How a diaphragm compressor works
A crankshaft drives a piston in an oil-filled cylinder. The piston does not touch the gas. Instead, it pushes hydraulic oil against a set of thin metal diaphragms, usually three layers, clamped between two shaped cavity plates. As the diaphragm flexes, it sweeps the gas side of the cavity, drawing gas in through a suction valve and pushing it out through a discharge valve.
Because the gas space is sealed by static clamping rather than by a moving piston ring or rod packing, the gas path is hermetic. There is no lubricant in contact with the gas and essentially no leakage to atmosphere through a moving seal. An oil-side compensating pump and pressure-limiting valve keep the hydraulic side filled and limit the diaphragm stroke.
- Gas side: cavity plate, gas valves and the gas face of the diaphragm set.
- Hydraulic side: piston, oil, distribution plate, compensating pump and relief valve.
- Middle layer: in a three-layer set, a leak-detection groove between layers carries any leakage from a cracked layer to a pressure switch or sensor, giving warning before gas and oil can mix.
Where diaphragm compressors are used
| Application | Why a diaphragm machine |
|---|---|
| Hydrogen refuelling and storage | High pressures, hermetic path, no oil contamination of fuel-cell gas |
| Specialty and electronic gases | Very high purity, no particulate or hydrocarbon pick-up |
| Oxygen and oxidising gases | No lubricant in the gas path (with oxygen-compatible design and cleaning) |
| Toxic or corrosive gases | Hermetic containment, suitable wetted materials |
| Helium, argon and other valuable gases | Minimal loss through leakage |
| Laboratory and pilot plants, gas cylinder filling | Small flows at high pressures with clean gas |
| Nuclear and radioactive gases | Containment with leak detection |
Diaphragm compressors are typically small to medium in flow. Where flows are large and purity requirements are moderate, a non-lubricated reciprocating, oil-free screw or centrifugal machine may be more practical. The choice is a trade-off between purity, containment, flow and maintenance.
Diaphragm versus other gas compressors
| Feature | Diaphragm | Non-lubricated piston | Lubricated piston | Oil-injected screw |
|---|---|---|---|---|
| Gas contact with oil | None | None in cylinder; risk from rod packing area | Yes | Yes, then separated |
| Leakage to atmosphere | Very low, static seals | Rod packing leakage | Rod packing leakage | Shaft seal leakage |
| Flow range | Small to moderate | Wide | Wide | Moderate to large |
| Pressure capability | High, multi-stage to very high | High with stages | High with stages | Moderate |
| Main wear items | Diaphragm set, gas valves, hydraulic oil | Rings, rider bands, packing, valves | Rings, packing, valves | Bearings, seals, oil and separator |
Defining the duty correctly
Diaphragm compressors are often bought for difficult gases, so the enquiry must be precise. The maker needs:
- Gas name and full composition, including trace components, moisture and any particulates.
- Molecular weight of the mixture and its range, because capacity and temperature depend on it.
- Suction pressure range in absolute terms, including the lowest credible value, for example from a falling storage or cylinder pressure.
- Discharge pressure in absolute terms, with any range for cylinder or storage filling.
- Required mass flow (kg/h) or normal volume with stated reference conditions.
- Suction temperature and the cooling water or ambient conditions available.
- Duty cycle: continuous, intermittent, frequent starts or batch filling.
- Hazardous-area classification and venting arrangements.
A note on units: pressures in psig and bar(g) should always be converted to absolute before calculating ratios. For example, 150 bar(g) is 2,175.6 psig; at sea level it is about 151 bar(a).
Stage ratios, temperature and cooling
Diaphragm compressors handle high pressure ratios per stage compared with many piston machines, because the thin gas layer is cooled well through the cavity plates. Even so, every stage heats the gas, and temperature limits for diaphragms, valves and seals govern how many stages are needed. A screening estimate uses T2s = T1 × r^((k−1)/k) in kelvin with an assumed efficiency, but the maker’s calculation for the actual gas decides.
Cooling usually comes from water through the heads and intercoolers. In Indian summers, cooling-water temperatures from cooling towers can be high, which raises gas temperatures and reduces margins. Specify the warmest credible cooling-water supply, and check for scaling and fouling, which slowly erode cooling performance.
Diaphragm integrity and leak detection
The diaphragm set is the heart of the machine. It is designed for long service, but it can fail from:
- Particles or liquid droplets entering the gas cavity and denting the diaphragm.
- Over-stroking caused by wrong hydraulic oil pressure or a faulty compensating system.
- Gas valves leaking, raising temperature and stress.
- Corrosion from incompatible gas components or moisture.
- Fatigue after long service.
With a three-layer set, a crack in the gas-side or oil-side layer lets gas or oil into the leak-detection path, and the detector raises an alarm or stops the compressor. The response must be engineered for the gas: where the leakage goes, how it is vented safely and whether the machine stops automatically. Suction filtration, clean piping and correct start-up after maintenance are the best protection for diaphragm life.
Capacity control and duty cycling
Diaphragm compressors are positive-displacement machines, so flow is set by speed, displacement and volumetric efficiency. Common methods of control include start/stop with a buffer volume, bypass or recycle from discharge to suction, variable-speed drives within the maker’s limits, and for multi-stage machines, maintaining interstage pressure. Frequent starts increase wear on valves and diaphragms, so a buffer vessel sized to reduce cycling is often worthwhile. Recycle wastes energy and adds heat, so it should be cooled and sized carefully.
Valves, pulsation and piping interfaces
Gas valves are frequent maintenance items. Leaking valves reduce flow and raise temperature, so stage pressures and temperatures should be logged as an early warning. Pulsation from the reciprocating action needs suitable pulsation dampers or buffer volumes, and piping must be supported against vibration. Suction filters should be fine enough to protect the diaphragm and easy to change. For high-pressure discharge piping, use the correct pressure-rated tubing and fittings, and keep relief valves and vent routes in line with the process safety design.
Commissioning and troubleshooting
Commissioning normally includes leak testing with an inert gas, purging air from the gas path, checking hydraulic oil level and pressure settings, confirming leak-detection alarm function and running through the operating cases. For oxygen or high-purity service, cleanliness checks are part of acceptance.
| Symptom | Possible causes | First checks |
|---|---|---|
| Reduced flow | Leaking gas valves, low suction pressure, oil-side fault limiting stroke | Stage pressures and temperatures, suction pressure, hydraulic pressure |
| High discharge temperature | Valve leakage, poor cooling, higher ratio than designed | Cooling-water flow and temperature, valve condition |
| Leak-detection alarm | Cracked diaphragm layer, detector fault | Stop safely, check detector, inspect diaphragm set |
| Oil-side pressure faults | Compensating pump wear, relief valve setting, air in oil | Oil level, pump, valve setting, bleed oil side |
| Knocking noise | Over-stroke, loose components, liquid in gas side | Stop and investigate before restarting |
How Reddy Compressors helps
Reddy Compressors sells new and used/refurbished gas compressors, including CNG, nitrogen, process-gas and booster machines, rents them, services them and supplies their spares. We work as an independent multi-brand specialist from our office at Tarnaka, Secunderabad, and have served industry since 2002. We are not an authorised dealer of any compressor brand. We have tie-ups with all major and reputed brands.
- We help you define the duty: gas data, absolute pressures, mass flow and duty cycle, so suppliers can quote the right machine.
- We compare diaphragm and other gas compressor options neutrally for your purity, flow and pressure needs.
- Our team of more than 20 service engineers and technicians carries out servicing and repairs across Telangana and Andhra Pradesh; equipment and spares are supplied across India.
- Spares such as diaphragm sets, valves and seals are identified from the nameplate, serial number and old part, and fit is confirmed before quoting.
Oxygen, hydrogen and toxic-gas duties still follow the gas-specific codes and the maker’s design rules, and we work within them.
What to send us
- Gas name, composition and purity requirement.
- Suction and discharge pressure ranges, stating gauge or absolute.
- Required flow and duty cycle.
- Cooling water or ambient conditions and site location.
- Nameplate photo and service history for an existing machine.
- Description of the fault or the project timeline.
Frequently asked questions
What is a diaphragm compressor used for?
Diaphragm compressors are used where the gas must not be contaminated or must not leak: hydrogen for fuel cells and refuelling, specialty and electronic gases, oxygen, toxic or corrosive gases, helium and other valuable gases, and laboratory or cylinder-filling duties. They suit small to moderate flows at high pressures, where a hermetic, oil-free gas path is worth more than the higher cost per unit of flow.
How does a diaphragm compressor differ from an oil-free piston compressor?
In an oil-free piston compressor, a piston with dry rings moves inside the gas cylinder and a rod packing seals the crankcase side, so some leakage and wear particles are possible. In a diaphragm compressor, the piston only moves hydraulic oil, and a clamped metal diaphragm separates the gas, giving a static-sealed, hermetic gas path with very low leakage and no wear debris from rings.
How long does a compressor diaphragm last?
Diaphragm life depends on gas cleanliness, correct oil-side pressures, valve condition, temperature and the number of starts, so there is no single figure. Particles and liquid in the gas are the most common causes of early failure. Good suction filtration, clean piping after maintenance and logging of stage pressures and temperatures help achieve the life the maker designs for, and the leak-detection system warns of a failing layer.
What happens if a diaphragm ruptures?
In a three-layer diaphragm set, a crack in one layer lets gas or oil into the leak-detection groove between layers. The detector signals an alarm or stops the compressor before gas and hydraulic oil can mix. The leakage must be vented to a safe place suited to the gas. The diaphragm set is then replaced, and the cause, often contamination or over-stroking, should be found before restarting.
Can a diaphragm compressor be used for hydrogen?
Yes, diaphragm compressors are widely used for hydrogen at high pressures because the gas path is hermetic and oil-free. Hydrogen service needs materials resistant to embrittlement, suitable sealing, hazardous-area electrical equipment and safe venting. These requirements follow hydrogen-specific codes and the maker’s design rules, so the machine must be specified and approved for hydrogen, not adapted from another gas.
Why is my diaphragm compressor losing capacity?
The usual causes are leaking gas valves, lower suction pressure than designed, a problem on the hydraulic side that limits diaphragm stroke, or a blocked suction filter. Log suction, interstage and discharge pressures and temperatures and compare them with the commissioning record. Rising stage temperature often points to a leaking valve. Check the hydraulic oil level and pressure settings before opening the machine.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Gas compression stage screening
Convert suction and discharge to absolute pressure, find the overall and per-stage pressure ratio, the interstage pressures for an equal split, the ideal discharge temperature by the gas’s k value, and, from the mass flow, the actual suction volume, normal volume and mass transferred.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Suction pressure (in the unit below): 10
- Suction pressure unit: bar a
- Discharge pressure (in the unit below): 200
- Discharge pressure unit: bar a
- Site atmospheric pressure (bar a; for gauge entries): 1.013
- Gas: Hydrogen (k 1.41, MW 2)
- Other gas: k = cp/cv (used only for Other): 1.3
- Other gas: molecular weight (kg/kmol; used only for Other): 17
- Suction temperature (°C): 30
- Number of stages (screening, equal ratio): 3
- Stage ratio for the temperature check (0 = equal split): 0
- Isentropic efficiency (%; 100 = ideal): 100
- Maximum allowable discharge temperature (°C; 0 = not checked): 0
- Mass flow (kg/h; 0 = not used): 20
- Compressibility factor Z at suction (1 = ideal gas): 1
- Running hours for the transferred mass (h; 0 = not used): 8
- Lowest credible suction pressure (same unit as suction; equal to suction = no comparison): 10
Result
- Suction, absolute: 10 bar a (8.987 bar g, 145 psia)
- Discharge, absolute: 200 bar a (198.99 bar g, 2,886 psig)
- Overall pressure ratio: 20 absolute discharge ÷ absolute suction
- Equal stage ratio: 2.714 over 3 stages
- Interstage pressures (equal split): 27.1, 73.7 bar a estimated operating pressures only, not design pressures
- Exponent (k − 1)/k: 0.291 k = 1.41 for hydrogen; R = 4,124.2 J/(kg·K)
- Ideal discharge temperature per stage: 132.1 °C (405.3 K) at a stage ratio of 2.714, suction 30 °C
- Actual suction volume: 25 m³/h (0.417 m³/min, 14.7 acfm) at 10 bar a, Z = 1
- Normal volume: 222.4 Nm³/h (0 °C, 1.013 bar a; density 0.0899 kg/Nm³)
- Mass transferred: 160 kg in 8 h (a logistics check, not storage sizing)
Method. P(abs) = P(gauge) + site atmospheric pressure. Overall ratio r = P₂(abs) ÷ P₁(abs); dividing gauge figures gives a wrong ratio, worst at low suction pressure. Equal split: stage ratio = r^(1/n); interstage pressure after stage i = P₁ × (stage ratio)^i. Ideal (isentropic) discharge temperature T₂ = T₁ × (stage ratio)^((k−1)/k), kelvin, assuming intercooling back to the suction temperature before every stage; with an isentropic efficiency η, T₂ = T₁ + (T₂ideal − T₁) ÷ η. Gas constant R = 8,314.46 ÷ molecular weight. Actual suction volume = ṁ × Z × R × T₁ ÷ P₁; normal volume uses 0 °C and 1.01325 bar a with Z = 1. Transferred mass = mass flow × running hours. Rounded textbook k values near ambient; real-gas Z and the maker’s calculation for the actual composition govern. These are screening values, not equipment ratings or settings.
What to take from it. Always work in absolute pressure, and check the lowest credible suction pressure: it gives the highest ratio and discharge temperature and the largest actual suction volume for the same mass flow.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Pressure unit and gauge/absolute conversion
Convert between bar, psi, kPa and MPa and between gauge and absolute pressure. A mix-up between gauge and absolute is a classic cause of wrong sizing.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Pressure: 7
- Unit: bar g
Result
- bar gauge: 7
- bar absolute: 8.01
- psi gauge: 101.5
- psi absolute: 116.2
- kPa gauge: 700
- MPa gauge: 0.7
Method. Absolute pressure = gauge pressure + atmospheric (1.01325 bar at sea level). 1 bar = 14.504 psi = 100 kPa = 0.1 MPa. Hyderabad is about 540 m above sea level; atmospheric pressure there is roughly 0.95 bar, so enter the local value in the flow tool if it matters.
What to take from it. A 7 bar gauge system is 8.01 bar absolute. Ratio and flow-conversion work uses absolute pressure.