Centrifugal compressor turbine axial displacement increase

[1906]

Hi, we started recycle gas comp. driven by high pressure steam turbine this july after coupling inspection and routine maintenance during turn-around period in our refinery hydrocracking unit. After start up, turbine side axial displacement(to active side) has been increasing step by step and recently almost reaching its trip set 0.6mm. Furthermore, recent trend shows comp. side axial displacement also increasing to inactive side, but lube oil analysis data and color is good condition, temperature of bearings are also in reasonable pigures. With this situation, we are on the verge of deciding somthing to do such as shutting down the unit or operating compressor more. So,If any body experienced something similar to our situation, please let me know.

 

[byrdj]

May not be related, but something to think about.

What type of coupling is between ST and compressor? Does the compressor have its own thrust?

I would think that the coupling should allow for axial growth of the two elements. for example if the coupling is a gear/spline type, there should have gear lenght to allow the two elements to float. The flexiable plate type couplings should have cold preloading axially that when elements grow, will reach a nuetral position.

I have seen these couplings misassembled and apply addition thrust loading (and reverse thrusting situations)

Does your maintance records include the element's thrust "bump" dimension and an external position referance. A planned shutdown and checking the ST's thrust bump sounds warrented if instrumentation show excessive wear.

It may be that thrust bump is ok and instrumentation is vibrating out of adjustment

 

[CESSNA1]

1906: Sounds like the thust bearing is wearing. If instrumentation is ok and procedure for measuring is being followed then I would suspect that. Bear in mind that normal wear will not show up in oil analysis. Oil analysis only detects abnormal or excessive wear. Many times the oil analysis confirms that the machine did fail.

Regards
Dave

 

[abeltio]

have you checked that the probes are properly assembled and are not giving erroneous readings?

saludos.
a.

 

[JJPellin]

I realize that quite a bit of time has passed since this original posting. But I would suggest shutting the machine down. For a hydrocracker recycle machine, I assume that the bearings are hydro-dynamic (Kingsbury type) and the coupling in non-lubricated (probably diaphragm). If the thrust possition of the turbine moved active by almost 0.6 mm and the compressor is starting to move inactive, this seems to confirm that something is happening. Depending on what surface the thrust probes are looking at, it could be a couple of pontential causes. The thrust bearing could be failing. But analysis of the oil in the drain line from that housing should show wear metals or high partical count. Failure of the babbit surface should show as increased temperature. But it could be a failure of the pivot on the backs of the bearing pads. The other main option would be a loose thrust disk allowing the shaft to move without a bearing failure. We had a machine that failed in thrust when the shaft cracked under the thrust disk. In our machine, there was an axial face on the balance piston that rubbed after only about 0.015 inch of movement. But your compressor probably has a purely radial balance piston with no face to rub. I would be very curious to hear what you find, so please post a reponse when the problem is found.

 

[1906]

Re the JJPellin comment, I'll post without any delay when some problem is found. For reference, recently we are carefully monitoring the trends which are standstill at 0.55mm for turbine, 0.27mm for comp. side. Thanks again JJPellin.