Pulsation Frequencies · Acoustic Resonance · API 618 Screening
Reciprocating compressors create pulsating flow because gas is delivered in discrete pulses rather than continuously. Key concepts:
Pulsation bottles dampen these pressure waves through volume expansion and acoustic impedance mismatch. API 618 specifies acceptable pulsation levels and when formal studies are required.
The fundamental pulsation frequency is f = (RPM × Nevents) / 60 Hz, where Nevents is the number of compression events per revolution: 1 per cylinder for single-acting, 2 per cylinder for double-acting. A 2-cylinder double-acting compressor at 900 RPM produces a 60 Hz fundamental, plus integer-multiple harmonics at 120, 180, and 240 Hz.
Acoustic velocity in gas is c = 223.0 × √(k × T / MW) ft/s, where k is the specific heat ratio (Cp/Cv), T is absolute temperature in °R, and MW is molecular weight in lb/lbmol. The coefficient 223.0 = √(gc × Ru) = √(32.174 × 1545.35). For natural gas at 100 °F, c is about 1,400 ft/s. Real-gas effects (Z < 1) typically reduce this 5–10%.
Half-wave resonance occurs at L = c / (2f) for open-open or closed-closed pipe segments, and quarter-wave resonance at L = c / (4f) for open-closed segments. Pipe lengths within ±20% of either resonant length should be avoided per API 618 §7.9.4.2.7. Common mitigations are changing pipe length, adding orifice plates, installing pulsation bottles, or reducing operating speed.
Industry rules of thumb size pulsation bottles at 5–25× cylinder swept volume, with the higher end for low-pressure suction (<50 psia) and the lower end for high-pressure discharge (>1000 psig). This calculator uses a conservative 10–25× screening band, expanding to 20–30× at low suction pressure and tightening to 8–15× at high discharge. Final bottle sizing requires API 618 Approach 2 digital acoustic simulation.