A pig sitting in the oversized barrel of a launcher will not move on its own - the flow that will eventually drive it down the pipeline runs through the mainline, not through the barrel where the pig waits. The kicker line is the piece of piping that fixes this: a bypass that diverts some mainline flow around and into the back of the barrel to get behind the pig and push it forward into the mainline. This guide defines the kicker line, walks through the valve-sequencing and differential-pressure steps of a launch, and covers the pressure readings and interlocks operators rely on to launch a pig safely.
Kicker Line (Pig Launcher) in one line: A kicker line is a small bypass pipe on a pig launcher that routes a portion of mainline flow into the rear of the launcher barrel, behind a loaded pig, so the flow builds up behind the pig and pushes it out of the barrel and into the mainline. Because the barrel is larger than the mainline, product does not naturally flow through it, so the kicker line supplies the driving flow to start the pig moving. Launching is a controlled valve sequence in which operators watch differential pressure across the pig and confirm it has left the barrel.
A pig launcher is a barrel wider than the pipeline it feeds, with an oversized section that holds the pig and a reducer that tapers down to mainline diameter at the exit. That geometry is what lets the pig be loaded and sealed behind a closure door, but it also means the main product flow, taking the path of least resistance, runs straight through the mainline and past the launcher rather than through the barrel. With no flow moving through the barrel, the pig has nothing pushing on it and simply sits where it was loaded.
The kicker line solves this by tapping the mainline and delivering flow into the barrel behind the pig, downstream of the closure. Fluid entering there builds pressure against the back of the pig, and because the front of the pig sees the lower mainline pressure ahead, a differential develops across it. Once that differential is large enough to overcome the pig's seal friction and static resistance, the pig starts to move forward, through the reducer, and out into the mainline where the normal product flow takes over and drives it down the line.
The kicker line is deliberately smaller than the mainline - it only needs to supply enough flow to nudge the pig out of the barrel, not to carry the full pipeline throughput. Its size and the valve on it give the operator a way to meter how forcefully flow is applied behind the pig, so the launch can be a controlled push rather than a sudden slam.
Launching follows a defined valve sequence, and the exact steps depend on the trap arrangement, but the logic is consistent. With the pig loaded and the closure door secured, the operator first equalizes and pressures up the barrel so it is at line pressure before any isolation valve is opened against a pressure difference - opening a valve with the barrel empty and the mainline live would be both unsafe and hard on the valve. Only once the barrel is confirmed at pressure does the operator open the kicker valve and the main trap (launcher isolation) valve in the correct order to establish flow behind the pig.
As flow is routed through the kicker line, the operator throttles the mainline valve to force more of the product through the barrel and behind the pig, raising the differential pressure across it. The pressure gauge on the barrel is the key indicator here: the differential has to climb to the point where it overcomes the pig's resistance, and the operator watches for the pressure to build and then break - a characteristic drop as the pig releases and moves out of the barrel into the mainline. That pressure signature, together with a pig-passage indicator downstream of the launcher, confirms the pig has actually launched rather than remaining stuck in the barrel.
After launch, the sequence reverses toward normal operation: flow is returned fully to the mainline, the kicker and trap valves are closed in order, and the barrel is isolated, depressured, and vented before the closure is opened for the next pig. Doing these steps out of order - opening a closure on a pressured barrel, for instance - is the classic pigging hazard the procedure exists to prevent.
Because a pig launcher is a pressure vessel that operators open by hand, the safety case rests on making sure the barrel is fully depressured before the closure can be opened and on preventing valves from being operated in a dangerous order. The core instrument is the barrel pressure reading, which tells the operator whether the trap is at line pressure, being vented, or safely at zero. Many launchers add a mechanical or key interlock scheme that will not release the closure until a bleed valve has been opened and pressure confirmed relieved, so a stuck or misread gauge alone cannot lead to opening a live barrel.
During the launch itself, the barrel pressure and the differential across the pig are what the operator reads to know the pig is moving, and mainline pressure and flow provide the context for how hard the line is being pushed. A cloud SCADA such as Merobix brings these readings - barrel pressure, mainline pressure, flow, and the downstream pig-passage signal - onto one screen and time-stamps them, so a launch that goes to plan is visible as a clean pressure build, break, and passage, and one that does not is visible as a barrel that pressures up but never shows a passage downstream.
That remote visibility matters most where launchers sit at unmanned or lightly staffed sites across a gathering field. An operator in a control room can confirm from the pressure and passage trail that a pig launched and is on its way, and can flag the abnormal case - a barrel that held differential without the pig releasing - for a field crew to investigate. The physical valve sequence and interlocks still govern the launch on site, but SCADA turns each launch into a recorded, monitorable event rather than a report that arrives only by radio.
The kicker line diverts a portion of mainline flow into the rear of the launcher barrel, behind the loaded pig. Because the barrel is larger than the mainline, product does not naturally flow through it, so without the kicker line there is nothing to push the pig. The flow it supplies builds pressure behind the pig, creating the differential that drives the pig out of the barrel and into the mainline.
The barrel pressure shows a characteristic build and then a break - a drop that occurs as the pig releases and moves out of the barrel. This pressure signature, confirmed by a pig-passage indicator located downstream of the launcher registering the pass, tells the operator the pig has left the barrel rather than remaining stuck inside. In SCADA both the pressure trace and the passage event are time-stamped so the launch is a recorded event.
The barrel is a pressure vessel, and opening its closure door while it holds line pressure could release stored energy violently, which is one of the most serious pigging hazards. The procedure requires venting the barrel to zero and confirming it on the pressure gauge before the closure is opened, and many launchers add a mechanical or key interlock that will not release the door until a bleed valve is open and pressure is confirmed relieved. Following the valve sequence in order is what prevents opening a live barrel.
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