Trimming Compressor Blades
Trimming Compressor Blades
(OP)
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!
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!





RE: Trimming Compressor Blades
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
RE: Trimming Compressor Blades
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
RE: Trimming Compressor Blades
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?
RE: Trimming Compressor Blades
Johnny Pellin
RE: Trimming Compressor Blades
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.