9.3 Pigging, Cleaning & Drying
Key Takeaways
After the hydrotest and before turnover, pigs are run through the line to remove water and debris — cleaning pigs (brush or scraper), gauging pig, and drying pigs (foam) — using a pig launcher and receiver
Drying methods include air blow, vacuum drying, foam-pigging, and a methanol sweep for gas lines; the method is selected to meet the moisture specification for the service
Dewatering and drying are especially critical for gas lines because residual water forms hydrates (ice-like solids) with natural gas at operating pressure and temperature and accelerates internal corrosion
The inspector verifies the pigging sequence, the gauge plate condition after each run, drying to the specified moisture dew point or criterion, and the complete pigging and drying documentation
API does not publish official domain weights; pigging, cleaning, and drying fall under Back-End Construction (BOK area 4) with an unofficial practice estimate near 22%
Pigging, Cleaning & Drying
Quick Answer: After the hydrotest and before turnover, the pipeline is pigged to remove test water and construction debris, then dried to the moisture specification for its service. Cleaning pigs (brush or scraper), a gauging pig, and drying pigs (foam) are launched from a pig launcher and received in a pig receiver. For gas lines, drying is especially critical because residual water forms hydrates (ice-like solids) with natural gas and drives internal corrosion.
Why Pig After a Hydrotest?
A hydrotest leaves the pipeline full of water and any debris dislodged during the test — weld slag, dirt, rocks, tools, and scale. Turning the line over to service in that condition would carry that water and debris into the product stream, foul valves and meters, and — in a gas line — create hydrates and corrosion. Pigging, cleaning, and drying are the bridge between a successful hydrotest and a line ready for product.
Pig Types and Their Post-Hydrotest Roles
| Pig Type | Construction | Purpose in Post-Hydrotest Sequence |
|---|---|---|
| Cleaning pig (brush) | Poly body with wire or nylon brushes | Remove weld slag, scale, and loose debris from the pipe wall |
| Cleaning pig (scraper/cup) | Poly body with urethane cups and scraper blades | Scrape stubborn deposits and push free water ahead |
| Gauging pig | Pig carrying a gauge plate sized near pipe ID | Confirm the bore is clear and round after the test (see 9.2) |
| Foam pig (swab) | Open-cell polyurethane foam cylinder | Push remaining water out and absorb moisture; repeated runs dry the line |
| Sphere | Inflatable elastomeric sphere | Batch and displacement duties, often in liquid lines |
| Smart pig (ILI tool) | Instrumented pig with sensors | Not typically a post-hydrotest tool; used for in-line inspection of in-service lines |
The post-hydrotest pigging sequence typically runs: cleaning pig(s) → gauging pig → foam (drying) pigs, repeated as needed until the receiver shows no debris and the foam pigs return dry.
Pig Launcher and Receiver
Pigs are introduced through a pig launcher at one end of the section and removed through a pig receiver at the other. Both are pressure-rated vessels with a barrel larger than the pipe ID so the pig can be loaded or removed, kicker lines to drive the pig, and isolation valves. The inspector verifies:
- The launcher and receiver are correctly installed and isolated before opening.
- The pig is loaded in the correct orientation (cups/blades pointing in the direction of travel).
- The receiver is depressurized and vented before the pig is removed.
- No one stands in front of the open receiver barrel — a pig can exit with significant force if the receiver is inadvertently pressurized.
Drying Methods
Once the free water is pushed out by pigs, the line must be dried to the moisture specification for the service. The four common methods are:
| Method | How It Works | Typical Use |
|---|---|---|
| Air blow (dry air purge) | Forced dry, filtered air is blown through the line until the outlet dew point meets spec | General service; simple and widely used |
| Vacuum drying | The line is evacuated with a vacuum pump; water boils off at low pressure | Lines with long or complex profiles where air flow is uneven |
| Foam-pig drying | Repeated foam pig runs absorb residual moisture until the pigs return dry | Supplement to air blow or vacuum; polishing step |
| Methanol sweep | A batch of methanol is pushed through the line to absorb water, then displaced | Gas lines, especially where hydrate risk is high and dew-point drying is insufficient in the available time |
The acceptance criterion is usually a dew point (e.g., a specified dew point at line pressure) or a moisture content limit in the project specification. The inspector verifies the criterion, the method, and the recorded dew-point or moisture readings.
Why Gas Lines Demand More
In a gas line, residual water combines with natural gas at certain pressures and temperatures to form hydrates — ice-like crystalline solids that can block valves, restrict flow, and damage equipment. Water also drives internal corrosion, particularly in the presence of CO2 or H2S. For these reasons, gas-line drying specifications are typically far more stringent than liquid-line specifications, and a methanol sweep is often used in addition to air blow or vacuum drying to suppress hydrate formation during commissioning. The inspector confirms the drying method and acceptance criterion match the service (gas vs. liquid) and the project specification.
Dewatering to Moisture Spec
"Dewatering" covers both removing the bulk test water and drying the residual moisture. The sequence is:
- Bulk removal — run cleaning and foam pigs to push the free water out through the receiver.
- Drying — apply the selected drying method (air blow, vacuum, foam, methanol sweep) until the outlet meets the dew-point or moisture criterion.
- Verification — record dew-point or moisture readings at intervals until the criterion is met and stable.
- Documentation — record the pigging sequence, pig runs, debris observed, drying method, and final moisture readings in the turnover package.
Inspector Verification and Documentation
The API 1169 inspector's post-hydrotest pigging and drying responsibilities are to verify:
- The pigging sequence was followed (cleaning → gauging → drying).
- The gauge plate condition after each gauging run (see 9.2).
- The drying method and acceptance criterion match the service and project spec.
- The final moisture or dew-point readings meet the criterion and are stable.
- The pigging and drying records are complete and included in the turnover package.
- The launcher and receiver were operated safely (isolation, venting, no personnel in front of the open barrel).
A line is not ready for turnover until it is clean, dry to spec, and documented. A common field finding is a line turned over with foam pigs still returning damp — the drying criterion has not been met, and the turnover is premature.
Why is drying a gas pipeline to a stringent moisture specification more critical than drying a liquid pipeline?
Liquid pipelines do not contain any water after the hydrotest
Residual water in a gas line forms hydrates (ice-like solids) with natural gas and drives internal corrosion, especially with CO2 or H2S present
Gas lines operate at lower pressures so water is harmless
Water improves the flow characteristics of a gas line
What is the correct orientation for loading a pig into a launcher, and what safety rule applies at the receiver?
Load the pig with cups/blades pointing opposite the direction of travel; stand directly in front of the open receiver barrel to catch the pig
Load the pig in any orientation; the receiver can be opened while pressurized
Load the pig with cups/blades pointing in the direction of travel; no one stands in front of the open receiver barrel because a pig can exit with significant force if inadvertently pressurized
Pigs do not have an orientation; receivers do not require isolation
Which drying method is most commonly associated with gas-line commissioning where hydrate risk is high and dew-point drying alone may be insufficient in the available time?
Air blow with no dew-point target
Vacuum drying only
Methanol sweep, in which a batch of methanol is pushed through to absorb water and then displaced
Leaving the line full of test water indefinitely
A foam pig is run repeatedly through a line after bulk water removal. What indicates that the line has met the drying criterion?
The foam pig returns the same color it was loaded
The pig receiver is full of water
The pig launcher will not open
The foam pigs return dry, and the recorded dew-point or moisture readings meet the specified criterion and are stable
Which post-hydrotest pigging sequence is correct?
Foam drying pigs, then cleaning pigs, then gauging pig
Gauging pig, then foam drying pigs, then cleaning pigs
Cleaning pig(s), then gauging pig, then foam drying pigs, repeated as needed until the receiver shows no debris and the foam pigs return dry
A single smart pig run, with no cleaning or drying
Sections you finish are checked off in the contents.