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Multi-Stage Centrifugal Compressor General Questions

lube oil dry gas seal centrifugal compressor hot gas bypass antisurge valve flow element motor

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#1 rha257


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Posted 05 January 2020 - 08:06 AM

Hello All,


I have a couple of general questions that are not really related to each other, however, I thought that making separate posts for each question will not be efficient. I tried to find answers on the web for these questions but I either failed to find anything or I found conflicting responses. 


1. The end user was specifying a limit on the nozzle inlet velocity (i.e. 28 m/s) for all the centrifugal compressors regardless of the process parameters. What is the main concern on the fluid inlet velocity (pressure drop at the first stage impeller?) and what significance does 28 m/s have?


2. The end user was specifying a limit on the allowable discharge temperature of the compressor to be 180 C under any circumstance. Any specific reason on why exactly 180 C is chosen? Is it an API standard?


3. Lube Oil System to be fully constructed from Stainless steel 316L. My understanding is that lube oil is not corrosive but I have read that the surface finishing of the carbon steel item like tanks and pipes will start to disintegrate on a micro level after a couple of years in to service and the disintegrated particles will accumulate on the bearing surface while stainless steel has smooth surface finishing and will not have this issue. Is this statement correct? Also, shouldn't the process of pickling solve this issue of surface finishing? Is the pickling process only applicable for stainless steel?


4. The end user specified that the overhung/cantilever configuration of dry gas seal panel relative to the main compressor skid is not acceptable under any circumstance and the dry gas seal panel must be located next to the compressor skid. What is the reason for such restriction?


5.  Rupture disks on the primary vent line are not acceptable and the use of PSV is required. Why is a rupture disk or PSV required anyway, does the primary vent line ever over-pressurized which requires pressure relief? If a relief device is required, where is the relief gas going to be routed?


6. End user stated that a barring device (i.e. turning gear) is required to be installed on the main gearbox because they received a recommendation from the dry gas seal vendors that the seal faces tend to rub against each other upon a compressor start-up from 0 RPM and until the lift-off RPM is reached (around 100 RPM) so a barring device which will rotate the compressor shaft at 100 RPM until lift off is achieved and then the device will dis-engage. How credible is this recommendation? The compressor MCS is around 12,000 RPM.


7. (This might be a very simple question) Why is the starting torque of an induction motor higher than the full load required by the compressor?


8. The end user wants to add a flow element on the secondary vent (vented to atmosphere) of the DGS and the compressor vendor is refusing to do so because of safety reasons (i.e. clogging of the vent line), however, the vendor does not have a problem with adding a flow element on the primary vent line (vented to flare) of the DGS. Why is it a safety concern on the secondary line but not a concern on the primary line?


9. The end user wants to add temperature gauges on the lube oil system return lines from the bearings although the bearing has 3-4 RTDs connected to it. Is there any valid technical justification for this requirement.


10. The end user specifies, that the flow characteristics for the anti-surge valve should always be linear characteristic and not quick opening. Why linear characteristic? Also, the hot-gas bypass shall be equipped with an electric actuator and shall be a ball valve. Is this the industry standard for the selection of HGBV?



I have already discussed with the end user and the compressor vendors most of these points but I did not fully understand the technical justification to most of these points. I thought to post these points on this forum and have some discussions with the experts on the forum.



Thank you very much  in advance.


Edited by rha257, 05 January 2020 - 08:07 AM.

#2 breizh


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Posted 06 January 2020 - 03:27 AM

Hi ,

You may consider this resource to support your work .

Note : Probably a good idea to get a copy of API 617 .

Good luck


#3 sgkim


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Posted 07 January 2020 - 05:08 AM



The end user seems to have his(her) experience on the previous compressor operation.  Most of end-user-specific requirements can be met at their own cost, but the real story should be paid attention.   Vendor's standard design practices of the compressor shall be informed to the vendor, the discrepancies from the end-user's RFQ shall be discussed and compromised against the additional costs.     


1.  Inlet gas velocity:  The maximum inlet nozzle velocity shall be limited either by noise problem and/or pressure drop.  Gas velocity and mach number can be calculated from the inlet flow, nozzle size,  operation condition and composition of the gas  But there would be no clear cut limitation.    


2. The allowable outlet temperature:  Each stage outlet temperature shall be limited by the maximum service temperature of elastomeric sealing material to be used for gaskets, valves, and fittings on the discharge side.    


3. MOC for lube oil system:  Initial lube oil may be non-corrosive, but the degraded or contaminated oil may be corrosive. Vendor standard material of construction(MOC) would be type 304SS . Cost increase to select 316LSS would be negligible.


4. Lube oil panel type and location:  Standard design type and location shall be informed to the user, the real story on this subject shall be listened and discussed for a settlement.


5. Safety device type and vent gas destination:  Standard practices of gas seal vendor (Rupture disc with <burst-alert> or preferably PSV) may be recommended for this subject.  Refer to the following site for this subject:    https://www.flowcont...eal-performance


The destination of the vent gas shall be determined by the user.   


6. Barring device for initial start up:   Dry gas seal vendor's standard practices would be highly recommended for long time service of the shaft seals.  Also refer to the site material as item "5".


7. Stating torque vs. full-load torque:  Maximum torque of the compressor shall be located near the full-load state of the compressor. Starting torque of the motor is much lower than that at full load since the motor shall be started under zero compression load. But initial torque required for the acceleration at start-up shall be compensated by the motor starting device if any.  Inform the user of the typical performance data with the start-up procedure. 


8. Flow element on the secondary vent:   Providing  the flow elements on the secondary vent line would also be a good recommendation; but be sure to maintain at an appropriate differential pressure by installing < a back-pressure regulator> on the discharge side.  Reverse flow from the vent header should be avoided.


9. Additional temperature alarms for lube-oil system:   If each of the bearing temperature sensing element with alarm is properly installed, no additional TIA's may not be required because the responses of the oil temperature indicators will surely slower than the bearing temperature sensors.  Too much alarms may not make the mare go, they also can be added at additional user's cost.       


10. Liner type anti-surge valve & motorized hot-gas ball valve: For the large size valves,  I understand,  motorized globe or butterfly valves are common and not expensive.   Please review the user's previous installation reference.        


~Stefano Kim

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