High compressor axial displacement

Dear all,
I need help, we have 2 BCL 406 centrifugal compressors which triped by high axial displacement at threshold of 0.5 micrometers despite the axial float is set to a minimum value of 0.25 micro. when we open the compressor we do not find a big change in float zone 0.26 micro. the two compressors are a subject of revamping of the vibration monitoring system. The old instrument for monitoring thrust
is a gauge bridge where the resistance of the gauges varies according to the axial thrust applied on the rotor. the new monitoring system presents two manitic probes from Bntely nevada installed at the suction side of compressor rotor. after starting the turbocomressor and when it reaches the complete sequence, the operator begins to increase the compressor speed (TNL), in order to move away from the surge zone, at this time the axial thrust increases up to exceeds the alarm threshold. when we start closing the recycling valve to load the machine, the axial thrust increases each time we close the recycling valve until the trip of the machine by high axial displacement. is there any explanation of this problem ?

Thank you


I think you have written recently about this issue, but may not have received a response. There haven't been many posts about large centrifugal compressors, and this is a controls-related forum though we do occasionally drift into mechanical issues--which this issue seems to be.

However, it may also be an instrumentation issue because you wrote this problem started after new ”manitic” probes (I think you meant to write 'magnetic' probes??? proximity probes???) were installed to monitor axial position. I suspect something about the set-up and/or configuration of the Bently-Nevada system and/or the probes is the issue here, because if I understand correctly you said the rotor can't move as far as the trip setpoint is configured to be. That suggests the setpoint is incorrect, for one thing. And that leads me to believe other B-N settings and/or configurations are probably also incorrect.

Can you confirm you meant to write that the rotor physically can't move as much as the trip setpoint is currently configured to be?

I presume when you measured the ”float” of the rotor upon opening the compressor cases you used feeler gauges or a dial indicator to measure the rotor float; please confirm how the rotor float was measured.

When you measured the rotor float using a dial indicator or feeler gauges did you also observe the Bently-Nevada probe readings as the rotor was shifted (”thrusted”)? If so, did the B-N readings match the dial indicator/feeler gauges measurements?

Are you working with the supplier of the B-N equipment to understand and resolve this problem? Or did the company purchase the equipment and install and configure it with internal engineering resources?

If the trip setpoint is intentionally set for a distance that is much higher than normal rotor float in order to detect a worn thrust bearing or thrust face have you asked the compressor manufacturer for assistance with this problem? Surely the compressor manufacturer has experience with magnetic (proximity???) probes being used to monitor axial thrust/position and may be able to offer some suggestions.

I, personally, don't have experience with axial position monitoring set-up and configuration, and it would seem no other possible responders can offer any help or solutions based on the information provided. It may also be because your post is somewhat unclear.

If you can answer the request for clarification someone may be able to offer advice. If the equipment and installation and commissioning was purchased from a supplier such as GE or Baker-Hughes-GE or Bently-Nevada you should be working with them to understand the installation and configuration of the system that they provided and installed and commissioned.

Best of luck. If you can provide clarification someone might be able to offer advice, but this really seems like it's not a controls problem--other than the installation or set-up or configuration is incorrect. It might also be that the magnetic probes are not the correct ones for the equipment/application, or the probe cables or proximitors are incorrect.

Please write back with clarification and to let us know how you fare in resolving the problem!
hi CSA;
thank you for replying

you are right, the rotor physically can't move as much as the trip setpoint
we used the dial indicator and the Bently-Nevada probe readings to measure the rotor float, the two methods gave the same results !!
actually the company purchase the equipment and install and configure it with internal engineering resources.


Please have an experienced person come to site to review the equipment and configuration and make recommendations.

It's probably going to be some innocuous setting, or incorrect components.

Please write back to let us know what you find and how you resolve the problem. Best of luck.