Multi-Stage Compressor Issue for LNG 1

[HamzaNossair]

Hello Everyone,

I have LP Casing-MP Casing-HP Casing of Mix Refrigerant Centrifugal Compressors for a LNG Project. I had a problem in the Mid Compressor casing.
Vendor quotes a suction pressure of (19.6 bar) for this MP Casing. The Licensor requires (23.77 bar), Also due to this higher MP stage pressure ratio, The cooler downstream this MP Casing will have an inlet temperature (MP Compressor Discharge Pressure) of 163F instead of 148F per the process Design.

The Vendor Replied and told us that the inter stage pressure of MR is not critical and can be adjusted without creating issues from process point of view.
The inter stage pressure between the LP and MP casing has been lowered to have a better rotor dynamic behavior of LP Casing MR as to reduce the head that this casing will handle.

Any one understands the details of these types of Cent Compressors TRAINS can advise??

 

[dcasto]

i suggest you get familiar with the fan laws. Centrifugal compressors only have inlet CFM and head that must be satisfied from process side. They mechanical side has many problems that could crop up like thrust, rotor tip speed, critical speeds, bearing cooling and more.

The centrifugal system you just barely describe will search equilibrium with each casings ability (each compressor case has multiple stages I assume). One way to adjust the pressures in between cases is to change the mixed refrigerant composition. You need to have your compressor modeled and watch what happens as you changed the composition. Another way is to change the duty on the interstages by artificially loading or unloading the chillers.

Discharge cooler load are more likely to be a function of condensing duty and slightly higher inlet temps will have a small effect on the next stage (higher temp equals higher acfm and with a fixed acfm a higher pressure is required to compensate, but a higher pressure means more condensing liquids and a lower ACFM and back to equilibrium). You can go nuts trying to solve the equilibrium point.