Gas compressor pulsation and vibration: causes and control

Pulsation and vibration: what is the difference?

A reciprocating compressor draws and delivers gas in pulses as each valve opens and closes. Those pulses travel through suction and discharge piping as pressure waves. Where pipe bends, reducers, closed ends and vessels reflect the waves, they can build up into standing waves at certain frequencies. That is pulsation.

Pulsation creates shaking forces at bends and changes of section. The pipe, its supports and the compressor frame respond to those forces, and to the machine’s own unbalanced forces, by vibrating. If a forcing frequency matches a natural frequency of the pipe span or structure, the response is amplified and fatigue cracks can follow, typically at small-bore connections, welds and nozzles.

Pulsation also has process effects: it upsets flow meters, makes pressure gauges read wrongly, causes valve flutter and can reduce compressor capacity or raise power. Measuring vibration alone does not tell you whether pulsation is the cause.

Why the gas changes everything

Acoustic natural frequencies depend on the speed of sound in the gas, which depends on molecular weight, temperature and the ratio of specific heats. Hydrogen-rich gas has a much higher speed of sound than natural gas; carbon dioxide has a lower one. A piping system that is quiet on one gas can resonate on another, or when composition, pressure or temperature shift. Analysis must use the gas properties for each real case, and liquid carry-over or two-phase flow invalidates simple screening.

Where the excitation comes from

For example, a compressor running at 990 rpm has a running frequency of 16.5 Hz. Its main pulsation harmonics then sit at 16.5, 33, 49.5 and 66 Hz and above. A pipe span whose natural frequency is near 33 Hz would be a concern on a double-acting machine. If the same machine runs on a variable-speed drive from 600 to 1,000 rpm, the harmonics sweep from 10 to 16.7 Hz, 20 to 33.3 Hz and so on, so the whole band must be checked.

How is pulsation controlled?

• Pulsation bottles (dampers) close to the cylinder flanges, sized to absorb pressure waves before they enter the piping.
• Choke tubes and internal baffles in bottles, forming acoustic filters for low frequencies.
• Orifice plates at selected points to add damping to standing waves, at the cost of a small pressure drop.
• Avoiding piping lengths that match the acoustic resonance at running-speed harmonics.
• Correct location and volume of receivers and separators, which also act acoustically.

Industry practice for reciprocating gas compressors commonly follows recognised standards such as API 618, which describes design approaches from simple bottle sizing through full acoustic and mechanical analysis. The level of study depends on pressure, power and gas hazard; the purchaser and the maker agree which approach applies.

How is vibration controlled?

• Pipe supports and clamps spaced and stiffened so span natural frequencies sit well away from forcing frequencies.
• Supports clamped to the pipe, not simple resting shoes, on lines near the compressor.
• Bottles supported so they do not cantilever from cylinder nozzles.
• Small-bore branch connections braced back to the main pipe, as they are the most common fatigue failure point.
• A foundation or skid stiff and heavy enough for the machine’s unbalanced forces, with proper grouting and anchor bolts.
• Avoid mounting gauges and instruments on long unsupported stems.

A practical study and commissioning workflow

1. Collect machine data from the maker: cylinder sizes, speeds, valve details, unloading steps and allowable nozzle loads.
2. Define every operating case: gas compositions, suction and discharge pressures and temperatures, speed range and load steps.
3. Collect accurate piping isometrics, support details and vessel dimensions, ideally as built.
4. Carry out acoustic analysis to predict pulsation levels and shaking forces.
5. Carry out mechanical analysis of piping and supports to find natural frequencies and stress.
6. Apply fixes: bottle changes, orifices, support changes or re-routing, then re-check.
7. At commissioning, measure pulsation and vibration at all speeds and load steps and compare with the agreed limits.
8. Record the baseline for future condition monitoring.

Machine-side checks before blaming the piping

Not every vibration problem is a pulsation problem. Before ordering an expensive study, rule out the mechanical causes that a service engineer can find on site.

• Foundation and grout: cracks, voids and oil-soaked grout lose stiffness and let the frame move.
• Anchor bolts: loose or stretched bolts and missing washers allow rocking.
• Soft foot and alignment between driver and compressor, checked hot and cold where possible.
• Coupling condition, belt tension and pulley alignment on belt-driven units.
• Crosshead, crankpin and main bearing clearances, and piston rod run-out.
• Valve condition: broken plates or springs alter the forcing pattern and pulsation.
• Skid-mounted packages: welds, sub-base stiffness and how the skid is supported on its plinth.

Mechanical faults and pulsation often combine. A support loosened by a slightly resonant pipe allows larger movement, which loosens it further. Fixing the mechanical defect first, then measuring again, avoids treating the wrong cause. Keep records of each change and its effect so the next investigation starts from evidence rather than memory.

Field symptoms and likely causes

A change of gas, a new speed range from a variable-speed drive, added unloading steps or modified piping can each move a previously quiet system into resonance. Treat any such change as a reason to review pulsation and vibration.

Measurement in the field

• Use accelerometers or velocity probes on the frame, cylinders, bottles and pipe spans, in three directions.
• Use dynamic pressure transducers on bottle and piping tappings to measure pulsation directly.
• Record frequency spectra, not just overall levels, so each peak can be matched to a source.
• Test at each speed and load step and with the actual gas.
• Bump or impact tests on stationary pipe spans identify their natural frequencies.

How Reddy Compressors helps

We sell new and used/refurbished gas compressors for CNG, nitrogen, process gas and booster duties, rent them, service them and supply spares, as an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands. On site, our engineers look at vibration problems practically: checking foundations, grouting and anchor bolts, alignment, soft foot, supports and clamps, small-bore bracing, valves and unloaders, and how vibration changes with speed and load. Where detailed acoustic or mechanical analysis is required, it is arranged with specialists and worked to the maker’s data.

Our service teams cover the whole of Telangana and Andhra Pradesh from Tarnaka, Secunderabad, and we supply machines and spares across India. Allowable vibration and pulsation limits, piping stress and hazardous-area compliance remain with the maker, the piping designer and your engineering authority.

What to send us

• Compressor make, model, serial number, speed and number of cylinders or stages, with nameplate photos.
• Gas type, composition and pressure range.
• Photos or a sketch of suction and discharge piping, bottles and supports.
• Where the vibration or cracking occurs, and at which speeds or load steps.
• Any vibration readings, spectra or failure history you have.

Frequently asked questions

What causes pulsation in a reciprocating compressor?

Pulsation comes from the intermittent way a reciprocating compressor draws in and pushes out gas as valves open and close each stroke. These pressure pulses travel through the suction and discharge piping. Where reflected waves line up at certain frequencies, standing waves build up and amplify the pulsation, creating shaking forces at bends and changes of pipe size.

What is the difference between pulsation and vibration?

Pulsation is a pressure fluctuation in the gas inside the pipe. Vibration is mechanical movement of the compressor, pipe or structure. Pulsation creates forces that cause vibration, but vibration can also come from machine unbalance, misalignment or loose foundations. Measuring both, with frequency spectra, shows which is driving a particular problem.

Why do small-bore pipe connections crack on compressor piping?

Small-bore branches, such as vents, drains, gauge connections and thermowells, are light and often unsupported, so they have natural frequencies that can match compressor harmonics. Shaking of the main pipe drives them into resonance and fatigue cracks form at the weld. Bracing each branch back to the main pipe and keeping them short are the usual remedies.

What does a pulsation bottle do?

A pulsation bottle is a vessel fitted close to the cylinder suction or discharge flange. Its volume, and sometimes internal baffles or choke tubes, absorbs and filters pressure pulses before they reach the piping. That reduces pulsation in the system, the shaking forces it causes, and its effects on meters, valves and compressor capacity.

Can a VFD cause vibration problems on a gas compressor?

It can. A variable-frequency drive (VFD) makes the machine sweep a range of running speeds, so every harmonic sweeps a frequency band. A piping span or acoustic mode that was safely away from a fixed speed may fall inside that band. Before adding a VFD, check pulsation and mechanical natural frequencies across the full speed range and consider lockout bands where necessary.

How do I find the source of compressor piping vibration?

Measure vibration in three directions at several points, and pulsation where tappings exist, at each speed and load step. Compare the frequency peaks with running speed and its multiples, lobe or blade passing frequencies and pipe-span natural frequencies found by impact testing. Check foundations, supports and alignment as well. The match between peaks and sources points to the fix.

Related pages

• CNG and process-gas reciprocating compressors
• Gas compressor capacity control
• Gas compressor interstage cooling and separation
• Positive-displacement blower pulsation and silencing
• Condition monitoring
• Reciprocating compressor service and reliability