ASME PTC 10
Understand polytropic vs isentropic efficiency, conversion methods, and their significance
Each compression stage reheats the gas; that reheat raises the work needed to reach the same discharge pressure compared with a hypothetical reversible-adiabatic path. Polytropic efficiency is the small-step (differential) efficiency before reheat is integrated, so it is always higher than the lumped isentropic efficiency. The gap grows with pressure ratio — typically 2-3 percentage points at PR=2 and 5-7 points at PR=10 for k=1.4.
For centrifugal compressors, always compare on a polytropic basis. API 617 and ASME PTC 10 specify ηp as the acceptance metric because it is independent of pressure ratio and lets you compare stages or whole machines on equal aerodynamic terms. If one vendor quotes ηisen, convert it to ηp (using k and the rated pressure ratio) before benchmarking.
Use the average k between suction and discharge conditions. For lean natural gas at typical pipeline conditions k ≈ 1.27-1.30; rich gas with significant C3+ runs 1.20-1.26; CO2 streams 1.28-1.32; air 1.40; hydrogen-rich recycle 1.40-1.41. For high-ratio or near-critical service, use a process-simulator kavg rather than a single end-point value.
Per GPSA §13 and field data: small overhung machines 72-78%, multi-stage beam compressors 75-85%, large pipeline-class machines 82-87%, and integrally-geared compressors up to 86%. Below 70% indicates fouling, internal recirculation, or excessive seal clearance — a maintenance trigger. Above 88% is uncommon outside axial machines.