Flare Gas and Sour Gas Compression: What Changes

In this category the machine does not choose the duty. The gas composition, the contaminants and the swing in flow do.

Two machine families carry most of the flare gas and sour gas duty: a dry screw machine up to 45 barg with a 15 bar differential-pressure limit, and a liquid ring machine up to 13 bara. Both tolerate entrained liquid and solids, which is what these streams contain. The published 15 bar differential limit — not the pressure ceiling — is what usually decides between them, and it is the number most often left out of an enquiry.

That is the short answer. The rest of this article is about why the category behaves differently: the suction conditions, the contaminants, the swing, and what hydrogen sulphide adds on top.

What flare gas actually is, physically

A flare gas recovery duty is usually described by its source — a refinery flare header, a delayed coker, an offshore platform, a landfill — and almost never by an analysis. The physical consequences of that are consistent:

Suction pressure near or below atmospheric. A recovery compressor pulls off a flare header that is held close to atmospheric by the flare system's own design. That makes the machine a vacuum-side machine, which changes seal arrangements, casing pressure ratings and the meaning of "suction pressure" on the datasheet — 0.02 bar(g) and 1.02 bar(a) are the same condition written two ways.
The flow swings, often by a lot. A flare recovery stream follows process upsets rather than production schedules. A machine sized on the average flow and the clean case will not cover both.
The composition swings with it. Molecular weight is the number that moves, and it moves the discharge temperature and the volumetric capacity with it.
Liquids and solids arrive with the gas. Hydrocarbon condensate, water and rust scale are normal, not exceptional, and the machine has to be built to survive all three.

None of that is a reason to avoid the duty. It is a reason to write the analysis, the swing envelope and the contaminant list on the enquiry rather than leaving them to inference.

What sour gas adds

Hydrogen sulphide changes the problem by adding a second failure mode on top of the mechanical one.

Toxicity. A leak of sour gas is a different event from a leak of methane, because the gas is toxic as well as flammable. That is why this category is specified for containment rather than for acceptable leakage: zero-leakage construction, static seals where possible, and a sealing arrangement that forms part of the specification rather than being left to the bid.

Corrosion, and the wet/dry distinction. Wet sour service and dry sour service are two different material problems. Water content, H₂S partial pressure and operating temperature together decide which one applies, so all three belong on the enquiry. Hydrogen sulphide does not sit in our data set as a named gas, so a duty that names it goes to a human rather than an automatic answer — toxicity and corrosion arrive together, and a screening tool cannot take responsibility for either.

If the stream also carries carbon dioxide and water, the combination is worse than either alone, and the material call belongs to the supplier's engineering team with the full analysis in hand.

Which families take these duties

Family	Published pressure limit	Why it fits this category	Where it stops
Dry screw (API 619)	45 barg, with a Δp ≤ 15 bar limit	No lubricant in the compression chamber, so contamination does not degrade an oil charge; tolerates dirty, dusty and liquid-carrying gas; used for flare gas, gas gathering, sour gas and steam recompression	Above 45 bar the duty moves to reciprocating or diaphragm; above 15 bar differential the duty needs review before it is ruled out (1)
Liquid ring	13 bara on the dedicated compressor line	The working liquid absorbs contamination and condensate instead of passing it through clearances; the liquid can be chosen to suit the gas; used for refinery overhead, flare gas and offshore duty	The pressure ceiling is low; above 13 bara the family drops out (1)
Reciprocating (API 618)	≤ 400 bar	Covers the high-pressure end and the largest flow range; oil-free construction exists for the clean end of this category	Needs a fundamentally clean gas — this family is the one that suffers from liquid carry-over and solids
Roots	Δp ≤ 1.0 bar single-stage, 2.0 bar two-stage	Large-volume micro-boost with no oil in the gas path	Differential pressure, not pressure ratio, is the whole constraint
Diaphragm	1–900 bar	The zero-leakage answer for high pressure and valuable, toxic or high-purity gas	Requires a clean gas; a diaphragm is not a machine to run on tar, naphthalene or scale
Oil-injected screw	50 barg	Covers fuel-gas and process-gas boosting where the gas is clean enough: natural gas, boil-off gas, coke-oven gas, coal-bed methane, biogas, carbon dioxide	Excluded outright for purity-critical gas; the oil contact rules it out for several streams in this category

Two of those limits are worth separating, because they are confused constantly. The 45 barg figure for dry screw machines is a pressure ceiling. The 15 bar figure is a differential limit — discharge minus suction. For a flare gas duty with suction near atmospheric, the differential limit is rarely reachable: reaching 15 bar of differential from 1 bar(a) suction means a discharge above 16 bar(a), which is already high for a flare recovery machine. For a gas-gathering duty with 20 bar suction, the same 15 bar limit binds hard and the machine cannot go past about 35 bar discharge however low the pressure ceiling sits. State both the suction and the discharge pressure, or the differential cannot be computed and the answer cannot be checked.

What changes when the gas is genuinely dirty

There is a point at which the family comparison stops mattering, because no machine in the comparison will run on the stream as it arrives:

Scale and rust from the flare header, and any polymerised or tar-like deposit, will destroy a diaphragm and score a reciprocating cylinder. Filtration and knockout have to be designed before the machine is chosen.
Naphthalene in coke-oven gas deposits on cool surfaces as the gas cools, so the coldest point in the machine becomes the failure point.
Ammonia washing is a routine step downstream of the compression unit in coke-oven gas plants because the ammonia has to come out before the gas is compressed further or cold enough to liquefy — the compression unit and the purification train are designed together.

A live example of that ordering: in the coke-oven gas to LNG project approved in Henan in September 2026, the compression unit is the second stage of the train, with the pretreatment unit ahead of it and tar, naphthalene, ammonia and sulphur removal all behind it (4). The machines see the raw gas. That is the normal arrangement in this category, and it is the reason the contaminant list matters as much as the pressure and flow.

What the Chinese supply side has in the category

Public product material and project records put domestic machines in this category in three places.

Dry screw. A central state-owned research institute in Shanghai publishes a dry and liquid-injected screw programme with over 700 units in operation across crude oil stabilisation, flare gas, coke-oven gas, reforming PSA and propane dehydrogenation, with hydrogen, hydrogen sulphide, vinyl chloride and butadiene in the medium list, and instanced machines on a chlorination tail-gas duty at 2.5 MPa. The same institute's records include a hydrogen compressor package for a zero-carbon hydrogen-ammonia project and a 500,000 Nm³/h reforming hydrogen tail-gas machine (5).

Liquid ring. A Foshan-based pump builder publishes a dedicated compressor line to 0.7 MPa(G), and a large-flow series covering refinery overhead gas, dry chlorine, dry hydrogen, nitroso gas, flare gas and offshore platform gas, with published flows to 1,400 m³/min — which is above the figure the best-known international liquid ring name publishes for the same class of machine (6).

Reciprocating. A compressors plant with a 1956 heritage publishes an instanced six-column, four-stage oil-free coke-oven gas machine at 12,370 Nm³/h, 1.6 MPa, 2,000 kW, and its parent company's Japanese line covers flare gas and coke-oven gas among others (7).

The point of listing these is not that the machines are equivalent to any particular imported machine. It is that the category is served domestically, and that the published figures exist to be checked against a duty.

What to put on the enquiry

For this category specifically, five lines carry more weight than the rest:

Suction pressure in bar(a), since a near-atmospheric suction is where the absolute/gauge distinction is decisive.
The swing envelope — minimum and maximum flow, and how fast it moves.
A composition, including molecular weight, water content and H₂S concentration if present.
The contaminant list — condensate, water, scale, tar, naphthalene, ammonia — with the particle sizes if filtration is already designed.
Both pressures, so that the differential pressure can be computed and the dry screw limit checked.

Where this goes to a human

Duties in this category that go to engineering review rather than an automatic answer: any gas with an estimated discharge temperature above 135 °C; any duty with more than four stages; any gas carrying a significant solids load; hydrogen sulphide as a named gas; and any duty where the flow is entered as actual m³/h. None of those are refusals. They are the cases where the numbers above are not enough on their own, and where the analysis has to lead.

What this article does not cover

Note that this article does not calculate the differential pressure limit in terms of a specific duty, does not design knockout, filtration or sealing arrangements, does not select materials for sour service, and does not assess compliance with any code. It sets out which families take flare gas and sour gas duty and which published limit usually decides between them.

Related on this site

Sources

#	Basis
1	Family boundaries and constraints, and the published envelopes behind them: dry screw ≤45 barg with Δp ≤15 bar (Howden's own published limit for its oil-free machines; Kobelco publishes the same pressure ceiling), liquid ring ≤13 bara on the dedicated compressor line, reciprocating ≤400 bar, Roots Δp ≤1.0 bar and 2.0 bar, diaphragm 1–900 bar, oil-injected screw ≤50 barg — GasCompressorSelect boundary and gas data set v1.3 §2 and its appendices A and B
2	Gas data: hydrogen sulphide toxicity and corrosion, wet and dry sour service, carbon dioxide with water, T2 = T1 × r^((k−1)/k), discharge-temperature line 135 °C, differential-pressure review above 15 bar, the handover rules M1–M10 — GasCompressorSelect gas data set v1.3 §3.3, §4, §5 and §6
3	Dry screw performance on dirty and liquid-carrying gas, and the four medium families it is applied to — published manufacturer envelopes for oil-free screw machines in the data set's appendix A
4	The process ordering of a coke-oven gas to LNG plant, with the compression unit ahead of tar, naphthalene, ammonia and sulphur removal — 平顶山市生态环境局拟批准公示, 30 August 2026, https://sthjj.pds.gov.cn/contents/11426/753546.html; see A Coke-Oven Gas Plant in Henan
5	Dry screw programme scale, medium list, 700+ units, chlorination tail-gas instance at 2.5 MPa, hydrogen-ammonia project package, 500,000 Nm³/h reforming tail-gas machine — 中船 711 所 published material, as recorded in the data set's appendix B
6	Liquid ring 0.7 MPa(G) dedicated line, large-flow series to 1,400 m³/min, gas list including refinery overhead gas, dry chlorine, dry hydrogen, flare gas and offshore gas — 广东肯富来泵业 published product data, as recorded in the data set's appendix B
7	Six-column, four-stage oil-free coke-oven gas machine at 12,370 Nm³/h, 1.6 MPa, 2,000 kW — 神钢无锡压缩机 instanced operating data, as recorded in the data set's appendix B