To understand why pulsation analysis is necessary, it helps to understand the fundamental mechanism by which reciprocating compressors generate pressure pulsations in the connected piping. This is not a complicated phenomenon, but it is frequently misunderstood.
The Cylinder Pressure Cycle
A reciprocating compressor works by cyclically compressing gas in a cylinder using a piston driven by a crankshaft. During each revolution of the crankshaft, the piston completes one cycle: suction stroke (gas drawn in from the suction header at low pressure), compression stroke (gas compressed to discharge pressure), and discharge stroke (compressed gas pushed out to the discharge header).
This intermittent flow into and out of the cylinder is the fundamental source of pulsation. Unlike a centrifugal compressor, which moves gas continuously, a reciprocating compressor moves gas in discrete pulses — one pulse per cylinder per revolution for a single-acting cylinder, two pulses per cylinder per revolution for a double-acting cylinder.
How Pulsations Propagate
Each pulse of flow creates a pressure wave that propagates through the connected piping at the acoustic velocity of the gas — typically 300 to 500 meters per second for natural gas at operating conditions. When these waves encounter changes in pipe geometry — elbows, tees, diameter changes, or closed ends — they reflect and combine with incoming waves.
The superposition of incident and reflected waves creates standing wave patterns in the piping. Where the standing wave has a pressure antinode, pulsation amplitudes are high. Where it has a pressure node, they are low. The locations of antinodes and nodes are determined by the pipe geometry and the acoustic wave frequency.
Why This Matters for System Design
The acoustic behavior of the piping system is determined by both the excitation source — the compressor cylinder — and the boundary conditions — the pipe geometry, bottle sizes, and terminations. Changing either changes the standing wave pattern and therefore the pulsation distribution.
This is why pulsation analysis is essentially an acoustic simulation: it calculates the standing wave patterns in the piping system and identifies locations where pulsation amplitudes exceed allowable limits.
