What is hydrostatic testing?

Hydrostatic testing is the way a pipeline, a vessel, or a section of piping proves it can hold pressure before it is trusted with product. The line is filled with water, pressurized above its intended operating pressure, and held there while the pressure and temperature are recorded. If the pressure holds within the allowed tolerance for the required time, the test passes. If it does not, there is a leak or a weak point, and it has to be found.

Why water

Water is nearly incompressible. That matters for safety: if a line fails under a hydrostatic test, the stored energy is small and the water simply runs out. The same failure under compressed gas would release far more energy. That is why hydrostatic testing is the default for new construction and for proving repairs, and why pneumatic tests are reserved for cases where water cannot be used.

How a hydrotest runs

• Fill. The section is filled from a water source through a fill pump, with pigs or vents used to push air out ahead of the water. Trapped air makes a test unreliable, so a clean fill matters.
• Pressurise. A high-pressure pump raises the section to the test pressure, which is set by code and by the design of the line, typically a multiple of the maximum allowable operating pressure.
• Stabilise. Pressure and temperature are allowed to settle. Water expands and contracts with temperature, so a line warming in the sun can look like it is gaining pressure and a line cooling overnight can look like it is losing it.
• Hold and record. The test period, often four to eight hours for a strength and leak test, is recorded on calibrated instruments: a chart recorder or a digital logger, a deadweight tester for the reference pressure, and temperature probes on the pipe and in the air.
• Dewater and dry. After a pass, the water is pushed out, usually with pigs and compressed air or nitrogen, and the line is dried to the specification the product requires.

What decides the cost

Length and diameter set the water volume, which sets fill time and disposal. Elevation change adds pressure at the low points and has to be accounted for in the test plan. Access, water availability, and what has to happen to the water afterwards are usually the biggest variables. A crew’s equipment and experience decide how many test sections a day it can turn.

Related terms: nitrogen purging, pigging, dewatering and drying.