Pigging Operations

Gauge pigs, and reading the plate on receipt

A gauge plate returns as either intact or bent. Crews who record only which one have thrown away most of the information.

What the plate is for

A soft aluminium disc, cut to a percentage of nominal internal diameter and carried on the front of a mandrel pig. Anything in the bore smaller than the plate deforms it.

The purpose is to prove that the line is clear enough for what comes next — most often an in-line inspection tool, which is expensive to run and easy to damage.

Sizing

The plate is sized as a percentage of internal diameter, commonly in the 90 to 95 percent range depending on what the run must prove and what the following tool requires.

Undersize the plate and it passes everything, proving nothing. Oversize it and it bends on ordinary bend ovality, which tells you nothing either. The sizing is a decision, and it should be recorded with its reasoning.

Reading the deformation

Uniform reduction around the circumference. A general bore restriction — scale, wax or deposit.
Localised bend at one point. A dent, a projecting weld or a partly closed valve. The angular position tells you where on the clock.
Smooth symmetric ovality. Often ordinary bend ovality rather than a defect.
Scoring and gouging. Debris, or contact with something sharp.

The clock position is the part that gets discarded most often, and it is what allows the finding to be located rather than merely reported.

Record it properly

Photograph the plate against a scale, with the orientation marked before it is removed from the pig. Record which pig, which run, which direction, and the receipt time.

A plate photographed loose on a tailgate has lost its orientation and therefore most of its value.

Where it sits in the sequence

Gauging follows cleaning and precedes the tools that matter. Running a gauge pig through a line that has not been properly cleaned produces a bent plate that tells you about the debris you already knew about.

Order the runs so that each one answers a question the previous run could not.

Standing noteThis is educational reference material, not engineering advice. Regulations are revised continually; the editions in force at the time of a project govern, and every project requires its own qualified engineering analysis and the operator’s governing specifications.

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