GPSA · API 617 · Lee-Kesler · Cryogenic NGL Recovery
| Gas Type | k | MW | SG |
|---|---|---|---|
| Lean NG | 1.27–1.30 | 17–19 | 0.59–0.66 |
| Rich NG | 1.20–1.25 | 19–24 | 0.66–0.83 |
| 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 |
| 80–88% | Normal range |
| 70–80% | Below optimal |
| <70% | Requires attention |
Per API 617 and OEM specifications for radial inflow expanders
| Parameter | Typical Range |
|---|---|
| Expansion Ratio | 2:1 – 5:1 (single stage) |
| Tip Speed | 800–1200 ft/s |
| Specific Speed | 60–90 (optimal) |
| Power Range | 100 kW – 25 MW |
| Condition | Recommended Method |
|---|---|
| P < 300 psia, lean gas | Ideal Gas ✓ |
| P > 400 psia | Real Gas |
| Rich gas (MW > 22) | Real Gas |
| Cryogenic (<-100°F outlet) | Real Gas |
| Efficiency shows >100% | Real Gas |
| Quick estimate | Ideal Gas ✓ |
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 error—it indicates real gas effects. Switch to the Real Gas method for accurate results.
| Scenario | What's Happening |
|---|---|
| η = 85% | Actual temp is warmer than isentropic (normal losses) |
| η = 100% | Actual temp equals isentropic (perfect, theoretical) |
| η > 100% | Actual temp is colder than isentropic prediction |
Expanders: η = (actual work out) / (ideal work out) — higher is better
Compressors: η = (ideal work in) / (actual work in) — inverted ratio
| NGL/LPG Recovery | 80–88% |
| LNG Processing | 85–88% |
| Fuel Gas Letdown | 75–82% |
✓ Helps: Clean gas, design-point operation, good bearings/seals, proper speed
✗ Hurts: Fouling, erosion, liquid ingestion, off-design turndown, worn seals