Back pressure is the difference between a controlled run and a pig that arrives at the receiver at fifteen metres a second. It is also the number most often pulled out of the air.
Back pressure is the pressure held at the receiving end of the line while the pig travels. Three separate requirements set it, and the governing one is simply the largest.
A liquid ahead of the pig has to get over every hill on the route. At the crest, the pressure is the receiver pressure plus whatever friction is left, minus the static head of the downhill leg. Let that go below the vapour pressure of the liquid and the column separates: the pig runs free into the void and slams back when the column rejoins.
Static head is ΔP = ρ g Δh. In field units, water costs about 0.098 bar for every metre of rise:
| Liquid | ρ (kg/m³) | bar per m | 20 m rise | 60 m rise | 120 m rise |
|---|---|---|---|---|---|
| Water / hydrotest water | 1000 | 0.0981 | 1.96 | 5.89 | 11.77 |
| Diesel | 840 | 0.0824 | 1.65 | 4.94 | 9.89 |
| Light crude (API 35) | 850 | 0.0834 | 1.67 | 5.00 | 10.01 |
| Heavy crude (API 20) | 930 | 0.0912 | 1.82 | 5.47 | 10.95 |
| Condensate | 750 | 0.0736 | 1.47 | 4.41 | 8.83 |
| MEG / glycol mix | 1110 | 0.1089 | 2.18 | 6.53 | 13.07 |
A 60-metre crest on a water-filled line needs roughly 5.9 bar at the receiver before you even count friction or the margin you want above vapour pressure. That is the whole reason back pressure exists on hilly routes.
With gas behind the pig, the pressure ratio across the run controls the surge. If the pressure at the pig falls to a fraction of what it was at the launcher, the gas column behind expands by the inverse of that ratio — and all of that expansion turns into pig velocity.
The practical control is to keep the expansion ratio low: hold the receiver pressure high enough that the absolute pressure at the pig never drops below roughly half the launch pressure, and tighter than that on an ILI run. A line vented to atmosphere at the receiver end has an expansion ratio equal to the full launch pressure in bar absolute — which is why "just open the receiver vent" ruins tools.
Most surge damage happens in the final few hundred metres, when the gas column is at its longest and the remaining friction ahead of the pig is at its smallest. Back pressure has to be held all the way into the trap, not released when the pig signal is seen.
A pig moves only once the pressure behind it exceeds the pressure ahead by enough to overcome sealing friction — roughly 0.3 bar for a light foam pig, 0.5 to 1.5 bar for a cup or disc pig with interference, more for a train of two or three, and more again for an ILI tool with magnetiser drag. Whatever back pressure you hold, the launch pressure must exceed it by this differential plus all the line friction, and that sum must stay under MAOP.
Excess back pressure stalls the pig, pushes launch pressure toward MAOP, and can exceed what the compressors or pumps can deliver. If the number you calculate cannot be reached without breaking one of those limits, the answer is a slower run, a staged schedule, or a different medium — not more pressure.
The back-pressure planner takes your elevation profile and gives you a staged schedule, the choke Cv, the pressure at each high point and the launch pressure needed to get there.
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