Magnetic Pigs: What They Are and How They’re Used?
When it comes to maintaining safe, efficient, and inspection-ready pipelines, removing ferrous debris is one of the most overlooked yet critical steps. Small fragments of welding rods, mill scale, or rust particles left behind after construction or normal operations can compromise inspection data, damage internal coatings, and cause unnecessary downtime. That’s where magnetic pigs come in — specialized pipeline cleaning tools designed to collect and retain magnetic debris before it becomes a problem.
At American Pipeline Solutions (APS), magnetic pigs are an integral part of our broader Cleaning & Conditioning and Inspection & Integrity Support services. We use them to prepare lines for in-line inspection (ILI), maintain flow efficiency, and support integrity management programs across oil, gas, water, and industrial systems nationwide.
What Is a Magnetic Pig?
A magnetic pig is a type of cleaning or conditioning tool equipped with high-strength magnets — usually neodymium or rare-earth types — that attract and capture ferrous materials inside a pipeline. The magnets are either mounted in boxes on a steel mandrel body or molded into polyurethane cups and discs for smaller diameters.
As the pig travels through the line, driven by product flow or compressed medium, the magnetic field attracts and holds metallic particles like scale, rust flakes, welding slag, and stray hardware. When retrieved, these contaminants adhere to the pig body or magnet housings, keeping them out of downstream equipment and preventing interference with future inspections.
It’s important to distinguish magnetic pigs from smart pigs. Smart pigs, often called MFL (Magnetic Flux Leakage) tools, use magnetization to detect wall loss and corrosion during inspection. Magnetic cleaning pigs, on the other hand, use magnets to collect debris, not to measure defects. They serve different roles within the same integrity workflow.
How Magnetic Pigs Work?
Magnetic pigs operate on a simple but powerful principle — magnetic attraction. As the pig moves through the line, it creates a strong magnetic field that draws ferrous particles away from the pipe wall and holds them on the pig’s surface.
This process serves two key purposes. First, it removes debris that can restrict flow or damage equipment. Second, it improves ILI accuracy. If iron particles remain on the pipe wall, they can distort magnetic readings or trigger false corrosion signals during inspection runs. A properly executed magnetic pig run significantly improves data quality and helps integrity engineers make better-informed decisions.
Many magnetic pigs also support above-ground tracking. Because the pig body is magnetized, its location can be detected by an AGM (Above Ground Marker) or magnetometer as it passes, allowing technicians to verify progress and confirm arrival without interrupting operations.
When Magnetic Pigs Are Used?
Magnetic pigs can be applied in nearly every stage of a pipeline’s lifecycle, but they deliver the most value in three scenarios:
1. Pre-Commissioning and Post-Construction
After welding, hydrotesting, or drying, pipelines often contain metallic residues — small offcuts, welding slag, or mill scale — that can pose long-term risks if left behind. A magnetic pig pass collects these remnants efficiently, protecting valves, meters, and inspection tools downstream.
APS routinely incorporates magnetic runs into our commissioning programs, especially for newly built or rehabilitated lines, to ensure a clean and safe transition to service.
2. Routine Operations and Maintenance
During normal operations, corrosion and wear can still generate ferrous debris. Over time, this material accumulates in low spots or slow-flow sections, contributing to pressure drop and inconsistent throughput. Periodic magnetic pig runs keep these systems clear, extend cleaning intervals, and prevent issues before they escalate.
3. Pre-Inspection Preparation
Before deploying a high-value MFL or UT inspection tool, it’s crucial to remove any remaining magnetic debris that could interfere with the sensors. APS integrates magnetic pigs into inspection readiness sequences to deliver the cleanest possible line environment, ensuring accurate readings and efficient inspection scheduling.
Common Configurations and Sizes
Magnetic pigs come in multiple configurations depending on the pipeline’s design and the task at hand.
Mandrel-body pigs are most common for diameters above 6 inches. They feature steel cores fitted with polyurethane cups or discs for sealing and drive, along with magnet boxes bolted to the body.
Foam or urethane pigs with embedded magnets are used in smaller or more complex systems where flexibility and tight bends are a concern.
Typical sizes range from 4 inches up to 56 inches, with magnet strength and placement tailored to the line’s diameter, wall material, and debris profile. APS evaluates these parameters during project scoping to choose the configuration that captures the most debris with minimal friction or risk of hang-up.
Magnetic Pigs vs. Other Cleaning Pigs
Magnetic pigs are part of a larger family of cleaning tools, each designed for a specific function. Brush pigs use steel or wire brushes to remove scale and hardened deposits; cup or disc pigs provide strong sealing for liquid displacement and scraping; foam pigs help dry or displace product after testing.
A magnetic pig doesn’t replace these tools — it complements them. In many projects, APS runs a cleaning sequence that might include a brush pig to loosen debris, followed by a magnetic pig to collect it, and finally a gauging or verification pig to confirm line clearance before inspection. This sequence provides the best combination of mechanical cleaning, debris removal, and verification.
Integration in a Typical Inspection Plan
For large-diameter pipelines preparing for in-line inspection, APS often deploys magnetic pigs as part of a defined pre-ILI workflow.
A standard sequence might look like this:
Initial mechanical cleaning: brush or scraper pig to remove bulk debris.
Magnetic pig run: collect ferrous particles dislodged by earlier passes.
Gauging pig run: confirm internal diameter and ensure obstruction-free passage.
Inspection tool run: MFL or UT tool collects data for analysis.
This structured approach delivers cleaner signals, reduces tool wear, and improves overall inspection efficiency.
Tracking and Locating Magnetic Pigs
Because of their inherent magnetization, magnetic pigs are easy to track. As the tool moves through the pipeline, it generates a magnetic signature that can be detected by Above Ground Markers (AGMs) or handheld magnetometers. Field crews use these readings to confirm the pig’s progress, identify any delays, and validate arrival times at each trap.
Tracking also adds a layer of safety and operational control — especially in multi-segment lines or areas with elevation changes — by ensuring that the pig is accounted for at all times during the run.
Benefits of Magnetic Pigs
Incorporating magnetic pigs into a maintenance or inspection plan offers multiple advantages:
Improved flow efficiency: removing metallic debris restores the internal diameter and consistent flow.
Enhanced ILI accuracy: clean walls lead to better magnetic flux readings and fewer false indications.
Reduced equipment damage: prevents abrasive particles from damaging valves, meters, or sensors.
Cleaner operations: debris collection reduces contamination risk during product changeovers.
Regulatory compliance: documented cleaning and debris reports strengthen integrity and safety records.
For operators managing extensive pipeline networks, these benefits translate into fewer disruptions, more reliable inspection data, and lower total maintenance costs.
Limitations and Considerations
Like any tool, magnetic pigs must be used correctly to deliver the expected results. They are not designed to replace aggressive mechanical cleaning when heavy scale or hardened deposits are present. In such cases, APS typically precedes the magnetic pass with brush or bi-directional pigs to break up material first.
Operators should also account for coating condition and pipeline geometry. Poorly bonded coatings can be damaged by strong magnetic contact, and tight bends or internal features can trap heavily loaded pigs. APS evaluates these risks during scoping and includes control measures such as differential pressure limits, bypass ports, and real-time tracking.
After each run, collected debris must be removed and disposed of safely. Because magnets retain the particles strongly, crews follow strict cleaning and handling procedures to avoid injuries or contamination.
How to Select the Right Magnetic Pig?
Selecting the correct magnetic pig depends on multiple factors: pipeline diameter, geometry, product type, debris characteristics, and inspection objectives. APS engineers evaluate these variables as part of every cleaning and inspection plan, ensuring each tool is optimized for the line’s design and condition.
We also consider the expected debris load and propulsion method — whether the pig will be driven by liquid, gas, or air — and confirm compatibility with AGM tracking systems when location verification is required. This process helps prevent delays and ensures that each run delivers the expected outcome safely.
Frequently Asked Questions
Are magnetic pigs the same as smart pigs?
No. Smart pigs collect data using magnetic flux leakage (MFL) technology, while magnetic cleaning pigs use magnets to remove ferrous debris. They are often used in sequence — magnetic cleaning first, smart pig inspection second.
Can magnetic pigs damage internal coatings?
When used correctly and selected based on coating type and line condition, magnetic pigs do not damage coatings. APS assesses coating adhesion and wall condition before recommending magnetic cleaning.
What sizes are available?
Standard magnetic pigs cover diameters from 4 to 56 inches, with custom designs available for larger or specialized applications.
Can magnetic pigs be tracked above ground?
Yes. Their magnetic signature can be detected by AGMs or magnetometers, allowing real-time confirmation of the pig’s position during a run.
Do I still need brush or foam pigs?
Often yes. Magnetic pigs target ferrous debris, while brush or foam pigs remove organic or non-magnetic materials. APS typically uses a combination for complete cleaning.
Where do magnetic pigs fit in an ILI preparation plan?
They are generally run after mechanical cleaning and before the inspection tool to maximize ILI accuracy and protect sensors.
Partner with APS for Pipeline Cleaning and Inspection Support
Magnetic pigs are just one part of a comprehensive pipeline cleaning and inspection program. The key to success lies in selecting the right tools, sequencing them properly, and documenting each step to meet both operational and regulatory standards.
At American Pipeline Solutions, we combine decades of field experience with the latest technology to deliver reliable, safe, and efficient pipeline services. Whether you need a one-time magnetic cleaning run, a full inspection-readiness program, or ongoing maintenance support, APS provides the expertise, equipment, and documentation to keep your system flowing and compliant.
Contact APS today to discuss your upcoming project or request a quote
