Pipelines operating at low flow rates (around 2 ft/sec or less) may allow solids to settle in the pipe invert. PE has a smooth, non-wetting surface that resists the adherence of sedimentation deposits. If the pipeline is occasionally subject to higher flow rates, much of the sedimentation will be flushed from the system during these peak flows. If cleaning is required, sedimentation deposits can usually be flushed from the system with high pressure water. Water-jet cleaning is available from commercial services. It usually employs high pressure water sprays from a nozzle that is drawn through the pipe system with a cable. Pressure piping systems may be cleaned with the water-jet process, or may be pigged. Pigging involves forcing a resilient plastic plug (soft pig) through the pipeline. Soft pigs must be used with PE pipe. Scraping finger type or bucket type pigs may severely damage a PE pipe and must not be used. Usually, hydrostatic or pneumatic pressure is applied behind the pig to move it down the pipeline. Pigging should employ a pig launcher and a pig catcher. A pig launcher is typically a tee assembly or a removable spool. In the tee assembly, the main flow is into the tee branch and out through a run outlet. The opposite tee run outlet is used to launch the pig. The pig is fitted into the opposite tee run; then the run behind the pig is pressurized to move the pig into the pipeline and downstream. In the removable pipe spool, the pig is loaded into the spool, the spool is installed into the pipeline, and then the pig is forced downstream (Note - Fully pressure rated wyes suitable for pig launching are generally not available.) A pig may discharge from the pipeline with considerable velocity and force. The pig catcher is a basket or other device at the end of the line to safely receive or catch the pig when it discharges from the pipeline.