The mill pressure test, or hydrostatic test, samples pipe quality. Pipe integrity is tested by sealing a segment of pipe, filling it with water, and systematically raising the water pressure as specifications require.
The mill test decreases the likelihood of hazardous spills in environmentally sensitive areas, as marine pipelines used to transport petroleum-based products are regularly tested. Optimum test pressures are calculated by taking into account the current status of the pipe, maximum operating pressures and applicable federal, state, and local regulations.
A segment of pipe is designated to be tested and is cleared of air to ensure accurate results. Valves are sealed to control water pressure, and whether the pipe is above or below ground is factored into determining test procedures. Additional factors include pipe lengths, locations of fittings and valves, and pipe diameters.
Test results center around whether all water was contained within the pipeline during a mill test. The leading indicators that water was lost are unforeseen changes in water pressure, temperature, or volume.