Pipelines are cleaned with line scrapers forced through the line by the liquid being pumped. Intervals between cleanings vary with the size of the pipe and the type of liquid. A drop in the flow rate, the continual presence of dirt, rust, or particulate in basket strainers, and or shortened filter life may indicate a need for cleaning. Batching pigs are used to separate fuels and prevent contamination. Treatment of batching pigs is the same as for line scrapers. Water slugs are not permitted to separate batches.
Decide on the scraper best suited for the operation. Check specifications to be sure it will pass through all valves and bends. Keep accurate records of the time the scraper is started and quantity of fuel pumped to trace the progress of the scraper and find the time of its arrival at the receiving station. It is good practice to bypass meters while scraper sediment is in the line. The scraper should be run at the minimum velocity (3.2 kilometers per hour [two miles per hour]) with no shutdowns while the scraper is in the line. Shutdowns will permit the scrapings to settle in front of the scraper, causing it to become stuck (this usually requires cutting the line to retrieve it).
Several methods are used to find scrapers stuck in lines. The knife-type scrapers make sufficient noise to be followed by line walkers. Brush-type scrapers are relatively silent and require a transmitting device to reveal their exact location. Their general location can be found from the time and quantity of fuel pumped before the stoppage occurred. Special devices include:
1. Noisemakers fastened to the scraper.
2. Directional antennas.
3. Radioactive material that can be found with a Geiger counter.
4. Magnetized core in the scraper that can be detected with a magnetometer.