Gas Compressor Pulsation and Vibration Interfaces

Gas Compressor Pulsation and Vibration Interfaces

1. Direct engineering answer

Pulsation is a time-varying pressure disturbance generated by cyclic gas displacement, valve events and interactions with connected volumes. Vibration is the structural response to dynamic forces; the two can be coupled but are not interchangeable. A defensible installation identifies forcing frequencies, acoustic resonances, shaking forces, mechanical modes, support stiffness and acceptance criteria across all credible operating cases. This page defines the interface data and decision workflow; a qualified pulsation/acoustic and mechanical specialist owns final calculations and acceptance.

2. Scope, exclusions and accountability

Package OEM supplies forcing, valve/cylinder details, allowable nozzle loads and machine limits; process engineer supplies gas states and transient cases; piping designer supplies isometrics, supports and as-built dimensions; acoustic specialist evaluates pressure pulsations; mechanical analyst evaluates vibration/stress; site owner approves operating limits and mitigations. No universal vibration or pulsation threshold is claimed here.

3. Why the gas changes the result

Sound speed, density, compressibility, viscosity and heat-capacity ratio change with composition, pressure and temperature. A natural-gas-rich stream cannot be assumed to behave like air, and a carbon-dioxide-rich or hydrogen-containing stream requires property evaluation over its own range. Liquid carryover and two-phase flow invalidate simple single-phase screening. Record gas analysis, phase margin and property source/validity at each scenario; distinguish suction and discharge properties.

4. Excitation sources and frequency inventory

For a reciprocating compressor, crankshaft frequency equals rpm/60 and harmonics are integer multiples; cylinder configuration, crank phasing, single/double-acting operation and valve events determine actual forcing amplitudes. For a rotary machine, rotor/impeller passage and gear-mesh frequencies may dominate; these require OEM data rather than generic assumptions. Driver, coupling, motor electrical and auxiliary equipment can add independent excitation. Build a frequency ledger, including variable-speed minima/maxima and operating transitions.

5. Compressor cylinder and nozzle interfaces

Request bore, stroke, rod diameter, clearance, operating end configuration, crank angles, cylinder pressure traces or OEM forcing spectrum, valve losses and nozzle sizes/orientation. Confirm which nozzle loads are permissible and which vibration probes are supplied. Pulsation bottles must be considered as designed pressure equipment with documented internal details; a bottle drawing without baffles, choke tubes, nozzles and internal volumes is insufficient for an acoustic model.

6. Geometry fidelity and boundary conditions

A study-ready piping model needs pipe internal diameters, schedule, material, centerline lengths, elbows, tees, reducers, branches, valves, restrictions, filters, coolers, vessels, bypasses, control valves, drains and dead legs. Acoustic reflection changes at area discontinuities and impedance boundaries. Piping support positions and actual restraint directions must be captured separately for mechanical response. Survey differences between issued-for-construction and as-built drawings before acceptance.

7. Screening equations and limitations

Running frequency f₁ = N/60, where N is rpm. Order frequency fₙ = nN/60. Idealized acoustic wavelength λ = a/f, where a is local speed of sound; a simple quarter-wave estimate f ≈ a/(4L) is valid only for a suitable uniform, approximately closed–open segment and is not a substitute for a network acoustic analysis. For a uniform gas with ideal-gas approximation, a ≈ √(kRT/M); use gas-property software for non-ideal, multicomponent high-pressure conditions. Pressure amplitude, dynamic stress and fatigue life cannot be calculated from frequency coincidence alone.

8. Worked case A — order and wavelength screening

ILLUSTRATIVE ONLY. A four-cylinder reciprocating gas package operates at 900 rpm; nominal discharge gas acoustic velocity is assumed 420 m/s for this simplified screening, not a verified project property. First order is 900/60 = 15 Hz; second order 30 Hz; fourth order 60 Hz. Wavelengths at these frequencies are 28.0 m, 14.0 m and 7.0 m. A hypothetical 3.5 m closed–open equivalent segment gives a quarter-wave estimate of 420/(4×3.5) = 30 Hz, coincident with second order. This is a REVIEW FLAG, not proof of resonance: effective length, boundary impedance, branch coupling, losses and forcing amplitude must be modelled.

9. Worked case B — speed range and resonance crossing

Assume variable-speed operation 600–1,200 rpm. Second-order forcing traverses 20–40 Hz. A modelled acoustic mode near 30 Hz would be crossed near 900 rpm, even if the unit is acceptable at the 600 rpm and 1,200 rpm endpoints. Therefore test at intermediate speeds and speed ramps, not only minimum/maximum. The analyst must determine whether a restricted operating band, bottle geometry change, pipe reroute, added support or different control strategy is technically justified; no restriction is implemented without OEM and process approval.

10. Worked case C — mechanical mode screening

Illustrative mechanical model: a supported spool has an estimated first bending natural frequency of 31 Hz and the compressor second-order forcing at nominal speed is 30 Hz. The frequency proximity indicates a potential response issue, but dynamic force, modal mass, damping, support boundary conditions and fatigue hotspots are unknown. A mechanical specialist should calculate forced response and local stress at welded small-bore branches, instrument connections and restraint points. Increasing a support stiffness may shift a mode but can increase local loads; redesign must consider both vibration and piping thermal flexibility.

11. Pulsation control choices

Pulsation vessels, acoustic filters, choke tubes, tuned side branches, revised line lengths and nozzle placement can reduce excitation transmission, but can also add pressure loss or maintenance complexity. Specify the desired mitigation function, allowed pressure drop, operating range, inspection access, liquid drainage and failure consequences. Avoid promising a universal bottle volume or line-length fix; selection is the outcome of specialist analysis and OEM review.

12. Mechanical response and fatigue-critical features

Small-bore connections, unsupported instrumentation, bottle attachments, drain branches, dead legs and weld transitions often merit specific attention because local stiffness and stress concentration differ from main-pipe behavior. Include restraint loads, thermal expansion, settlement, nozzle flexibility and compressor foundation vibration. A low overall velocity reading does not automatically exclude a local high-frequency fatigue risk. Conversely, a local vibration peak does not alone prove unacceptable stress.

13. Operating cases and transients

Analyse maximum flow, minimum flow, maximum pressure ratio, lowest suction density, highest discharge density, recycle open/closed, parallel machines, alternate gas compositions and speed sweep as applicable. Evaluate startup, shutdown, emergency depressurisation, valve switching and unloading where relevant. Document excluded cases explicitly; operating-case coverage is an owner-approved boundary, not an implicit claim of universal applicability.

14. Instrumentation and measurement plan

Specify accelerometer and velocity-sensor locations, mounting, orientation, frequency range, calibration, acquisition sample rate, anti-aliasing, spectral resolution, pressure transducer response, reference tachometer and simultaneous channel timing. Record background vibration and operating condition at every measurement. Trending should retain time waveform, spectrum, order tracking and process tags where relevant. A single handheld overall-vibration number cannot validate a full pulsation study.

15. Acceptance criteria ownership

Do not import generic machine vibration limits as automatic acceptance limits for process piping or acoustic pulsation. The contract must name the applicable edition and clause of any agreed industry or OEM criteria, the calculation/measurement method, frequency range, operating cases, allowable pulsation, shaking force, dynamic stress and corrective-action thresholds. Owner and specialist must approve any deviation. Distinguish design criteria, factory test, commissioning baseline and long-term monitoring trigger.

16. Study lifecycle and revision control

Freeze a study input register with revision identifiers. Preliminary study uses process and layout assumptions; detailed study incorporates OEM forcing and final vessel internals; as-built verification reconciles site geometry and supports; commissioning checks representative cases. A compressor speed change, gas composition change, piping reroute, bottle internal modification or control valve replacement may require revalidation. The analyst should issue a change-impact statement before operating outside the approved envelope.

17. Pulsation Study Input Schedule — mandatory engineering asset

Complete each line with evidence ID, operating case, units, revision, responsible owner, technical status, open item and disposition. Blank cells are not assumed acceptable. This schedule is a pre-study readiness tool, not the specialist report itself.

18. Responsibility and interface decision register

Use one accountable technical owner for each disposition; supplier review does not replace plant approval. The owner signs off operating case coverage and acceptance criteria; OEM signs off forcing, bottle internals, nozzle limits and modifications; piping/structural specialists sign off mechanical modifications; the acoustic specialist signs off model and response. Record deviations with date, revision and affected drawings.

19. Troubleshooting decision paths

A narrow-band peak tracking shaft order suggests synchronous forcing; a fixed-frequency peak across changing speed suggests a structural or acoustic mode; broad-band impacts may point to valve or mechanical faults. These are hypotheses, not diagnoses. Compare process pressure pulsations with vibration spectra and tachometer reference; verify sensor integrity and support condition before proposing hardware changes. If severe or rapidly increasing vibration is observed, follow the site safety and OEM stop-work rules.

High vibration at one speed

Confirm order tracking, identify mode crossing, compare support condition and process state; request specialist forced-response review.

Bottle or branch cracking

Preserve evidence, isolate per approved procedure, examine fatigue origin and local stress, reconcile as-built geometry; do not simply replace and restart.

Changed gas or pressure

Recalculate sound speed/density, update compressor forcing and acoustic model; document revised operating envelope.

Post-maintenance vibration

Check valve replacement, bottle internals, flange assembly, supports, machine alignment and data baseline; escalate unresolved differences.

20. Commissioning and site validation

Before startup confirm drawings, mechanical completion, pressure-system integrity, instrument calibration, emergency procedures, trip functions and safe measurement access. At approved steady operating points capture pressure, temperature, flow, speed, valve state, pressure pulsation and vibration simultaneously. Perform controlled speed sweeps only where OEM/site procedures permit. Compare measured spectral peaks and amplitudes to model predictions, identify unexplained modes and close deviations before accepting the package.

21. Procurement and supplier evaluation

Require bidders to state the exact study scope, model software/method, included operating cases, deliverables, assumed inputs, treatment of missing OEM forcing data, allowable criteria, number of review cycles, as-built updates and commissioning support. A proposal offering only a generic vibration check does not satisfy an acoustic pulsation and piping interaction study. Record assumptions and commercial exclusions separately from technical compliance.

22. Failure-mode and consequence register

For each plausible failure, record initiator, detectable symptom, safety or production consequence, preventive control, protective response, responsible owner and verification record. Include small-bore fatigue, bottle/nozzle cracking, instrument failure, unsupported pipe resonance, pressure drop after mitigation, altered acoustic boundary after valve replacement and speed-band operation outside the approved range. Site-specific hazard review determines risk ranking.

23. Frequently asked engineering questions

Q: Is low compressor vibration sufficient proof of safe piping? No. Local piping modes and small-bore fatigue need their own evaluation. Q: Can a bottle eliminate all pulsation? No. Its effectiveness depends on gas, geometry, operating range and the connected network. Q: Is frequency matching a failure prediction? No. It is a screening flag until forcing amplitude and response are analysed. Q: Should a VSD unit be checked at every speed? Evaluate the full speed range, with appropriate model resolution and selected field points. Q: Who approves acceptance criteria? The named plant technical authority with OEM and specialist input. Q: Can air-property calculations be reused for methane-rich gas? Not without validated property equivalence. Q: Can support additions be made on site? Only after piping flexibility, loads and dynamic response are reviewed.