Pigging Operations
Cleaning runs before a hydrostatic test, and how to know when to stop
Cleaning is the phase with the least glamour and the most influence on whether the schedule holds.
Why clean before a test at all
Debris left in a line does three things to a pressure test: it fouls the fill, it obstructs the instruments and drains, and it makes the dewatering and drying that follow far longer than planned.
On a new-construction line, the debris is construction debris — weld slag, dirt, water, and whatever was left in the pipe before it went in the ditch.
Progression
Cleaning is a progression, not a single run. Typically bare or lightly-fitted pigs first to sweep bulk debris, then brush-fitted pigs to work the wall, then a proving run.
Each pass should be less eventful than the last. If run four returns as much as run two, something is wrong with the assumption rather than with the pigging.
The stopping criterion
This is where crews and operators disagree, and the disagreement is nearly always because no criterion was written down.
- Debris returned by mass or volume, falling below an agreed threshold
- Two consecutive runs returning comparable and acceptable quantities
- A clean gauge plate at the agreed percentage
- Whatever the following tool vendor requires, in writing
What the returns tell you
Weigh and photograph what comes out. Mill scale, construction debris, water and product residue each say something different about the line’s history and about what the drying phase will demand.
Heavy water return on a supposedly dry line is a schedule warning worth raising immediately.
Cost of getting it wrong
An under-cleaned line does not usually fail the pressure test. It fails the drying acceptance weeks later, after the water is already in and the schedule is already committed.
That is the expensive way to discover that two more cleaning runs would have been cheap.
From Proper Pigging, the hydrostatic and nitrogen test field book, and the HydroTech certification courses built on it.