GPSA Section 13 / API 617 · 618 · 619
Compressor thermodynamics, power equations, efficiency conversions, and API selection criteria
Polytropic head (GPSA Eq. 13-22) represents the actual aerodynamic work along the real compression path and its ηp is independent of pressure ratio, so it gives a stable basis for comparing machines and sizing multi-stage units. Isentropic head is the ideal-process reference; the calculator reports both and uses BHPg = ṁ·Hp / (33,000·ηp·ηmech) per GPSA Eq. 13-30.
Limits are driven by lubricant breakdown, valve life, and seal materials: API 618 reciprocating ≈ 350°F (valve life), API 617 centrifugal ≈ 450°F (rotor metallurgy), API 619 screw ≈ 300°F (oil-flooded lube). When the calculator predicts T₂ above the type limit it flags the result and you must add intercooling, increase stages, or pre-cool suction.
Equal-work staging gives rstage = rtotal1/N. Practical per-stage ratio limits are ≈ 3.5 (centrifugal), 4.5 (reciprocating), 6.0 (screw). The calculator computes the per-stage ratio and warns if it exceeds the type limit; it also recommends a stage count when overall ratio > 4. Intercooling between stages typically saves 10–20% power vs. uncooled.
API 617/618/619 require a minimum 10% margin over rated BHP to cover startup transients, gas-property excursions, and motor service factor. After applying the 10% margin the calculator rounds up to the next NEMA MG-1 standard size (1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100, 125, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800, 900, 1000, 1250, 1500, 1750, 2000, 2250, 2500, 3000 HP…).
Sales-quality natural gas is roughly MW = 17–20 lb/lbmol, k = 1.26–1.31, Z ≈ 0.95–0.98 at 100 psia and 0.85–0.92 at 500 psia (60°F). Richer associated gas: MW 22–26, k 1.20–1.25. For sour gas, use Wichert-Aziz to correct pseudocritical T and P before reading Z. The defaults shipped with the calculator (MW=19, k=1.28, Z=0.90) approximate medium-pressure NG.