Bypass is the one setting on a cleaning pig that changes both how fast it travels and how well it cleans, and the two pull in opposite directions.
A bypass pig has holes through it. Some of the flow that would push the pig instead passes through those ports and jets out in front, doing two useful things:
Port flow is ordinary orifice flow driven by the differential across the pig:
Qbypass = Cd × Aport × √(2ΔP / ρ) with Cd around 0.6 for a drilled port.
The pig then travels at (Qtotal − Qbypass) / A. Note the feedback: a pig that starts to slow builds more differential, which pushes more flow through the ports, which slows it further. Over-porting a pig is how a run stalls at kilometre three.
The table below is a 12-inch line pumping 263 m³/h of water with 1 bar across the pig:
| Bypass | Port area (cm²) | Bypass flow at 1 bar ΔP (m³/h) | Pig speed at 263 m³/h total (m/s) |
|---|---|---|---|
| 0 % | 0 | 0 | 1.00 |
| 1 % | 7 | 23 | 0.91 |
| 2 % | 15 | 46 | 0.83 |
| 3 % | 22 | 69 | 0.74 |
| 5 % | 36 | 115 | 0.56 |
Port area is quoted as a percentage of the pipe bore area. Small percentages move a lot of flow — which is exactly why bypass is quoted in single figures.
| Pig | Bypass | Reason |
|---|---|---|
| Foam, light cleaning | 0–2 % | Often none; the foam itself is permeable. |
| Cup or disc cleaning pig | 1–3 % | The working range for wax and scale. |
| Heavy debris / first pass | 2–5 % | Fluidising a known deposit, accepting a slower run. |
| Gauging plate run | 0 % | The plate is the measurement; nothing should disturb it. |
| ILI tool | 0–5 % | Vendor-set, fixed or actively controlled for speed. |
| Batching / sealing pig | 0 % | The whole job is to separate two fluids. |
Fitting more ports to slow a pig also softens the seal and reduces the sweeping efficiency: the same jet that fluidises debris also reduces how much liquid the pig pushes ahead of it. If the run needs both a slow pig and a clean sweep, reduce the flow rate rather than open up the pig.
Where the ports point matters as much as their area. Ports aimed at the pipe wall lift deposit off the bottom of the line; ports aimed straight ahead simply push the pile along. A pig that comes back clean from a line that is known to be dirty usually means the flow went through it rather than past the wall.
Set bypass as a percentage and it shows the split, what the pig speed becomes, and whether the run still lands in the window for the tool you picked.
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 calc & planner