1. Pigging Overview
Pipeline pigging is the practice of inserting a device (pig) into the pipeline and propelling it through the line using pipeline flow or pressure. Pigging serves multiple purposes including liquid removal, cleaning, batching, and in-line inspection. The pig launcher and receiver (collectively called pig traps) are the vessels that allow pigs to be inserted into and retrieved from a pressurized pipeline.
Launcher vs. Receiver
A pig launcher is installed at the upstream end of the pipeline section to be pigged. The pig is loaded into the barrel at atmospheric pressure, the closure is sealed, the barrel is pressurized, and flow is diverted behind the pig to propel it into the pipeline. A pig receiver is the reverse: flow arrives with the pig, the pig enters the oversized barrel, flow continues through the mainline, the barrel is depressurized, and the pig is retrieved. Both require the same basic components but differ in piping configuration.
Reasons for Pigging
| Purpose | Pig Type | Frequency |
|---|---|---|
| Liquid removal | Foam, solid cast, or bi-directional | Daily to weekly in wet gas pipelines |
| Cleaning | Brush pigs, scraper pigs | Monthly to annually; depends on product and corrosion |
| Batching | Sphere or solid cast separating pig | As needed for product changes |
| In-line inspection (ILI) | Smart pigs (MFL, UT, caliper, geometry) | Every 5–10 years per 49 CFR 192/195 |
| Dewatering / drying | Swab pigs, foam pigs | Post-hydrostatic testing; commissioning |
| Gauging | Gauging plate pig | Pre-ILI verification of minimum bore |
Governing Standards
| Standard | Scope |
|---|---|
| ASME B31.8 | Gas transmission and distribution piping (design, fabrication, inspection, testing) |
| ASME B31.4 | Pipeline transportation of liquid hydrocarbons and other liquids |
| ASME BPVC Sec. VIII | Pressure vessel code for pig trap barrels designed as vessels |
| 49 CFR 192 | Federal pipeline safety regulations for gas pipelines (ILI requirements) |
| 49 CFR 195 | Federal pipeline safety regulations for hazardous liquid pipelines |
| API 1163 | In-line inspection systems qualification standard |
2. Pig Types and Dimensions
The pig type determines the required launcher and receiver dimensions. Smart pigs (ILI tools) are typically the longest and most dimensionally demanding, so pig trap sizing should accommodate the largest pig expected to be used over the pipeline's operating life.
Pig Dimensions by Type
| Pig Type | Typical Length | OD vs. Pipeline ID | Weight (approx.) |
|---|---|---|---|
| Foam pig | 1–2 × pipe diameter | Slightly oversized (2–5%) | Light (2–10 lb) |
| Solid cast utility pig | 1.5–2.5 × diameter | Nominal pipe ID with sealing cups | 10–50 lb |
| Brush / scraper pig | 2–3 × diameter | Nominal with brush oversize | 20–100 lb |
| Bi-directional pig | 1.5–2 × diameter | Nominal with dual-direction cups | 15–60 lb |
| Gauging pig | 2–3 × diameter | Gauging plate at 95% of nominal ID | 20–80 lb |
| MFL smart pig | 4–8 × diameter | Nominal with sensor arrays | 200–5000 lb |
| Caliper / geometry pig | 3–5 × diameter | Nominal with mechanical arms | 100–1000 lb |
Future ILI Accommodation
Even if initial pigging operations only require utility pigs, the pig trap should be sized to accommodate future ILI tools. Retrofitting a pig launcher to accept a longer smart pig is extremely expensive and may require pipeline shutdown. Industry best practice is to size pig traps for the longest commercially available ILI tool for the pipeline diameter at the time of design, plus a safety margin.
3. Barrel Sizing
The pig trap barrel must be large enough in diameter to accept the pig and long enough to fully contain the pig during loading and retrieval operations. Both the launcher and receiver barrel dimensions follow similar sizing criteria.
Barrel Diameter
Barrel ID ≥ Pipeline Nominal OD + clearance
Typical: Barrel NPS = Pipeline NPS + 2 sizes (e.g., 12" pipeline → 16" barrel)
For ILI tools: Barrel NPS ≥ Pipeline NPS + 2" minimum clearance on diameter
| Pipeline NPS | Typical Barrel NPS | Barrel ID (inches) | Notes |
|---|---|---|---|
| 6" | 8" | 7.981 | Standard wall barrel |
| 8" | 10" | 10.020 | Standard wall barrel |
| 10" | 12" | 12.000 | Standard wall barrel |
| 12" | 16" | 15.000 | Two-size jump common |
| 16" | 20" | 18.812 | Two-size jump |
| 20" | 24" | 22.624 | Two-size jump |
| 24" | 30" | 29.000 | Oversized for large ILI tools |
| 30" | 36" | 34.500 | Major trunk lines |
Barrel Length
Lbarrel = Lpig + Lclosure + Ltransition + Lmargin
Where Lpig = longest pig to be accommodated, Lclosure = closure depth, Ltransition = reducer length, Lmargin = operational margin (typically 12–24 inches)
Length Rules of Thumb
For utility pigging only, a barrel length of 1.5 times the pipeline diameter or 5 feet minimum (whichever is greater) is generally adequate. For ILI-capable launchers, the barrel length typically ranges from 1.5 to 2.5 times the longest ILI tool length. Smart pig vendors should be consulted for specific tool dimensions, as tool lengths vary significantly between manufacturers and inspection technologies.
Reducer / Transition Piece
A concentric or eccentric reducer connects the oversized barrel to the pipeline diameter. The reducer must be gradual enough to guide the pig smoothly into the pipeline.
| Reducer Type | Application | Notes |
|---|---|---|
| Concentric | Vertical pig traps; centered flow path | Pig centered on entry; common for gas service |
| Eccentric (flat on bottom) | Horizontal launchers with liquid service | Maintains flat bottom for pig support and drainage |
4. Closure Types
The closure is the end cap of the pig trap barrel that opens to allow pig loading (launcher) or pig retrieval (receiver). Closure selection affects operational safety, convenience, and cost.
Closure Comparison
| Closure Type | Opening Time | Pressure Rating | Advantages |
|---|---|---|---|
| Quick-opening | 2–5 minutes | Up to ANSI 1500 | Fastest access; interlock safety features; most common for frequent pigging |
| Threaded (screw-type) | 10–30 minutes | Up to ANSI 2500 | High pressure capability; compact; lower cost for small diameters |
| Flanged (blind flange) | 30–60 minutes | Up to ANSI 2500 | Simple; standard components; used for infrequent pigging |
| Hinged (davit arm) | 5–15 minutes | Up to ANSI 900 | Closure swings clear; no lifting required; good for heavy closures |
Closure Safety Interlocks
Quick-opening closures must incorporate safety interlocks to prevent the closure from being opened while the barrel is pressurized. Per ASME B31.8 and most operator standards, the interlock system must: (1) prevent closure rotation until barrel pressure is at or near atmospheric, (2) include a pressure-relief device that vents residual pressure during the initial stage of opening, and (3) be designed so that the closure cannot be fully removed until all pressure is relieved. Closure-related incidents are a leading cause of pigging injuries.
5. Kicker Line Design
The kicker line (also called the bypass line) diverts mainline flow behind the pig in a launcher to propel it into the pipeline. In a receiver, the bypass allows flow to continue past the received pig. Proper kicker line sizing ensures adequate differential pressure to launch the pig reliably.
Kicker Line Sizing
Kicker Line NPS ≥ 0.5 × Pipeline NPS (minimum)
Typical sizing: 50–75% of mainline diameter
For smart pig launching: ≥ 4" NPS minimum regardless of pipeline size
| Pipeline NPS | Min Kicker NPS | Recommended Kicker NPS |
|---|---|---|
| 6" | 3" | 4" |
| 8" | 4" | 4"–6" |
| 10" | 4" | 6" |
| 12" | 6" | 6"–8" |
| 16" | 8" | 8"–10" |
| 20" | 10" | 10"–12" |
| 24" | 12" | 12"–16" |
Differential Pressure for Pig Launch
The differential pressure required to launch a pig depends on pig type, sealing cup material, pipe surface condition, and any liquid head in the barrel. Typical launch pressures range from 5–15 psi for foam pigs to 15–50 psi for solid cast pigs with polyurethane cups. Smart pigs may require 25–75 psi differential to overcome the friction of multiple sensor arrays and guide wheels. The kicker line must be sized to deliver sufficient flow to create this differential pressure behind the pig.
6. Pressure Rating
The pig trap barrel, closure, fittings, and all associated piping must be rated for the maximum allowable operating pressure (MAOP) of the pipeline system. The design code (ASME B31.8 for gas or B31.4 for liquids) governs wall thickness calculations and material selection.
Barrel Wall Thickness
t = P × D / (2 × S × F × E × T) (ASME B31.8)
Where P = design pressure (psig), D = barrel OD (inches), S = SMYS (psi), F = design factor, E = longitudinal joint factor, T = temperature derating factor
Design Factors for Pig Traps
| Component | Design Factor (F) | Code Reference |
|---|---|---|
| Pig trap barrel (fabricated) | 0.60 | ASME B31.8, Table 841.114(b) |
| Pig trap barrel (at compressor station) | 0.50 | ASME B31.8, Class 3 location |
| Mainline pipe (Class 1) | 0.72 | ASME B31.8 |
| All fittings and closures | Match barrel rating | ASME B16.5, B16.47 as applicable |
ASME BPVC vs. B31 Design
Some operators and jurisdictions require pig trap barrels to be designed and fabricated under ASME Boiler and Pressure Vessel Code Section VIII Division 1, rather than ASME B31.8, particularly when the barrel diameter exceeds the mainline pipe diameter by more than one or two sizes. BPVC-designed barrels require stamping, third-party inspection (AI), and a U-1 data report. The design approach should be established early in the project based on operator standards and jurisdictional requirements.
7. Operational Procedures
Safe pigging operations require well-defined procedures for loading, launching, tracking, receiving, and retrieving pigs. Each step involves specific valve sequencing, pressure management, and safety precautions.
Pig Launching Sequence
| Step | Action | Safety Check |
|---|---|---|
| 1 | Verify barrel is depressurized and drained | Check pressure gauge at zero; open drain valve |
| 2 | Open closure and load pig | Inspect pig condition; verify correct orientation |
| 3 | Close and secure closure | Verify interlock engagement; check seal |
| 4 | Slowly pressurize barrel through kicker valve | Monitor pressure rise; check for leaks |
| 5 | Equalize barrel to mainline pressure | Confirm pressure equalization |
| 6 | Open mainline valve downstream of trap | Verify valve fully open |
| 7 | Close mainline bypass valve to divert flow behind pig | Monitor pig signaler for pig passage |
| 8 | Confirm pig has entered pipeline | Pig signaler activates at barrel exit |
| 9 | Return valves to normal operation | Open bypass; close kicker |
Pig Tracking
Pig passage is confirmed using pig signalers (indicators) installed at key locations along the pipeline. Common signaler types include:
| Signaler Type | Detection Method | Application |
|---|---|---|
| Mechanical (intrusive) | Spring-loaded trigger activated by pig passage | Most common; visual flag indicator; weldolet-mounted |
| Magnetic (non-intrusive) | Detects magnetic field of pig-mounted magnets | No pipeline penetration required; suitable for high-pressure systems |
| Acoustic | Listens for pig noise through pipe wall | Portable tracking; locating stuck pigs |
| AGM (above-ground marker) | Electromagnetic transmitter on pig detected by receiver | Smart pig tracking; precise location determination |
8. Design Details
Beyond the barrel, closure, and kicker line, several additional design elements are essential for safe and efficient pig trap operation.
Essential Components
| Component | Purpose | Design Notes |
|---|---|---|
| Pig signaler connection | Detect pig passage at barrel exit/entry | 2" weldolet or flanged connection; located at reducer |
| Pressure gauge | Monitor barrel pressure for safe operation | Minimum two gauges; one at closure end, one at mainline tee |
| Drain connection | Drain liquids before opening closure | Low point drain at closure end; sized for liquid volume |
| Vent connection | Depressurize barrel safely | High point vent; sized for controlled depressurization rate |
| Safety relief | Overpressure protection | PSV sized per ASME B31.8; set at MAOP |
| Barrel support | Support barrel and pig weight | Saddle or structural supports; allow thermal expansion |
Valve Requirements
| Valve | Location | Type |
|---|---|---|
| Mainline isolation (upstream) | Upstream of barrel tee | Full-bore ball valve |
| Mainline isolation (downstream) | Downstream of barrel tee | Full-bore ball valve |
| Kicker valve | Kicker line to barrel | Ball valve; throttling capability preferred |
| Bypass valve | Mainline around barrel | Ball valve for normal flow path |
| Barrel isolation | Barrel neck adjacent to mainline | Full-bore ball valve to isolate barrel |
Full-Bore Valve Requirement
All valves in the pig passage path must be full-bore (full-opening) to allow unrestricted pig passage. Reduced-bore valves will obstruct the pig and potentially cause a stuck pig or valve damage. Ball valves with full port openings are the standard choice. Gate valves may be used but must be confirmed full-bore with no internal obstructions when fully open.