Is the line piggable? The checks before you launch

Bends

Bend radius is quoted in pipe diameters: a 3D bend has a centreline radius of three times the pipe diameter, a 1.5D bend half of that. Everything about pig selection follows from the tightest bend on the route.

Two traps: back-to-back bends with no straight between them are harder than either bend alone, and a mitred bend is not a radius at all — treat it as an obstruction until the pig vendor says otherwise.

Valves

• Full-bore ball valves — fine, provided they are fully open and the bore matches the line.
• Reduced-bore ball valves — a hard stop. So are most plug valves, globe valves, swing check valves and any valve with a seat that protrudes into the bore.
• Gate valves — only if through-conduit and fully open; a partially open gate is the classic pig trap.
• Valve position confirmation matters as much as valve type. "Believed open" has stopped more pigs than reduced bore.

Tees, wyes and offtakes

An unbarred tee with a branch larger than about 30 % of the run bore can swallow a pig or let a cup extrude into the branch and tear. Barred tees solve it, and any line intended to be pigged should have them. Where an old line has unbarred offtakes, the practical answers are a longer pig body that bridges the opening, a foam pig that cannot jam, or isolation of the branch during the run.

Bore changes and reducers

Bore changes come from a change of schedule, a change of nominal size, or a fitting. A pig sized for the larger bore will not pass the smaller; a pig sized for the smaller will not seal in the larger. Where the line genuinely steps, the options are a dual-diameter pig designed for that pair of sizes (10 in / 12 in, say) or two separate runs with an isolation point between them.

Ovality, dents and deposits

Dents, buckles and ovality shrink the effective bore. On an unpigged line of unknown condition the sequence is a foam pig first, then a gauging pig with a plate at 90 95 % of the nominal minimum bore, and only then a cleaning train or a tool. The bent plate that comes back is cheap information about how much bore you actually have.

Launcher and receiver

• Barrel length at least the length of the pig train plus one more pig length, so the train sits fully behind the kicker connection.
• Kicker line properly sized and positioned; without kicker flow the pig will not leave the oversize barrel.
• Reducer between barrel and line — eccentric, invert flat, so the pig does not have to climb a step.
• Valve sequence, vents, drains and the closure checked and proved before loading, with the pressure-relieving device on the door verified.