Trimming Compressor Blades 1

[JamesDUK]

Hello all.

Background - Am purchasing a new 4 stage (single shaft) GT driven centrifugal compressor, approx 17 MW shared between the 4 stages. We have a number of side streams coming in an out, result the majority of the work is done by the 3rd stage, and 3rd stage discharge pressure sets the GT speed. We included a PTC 10 perfromance test in the purchase order, and when this was conducted the 2nd & 3rd stages produced much more head than design at the certified point. The 1st & 4th were close (within 4%). The vendor proposed slowing the rated speed to compensate, giving the same overall inlet and discharge pressure but that screws our interstage pressures.
So we're reluctantly looking at trimming the 2nd & 3rd stage impellers to get them more in line with design.

This is a general request for information from anyone who's experience of doing such a modification. How the modifications would be done, what are the potential pitfalls, what questions do I need to be asking our vendor etc?

Thanks in advance!

 

[pmover]

basically, the compressor mfg is asking if process conditions can be changed to fit the compressor. highly recommend you review the specifications and requirements for the test. if specs require head for 2/3rd stages to be within a specified tolerance at a specified speed, then the compressor mfg needs to comply.

making adjustments to process conditions is a matter worthy of review. highly recommend that the process engineers review the change and decide. consult them and seek their recommendations. sometimes, making process changes can be acceptable. the idea/design is to fit the compressor to the process conditions and not vice versa. then again, making process changes to fit compressor operation can be made. a thorough analysis needs to be done.

trimming impellers is a practice that has been done to meet process conditions or specifications. it is more work for the compressor mfg to do (i.e. less profit margin). they certainly have the skills, knowledge, resources, etc. to trim the impellers. think of having the impellers trimmed now and matching the process conditions rather than having the control system continuously make adjustments (sine wave or batch process) in order to meet process requirements.

good luck!
-pmover

 

[JJPellin]

I see no immediate problems from trimming impellers. But you need to consider a few items. If you did a rotor-dynamics analysis on this compressor, it should be reviewed in light of the change. Trimming impellers will reduce mass and raise the natural frequency of the rotor. It is unlikely to cause a problem unless there was a very small separation between the rotor critical speed and running speed in that first analysis. If the separation margin for rotor resonance was within API requirements, then you should be just fine. If you feel uncomfortable with this risk, you could arrange to re-run the analysis. The same thing could be applied to possible torsional resonance, but the probability is much lower.

Much of the documentation for the compressor has probably already been prepared. You will need to make sure that the manufacturer corrects all documentation to the new design. We have had changes made on the test stand that didn't get into the documentation (bill of material, data-sheets, curves) and later a mistake was made because of the incorrect documentation.


Johnny Pellin

 

[dcasto]

I'd like to see a process compressor with multiple feeds and interstage pressures that are so exact that I'd risk going in and trimming the wheels. What will the new efficencies be after the trim? What about upsets in the process? How will the anti-surge system work?

I'd look at several other aspects before going down that route. Look at the process and see what happens to it with changes in those interstages. Could you use some external control valves that will adjust the flows in recylce or induce some back pressures or reduced suction pressures?

 

[JJPellin]

I feel a need to point out that we are not talking about modifying an installed machine that has been providing good service. It is on the test stand. And we are not talking about risky tweaking to meet unnecessary specifications. It is described as "much more head than design." Modifications to a machine this complex on the test stand are not unusual, scary or particularly risky. All of the items that you list that could be potential problems could all be problems if they don't do the modification. The compressor company that designed and built this machine knows best how to get the proper performance out of it. And if you decline to let them do the modifications that they want, they may decline to honor their performance guarantees.

Johnny Pellin

 

[JamesDUK]

Hi again,

Thanks for the replies, you picked up some stuff I didn't think about which was great.

As extra info we have looked at and are still looking at alternatives to the trimming. Also the vendor denying liability, due to use not agreeing tolerances on the interstage pressures before purchase (valuable lesson). Also insisting that as the overall power consumption meets API 617 requirements (sorry can't remember para no. 4.3.3 something) even if the individual stages do not then it is not their problem. So repair cost would be to our account. It's getting commercially messy so I wanted to be clear on the potential technical pitfalls before we decide how to proceed.