Pigging: How We Clean and Inspect Pipelines from the Inside

Educational content only: procedures and operating values vary between companies and sites. Always follow your company’s approved procedures, your supervisor’s instructions and the permits in force at your site.

What is a pig and why use one?

A pig is a cylindrical or spherical tool with a diameter close to the pipe's inside diameter. It is loaded at the start of the line and pushed by the liquid or gas flowing behind it, travelling to the end of the line. The name is often said to come from the squealing noise early tools made inside the pipe.

Pigging serves many purposes:

• Cleaning: removing wax (paraffin), sand and deposits that narrow the pipe and raise pressure loss.
• Dewatering: water collecting at low points speeds up internal corrosion, and after a hydrostatic test the test water must be removed.
• Liquid removal from gas lines: accumulated liquids reduce capacity and can arrive suddenly at the plant as a large slug.
• Batching: in product lines, a pig separates two different products such as diesel and gasoline.
• Gauging and inspection: checking for dents, and measuring wall thickness and corrosion with intelligent tools.

Types of pigs

• Foam pig: light and flexible, it passes bends and restrictions. Good for the first run in a line of unknown condition.
• Mandrel pig (cups or discs): a steel body fitted with polyurethane cups or discs, sometimes with steel brushes or blades to scrape wax.
• Solid cast pig: a single piece of polyurethane, economical and common for routine cleaning.
• Sphere: widely used in gas lines to remove liquids, and able to pass branches other pigs cannot.
• Gauging pig: carries an aluminium plate, typically 90 to 95% of the smallest inside diameter. If the plate comes out undamaged, the line has no major dents.
• Intelligent pig (ILI tool): carries sensors, batteries and memory. The best-known technologies are magnetic flux leakage for metal loss, ultrasonic for wall thickness, and caliper for geometry.

The pig trap

A trap is a section of pipe wider than the line, installed at the start (launcher) and end (receiver). Its parts:

• Major barrel: one or two sizes larger than the line, so the pig can be loaded easily.
• Reducer: guides the pig from the wide barrel into the line.
• Quick-opening closure: usually with a safety lock that prevents opening while pressure remains in the barrel.
• Main isolation valve: between trap and line; it must be full-bore so the pig can pass.
• Kicker line: diverts part of the flow behind the pig to drive it.
• Vent, drain and pressure gauge.
• Pig signaller: confirms the pig has left or arrived.

Launching steps

These steps are typical; the exact sequence is set by your company procedure and the work permit:

1. Confirm the trap is isolated from the line: main isolation valve and kicker closed.
2. Open the vent and drain to a closed or safe system, and release pressure and liquid.
3. Confirm zero pressure by two independent means: the pressure gauge, and an open vent showing no flow.
4. Open the closure, insert the pig and push it until it seats snugly in the reducer.
5. Inspect and clean the O-ring, close the door and engage the safety lock.
6. Close the drain, fill the trap slowly from the kicker with the vent open to expel air, then close the vent and equalize with the line.
7. Open the main isolation valve, then open the kicker.
8. Gradually throttle the mainline valve so flow passes behind the pig and drives it into the line.
9. When the signaller confirms the pig has left, return the valves to normal and log the time.

Receiving steps

The receiver is lined up to the pipeline well before the pig is due. On arrival the flow pushes the pig into the barrel and the signaller confirms it. The trap is then isolated and depressurized, zero pressure is confirmed by two means, and the door is opened to remove the pig. Behind the pig there is usually a quantity of debris, wax and liquid, so be ready for it.

Hazards and safety

• Trapped pressure at the door: the worst pigging accidents happened when a trap door was opened with pressure still inside, and the door or pig was released with great force. Never stand in front of the door; use the safety lock and double verification.
• H2S and gas: sour gas may escape when the trap is opened. Use a gas detector, and breathing apparatus if required.
• Iron sulfide (FeS): black debris can ignite by itself as it dries in air. Keep it wet and dispose of it by the approved method.
• NORM: naturally occurring radioactive material can concentrate in production line debris and needs measurement and special handling.
• Stuck pig: do not try to free it by raising pressure beyond limits. Follow the pigging contingency plan.

Takeaway for the technician

Pigging is routine, but its most dangerous step is very simple: opening a door. Double-checking zero pressure, never standing in front of the door, and preparing for hazardous debris are three rules that protect the crew every time.

Test your understanding

Related lessons

• Pipeline hydraulics and pump stations
• Corrosion, cathodic protection and integrity
• Why does production tubing get choked by wax? All about paraffin

Open the calculators

Sources

1. What Is Piping - Pipeline pigging: pig types, launcher and receiver
2. What Is Piping - Pig launchers, receivers and traps
3. Hydrotech - Pig receivers and their role in safe pipeline operations
4. PHMSA Fact Sheet - Smart pigs (archived)
5. University of Bristol - A review of fluid driven pipeline pigs and their applications