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Damaged Compressor Stator Parts–
Resulting from Operationin Deep Choke
Jean-Claude Pradetto & Urs BaumannMAN Diesel & Turbo Schweiz AG
Urs Baumann is Manager of the R&D departmentof MAN Diesel & Turbo Schweiz AGin Zurich, Switzerland.Urs has a diploma (MechanicalEngineering, 1987) from the SwissFederal Institute of Technology inZurich.Urs Baumann is member of theTechnical Advisory Committees ofthe Turbomachinery SymposiumATPS in Singapore.
Jean‐Claude Pradettois the Head of the In ServiceDevelopment group of MAN Diesel& Turbo Schweiz AG in Zurich,Switzerland.Jean‐Claude is responsible fortrouble shooting issues as well asfor the continuous improvement ofthe turbocompressors.He has a diploma (MechanicalEngineering, 1988) from the SwissFederal Institute of Technology inZurich.
Damaged Compressor Stator Parts–
Resulting from Operation in Deep Choke
The internal stationary parts of a high‐speed, oil‐free, integrated motor compressor used in gas storage service were found severely damaged. The analysis of the operational trend data suggested prolonged operation in deep choke condition. Additional CFD calculations for these operating points confirmed that severe flow separations and flow reversals (at exactly the damaged stages) had to be expected.
Concluding from the RCA the following measures were taken:• Recalibration of the anti‐choke control to move the response line to higher pressure ratios • Instruction to the operators to switch from serial mode to parallel mode operation if the
compressor is operated beyond the response line of the anti‐choke controller.
After repair (no design changes) the machine is now in service for more than one year without any problems.
Description of the System Tandem HOFIMTM – Operating Modes
Serial Operation Parallel Operation
The machine is able to operate in serial parallel modes, according to the requirementsof the process
fullcavern
highestpr.ratio
serialoperation
Gas Storage Application Operating Conditions
emptycavern
freeflowintocavern
emptycavern
verylow
pr.ratio
paralleloperation
backpressurethrottling
fullcavern
freeflowfromcaverninto
pipeline
emptycavern
verylow
pr.ratio
paralleloperation
backpressurethrottling
max. storage pressure
min. storage pressure
pipeline pressurepressure
storage recovery
The backpressurethrottle ensuresthat the minimumpressure ratio ofthe compressor isalways maintained.
RCA Vibration Data Analysis
attemptto restart
failed due toexcessivevibrations
the machinetripped on
high vibrations
RCA Vibration Data Analysis
event #1(120 um)
event #3(>330 um)
HP-pos. X
HP-pos. Y
event #2(220 um)
~4.5 sec
Trip
Step‐increase of thesynchronous vibration
axial pos.
speed
S2M Datalogger
3 distinctpeak eventsbefore trip
RCA Vibration Data Analysis
the waterfall plotreveales a stepincrease of theunbalance (1x)
and
an excessive shaftdisplacement(impact or the like)
Investigation Visual Inspection on Site
impeller 5
impeller 4
impeller 3
broken locking screwpiece of 4th stage hub labyrinth
trapped piece of 4th stage hub
labyrinth
impeller 5(shroud)
impeller 4(hub)
5th stage shroud
laby
4th stagehub laby (missing)
Investigation Inspection at OEM’s Facility
damaged diffuser vanes found in
stages 3, 4 & 5 onlydamaged hub labyrinthsfound in stages 3, 4 & 5 only
damages found in HP compressor only
4th stage hub laby(upper half) withswirl brakes
RCA Interpretation of Damaged Laby’s
This side must havebeen loaded, hammeringcontact
completely batteredsurface, material missing.
cracks on both sidesimpeller
5 (shroud)impeller 4 (hub)
5th stage shroud laby
4th stagehub laby(missing)
Normal loadingdirection andloaded surface
Conclusion:Reversing flow directioncausing alternating loadon both faces
RCA Evidence for Operation at Choke
flowseparationradial
flow
• clearly visible flow patterns in thediffusers of the stages 3, 4 & 5
• the comparison with CFD simulationsof typical operating conditions evidences repeated
or continuous operation at choke.
Calculated Flow Patterns Typical for Operation at:
RCA Repeated or Continuous Operation at Choke
In choke conditions the diffuser of the last stage(s) do not increase pressure. Therefore the hub labyrinth has a reversed flow.
Dynamic flow and pressure fluctuations duringchoke operation caused the damages in the last stages of the HP compressor.
Operating point in serial operationEffective choke line
series parallel
Conclusion .• According to the production profile
prolonged operation at very low pressureratios was a fact (pi HP < 1.05, or evenbelow 1.00).
• The choke controller should haveprevented the machine from running in this condition.
• The choke controller response line was not set properly. Therefore the machinewas running in deep choke.
• The two compression sections werealways running in serial mode. With theselow pressure ratios parallel mode wouldhave been appropriate.
max. storage pressure
min. storage pressure
pipeline pr.pressure storage recovery
pressure
ratio
PI
volume flow
actual (measured) operatingpoints with pressure ratios
below 1.05
Measures Correct Implementation of theChoke Controller Response Line
Operating point in serial operationChoke controller response lineEffective choke line
• Prevent machine fromoperating in deep choke.
• Implementation of a correctchoke controller response line.
• Encourage the operator to switchfrom serial to parallel operationif the choke response line is reached.
• In this case the efficiency will besignificantly higher than in serialoperation.
Thank You for your kind attention
Questions ?
Jean‐Claude PradettoHead of In‐Service DevelopmentMAN Diesel & Turbo Schweiz AG+41 (44) 278 3820jean‐[email protected]
Urs BaumannHead of R & DMAN Diesel & Turbo Schweiz AG+41 (44) 278 [email protected]
