Reciprocating or Diaphragm for High-Pressure Hydrogen?

For high-pressure hydrogen, two families do the work: diaphragm machines from 1 to 900 bar and reciprocating machines up to 400 bar. For low-pressure, large-flow hydrogen below 45 bar, a dry screw machine is the routine industry answer, not a fallback. Centrifugal machines are excluded, and the reason is physics rather than preference.

This article sets out why hydrogen is treated as its own case, where each family's boundary sits, and which pressure grades are actually being delivered — including the grades that sit above the recommended ranges.

Work through it and you can tell which of the two families your duty point falls into, or whether it goes to a human.

Why does hydrogen need its own answer?

Three properties of the gas do the damage.

Molecular weight 2.02. Hydrogen is the lightest molecule in industrial service. In a dynamic machine, the head produced per stage does not scale with molecular weight the way people expect — but the pressure ratio does, and it falls sharply as molecular weight drops. At a tip speed of 350 m/s (1,150 ft/s), the pressure ratio of a single stage on pure hydrogen is roughly 1.1. Getting to a useful pressure would need a stage count that a single-casing rotor cannot carry, with a practical rotor-dynamics limit around 10 to 12 stages, and refining standards require material yield strength below 120 ksi in hydrogen service to guard against embrittlement, which limits tip speed further. The industry conclusion recorded in our data set is the same one you will find in the field: high-pressure pure hydrogen goes to reciprocating or diaphragm machines. A centrifugal machine can serve a refinery's recycle hydrogen loop at high flow and low pressure ratio. It does not serve a 500 bar dispensing duty.

Embrittlement. Hydrogen diffuses into high-strength steels and reduces their ductility. Our gas data carries an embrittlement flag for hydrogen, which constrains the material selection away from high-strength steels and towards grades such as 316L and aluminium alloys. This is a material rule, not a coating or a derating factor.

Autoignition at 560°C (1,040°F) and a very wide flammability range. This is why sealing and leakage get treated more strictly for hydrogen than for most gases, and why zero-leakage construction is often specified rather than preferred.

Which of the two families wins, and on what?

The dividing line in practice is flow, not pressure. Both families reach 400 bar (5,800 psi); only the diaphragm family is recommended above it, up to 900 bar (13,050 psi). Below 400 bar, the choice comes down to how much gas is moving. A few hundred Nm³/h at several hundred bar is diaphragm territory. Tens of thousands of Nm³/h at a hundred bar is reciprocating territory.

Why is "screw compressors cannot handle hydrogen" wrong?

One correction in our own data set's history bears repeating, because it is a common misconception. Hydrogen is not a gas that screw compressors cannot handle. Dry screw machines are used for low-pressure, large-flow hydrogen as a matter of routine, and there is documented Chinese supply into a zero-carbon hydrogen-ammonia project.

What is excluded is centrifugal compression of high-pressure pure hydrogen. Two different statements, and conflating them leads buyers to rule out a family that would have fitted their duty.

What pressure grades do hydrogen stations actually use?

Hydrogen station machinery in our data is recorded in a 350–550 bar band (5,080–7,980 psi). Below that, a station's storage and dispensing stages typically operate at lower grades — a site delivered in Foshan in July 2026 used 22 MPa, 45 MPa and 90 MPa machines on one site, covering filling, storage and dispensing respectively. Trailer-filling packages are supplied at 515 bar (7,470 psi) and above, for the tube trailers that carry hydrogen by road.

The highest grades on that list are delivered hardware, not catalogue claims. High-pressure hydrogen diaphragm machines are in series production in China to 70 MPa (700 bar), with 90 MPa machines delivered — including the Foshan set above — and published ceilings at 100 MPa (14,500 psi). The recommended diaphragm window on this site follows those deliveries and runs to 900 bar (13,050 psi), which covers every grade a station architecture produces. Above 900 bar a duty goes to an engineering review; above 1,000 bar it is bespoke design.

The grades a station uses are set by its architecture rather than by the compressor. Storage and dispensing normally run at different pressures and tube-trailer filling runs higher again, which is why one site can carry three or four pressure grades in its equipment list.

How many stages does a hydrogen duty need, and how hot does the gas get?

Stage count. A single crankshaft carries at most four stages in our reference data, and the count depends on the pressure ratio, the gas temperature at each intercooler outlet and the discharge-temperature limit. A hydrogen duty that needs more than four stages is a design project rather than a selection, so it goes to a human.

Discharge temperature. It is estimated as T2 = T1 × pressure ratio^((k−1)/k), with k taken from the gas data — 1.41 for hydrogen. Above 135°C (275°F) the duty is handed over rather than solved. Discharge temperature, not pressure, is what most often decides how many stages a hydrogen machine needs, which makes it the first thing to estimate and the last thing to guess at.

When does this go to a human?

For hydrogen duties specifically, the duty goes to a human when it crosses any of six boundaries — pressure, flow, temperature, stage count, entrained contamination, or a flow unit the standardisation cannot carry:

• Discharge pressure above 900 bar — the 900–1,000 bar band is an engineering review rather than an automatic recommendation; above 1,000 bar is bespoke design
• Suction flow above 100,000 m³/h, or a low-molecular-weight high-flow continuous duty that belongs to a dynamic machine
• Estimated discharge temperature above 135°C (275°F)
• More than four stages
• Liquid droplets or solids in the gas
• Actual m³/h entered as the flow unit

What people assume, and what the data says

Questions we get about hydrogen duty

Which family should I choose at 200 bar? Both are available at 200 bar. The choice is flow: a few hundred Nm³/h points at diaphragm, tens of thousands of Nm³/h points at reciprocating.

Can I use a diaphragm machine at 700 bar? Yes. 700 bar sits inside the recommended diaphragm window, which runs to 900 bar following documented production. The engineering-review band starts above 900 bar, and bespoke design above 1,000 bar.

Why does the tool ask for suction temperature? Because it drives the discharge-temperature estimate, and because the unit conversion for your flow figure depends on it.

What materials do you specify for hydrogen? The data set carries an embrittlement flag that constrains the selection away from high-strength steels. Beyond that, the final material call is the manufacturer's engineering team's.

Does API 617 apply? API 617 covers centrifugal and axial compressors, which this tool does not recommend. Hydrogen service is one of the clearest cases where that exclusion matters, and it is documented as a decision with its rationale rather than as a gap.

What this article does not cover

Note that this article does not calculate stage count, intercooler duty, power or rotor dynamics, and it does not specify a sealing arrangement or a final material grade. It sets out which family covers a hydrogen duty point and where the tool hands over instead.

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