What Is Pipeline Pigging?
Pipeline pigging is the practice of inserting a device — called a pig — into a pipeline and propelling it through the line using differential pressure. Pigs are used to clean the inside of pipelines, remove liquids, inspect pipe walls, apply coatings, separate product batches, and perform dozens of other maintenance and operational functions. The word “pig” comes from the squealing sound early cleaning devices made as they scraped through pipes.
The propellant gas or liquid that pushes the pig through the line is a critical part of the pigging operation. In natural gas pipelines and during post-construction commissioning activities, nitrogen is the preferred — and often required — propellant.
Why Use Nitrogen to Propel Pigs?
Nitrogen is preferred over compressed air or product gas for pig propulsion in several specific situations:
Post-Hydrotest Dewatering
After a hydrostatic pressure test, the pipeline is full of water. Foam pigs are launched to push bulk water out of the line. Nitrogen — not compressed air — is used to propel these pigs for the same reason nitrogen is used for all post-hydrotest work: compressed air introduces oxygen into a system that will soon receive hydrocarbon gas. Nitrogen keeps the system inert throughout the dewatering process.
Gas Pipeline Maintenance Pigging
On operating natural gas pipelines, pigs are routinely launched using the pipeline’s own gas pressure as the driving medium — this is standard practice for in-service inspection pigs and scraper pigs on transmission lines. However, for maintenance pigging on sections that have been isolated and de-pressurized, nitrogen provides a convenient, clean, controllable pressure source for pig launching without bringing the system back to full operating gas pressure during maintenance.
Liquid Hydrocarbon Pipelines
On crude oil and NGL pipelines, nitrogen can be used to propel pigs when product is not available (during commissioning or pre-startup operations) or when a clean, non-contaminating drive medium is required between different product batches.
New Pipeline Commissioning
During the commissioning of a new pipeline, multiple foam pig runs may be performed with nitrogen to clean mill scale, construction debris, and water from the pipe interior before the drying and purging phase begins. Nitrogen maintains the inert pre-commissioning atmosphere while providing the pressure needed to move pigs through the line.
Types of Pigs Used with Nitrogen Drive
Foam Pigs
Open-cell polyurethane foam pigs are the most common type used for post-hydrotest dewatering and pre-commissioning cleaning. They compress slightly to seal against the pipe wall, sweep liquids and soft debris ahead of them, and are highly tolerant of variations in pipe diameter (bends, tees, weld beads). Foam pigs range from “bare” foam to foam covered in wire mesh, criss-cross wire, or abrasive material for more aggressive cleaning. Multiple foam pig runs of increasing firmness are common on construction pipe.
Gel Pigs
Gel pigs are cylindrical slugs of a cross-linked polymer gel injected directly into the pipeline. They flow as a viscous plug that sweeps the pipe bore. Gel pigs are excellent at removing water from low spots and complex geometries where foam pigs leave residual moisture. They are particularly effective in post-hydrotest dewatering of sections with elevation variation or dead legs. Nitrogen propels the gel slug through the line at controlled flow rate.
Mandrel Pigs (Scraper Pigs)
Mandrel pigs have a central steel body with polyurethane cups or discs that seal against the pipe wall. They are more rigid than foam pigs, more effective at removing wax, scale, and hard deposits, and can be fitted with magnetic or ultrasonic sensors for inline inspection. Nitrogen can propel mandrel pigs through sections of pipeline that have been isolated for maintenance.
Intelligent Pigs (ILI Tools)
Inline inspection (ILI) tools — commonly called smart pigs or intelligent pigs — carry sensors (magnetic flux leakage, ultrasonic, or geometric) that detect metal loss, corrosion, dents, and weld anomalies. Most ILI runs on gas pipelines are propelled by the pipeline’s operating gas pressure. Nitrogen propulsion is used for ILI tools in pre-commissioning scenarios where the line has not yet been gassed up, or for tool retrieval from isolated sections during maintenance.
Nitrogen Pig Launching Procedure
- Pig launcher inspection: Confirm the launcher barrel is clear, the pig is the correct size and specification, and all instrumentation (launch indicator, pressure gauges) is in place.
- Load the pig: Insert the pig into the launcher barrel with the correct orientation (pigs are directional). Close and torque the launcher door.
- Connect nitrogen supply: Connect the nitrogen trailer to the pig launcher inlet connection.
- Pressurize the launcher: Build nitrogen pressure behind the pig to the required differential. The pressure needed to launch a pig depends on pig type, pipe diameter, grade, and any back-pressure from liquid head at the receiver end.
- Open the kicker valve: Open the launcher kicker valve to allow nitrogen to pressurize behind the pig and push it into the mainline.
- Monitor pig tracking: Use acoustic pig signallers, magnetic pig detectors, or flow monitoring at the receiver to track pig progress and confirm arrival.
- Receive and retrieve: When the pig arrives at the receiver, reduce pressure, confirm zero pressure at the door, open and retrieve.
Calculating Nitrogen Volume for Pig Propulsion
For a dewatering pig run, the nitrogen volume required is approximately equal to the pipeline volume between the pig and the leading water slug — which at the start of dewatering is essentially the full pipeline volume. At near-atmospheric discharge (venting dewatered sections to atmosphere), the nitrogen needed is approximately 1–2 pipe volumes to maintain forward pig velocity through the full pig run.
For pig runs at elevated pressure (maintenance pigging with back-pressure), the nitrogen volume required must account for the pressure at which the pig is moving — Boyle’s Law applies, and significantly more standard cubic feet are needed to maintain pressure at the pig face throughout the run. Use the HydroTech Pocket Engineer app to calculate nitrogen volumes for pig propulsion operations.
NitroTech Pipeline Pigging Support
NitroTech provides nitrogen for pipeline pigging operations as part of our complete commissioning and pipeline services. Our field supervisors coordinate pig launches, monitor flow rates during pig runs, and integrate with your operations team through pig arrival at the receiver. We support dewatering pig runs, pre-commissioning cleaning runs, and maintenance pigging across all major U.S. basins. Learn more about our pipeline services or request a quote.