Process Gas Compressor Selection Requirements

1. Direct answer and purpose

A process-gas compressor can be selected only after the full gas composition range, suction state, flow basis, discharge requirement, operating cases, phase behaviour, contaminants, materials, sealing, utilities, controls and specialist responsibilities are defined. A machine suitable for atmospheric air does not thereby have suitable thermodynamic performance, materials, containment, lubrication, sealing or protection for a process gas.

Final selection, performance and safety remain with qualified process, machinery, controls, materials and supplier specialists using validated properties and project standards.

2. Controlled decision and package boundary

The process licensor or owner defines process cases and gas analyses. The compressor specialist selects technology and predicts performance. Materials, process safety, mechanical, electrical, controls, environmental and operations owners approve their interfaces.

Draw the boundary from suction isolation through scrubber, filter, compressor stages, intercoolers, separators, recycle, check and relief devices to discharge isolation. Include driver, coupling or gearbox, lubrication, sealing or distance-piece systems, cooling, drains, vents, purge, leak-gas recovery, flare or treatment, instruments, control panel, foundations and maintenance handling.

3. Gas composition range

Provide component-by-component molar composition for every case, with totals reconciled to 100%. State analytical method, sampling location, wet or dry basis, detection limits, uncertainty and date. Include trace components that control corrosion, toxicity, flammability, polymerisation, material attack, lubricant compatibility or environmental handling even when they have little effect on molecular weight.

Define minimum, normal and maximum for variable components and identify combinations that can occur together. Do not build a fictitious worst gas by independently combining maxima that cannot coexist; do not omit a credible start-up or upset mixture because its duration is short.

4. Flow and reference basis

Prefer mass or molar flow for process definition and also provide any required actual suction volume. If standard or normal volume is used, state reference pressure, temperature, dry/wet basis and compressibility convention. Never compare or sum volumes at different references without conversion.

For each case record minimum, normal and maximum continuous flow, turndown, recycle, start-up, depressurisation, blowdown, purge and intermittent peaks. Clarify whether quoted flow includes water, solvent or other condensables. Supplier guarantees must use the same boundary and basis as the process datasheet.

5. Suction state

State suction flange absolute pressure, temperature, composition, phase, superheat or margin from saturation, expected pressure and temperature fluctuation, piping loss and upstream control. Identify whether pressure is at battery limit, separator outlet or compressor nozzle. A small suction-pressure change can materially alter suction volume and positive-displacement cylinder requirements.

Define entrained liquid, aerosol, solids and slug scenarios. Provide upstream separator design conditions, level control, drains, demister and trip. The compressor specialist must review whether the gas is clean and dry enough for the proposed technology and how start-up debris is controlled.

6. Discharge and pressure cases

Specify required discharge absolute pressure at the compressor nozzle and downstream battery limit, including piping, cooler, separator, check valve and control losses. Define normal control target, maximum operating requirement, settle-out or blocked cases and relief basis under the project pressure-safety process.

Compression ratio is discharge absolute pressure divided by suction absolute pressure. It helps compare cases but does not determine stage count, temperature, power or suitability by itself. Use validated gas properties, compressor maps or reciprocating calculations and supplier limits.

7. Thermodynamic-property basis

Identify the approved equation of state or property package, version, binary interaction parameters where applicable and composition range. Provide molecular weight, compressibility, heat capacity ratio or appropriate real-gas properties, enthalpy, density, dew point and phase envelope at relevant states.

Property uncertainty can dominate performance near phase boundaries or for hydrogen-rich, carbon-dioxide-rich, polar or strongly non-ideal mixtures. Do not substitute generic air properties.

8. Phase risks and liquid management

Plot or calculate the compression path, intercooler outlets, separators, recycle and shutdown equalisation against water and hydrocarbon dew points or other phase boundaries. Identify condensation, hydrate, freezing, deposition, polymerisation and two-phase risks. State allowable liquid at each interface; “no free liquid” requires a measurement and protection basis.

Specify suction and interstage separators, heating, cooling limits, drains, level trips, demisting and liquid disposal. Recycle gas can cool or heat and can change composition. Review start-up and low-flow operation because they may have different phase risk from normal duty.

9. Gas hazards and containment

Document flammability, toxicity, asphyxiation, oxidising potential, pyrophoricity, reactivity, corrosivity and environmental significance. Define hazardous area, ventilation, gas detection, leak routing, purge, relief and emergency isolation under the project risk assessment.

Containment includes shaft, rod packing, distance pieces, seals, casing joints, drains, vents and maintenance opening. Specify acceptable leakage or recovery boundary and how it is monitored. Do not claim “gas tight” without a defined test, pressure, duration and acceptance criterion.

10. Materials and corrosion

Provide gas and condensate composition, water content, temperature, pressure, cleaning chemicals and expected contaminants to the materials specialist. Review casing, cylinders, valves, impellers, rotors, piston rings, packing, coatings, gaskets, elastomers, lubricant and instrumentation wetted parts.

Consider wet versus dry corrosion, sulphide or chloride effects, hydrogen interaction, oxygen service, solvent swelling, polymer deposits, erosion and low-temperature toughness where relevant. A generic material name does not replace grade, heat treatment, hardness, coating and fabrication requirements.

11. Lubrication, cleanliness and product purity

State whether process gas may contact lubricant and whether oil carryover is allowed. Define product-purity, catalyst, downstream membrane, food, medical or oxygen-service constraints only with project evidence. Oil-free compression refers to the process compression path or defined boundary and does not mean the entire package contains no lubricant.

Identify cross-contamination from seals, purge, cooling leaks, cleaning and maintenance. Specify flushing, preservation and cleanliness inspection. Where non-lubricated components are required, supplier wear, speed, load and maintenance limits remain part of selection.

12. Technology selection questions

Candidate technologies may include reciprocating, centrifugal, screw, diaphragm, labyrinth-piston, liquid-ring or other designs. Compare them using suction volume, pressure ratio, flow range, gas properties, contamination, phase risk, containment, pulsation, turndown, availability, maintenance and site utilities.

A reciprocating solution may suit high pressure ratio and flexible flow but needs pulsation, valves, cylinders, packing and rod-load review. A centrifugal solution needs operating-map, surge, choke, speed and molecular-weight review. Positive-displacement rotary technologies need internal temperature, lubricant/seal, liquid and built-in ratio assessment.

13. Cooling, utilities and environment

Record available cooling-medium inlet temperature, flow, fouling, pressure and seasonal range; electrical supply; steam; instrument air; nitrogen or other purge; drains; flare or recovery; ventilation and ambient range. Interstage cooling changes power, discharge temperature and condensation risk.

Define heat rejection, noise, vibration, hazardous-area interfaces, emissions, seal vents and waste fluids. Utility failure cases belong in start-up, shutdown and trip logic. Do not assume a utility is continuously available because it exists at battery limit.

14. Controls and ownership

List capacity-control method, suction or discharge pressure control, speed, unloaders, clearance pockets, recycle, bypass, anti-surge, sequencing and start permissives as applicable. Assign ownership of every setpoint, alarm, trip, valve, sensor and protection function between process control, package control and safety systems.

Define fail positions, voting, response time, override management, restart, black-start or power restoration, communication loss and local/remote control. Anti-surge or recycle design must use the selected compressor, gas cases and validated instrumentation; a generic control curve is not acceptable.

15. Required asset - Gas Compression Enquiry Datasheet

Complete every applicable row for every operating case.

16. Specialist responsibility matrix

17. Worked comparison - stated property basis

Consider a bounded hypothetical gas enquiry using components nitrogen, carbon dioxide, methane and water. Molar flows are wet. Suction and discharge pressures are absolute. For a simple suction-volume screen only, use the ideal relation V = nRT/P with R = 0.08314 bar m3/(kmol K) and Z = 1. The real project must use a validated property method.

This arithmetic compares enquiry cases; it does not calculate power, discharge temperature, stages, driver size or compressor suitability. Composition ranges and phase behaviour remain supplier/process-specialist inputs.

18. Normal, start-up and upset enquiry comparison

Calculated ideal suction volumes are rounded. Normal: 500 x 0.08314 x 303.15 / 2.0 = 6,301.0 m3/h. Start-up: 200 x 0.08314 x 288.15 / 1.2 = 3,992.8 m3/h. Upset: 650 x 0.08314 x 318.15 / 2.5 = 6,877.3 m3/h.

19. Interpretation of the case comparison

The normal case has the largest continuous duty duration but not the highest ideal suction volume. The start-up case has the highest pressure ratio and greater water content, so it may govern stage temperature, separator or condensation review despite lower flow. The upset case has the highest molar flow and ideal suction volume, so it may govern short-term capacity, driver or protective response.

No case can be discarded using one metric. Real-gas compressibility, mixture heat capacity, dew point, losses, recycle and equipment maps are absent. A supplier enquiry must retain all three cases and identify guaranteed, design and transient requirements.

20. Wet versus dry flow check

If the normal wet flow is 500 kmol/h and water is 0.2 mol%, dry non-water flow is 500 x (1 - 0.002) = 499 kmol/h. If a supplier interprets 500 kmol/h as dry gas and then adds water, the total wet flow changes. The datasheet must state the basis.

At start-up, 2.0 mol% water makes the distinction larger: dry non-water flow is 200 x 0.98 = 196 kmol/h. These arithmetic checks do not establish water saturation or condensation; use validated equilibrium at the stated pressure and temperature.

21. Measurement and data validation

Validate online or laboratory composition across normal and transient states. Reconcile flow meters with pressure, temperature, compressibility and wet/dry basis. Check suction pressure and temperature at the compressor nozzle, not only an upstream battery limit. Preserve raw analyses and instrument uncertainty.

During commissioning, compare measured case data with the supplier's approved performance correction method. Record recycle, unloaders, speed, cooling, separator levels, vibration, temperatures, power and leakage. Do not extrapolate beyond approved curves or property range.

22. Diagnostic matrix

23. Commissioning and acceptance

Verify installed package, tag, materials, rotation, alignment, piping cleanliness, separators, drains, cooling, lubrication, sealing, purge, vents, relief, instruments, cause/effect and documentation. Confirm temporary start-up screens or cleaning controls are handled under the approved procedure.

Commission with approved inert or process introduction sequence. Progress through allowed pressure, temperature, flow and composition cases. Test capacity control, recycle or anti-surge, trips, separator levels, utility failure and safe shutdown. Accept only against the agreed boundary, correction method, tolerances and evidence.

24. Change control

Reopen selection after changes to composition, trace contaminants, wet/dry basis, flow, suction pressure or temperature, discharge requirement, property method, phase envelope, cooling, purge, materials, lubrication, seals, control strategy, downstream loss, relief basis or operating duration.

A seemingly small component change can alter molecular weight, compressibility, corrosion, flammability or phase behaviour. Operations must not add a new gas or cleaning chemical without process, materials, safety and supplier review.