When a pig disappears into a pipeline it becomes invisible - the crew launching it has no direct view of where it is, how fast it is moving, or whether it is still moving at all. A pig tracking transmitter is the small device carried inside or attached to the pig that solves this, radiating a signal an above-ground crew can detect through the pipe wall and the soil above it. This guide explains how magnetic and extremely-low-frequency transmitters work, how above-ground markers and pig-passage indicators report a pig's progress, and why that timing information matters for cleaning runs and for finding a pig that has stopped.
Pig Tracking Transmitter in one line: A pig tracking transmitter is a battery-powered device mounted in or on a pipeline pig that emits a magnetic or extremely-low-frequency (ELF) signal, letting crews with above-ground receivers detect the pig as it passes each location along the line. Combined with fixed pig-passage indicators at launchers, receivers, and valve stations, it tells operators where the pig is, when it passes each point, and how fast it is travelling. That location and timing data is essential for pacing a run, confirming the pig arrived, and locating a stuck pig.
Two signal types dominate pig tracking. A magnetic transmitter uses a strong permanent or driven magnet whose field a handheld or vehicle-mounted receiver picks up as the pig passes directly beneath it; it is simple and battery-free in its passive form but has a short detection range, so a crew has to be almost on top of the line to catch the pass. An extremely-low-frequency transmitter is the more capable option: it radiates a low-frequency electromagnetic field, typically around 22 hertz, that penetrates soil, water, and even concrete far better than higher frequencies, so a receiver can detect the pig from a greater distance and through cover that would defeat a magnetic pass. Both are packaged to survive the pressure, vibration, and impacts inside a live line.
In the field, tracking is a coordinated exercise. Crews position above-ground markers - the receiver stations - at known points along the route, often at road crossings, valve sites, or wherever the pig's arrival needs to be timed. As the pig approaches, the receiver's tone or display rises to a peak directly over the transmitter and falls as it passes, giving the crew a precise passage time and confirming the pig is on the move. By logging passage times at successive markers and knowing the distance between them, the crew calculates the pig's speed and predicts its arrival at the next station, which lets them stage themselves ahead of it rather than chase it.
Fixed pig-passage indicators, sometimes called pig signalers, complement the mobile crew. These are mechanically or magnetically triggered devices mounted on the pipeline - often at the launcher, the receiver, and intermediate valve stations - that trip when the pig physically passes, giving a discrete local flag and, when wired in, an electrical contact. Unlike a transmitter that must be actively received, a passage indicator sits permanently at its point and reports every pass, which makes it the natural feed into automated monitoring.
During a routine run, tracking turns a blind operation into a managed one. Knowing the pig's speed lets the crew throttle flow to keep it within a safe velocity band - too fast and a cleaning pig can skip over deposits or damage itself, too slow and it may stall in an uphill section or against a heavy slug of liquid it has swept ahead. Passage times at each marker also confirm the pig is still travelling and has not hung up, so a run that goes quiet between two stations is an early warning rather than a surprise discovered only when the pig never reaches the receiver.
The stakes rise sharply when a pig stops. A stuck pig can block flow, trap a growing column of liquid or gas pressure behind it, and cost days of production while it is located and freed. Tracking is what narrows the search: the last marker that registered a pass and the first that did not bracket the pig between two known points, and a crew can then walk that segment with a receiver to pinpoint the transmitter's position before excavating or applying differential pressure to push it free. Without a live transmitter, the same search can mean guessing along miles of buried line.
Tracking data also feeds the decision of how hard to push. Operators watching the launcher and receiver pressures alongside the pig's last known position can judge whether a stalled pig is likely to move with a modest increase in differential pressure or whether pushing harder risks over-pressuring the line. That combination of position and pressure, read together, is the core of a controlled stuck-pig response rather than a blind escalation.
Pig-passage indicators and receiver-mounted signalers produce exactly the kind of discrete event a SCADA system is built to capture: a contact closes the instant the pig passes, and that transition can be time-stamped, logged, and turned into an alarm or a notification. Wiring these signalers into a remote terminal unit at each instrumented site lets an operator watch a run unfold from a control room or a phone rather than relying solely on radio calls from field crews, and it creates a permanent record of when the pig cleared each station.
A cloud SCADA such as Merobix reads these passage contacts along with the launcher and receiver barrel pressures, the mainline pressure, and flow, so an operator sees the whole run on one screen: the pig passing station three at a recorded time, the differential pressure across it, and the flow that is driving it. Rules can flag the abnormal cases automatically - a passage that is overdue against the expected travel time, or a barrel pressure that climbs without the corresponding downstream passage, both of which suggest the pig has slowed or stalled.
For an operator running gathering lines across a wide field, this remote visibility is what makes lean-crew pigging practical. Rather than staffing every station, the operator can dispatch a mobile receiver crew to the one segment where the SCADA event trail shows the pig went quiet, using the last confirmed passage as the starting point. The transmitter still does the physical locating on the ground, but the SCADA record of passage events tells the crew where to go and when the problem began.
A magnetic transmitter relies on a strong magnet whose field a receiver detects only at close range, so a crew must be nearly directly over the line to catch the pass. An extremely-low-frequency (ELF) transmitter radiates a low-frequency electromagnetic signal, commonly around 22 hertz, that penetrates soil, water, and concrete far better and can be detected from a greater distance. ELF is generally preferred for buried or submerged lines where a magnetic signal would be too weak to find reliably.
The last above-ground marker that registered the pig passing and the first one that did not bracket the pig between two known points. A crew then walks that segment with a receiver tuned to the transmitter's signal, watching for the tone or reading to peak directly over the pig's position. Once pinpointed, the location guides whether to excavate, apply differential pressure to push the pig free, or take another recovery step.
A pig-passage indicator, or pig signaler, is a device on the pipeline that trips mechanically or magnetically when the pig passes, and it can provide an electrical contact in addition to its local flag. That contact wires into a remote terminal unit, which time-stamps the passage and sends it to SCADA as a logged event or alarm. This lets operators track a run remotely and creates a permanent record of when the pig cleared each instrumented station.
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