Hydrostatic testing is the most used leakage checking method for piping networks. ASME B31.3 Process piping code specifies criteria for hydrotesting in a process industry.

Test Fluid

As per ASME B31.3 Section 345.4.1, the test fluid shall be water unless there is the possibility of damage due to freezing or if water will casue adverse effect on the piping or the process. In that case another suitable nontoxic liquid may be used. If the used test liquid is flammable, its flash point shall be at least 49°C (120°F), and consideration shall be given to the test environment.

Test Pressure

As per ASME B31.3 Section 345.4.2 the hydrostatic test pressure at any point in a metallic piping system shall be as follows:

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(a) not less than 11⁄2 times the design pressure;
(b) for design temperature above the test temperature, the minimum test pressure shall be calculated by Eq.
(24), except that the value of ST/S shall not exceed 6.5:

\[P_T = \frac{1.5\times{P}\times{S_T}}{S}\]

where

P = internal design guage pressure.
PT = minimum test guage pressure.
S = stress value at design temperature (see Table A-1 of B31.3)
ST = stress value at test temperature.

If the test pressure as defined above would produce a nominal pressure stress or longitudinal stress in excess of the yield strength at test temperature, the test pressure may be reduced to the maximum pressure that will not exceed the yield strength at test temperature.

Hydrostatic Test of Piping With Vessels as a
System

As per ASME B31.3 Section 345.4.3 where the test pressure of piping attached to a vessel is the same as or less than the test pressure for the vessel, the piping may be tested with the vessel at
the piping test pressure.

Where the test pressure of the piping exceeds the vessel test pressure, and it is not considered practicable to isolate the piping from the vessel, the piping and the vessel may be tested together at the vessel test pressure, provided the owner approves and the vessel test pressure is not less than 77% of the piping test pressure
calculated in accordance with para. 345.4.2(b).