Reciprocating compressors operate on the principle of positive displacement, where gas is compressed by reducing its volume through the reciprocating motion of pistons within cylinders. Below is a breakdown of the key operational phases and design features compliant with API 618 standards:

1. Compression Cycle

• Intake Stroke:

  • The piston moves downward, creating a vacuum that opens the suction valve.

  • Gas enters the cylinder at low pressure.

• Compression Stroke:

  • The piston reverses direction and moves upward, closing the suction valve.

  • Gas volume decreases, increasing pressure until it exceeds the discharge valve’s cracking pressure.

• Discharge Stroke:

  • The discharge valve opens, forcing compressed gas into the outlet piping/system.

  • The piston completes the stroke, resetting the cycle.

API 618 Compliance: Double-acting cylinders (compression on both piston strokes) are often used to maximize efficiency and minimize pulsation, as mandated by API 618.

2. Key Components

• Crankshaft & Connecting Rod:
  Converts rotary motion from the driver (e.g., motor, engine) into linear piston movement.

• Piston & Cylinder:
  Precision-machined pistons and hardened cylinders ensure leak-free compression, critical for handling toxic/flammable gases per API 618.

• Valves (Suction/Discharge):
  Pressure-actuated reed or plate valves designed for high-cycle fatigue resistance and gas compatibility.

• Pulsation Dampeners:
  ASME-coded pulsation bottles or dampeners suppress pressure fluctuations, meeting API 618’s pulsation control requirements.

• Piston Rod Packing:
  Multi-ring packing systems prevent gas leakage, adhering to API 618’s stringent emissions criteria.

3. API 618-Specific Design Considerations

• Dynamic Load Analysis:
  Forces, vibrations, and rod loads are rigorously simulated to avoid mechanical failure during continuous duty cycles.

• Material Selection:
  Cylinders, valves, and piping use API-specified materials (e.g., stainless steel, alloy coatings) to resist corrosion from aggressive gases like H₂S or CO₂.

• Cooling & Lubrication:

  • Jacketed Cylinders: Maintain optimal gas temperature to prevent degradation.

  • Non-lubricated Designs: Optional for applications requiring oil-free gas (e.g., oxygen, food-grade CO₂).

• Safety Systems:
  Integrated pressure relief valves, temperature sensors, and vibration monitors ensure fail-safe operation.

4. Performance Characteristics

• Isentropic Efficiency: Typically 75–90%, depending on gas properties and staging.

• Pressure Ratio: Achieves ratios up to 1:3 per stage; multi-stage configurations used for higher ratios.

• Pulsation Control: Meets API 618’s allowable pulsation limits (<2% peak-to-peak) to protect downstream piping.