Pipeline Pig Launcher

Overview

A pig launcher is the entry airlock of a pipeline. Pipelines must be swept by pigs, cylindrical tools that travel with the flow, to remove wax, water and debris, to gauge the bore after construction, and to carry in-line inspection (ILI) instruments that map corrosion. Since the line cannot be shut down and cut open each time, every piggable pipeline terminates in a pair of traps: a launcher at the upstream end and a receiver downstream. The launcher is a short, oversized pressure vessel (Launcher Barrel) hung off the main line through a full-bore Trap Isolation Valve, with a hinged Quick-Opening Closure at the back for loading.

Construction

The barrel has two bores. The Major Barrel is one to two pipe sizes larger than the line, 28 in on a 24 in line, so a pig with oversized sealing cups slides in easily; it is sized around 2.5 pig lengths to accept long ILI tool strings. A flat-bottomed Eccentric Reducer necks down to the line-size Minor Barrel (Neck), where the pig cups first engage the bore and seal. The flat bottom matters: a concentric reducer would leave a ledge that snags cup edges. Welded Branch Nozzles carry the kicker, vent, drain, relief and instrument connections, and Pig Guide Bars keep long tools centered during loading.

The Closure Door is a forged dished head locked by a rotating Locking Ring and sealed with an O-ring, rated to the full vessel design pressure. Its critical safety feature is the Pressure Warning Device: a hollow bleeder screw that must be backed out before the ring can rotate, venting any residual pressure audibly and visibly past the seal. Doors on large bores weigh hundreds of kilograms and swing on a Door Davit.

Launch sequence

1. With the Trap Isolation Valve and Kicker Valve closed, the barrel is blown down through the Vent Valve to the Vent Stack and emptied through the Drain Valve. The local Pressure Gauge confirms zero.
2. The closure is opened and the pig pushed in from the Pig Loading Tray until its front cups seat firmly in the reducer or minor barrel. The closure is shut and locked.
3. The barrel is purged of air, then slowly pressurized through the small-bore Balance Line and Balance Valve until it equalizes with the main line, opening the trap valve across a pressure difference would slam the pig or damage the valve.
4. The trap valve is opened fully. The pig does not move yet, because flow is still passing straight down the main line.
5. The Mainline Block Valve is throttled while the Kicker Valve opens, diverting flow through the Kicker Line into the barrel behind the pig. The differential across the pig builds until it breaks free, passes the Barred Tee (its branch barred so the pig cannot wander into it), and enters the line.
6. Both Intrusive Pig Signaller units trip as the pig passes, raising their Signaller Flag indicators and telemetry contacts. The trap is then re-isolated, vented, and left empty.

The mainline valve reopens and flow returns to normal; the pig rides the product at line velocity, typically 1–5 m/s.

Safety systems

Trap accidents are almost always sequence violations, opening the closure on a pressurized barrel. Besides the bleeder screw, modern stations fit a mechanical Valve Interlock Set: coded keys released by one valve operation are required to operate the next, making the vent-drain-open order physically enforceable. A small Thermal Relief Valve protects the blocked-in barrel from solar thermal expansion, and Earthing Boss bonding points control static during venting. Barrel supports are split between an anchored and a sliding Saddle Support so thermal growth does not load the nozzles.

Variants

Liquid-line launchers add closed-drain connections for hydrocarbon liquids; sour-service traps use NACE MR0175-compliant materials throughout. Subsea and offshore traps may be vertical or inclined to save deck space, and temporary launchers are bolted on for one-off ILI campaigns on lines without permanent traps.