What compressed air does in a pigging operation
A pig is a close-fitting plug – foam, cup, disc or gauging plate type – that is launched into a pipeline and pushed along by a pressure difference across it. Compressed air is one of the common driving media, alongside water, product and nitrogen. Air is used where it is safe and permitted: typical jobs are cleaning new lines after construction, gauging to prove there are no dents, pushing water out after a hydrotest (dewatering), and swabbing and drying with dry air afterwards.
The air does three things at once:
- Builds pressure behind the pig to overcome its friction against the pipe wall and any liquid head in front of it.
- Fills the growing volume behind the pig as it travels, so flow decides speed.
- Carries moisture out during drying runs, which is why air dryness matters so much in the final stage.
Is compressed air the right medium?
Not always. Compressed air contains oxygen, so it must not be used where it could form a flammable mixture with residual hydrocarbons in a line that has carried gas or product. In those cases the pipeline owner’s procedure will usually call for nitrogen or another inert medium, or a purge before air is introduced. Air is generally well suited to new-construction lines, water lines and lines that have been made gas-free under the pipeline owner’s procedure.
The decision belongs to the pipeline owner and their engineer. We can supply the compressed air, and the feed air for nitrogen generation where that is the chosen route; see oil and gas support: compressed air and nitrogen.
How much air does a pigging run need?
The governing quantity is the volume of pipeline behind the pig, at the driving pressure. A simple, transparent way to estimate it:
- Calculate the internal volume of the line: π × (internal diameter)² ÷ 4 × length. For example, 1 km of pipe with 300 mm bore holds about 70 m³.
- Multiply by the absolute pressure ratio to get free air. At 7 bar gauge (about 8 bar absolute) that 70 m³ needs roughly 560 m³ of free air, ignoring temperature effects.
- Divide by compressor free air delivery (FAD) to estimate filling time. A 600 cfm compressor delivers about 17 m³/min, so that example needs over half an hour of running to fill the line behind a pig that has reached the far end.
- Add allowances for hose and launcher volume, leakage and any bypass past the pig.
This simple arithmetic shows why long or large-bore lines often need more than one compressor, or a combination of compressors and receivers. It also shows that pig speed depends on flow: more FAD means faster travel at the same pressure, which the pigging procedure may limit.
Pressure: what drives the pig and what limits it
The pressure needed behind a pig is the sum of its friction (which depends on pig type and fit), any liquid column it is pushing uphill, and the back-pressure at the receiver. Lifting water adds roughly 1 bar for every 10 m of elevation, so dewatering a line that climbs a hill can need much more than the same line on flat ground.
The upper limit is set by the pipeline and the procedure, never by the compressor’s rating. Ask the pipeline engineer for:
- the maximum permitted driving pressure at the launcher;
- the expected normal driving pressure for the chosen pig;
- any speed limit, and how it will be controlled;
- the relief setting that protects the line and the temporary air system.
| Pigging task (typical) | Air pressure character | Air quality concern | Typical compressor class |
|---|---|---|---|
| Cleaning new lines (foam / brush pigs) | Low to moderate, mainly to overcome friction | Clean air; water and oil kept out | 250–600 cfm at 7–10 bar (101.5–145 psig), multiple units on large lines |
| Gauging (gauge-plate pigs) | Moderate, steady | Clean | As for cleaning |
| Dewatering after hydrotest | Higher, rising with elevation and line length | Clean air; enough flow to keep the pig moving | 400–600 cfm units or larger, often 10–14 bar (145–203.1 psig) or higher where elevation demands |
| Swabbing and drying | Low, long-duration flow | Dry air is the whole point: aftercooling plus a dryer | High flow with refrigerated or desiccant drying |
Air quality for pigging and drying
Wet or oily air defeats the purpose of cleaning and drying. A typical temporary train for pigging is compressor, aftercooler, moisture separator with drain, filtration and, for drying, a dryer. A refrigerated dryer typically delivers a pressure dew point around +3 °C; a desiccant dryer typically −20 °C or −40 °C. When the specification calls for a target dew point in the line at the end of drying, it is the dryer, not the compressor, that makes it achievable. See desiccant adsorption dryers and portable aftercoolers.
In the high ambient heat of Andhra Pradesh and Telangana summers, aftercooler sizing and cooler cleanliness matter: hot air carries much more water, and portable compressors at 40–45 °C ambient, and above 45 °C on open sites, need clear cooling airflow to avoid high-temperature shutdowns.
Safety boundaries we work within
Compressed air stores a large amount of energy, and a pipeline full of it behaves very differently from one full of water. A stuck pig that suddenly frees, a launcher door opened under pressure, or a whipping hose can cause serious injury. The pipeline owner’s or engineer’s procedure governs the operation. Within it, the air supplier is responsible for:
- supplying machines in sound condition with working safety shutdowns, relief valves and gauges;
- rated hoses, couplings with whip checks or restraints, and a clear isolation valve at the launcher;
- a pressure regulator or controlled loading so pressure can be raised gradually;
- operators who follow the permit-to-work, exclusion zones and communication plan set by the client;
- not opening, adjusting or venting the pipeline itself unless the procedure assigns that task.
Venting at the receiver must be planned: discharging large volumes of air takes time, is noisy, and can carry water, debris and pigs at speed. See safety, isolation and permit to work.
Planning the temporary air package
- Agree the task sequence (cleaning, gauging, dewatering, drying) and the pressure and flow limits for each.
- Calculate line volume and free-air requirement per run, and the target time for each run.
- Choose compressor number and class, with standby if interruption mid-run would be a problem.
- Specify treatment: aftercooler and separator always; dryer for drying runs.
- Specify the manifold, hoses, launcher connection, regulator, relief and gauges.
- Plan fuel, operators, shifts and servicing for the job duration.
- Commission the package before connecting it to the line: run it, check pressure, temperature and dew point, and confirm shutdowns.
On long round-the-clock hire, plan routine servicing in advance. On our hire jobs, compressors are serviced every 300 running hours, with an oil change and filter service.
How Reddy Compressors helps
Reddy Compressors has sold and hired out compressors for pipeline pigging work, and we supply, rent and support complete pigging air packages. From our base at Tarnaka, Secunderabad, we work as an independent multi-brand specialist, selling, hiring and servicing ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and other machines; we are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.
Our team has also delivered larger temporary air packages for pipeline work. On a major natural-gas development at Kakinada in Andhra Pradesh, we supplied and commissioned a package of high-pressure compressors – three rated 1,100 cfm at 300 psig (20.7 bar), two rated 1,000 cfm at 275 psig and one rated 750 cfm at 150 psig – together with generators, distribution boards, operators and a technician on site.
- Rental from our own fleet – diesel and electric compressors from 50–2,000 cfm and 2–30 bar, with hoses, aftercoolers, moisture separators and dryers.
- Sales – new, used and refurbished portables, and brand-new machines for long-term projects.
- Service and spares – more than 20 service engineers and technicians covering all of Telangana and Andhra Pradesh; equipment, rentals and spares supplied across India.
What to send us
- Pipeline diameter, wall thickness, length and elevation profile
- Pigging tasks (cleaning, gauging, dewatering, drying) and pig types
- Maximum permitted driving pressure and any speed limit from the procedure
- Target dew point for drying, if any
- Site location, duration, working hours and fuel arrangements
- Whether air is permitted, or nitrogen is required for any section
Frequently asked questions
What size compressor do I need for pipeline pigging?
Work from the line volume. Calculate internal volume, multiply by the absolute pressure ratio to get free air, and divide by compressor FAD to estimate run time. Small or short lines may use one 250–600 cfm portable; long or large-bore lines, or dewatering uphill, often need several high-pressure units or a larger package. The pipeline procedure sets the pressure limit.
Can compressed air be used to pig a gas pipeline?
Only if the pipeline owner’s procedure allows it. Air contains oxygen and can form a flammable mixture with residual gas or hydrocarbons, so lines in hydrocarbon service usually need purging or an inert medium such as nitrogen. Air is commonly used on new-construction lines, water lines and lines made gas-free under an approved procedure. The decision belongs to the pipeline owner’s engineer.
How much pressure is needed to push a pig?
It is the sum of the pig’s friction, any liquid head it is pushing (roughly 1 bar per 10 m of water lifted) and the back-pressure at the receiver. Foam pigs in clean lines may move at low pressure, while dewatering hilly lines needs far more. The maximum driving pressure always comes from the pipeline rating and procedure, not from the compressor’s capability.
How do you dry a pipeline with compressed air?
After dewatering, foam swabbing pigs are run with dry air to remove remaining films of water, followed by a sustained flow of dry air until the outlet dew point meets the specification. That needs an aftercooler, separator and dryer: refrigerated dryers typically give around +3 °C pressure dew point, and desiccant dryers typically −20 °C or −40 °C for tighter targets.
Do you hire compressors for pigging in Andhra Pradesh and Telangana?
Yes. We hire portable diesel and electric compressors from our own fleet, with aftercoolers, separators, dryers and hoses, and our service team covers all of Telangana and Andhra Pradesh. For larger projects we supply equipment across India, with project-based site support where feasible. Send us the pipeline data and the pigging procedure’s pressure limits so we can propose a package.
What safety precautions apply when pigging with air?
Compressed air stores much more energy than water, so the pipeline owner’s procedure governs. Key precautions include exclusion zones at launcher and receiver, never opening a trap under pressure, restrained hoses with whip checks, a relief valve set below the line’s limit, gradual pressurisation, planned venting, and clear communication between launcher and receiver teams throughout each run.
Related pages
Calculators and worked examples
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Pipeline pigging: Air volume, run time and driving pressure
Line volume behind the pig, free air to fill it at the driving pressure, run time with the compressors on site, pig speed, and an estimate of the driving pressure needed.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Pipe internal diameter (mm): 300
- Line length (m): 1000
- Driving pressure behind the pig (bar g): 7
- Atmospheric pressure (bar a): 1.013
- Compressor FAD, each (unit below): 600
- FAD unit: cfm (ft³/min)
- Number of compressors running together: 1
- Allowance for hoses, launcher, leakage and bypass (%): 0
- Pig friction pressure, from the procedure or pig maker (bar): 0
- Height of liquid column pushed uphill (m): 0
- Specific gravity of that liquid (water = 1): 1
- Back-pressure at the receiver (bar): 0
- Maximum permitted driving pressure (bar g; 0 if not given): 0
Result
- Line internal volume: 70.7 m³
- Absolute pressure ratio: 7.91 (8.01 bar a ÷ 1.013)
- Free air to fill the line behind the pig: 559 m³
- Total compressor FAD: 16.99 m³/min = 16,990 L/min = 600 cfm
- Run (fill) time: 32.9 min = 0.55 h
- Average pig speed: 0.51 m/s = 30 m/min (if the line ahead vents freely)
- Driving pressure needed (estimate): 0 bar = friction 0 + liquid head 0 + back-pressure 0
- Note: Friction, head and back-pressure not entered enter them from the procedure to check the driving pressure
Method. Line volume V = π × D² ÷ 4 × L (internal diameter). The space behind the pig starts empty and ends full of air at the driving pressure, so free air = V × (p_g + p₀) ÷ p₀, plus your allowance for hose and launcher volume, leakage and bypass past the pig. Run time = free air ÷ total FAD (1 m³/min = 35.31 cfm). Pig speed ≈ actual air flow at driving pressure ÷ pipe area = FAD × p₀ ÷ (p_g + p₀) ÷ area. Driving pressure needed ≈ pig friction + liquid head (ρ g h: 0.0981 bar per metre of water lifted × specific gravity) + back-pressure at the receiver. Temperature and pressure gradient along the line are ignored. The maximum permitted driving pressure and any speed limit come from the pipeline owner’s procedure, never from the compressor rating.
What to take from it. Long or large-bore lines need hundreds of cubic metres of free air per run: plan several compressors or compressors plus storage, and let the procedure, not the compressor, set the pressure and speed.
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.