Note: General information on in-line inspection can be found in GRI Report 91/0365, "In-Line Inspection of Gas Transmission Lines" ^[Crouch92]. This section provides information specifically relevant to MFL in-line inspection.

An MFL tool is inserted into a pressured pipeline through a pig launcher as shown below or through a pipeline spool piece. A pig launcher is typically a pipe attached to the main run pipe. Valves isolate the launcher from the line pressure when the launcher is opened and the tool installed. After the tool is in place, a set of valves is used to pressurize the launcher. Then gas pressure moves the tool from the launcher into the run pipe. After the tool is in the run pipe, flowing gas propels the tool through the line.

Running an MFL tool is similar to running any other maintenance or inspection tool. The tool moves through the line with the pressurized, flowing gas. While the tool is running, the velocity of the flowing gas is usually reduced from normal pipeline conditions to the range required by the tool. The velocity of a tool varies at bends, valves, tees, heavy wall sections, and elevation changes. Control of the gas and tool velocity is important in providing good inspection results.

Tool position can be monitored during the run with in-line or external sensors. Monitoring the tool's position is important in the event that a tool becomes stuck. Here, external monitors that sense when a tool has passed can be used to bracket the location where the tool is stuck.

Retrieving an MFL tool is similar to launching, and a pig receiver is similar to a pig launcher. The tool is stopped with a bypass flow system or with valves when it approaches the receiver. Sensors mounted through the pipe wall detect the arrival of the tool. Valves allow gas pressure from the line to move the tool into the receiver. After the tool is in place, the receiver is isolated from the line pressure and the pressure in the receiver is released. Finally, the receiver is opened and the tool removed.

After a tool is removed from a receiver, the inspection data record is removed. The tool is inspected to verify that all components were in working condition at the end of the run. In addition, some of the data often are examined to determine whether the tool operated successfully throughout the run.