How to calculate pig speed
Everything else on a pigging job is negotiable. Speed is not: too slow and the cleaning is poor and the tool sits in the line, too fast and the inspection data is worthless and the pig arrives like a bullet.
A pig has no engine. It goes at the speed the fluid behind it pushes it, so the calculation is the bulk velocity of the flow:
v = Q / A — where Q is the volumetric flow rate at line conditions and A is the internal cross-section, A = π/4 × ID².
Two things trip people up. The bore is the internal diameter, not the nominal size — a 12-inch line is 323.9 mm outside and 304.8 mm inside at standard wall, and the area follows the square of that difference. And on a gas line, Q must be the actual volume at line pressure, not the standard volume the flow meter reports.
Flow rate for 1 m/s, by line size
Read the number for your size and scale it. Want 2 m/s? Double it. Want 0.5 m/s? Halve it.
| Line | Bore (mm) | Area (cm²) | Flow for 1 m/s (m³/h) | US gal/min |
|---|---|---|---|---|
| 6" std wall | 154.1 | 186 | 67 | 296 |
| 8" std wall | 202.7 | 323 | 116 | 512 |
| 10" std wall | 254.6 | 509 | 183 | 807 |
| 12" std wall | 304.8 | 730 | 263 | 1,157 |
| 16" std wall | 387.3 | 1178 | 424 | 1,868 |
| 20" std wall | 488.9 | 1878 | 676 | 2,976 |
| 24" std wall | 590.5 | 2739 | 986 | 4,341 |
| 30" std wall | 742.9 | 4335 | 1,561 | 6,871 |
Gas lines: standard volume is not line volume
Compressors and membrane units are rated in standard cubic metres per hour. The pig responds to actual cubic metres per hour. The conversion is the gas law:
Qline = Qstd × (Pstd / Pabs) × (T / Tstd) × Z
At 8 bar g the absolute pressure is about 9.0 bar, so a cubic metre at line conditions costs you roughly 8.9 standard cubic metres of supply. The 263 m³/h that drives a 12-inch pig at 1 m/s becomes about 2,340 Sm³/h at the compressor — which is the number that decides how many units you mobilise.
Gas behind the pig is a spring. If the pressure ahead falls away — the pig crests a hill, or the receiver is open to atmosphere — the compressed gas expands and the pig accelerates, sometimes to ten times the planned speed. That is what wrecks tools and receiver doors. The cure is back pressure at the receiving end, not less flow at the launcher.
The windows to aim for
| Run | Typical window | Why the limit exists |
|---|---|---|
| Cleaning, liquid | 1–3 m/s | Below 0.5 m/s debris drops out ahead of the pig; above 3 m/s the brushes skate. |
| Cleaning, gas | 2–5 m/s | Higher speeds are tolerated, but surge control matters more than the average. |
| Gauging plate | 1–3 m/s | Fast impact on an obstruction destroys the plate and tells you nothing. |
| MFL inspection | 1–4 m/s | Above the vendor limit the magnetiser cannot saturate the wall and sizing degrades. |
| UT wall-loss inspection | 0.5–2 m/s | Shot spacing: too fast and the tool skips wall between readings. |
| Liquid batch between pigs | 1–2 m/s | Keeps the slug intact and limits mixing at each interface. |
The vendor's limit governs. These are the ranges you will meet in the field; the tool datasheet for the specific run always wins, and most ILI contracts specify both a window and a maximum percentage of the run allowed outside it.
Why the real speed differs from the calculated one
- Bypass. Flow through the pig's ports does not push it. The pig travels at (Q − Qbypass) / A, which is slower than the line velocity.
- Profile. On an undulating line the pig slows climbing and runs on the way down. The average over the run can be right while the instantaneous speed is well outside the window.
- Drag. A pig needs a pressure difference across it before it moves at all — typically 0.3 to 1.5 bar depending on type, size and sealing interference. Until that builds, the pig sits still and the line simply pressurises.
- Debris. A pig pushing a growing slug of wax or scale needs progressively more differential and slows down, sometimes to a stop.
Run time
Once the speed is set, the run time is just t = L / v. Ten kilometres at 1 m/s is 2 h 46 min. Add the launch sequence, the receiver work and contingency for a stall, and that is the window you ask the operations team to hold.
Pipeline pigging calc & planner
Put in the size, wall, medium and target speed and it gives you the flow rate, the run time, the pressure the spread has to deliver and the equipment to book — in SI or US units.
Open the pigging calculatorWho writes this. A mechanical engineer with twelve years in oil and gas — in-line inspection, fired heater and furnace inspection, and pipeline integrity. What is here comes from the published standards and from what those years in the field actually looked like. It is not written by an API-certified inspector.
This is not an assessment. Nothing on this site may be used to justify a decision about real equipment. Assessing plant requires the current editions of the applicable codes, data from a licensed source, and a competent engineer who signs for the answer. · Integrity Field Guide · Pipeline pigging calculator · Pigging equipment & vendor directory · Pipe & fitting dimensions
Pictures: “Acid-corrosion-in-a-flue-gas-duct-01” by CEphoto, Uwe Aranas, CC BY-SA 3.0, via Wikimedia Commons; “Anacortes Refinery 31911” by Walter Siegmund (talk), CC BY 2.5, via Wikimedia Commons; “United States Strategic Petroleum Reserve 072” by ENERGY.GOV, public domain, via Wikimedia Commons; “Trans-Alaska Pipeline System Visitors Center Scraper Pig” by Leijurv, CC BY-SA 4.0, via Wikimedia Commons; “Gulf raffinaderij, Rozenburg, Bestanddeelnr 924-8259” by Hans Peters for Anefo, CC0, via Wikimedia Commons.
