Hydrostatic Testing

Hydrostatic (Hydro) Testing is a process where components, such as piping or pressure vessels are tested for strength and leaks by filling the equipment with pressurized liquid. For pipelines, hydro tests are conducted while the pipeline is out of service. All oil and/or natural gas is typically vented off, and the line is mechanically cleaned prior to testing.

Hydrostatic testing works by completely filling the component with liquid (usually, but not always, water), until a specific pressure is reached. The hydro test pressure often exceeds the designed working pressure of the equipment, sometimes by over 150%, depending on the exact regulations and code requirements, as applicable. The pressure is then held for a specific amount of time to inspect visually for leaks. The visual inspection can be enhanced by applying either tracer or fluorescent dyes to the liquid, as required or needed.

Hydrostatic testing is often required as a final proof test after repairs are completed and equipment is returned to service. While it can tell you whether or not leaks are present, a hydrostatic test does not necessarily ensure the integrity of the component beyond the time period of the actual test. On-going equipment integrity is best managed by an effective, integrated fixed equipment mechanical integrity (FEMI) program.

There are two additional methods of hydrostatic testing: water jacket testing and the direct expansion method. These are more often used for cylinders or vessels.

  • With water jacket testing, the vessel to be examined is filled with water, after which it is placed in a sealed container which is itself filled with water and connected to a calibrated gas tube. It is at this point that the vessel is pressurized for a period of time before being subsequently depressurized. Pressurizing the vessel forces water out of the test jacket and into the tube. Operators can then determine how much the vessel expanded. This method does cause some slight, yet permanent stretching to the vessel.
  • With the direct expansion method, the vessel being examined is completely filled with water. Then additional water is pumped in until it reaches the test pressure. The amount and weight of the water forced into the vessel, along with the amount not expelled from the vessel upon the release of the pressure allows the inspector to determine how much the vessel expanded.

Hydrostatic testing can be used to examine many different types of equipment, including pipelines, fire extinguishers, storage tanks, and gas cylinders. It is particularly useful for pipelines in situations where the use of inline inspection tools are not feasible.

Prior to conducting a hydrostatic test, one should consider the specific gravity and chemistry of the hydro test fluid both in terms of loads and corrosivity (e.g., chloride content of water), and how this may impact the equipment. For example, some equipment foundations and piping supports may not be designed to handle the loads. When hydrostatic loads are unacceptable, alternative test methods should be considered such as pneumatic testing or other gas leak testing. When using gasses (e.g., air or nitrogen), special caution should be paid to safety as gas pressurization results in significantly higher amounts of stored energy in the test subjects, which can result in catastrophic failures. It is best to use a customized procedure, created by competent personnel, for this type of testing.


Related Topics

In-line Inspection (ILI) Nondestructive Testing (NDT) Visual Inspection


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