CNG and process-gas reciprocating compressors

Why is gas compression different from compressed air?

An air compressor takes in atmospheric air, compresses it and delivers it to plant users. Leakage is a cost, not a hazard, and the intake is free and unlimited. A gas compressor works on a medium that may be flammable, toxic, expensive or a product in its own right. That changes almost every part of the machine:

• Containment. The crankcase, distance piece and piston-rod packing must prevent gas leaking to atmosphere or into the crankcase. Packing vents and leak recovery are design items, not accessories.
• Gas properties. Molecular weight, ratio of specific heats and compressibility affect discharge temperature, power and staging. Hydrogen and natural gas behave very differently from air.
• Materials. Gases containing hydrogen sulphide, moisture or carbon dioxide need suitable cylinder, valve and seal materials.
• Hazardous area. Motors, instruments, panels and lighting must suit the zone classification of the installation.
• Purge and start-up. Air must be purged from the system before gas is introduced, usually with nitrogen, to avoid a flammable mixture inside the machine.

Converting a standard piston air compressor for CNG or another flammable gas is unsafe. The crankcase is open to the atmosphere, the seals are not designed for gas, and the electrical equipment is not rated for a hazardous area.

Where are reciprocating gas compressors used?

India’s city-gas distribution network has expanded across many districts, including in Telangana and Andhra Pradesh, and the Krishna–Godavari basin off the Andhra coast is an established natural-gas region. Both have increased the number of gas-compression enquiries in the region.

What information defines a gas-compression duty?

Before any machine is discussed, the duty must be written down. These are the inputs a gas-compressor manufacturer will ask for:

1. Gas composition, ideally a recent analysis, including moisture, hydrogen sulphide, carbon dioxide and any liquids or particles.
2. Suction pressure and temperature, including the minimum and maximum, because pipeline pressure can vary through the day.
3. Final discharge pressure needed at the point of use or storage.
4. Flow, stated in normal or standard cubic metres per hour or kilograms per hour, and how it varies.
5. Duty pattern: continuous, batch or cascade filling, and the number of starts per hour.
6. Site conditions: power supply, ambient temperature, altitude, cooling water availability.
7. Hazardous-area classification and any project or regulatory requirements, including licensing of storage and filling.
8. Oil tolerance: whether the gas can carry any lubricant downstream, which decides between lubricated, non-lubricated and diaphragm designs.

Pressure alone does not define the machine. A request for “a 250 bar compressor” says nothing about the gas, the flow or the suction conditions.

How are reciprocating gas compressors built?

Reciprocating designs are common in gas service because they handle high pressure ratios and varying suction conditions well. Typical features include:

• Multiple stages with intercoolers and separators between them to control discharge temperature and remove condensed liquids.
• Distance pieces between the crankcase and the cylinder, so the gas side and the lubricated running gear are kept apart.
• Rod packing with vents routed to a safe location or recovery system.
• Lubricated or non-lubricated cylinders, depending on whether oil is acceptable in the gas.
• Capacity control by speed variation, clearance pockets, suction-valve unloaders or recycle.
• Pulsation dampers on suction and discharge to protect pipework and instruments.
• Safety instrumentation: high discharge temperature per stage, low and high suction pressure, low lube-oil pressure, vibration and gas detection.

Where even trace leakage or contamination is unacceptable, as with hydrogen or high-purity gases, metal-diaphragm compressors are often used instead of piston designs.

Gas compressors are normally bought as complete engineered packages from a specialist manufacturer or packager, with the compressor, driver, coolers, scrubbers, controls and gas detection designed together and documented for the specific gas. That documentation, including datasheets, test records and certificates, becomes the basis for approvals, operation and later maintenance, so it should be specified at enquiry stage and checked at handover.

What safety boundaries apply?

Gas compression is governed by the project’s design basis, the manufacturer’s instructions, the site’s hazardous-area classification and the applicable regulations for storage and dispensing. Plant owners and their engineers set those boundaries; any supplier must work inside them. In practice that means:

• Only equipment designed and documented by its manufacturer for the specific gas and duty is used.
• Electrical equipment is certified for the zone in which it is installed.
• Work on gas systems follows permit-to-work, isolation, depressurisation and purging procedures.
• Pressure vessels, cascades and pipework are inspected and tested as required.
• Vent and relief discharges go to a safe location.

Where does compressed air fit in gas and process plants?

Alongside the gas compressors themselves, gas and process plants depend on ordinary compressed air: instrument air for control valves and actuators, plant air for maintenance tools, air for purging and drying under procedure, nitrogen generation feed air, and temporary air for shutdowns and construction. Instrument air is usually dried and filtered to a strict ISO 8573-1 class, often with a desiccant dryer giving a pressure dew point of −40 °C.

Reddy Compressors supplies this side as well. On one of our projects, for an international oilfield-services contractor working on a major natural-gas development at Kakinada in Andhra Pradesh, we supplied and commissioned a temporary power-and-air package with compressors rated 1,100 cfm at 300 psig, 1,000 cfm at 275 psig and 750 cfm at 150 psig, together with generators, distribution boards, operators and a technician on site.

Gas compressor vs air compressor at a glance

CNG station types and what each asks of the compressor

“CNG compressor” covers three quite different duties. The station layout decides suction pressure, and suction pressure decides almost everything else about the machine.

An online or mother station compressor is a multi-stage reciprocating machine designed for its suction range. A daughter booster is a different machine for a different curve. One cannot simply replace the other.

Stage pressures and ratio per stage: worked

Compression ratio is always worked in absolute pressure. Overall ratio R = (discharge + 1.013) ÷ (suction + 1.013). With n equal stages, the ratio per stage is r = R to the power 1/n.

The discharge temperature of each stage rises with r. For an ideal adiabatic stage, T2 = T1 × r to the power (k−1)/k, in kelvin. For natural gas, k is about 1.30. Predicted stage discharge temperature is commonly limited to about 150 °C (API 618 is the usual reference for reciprocating process-gas machines), so the stage count is chosen to keep each stage below that. How k and compressibility vary with gas composition is explained on gas properties and compression behaviour.

Worked check, 1 bar(g) suction, four stages

• Absolute pressures: suction 1 + 1.013 = 2.013 bar(a); discharge 250 + 1.013 = 251.013 bar(a).
• R = 251.013 ÷ 2.013 = 124.7. r = 124.7 to the power 0.25 = 3.34.
• Interstage pressures: 2.013 × 3.34 = 6.73 bar(a), that is 5.7 bar(g). Then 22.5 bar(a), that is 21.5 bar(g). Then 75.1 bar(a), that is 74.1 bar(g).
• Temperature: 313.15 K × 3.34 to the power 0.231 = 313.15 × 1.321 = 413.7 K, about 141 °C. This is ideal; real stages run a little hotter, so cooling between stages must bring the gas back close to 40 °C.

Two practical points follow. A machine designed for 20 bar(g) suction will overheat its first stage if site pressure falls to 10 bar(g), because the first-stage ratio climbs. In real machines the stages are not exactly equal: designers often give the last stage a slightly lower ratio, since its rod load and packing duty are highest.

Sizing flow from vehicles per hour

CNG is sold by mass (kg). Compressors are rated in Nm³/h at a stated suction pressure, so convert using the gas density. A methane-rich pipeline gas is about 0.72–0.78 kg/Nm³ at 0 °C and 1.01325 bar(a), so 1 kg is about 1.28–1.39 Nm³. Use your gas analysis. The example below uses 1.35 Nm³/kg.

Worked example (illustrative peak hour)

• Peak demand: 460 kg/h × 1.35 = 621 Nm³/h.
• Assume the peak lasts 2 hours: 2 × 460 = 920 kg.
• A stationary cascade of 3,000 litres water capacity at 250 bar and about 25 °C holds roughly 550–600 kg (gas density about 185–200 kg/m³ at that pressure, allowing for compressibility). Assume only about one-third, roughly 190 kg, can be drawn while still filling vehicles to full pressure.
• Compressor must supply (920 − 190) ÷ 2 = 365 kg/h, which is 365 × 1.35 = 493 Nm³/h, at the lowest suction pressure expected at peak.

The last point matters. A reciprocating machine’s mass flow falls roughly in proportion to suction density. If network pressure sags at peak hours, the rated Nm³/h at design suction is not what you get. Size for the low-suction case, and add a margin for one machine being down for maintenance.

Cooling, moisture and oil carry-over

• Cooling and short fills. A vehicle cylinder is filled to a pressure, but what it holds depends on temperature. If gas goes in at 45 °C and the cylinder is filled to 200 bar, then cools overnight to 25 °C, pressure falls to roughly 200 × 298 ÷ 318 = 187 bar (ideal-gas approximation). The customer gets a short fill. Aftercoolers sized for 45 °C ambient, plus dispensers with temperature compensation, address this. Coolers sized for 35 °C will not.
• Moisture. At 250 bar, a few milligrams of water per m³ can condense or form hydrates in regulators and cylinder valves. With any CO₂ or H₂S present, the water also drives corrosion. Stations therefore fit a gas dryer, usually a regenerative desiccant type at suction or at an intermediate pressure. The target is a water dew point at storage pressure safely below the lowest ambient the cylinders will see, with the figure taken from the gas specification that applies.
• Oil carry-over. Lubricated cylinders and packings put some oil into the gas. Oil in vehicle cylinders and regulators causes sticking and fouling. Control it with the minimum cylinder lube rate the maker allows, efficient interstage separators with automatic drains, and coalescing filters after the final stage. Check filter differential pressure and drain logs as routine. Cooler and separator design is covered in more depth on gas compressor interstage cooling and separation.
• Interstage separators. Liquids collected between stages must be drained to a closed system, not to open ground. They contain hydrocarbons.

Approvals, licensing and CNG compressors for home use

A CNG station is a licensed hazardous installation. Generically, it needs approval of the site layout and separation distances, pressure-vessel and cylinder cascade certification, a hazardous-area classification with certified electrical equipment, gas detection and emergency shut-down, fire protection, and periodic inspection and testing. These come from the national petroleum and explosives safety authority, the city-gas distribution regulator and the local authorities. Requirements change, so confirm current rules with them at the project stage, not after equipment is bought.

What about a CNG gas compressor for home?

Small home refuelling appliances exist in some countries. They connect to the domestic gas supply and fill a car overnight. In India, filling vehicle CNG cylinders is a regulated activity that is generally permitted only at approved, licensed stations. Refuelling at home is not generally permitted. Anyone considering it should check the current position with the petroleum and explosives safety authority and their city-gas supplier before buying anything.

The engineering is also demanding. Domestic piped gas arrives at only about 21 mbar(g). Compressing it to 250 bar(g) is an overall ratio of 251.013 ÷ 1.034 = about 243. That needs four or five stages (about 3.95 or 3.0 per stage), with a dryer and certified safety controls. It cannot be done safely with an air compressor or a converted one.

How Reddy Compressors helps

Reddy Compressors, Tarnaka, Secunderabad, has served industry since 2002 as an independent multi-brand compressor specialist, not an authorised dealer of any manufacturer. We have tie-ups with all major and reputed brands. For gas duties we:

• Help you write down the duty (gas, suction, final pressure, flow, site classification) before any machine is proposed.
• Sell new and used/refurbished gas compressors, including CNG, nitrogen, process-gas and booster compressors, matched to the gas and documented for it.
• Rent and service gas compressors, and supply their spare parts, identified from the make, model, serial number and nameplate.
• Supply, rent and service the compressed-air side of gas and process plants: instrument air, plant air, nitrogen feed air and temporary air, from our own fleet of 50–2,000 cfm and 2–30 bar.
• Supply generators for temporary power through Reddy Generators, which is under the same management.

Oxygen, hydrogen and toxic-gas duties follow gas-specific codes and the maker’s design rules. We never offer an air compressor for a gas service.

Our team of over 20 service engineers and technicians covers Telangana and Andhra Pradesh, and we supply equipment and spares across India.

What to send us

• The gas to be compressed and a recent gas analysis if available.
• Suction pressure and temperature range, and the final pressure required.
• Flow in Nm³/h or kg/h, and the duty pattern.
• Site location, power supply and hazardous-area classification.
• For the air side: instrument and plant air users, pressure, flow and air-quality class.
• For temporary air: project duration, pressure, flow and site access.

Frequently asked questions

Can a normal air compressor be used to compress CNG?

No. A standard air compressor is not designed to contain a flammable gas. Its crankcase is open, its seals and packing are meant for air, and its motor and controls are not rated for a hazardous area. Leaking natural gas could form an explosive mixture. CNG needs a compressor built, tested and certified by its manufacturer for natural-gas service.

What pressure does a CNG station compressor deliver?

CNG station compressors typically compress gas to around 250 bar for storage cascades, so that vehicles can be filled to their cylinder working pressure, commonly 200 bar. The exact figures depend on the station design, local regulations and the vehicle cylinders served. Suction pressure from the gas grid also varies, which affects the number of stages and the flow.

What is the difference between a gas compressor and an air compressor?

An air compressor draws atmospheric air and treats leakage only as energy loss. A gas compressor handles a specified gas at a given suction pressure, often flammable or valuable, so containment, materials, hazardous-area electrical equipment, purging and safety instruments are designed around that gas. Gas properties also change discharge temperature, staging and power compared with air.

Why do gas compressors have a distance piece?

A distance piece is a spacer between the crankcase and the compressor cylinder. It keeps gas that leaks past the rod packing away from the lubricated crankcase, and stops crankcase oil from travelling along the rod into the gas. Vents and drains on the distance piece route any leakage to a safe location, which is essential for flammable or toxic gases.

When is a diaphragm compressor used instead of a piston compressor?

Diaphragm compressors are used when the gas must not leak or pick up any contamination, for example hydrogen, high-purity or toxic gases. The gas is sealed by flexing metal diaphragms, so it never touches lubricated parts. They suit lower flows at high pressure. Piston compressors are usually chosen for larger flows where some lubrication is acceptable.

Do you supply and service CNG and process-gas compressors?

Yes. We sell new and used/refurbished CNG, nitrogen, process-gas and booster compressors, rent and service them, and supply their spares. We also supply instrument, plant and temporary project air, with rentals from 50–2,000 cfm and 2–30 bar. We have delivered a temporary air-and-power package for a natural-gas development at Kakinada. We supply across India and our service teams cover Telangana and Andhra Pradesh.

What is a CNG compressor and how does CNG compression work?

A CNG compressor is a multi-stage, gas-tight reciprocating machine. It takes natural gas from a pipeline or a mobile cascade and raises it to about 250 bar for storage and vehicle filling. Each stage compresses the gas by a ratio of about 2–4, then a cooler brings it back near ambient and a separator removes liquids before the next stage. A dryer removes moisture, filters limit oil carry-over, and the gas is stored in cylinder cascades that feed the dispenser.

How do I size a compressed natural gas compressor for a station?

Start from the peak hour. Add up the fills by vehicle type in kg, convert to Nm³ using your gas density (about 1.28–1.39 Nm³ per kg for methane-rich gas), and subtract what the stationary cascade can supply during the peak. In the illustrative example above, 460 kg/h at peak needs about 493 Nm³/h of compressor capacity. Rate it at the lowest suction pressure expected at peak, and allow for one machine being down.

How many stages does a CNG gas compressor need?

It depends on suction pressure. Work out the overall ratio in absolute pressure and keep each stage at about 3–3.5 or below, so discharge temperature stays under about 150 °C. From 1 bar(g) to 250 bar(g) the ratio is about 125, which needs four stages. From 10 bar(g) three stages suit, and from 20 bar(g) two to three. Daughter boosters work on a falling suction curve and are designed separately.

Can I install a CNG gas compressor at home in India?

In India, filling vehicle CNG cylinders is a regulated activity that is generally carried out only at approved, licensed stations. Refuelling at home is not generally permitted. Check the current rules with the petroleum and explosives safety authority and your city-gas supplier before considering any equipment. Technically, compressing 21 mbar(g) domestic gas to 250 bar(g) is an overall ratio of about 243, which needs four to five stages, drying and certified safety systems.

Related pages

• Gas compressors: CNG, nitrogen booster, biogas and process gas
• Process gas compressor selection requirements
• Natural gas booster system boundaries
• Gas compressor seal and leakage management
• Diaphragm process gas compressors
• Oil and gas support: compressed air and nitrogen
• Reciprocating air compressors