- Pipeline pigging is one of the most operationally demanding activities in pipeline management, and the valves and fittings that make up the pig launch and receive infrastructure — including full-bore isolation valves, kicker valves, pig signallers, and barred tees — must be specified with the same rigour as the pipeline valves themselves.
- The barred tee — a tee fitting with bars or grating welded across the branch opening — is a deceptively simple component that prevents a pig from entering a branch line instead of continuing down the mainline, and getting the bar spacing, bar geometry, and weld quality wrong causes pigs to become stuck or damaged in ways that are expensive and time-consuming to resolve.
- Selecting the correct valve configuration for a pig launcher or receiver is not just about bore size — it requires understanding the pig type, the pig’s physical dimensions, the pig’s flexibility and deflection characteristics, and how the pig transitions from the launcher barrel through the closure valve and into the live pipeline without sustaining damage or becoming jammed.
Why Pigging Infrastructure Matters
Pigs — pipeline inspection gauges, cleaning pigs, intelligent inspection tools, or any cylindrical device propelled through a pipeline by the flow or by injected fluid — are fundamental to the long-term integrity management of liquid and gas pipelines. They clean wax and scale deposits that reduce flow efficiency, inspect the pipe wall for corrosion, dents, and cracks, verify pipeline geometry, and deliver chemicals and monitoring devices to remote locations in the pipeline. Without pigging, pipelines deteriorate in ways that are invisible from the surface until they fail.
Every pig must have a way to enter the pipeline and a way to exit. The infrastructure that provides these functions — pig launchers, pig receivers, and all the associated valves and fittings — must be designed to accommodate the specific pigs that will be used over the life of the pipeline. Getting this infrastructure wrong means either that pigs cannot be used at all, or that every pigging operation becomes a high-risk, time-consuming event requiring manual intervention to recover a stuck pig. Neither outcome is acceptable for a pipeline that is intended to be pigged regularly as part of its integrity management programme.
Full-Bore Valves for Pigging Service
The most fundamental requirement for any valve in a piggable pipeline is a full bore — the valve bore must be at least equal to the pipeline internal diameter, with no reduction in bore diameter and no internal protrusions that would prevent a pig from passing through. This means full-bore ball valves conforming to API 6D, or gate valves where the bore is verified to be clear of any internal obstructions in the open position. Reduced-bore valves of any type cannot be used in any location where a pig must pass.
For intelligent inline inspection tools, the bore requirement is more demanding than simple full-bore. ILI tools contain sensor arrays, electronics, and centralising components that require the bore to be within tight tolerances of the nominal pipe bore, with smooth transitions between bore sections. A valve that technically meets the full-bore bore diameter requirement but has a bore that is slightly out-of-round, has a step at the seat ring, or has a non-uniform bore profile, can prevent certain ILI tools from passing or can damage tool components. The pig tool manufacturer’s bore requirements should be obtained and verified against the proposed valve bore at the design stage.
Valve bore cleanliness is important in pigging service. Deposits of wax, scale, or corrosion products that build up in valve bores reduce the effective bore and can cause pigs to jam. Valves in pigging service should be specified with smooth bore profiles that minimise ledges and recesses where deposits accumulate, and they should be included in the regular cleaning pigging programme to prevent buildup. Specifying valves with internal bore coatings (fusion-bonded epoxy or similar) reduces the adherence of wax and scale deposits and simplifies cleaning.
Pig Launcher and Receiver Valve Configurations
A pig launcher typically consists of a barrel — a large-bore vessel that accommodates the pig before it is launched — connected to the mainline through a reducing section and a full-bore kicker valve. The launch sequence involves inserting the pig into the barrel through the closure door, pressurising the barrel with line fluid or gas, opening the kicker valve to allow the propellant to enter behind the pig, and then opening the mainline valve to allow the pig to enter the pipeline. The kicker line introduces fluid behind the pig to provide the initial propelling force before the pig is in the pipeline and subject to the full line flow.
The mainline valve at the launcher connection is typically a full-bore ball valve or gate valve sized to match the pipeline diameter. This valve must open fully — to its fully open, full-bore position — before the pig passes through. Partial opening of the mainline valve while the pig is in the launcher barrel will cause the pig to wedge against the partially open disc or gate, potentially jamming the valve and the pig simultaneously. The valve operating sequence must be interlocked with the launcher pressure equalisation to prevent inadvertent partial-opening pig launches.
The pig receiver has a similar configuration in reverse: a receiving barrel where the pig is captured, a mainline connection valve, bypass piping to maintain flow continuity during pig receipt, and a closure door through which the pig is extracted after receipt. The trap closure valve at the receiver must be closed before the closure door is opened — this is a fundamental safety requirement because the trap may still be at line pressure when the pig arrives, and opening the door against pressure will eject the pig and any fluid in the trap violently. Interlocking systems that physically prevent the closure door from being opened unless the isolation valve is confirmed fully closed are mandatory on all pig receiver installations.
Barred Tees: Design and Specification
A barred tee is installed wherever a pig must be prevented from entering a branch connection while travelling along the mainline. Without a barred tee, a pig approaching a branch could enter the branch instead of continuing down the mainline, particularly if the flow velocity into the branch is significant or if the pig’s leading end is deflected toward the branch by flow asymmetry. A barred tee has a grating of bars welded across the branch opening, oriented perpendicular to the mainline flow direction, that physically prevents the pig from entering the branch while allowing full flow from the branch into the mainline (or from the mainline into the branch in the opposite direction).
The bar spacing in a barred tee must be small enough to prevent the smallest pig cup or module from entering the branch, while being large enough to allow unrestricted flow. The standard bar spacing is typically 50 percent of the pipeline nominal diameter for straight pig cups, though this requires verification against the actual pig dimensions for each pig type. Intelligent inspection pigs with complex leading end geometries may require smaller bar spacing to prevent leading-edge components from entering the branch. The pig manufacturer should be consulted when specifying barred tees for ILI pig service.
Bar geometry and weld quality in a barred tee are critical to both pig safety and pipeline safety. Bars must be smooth, without sharp edges or burrs that could damage pig cups or seals as the pig slides past. All welds must be full-penetration, post-weld inspected, and heat-treated where required by the pipeline specification. A failed bar weld that allows a bar to drop into the mainline will certainly jam the next pig and may cause serious damage to an intelligent inspection tool worth hundreds of thousands of pounds. Barred tees should be inspected by a qualified ILI tool operator before commissioning to confirm bar geometry and weld integrity using the same tools that will be used in service.
Pig Signallers and Detection Systems
Tracking pig location during a pigging operation requires detection devices at known points along the pipeline. Pig signallers — also called pig indicators or pig detectors — are fitted to the pipeline at strategic locations to detect when a pig passes. The most common types are mechanical signallers, where the pig’s passage physically deflects a target mounted in the pipeline bore through a spring-loaded mechanism connected to a flag or switch outside the pipe; and electronic signallers, where a magnetic pickup detects the passage of magnets embedded in the pig body.
For intelligent pigging tools, acoustic systems and electromagnetic systems can also track pig location in real time by picking up acoustic signals generated by the tool’s internal instrumentation or by detecting electromagnetic signals from active transmitter pigs. GPS-tagged surface markers correlate pig location data with geographic coordinates when combined with data from the tool’s internal odometer wheel.
Pig signallers must be installed in a bypass arrangement if they require pipeline shutdown for installation or maintenance — the bypass allows the signaller to be isolated and serviced without interrupting pipeline flow. Signallers installed directly in the pipeline bore (through a fitting) must be retractable under pressure, using a pressure-containing stuffing box arrangement similar to a valve stem seal, so they can be withdrawn and replaced without depressurising the pipeline. Any signaller with a retractable in-bore target must have a blowout-proof retraction mechanism — the same safety requirement that applies to valve stems.
Kicker Valves and Bypass Valves
The kicker valve introduces propellant fluid or gas into the launcher barrel behind the pig at the beginning of the launch sequence. In liquid pipelines, the kicker valve is typically a standard flanged globe or gate valve in the 2-inch to 6-inch range, connecting the mainline upstream of the launcher to the launcher barrel. In gas pipelines, the kicker connection may also incorporate a pressure-equalising function to gradually pressurise the launcher barrel to line pressure before the mainline connection valve is opened, preventing the pig from being shot violently into the pipeline when the mainline valve opens.
Bypass valves at launchers and receivers maintain pipeline flow continuity when the mainline isolation valve at the launcher or receiver is closed. Without a bypass, closing the mainline valve would interrupt all flow through the pipeline section — operationally unacceptable in most pipeline systems. The bypass valve connects the upstream and downstream mainline on either side of the launcher or receiver isolation valves, allowing continued flow while the pig is being loaded or unloaded. Bypass valve size must be adequate to maintain acceptable pipeline flow pressure during pigging operations — an undersized bypass causes excessive pressure drop across the bypass, which can affect pig propulsion velocity and may cause operational problems downstream.
Valve Selection for Pig-Compatible Crossover and Block Valve Stations
Block valve stations — periodically spaced along long pipelines for emergency isolation and sectionalisation — must be pig-compatible if they are in a piggable line. The configuration of valves at a block valve station includes the mainline isolation valves (full-bore, pigging compatible), bypass valves for flow continuity during isolation, vent and drain connections, and instrumentation. Every valve in the mainline through which pigs will pass must be full-bore and must open cleanly to full bore without any reduction in clear bore diameter.
The act of opening or closing a mainline block valve while a pig is passing through the station must be physically prevented. Interlocking — through pig signallers at the inlet and outlet of the station that confirm pig passage before allowing any valve closure — prevents valve operation from catching a pig mid-passage, which would certainly damage or destroy the pig and potentially damage the valve. For intelligent pigging operations, the pipeline operating procedure typically specifies a minimum time or distance margin before any block valve can be operated after the pig signaller at the upstream end of the station has confirmed pig arrival.
Magnetic and Gel Pig Compatibility with Valve Trim
Cleaning pigs — particularly gel pigs and foam pigs used for aggressive wax or hydrate removal — interact with valve trim in ways that can cause problems if not considered at the design stage. Gel pigs consist of a viscoelastic gel plug that conforms to the pipe bore and sweeps deposits ahead of it as it moves. When a gel pig passes through a ball valve, the gel material must flow through the ball bore without leaving gel residue in the valve body cavity behind the ball. A ball valve with a cavity between the ball and body that can trap gel will become blocked with hardened gel if the valve is operated before the gel is cleared. Full-bore ball valves with body cavity vent connections allow trapped gel to be flushed out after pig passage.
Magnetic flux leakage (MFL) pigs use powerful permanent magnets to magnetise the pipe wall and measure flux leakage through corrosion defects. These powerful magnets generate significant attractive forces to any ferromagnetic material in the valve bore — including gate valve gates, globe valve plugs, and check valve discs. Gate valves in MFL pig service must be confirmed to be locked fully open before the pig passes, as the magnetic attraction between the pig and a partially open gate can hold the gate down and prevent the pig from passing. Similarly, check valves in MFL pig service should be swing-check type with full-bore open position, not tilting-disc types where the disc may be attracted toward the pig and cause interference.