GPSA Β· API 617 Β· Lee-Kesler Β· Cryogenic NGL Recovery
| Gas Type | k | MW | SG |
|---|---|---|---|
| Lean NG | 1.27β1.30 | 17β19 | 0.59β0.66 |
| Rich NG | 1.20β1.25 | 19β24 | 0.66β0.83 |
| Methane | 1.31 | 16.04 | 0.554 |
| Ethane | 1.19 | 30.07 | 1.038 |
Understand turboexpander efficiency principles, calculations, and industry applications
| Efficiency | Assessment |
|---|---|
| >88% | Exceptional |
| 80β88% | Normal range |
| 70β80% | Below optimal |
| <70% | Requires attention |
Per API 617 and OEM specifications for radial inflow expanders
| Parameter | Typical Range |
|---|---|
| Expansion Ratio | 2:1 β 5:1 (single stage) |
| Tip Speed | 800β1200 ft/s |
| Specific Speed | 60β90 (optimal) |
| Power Range | 100 kW β 25 MW |
| Condition | Recommended Method |
|---|---|
| P < 300 psia, lean gas | Ideal Gas β |
| P > 400 psia | Real Gas |
| Rich gas (MW > 22) | Real Gas |
| Cryogenic (<-100Β°F outlet) | Real Gas |
| Efficiency shows >100% | Real Gas |
| Quick estimate | Ideal Gas β |
The ideal gas method assumes Z=1. At high pressure (>400 psia), real gases experience additional Joule-Thomson cooling beyond isentropic predictions. This isn't an errorβit indicates real gas effects. Switch to the Real Gas method for accurate results.
| Scenario | What's Happening |
|---|---|
| Ξ· = 85% | Actual temp is warmer than isentropic (normal losses) |
| Ξ· = 100% | Actual temp equals isentropic (perfect, theoretical) |
| Ξ· > 100% | Actual temp is colder than isentropic prediction |
Expanders: Ξ· = (actual work out) / (ideal work out) β higher is better
Compressors: Ξ· = (ideal work in) / (actual work in) β inverted ratio
| NGL/LPG Recovery | 80β88% |
| LNG Processing | 85β88% |
| Fuel Gas Letdown | 75β82% |
β Helps: Clean gas, design-point operation, good bearings/seals, proper speed
β Hurts: Fouling, erosion, liquid ingestion, off-design turndown, worn seals
Isentropic efficiency measures how closely a turboexpander approaches ideal (reversible) expansion. This calculator uses the Lee-Kesler equation of state and GPSA Engineering Data Book methods to compute actual vs. ideal enthalpy change across the expander.
Turboexpander efficiency calculations follow GPSA Section 13, API 617 for centrifugal compressors and expanders, and the Lee-Kesler thermodynamic correlation for real-gas enthalpy departures.
Higher inlet-to-outlet pressure ratios increase the available enthalpy drop, generating more shaft power. The calculator determines actual power output based on gas composition, inlet conditions, and isentropic efficiency.
Turboexpander isentropic efficiencies in gas processing plants typically range from 75% to 88%, depending on machine design, gas composition, and operating conditions per GPSA guidelines.