ).

Compressor componentsCompressor componentsfailure due to reversefailure due to reverse

rotationrotation

Bearings damageBearings damage

Uncontrolled gas releaseUncontrolled gas releasefrom the compressors.from the compressors.

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Mitigating Actions. Phase 1 Compressors

• Minimize the number of units starts and stops until modify the system.

• Future installation of check valves at the suction of the LP & MP.

ITVs

1ST

2N

D

3R

D

ASV 100% Close

Cooler

SDV Open

SDV Open

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Mitigating Actions. Phase 2 Compressors

• Interim - modified the I&C for the ITV’s of the MP & HP to fully close during machine stop.

• Final - install check valves at the suction of the LP, MP and immediately after the discharge.

ITVs, final position

1ST

2ND

3RD

ASV 100% Close

Cooler

SDV Open

SDV Open

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Shutdown with ITV closing before tuning

Shutdown With ITV closing after tuning

Shutdown with check valve installed and ITV closure removed

Mitigating Actions. Phase 2 Compressors

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Lessons Learned

• Dynamic simulation requirements for such process configurations.

• Mitigating actions to eliminate reverse rotation should consider:• ITV modification for complete closure during shutdown. • Installation of check valves at suction lines.

• Machines construction should include mitigating design:• Bidirectional seal will eliminate risk from reverse rotation.• Center pivot for tilting pad can accommodate reverse rotation

better than offset pivot design.• API standard is for 3 minutes run down tank design and so,

emergency DC pump is an advantage.

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Conclusions

• Reverse rotation phenomena have potential to occur in the LPG refrigeration cycles.

• Reverse rotation phenomena is more common in the motor driven compressors with relatively low machine inertia.

• Detailed surveillance of coast down speed trends of new units is recommended to ensure detection of Reverse Rotation:• Reverse rotation phenomena may not increase vibration or axial

movements above alarm values.

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Thank YouThank You

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Pressure trends for unit stop

171K001B BOG COMP SUCK001B 1ST SUCTION PRESSK001B 2ND SUCTION PRESSK001B 2ND SUCTION PRESSK001B 3RD SUCTION PRESSK001B 3RD SUCTION PRESSK001B DISCHARGE PRESS171K001B C3 BOG COMP OUT

171PI1221B.PV

BARG171PI1224BA.PV

BARA171PI1226B.PV

BARG171PI1228BA.PV

BARG171PI1231B.PV

BARG171PI1233BA.PV

BARG171PI1236BA.PV

BARG171PI1237B.PV

Plot-0

11/23/2006 6:33:38 PM 11/23/2006 6:41:06 PM7.47 minutes

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14

19

-1

22

4.40

4.38

4.41

4.41

4.41

4.41

4.40

0.20Pressure equalization between the three stages in conjunction with the reverse rotation

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• ASV improper function is potential cause for reverse rotation during coast down.• Experienced in LNG BOG compressor when opening time upon

shutdown is more than 2 seconds due to valves stuck or instruments failures.

71K051A BOG COMP DSCH 71K051A BOG DSCH PRES 71KX051A GR KYPHSRCOMP SD 71KX051A GR KYPHSRMTR SD

71FI3142A.PV

KNM3/H71PI3129A.PV

BARG71SI3508A.PV

RPM71SI3510A.PV

RPM

Plot-0

6/6/2007 11:37:03 AM 6/6/2007 11:47:08 AM10.08 minutes

5

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15

20

25

0

30

0

30

0

14000

0

3000

0.

0.

0.3953

0.

71K051A BOG COMP DSCH 71K051A BOG DSCH PRES 71KX051A GR KYPHSRCOMP SD 71KX051A GR KYPHSRMTR SD

71FI3142A.PV

KNM3/H71PI3129A.PV

BARG71SI3508A.PV

RPM71SI3510A.PV

RPM

Plot-0

10/6/2006 9:29:51 PM 10/6/2006 9:32:05 PM2.23 minutes

5

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25

0

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14000

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3000

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0.45781

0.

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