Pigging Operations

Dewatering after a hydrostatic test, and why it decides the schedule

Dewatering is judged by what it leaves behind, not by how much water it removes. The residue is what the drying phase has to deal with.

The train

Bulk water is displaced by a train of pigs driven by air or nitrogen. The train is designed rather than assembled: sealing pigs to hold the interface, spacing to manage slugs, and enough elements that a single compromised seal does not lose the separation.

A single pig pushing a full line of water over rolling terrain will not hold the interface. The water finds its way past, and you finish with a wet line and a false sense of progress.

Terrain again

The same elevation profile that governed the test governs the dewatering. Low points hold water after the bulk has gone, and those pockets are exactly what the drying phase then has to evaporate.

Where the profile is severe, the number of passes goes up.

Disposal is a permit question, not a pump question

Test water carries whatever was in the line: mill scale, construction debris, corrosion products and any additives used. Discharge is permitted, and the permit conditions are set long before the water is in the pipe.

Where the water came from, and whether it may be returned there
What treatment is required before discharge
Sampling and reporting obligations
Where it goes if it fails a sample

The handover to drying

The dewatering phase should end with a stated condition, not merely with the pigs received. How much water is estimated to remain, where, and on what basis.

This is the handover crews skipDrying crews who are handed a line with no statement of residual condition are starting blind, and the first dew point readings then come as a surprise to everyone.

Why it matters commercially

Water, stabilisation and drying are the true schedule drivers on most commissioning programmes — not the eight-hour hold that everyone plans around.

Operators who understand that plan their tie-in dates differently.

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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From Proper Pigging, the hydrostatic and nitrogen test field book, and the HydroTech certification courses built on it.