What is HYDROSTATIC LEAK TESTING ? Leak Test for Fluids and Pressures

Hydrostatic (Hydro) Testing is a process where components such as piping systems, gas cylinders, boilers, and pressure vessels are tested for strength and leaks It can be done on your home’s sewer line to see if any leaks are present.

A hydrostatic test involves filling a system with a liquid. Once the system is filled with liquid, it’s pressurized to test the system’s integrity. The next step is observation. A drop in the liquid level is a clear sign the liquid is coming out somewhere, even if no one can see the leak.Hydro tests are often required after shutdowns and repairs in order to validate that equipment will operate under desired conditions once returned to service.

Although hydrostatic testing is considered to be a nondestructive testing method, equipment can rupture and fail if the inspection exceeds a specified test pressure or if a small crack propagates rapidly.

In these article we will learn about some leak tests for Fluids and Pressures :-

TEST FLUID :-
1) The test fluid shall be water unless there is a possibility of damage due to freezing, or if the process or piping material would be adversely affected by water. In that case other suitable test fluids may be used. Special precautions are required if the test fluid is toxic or flammable.

2) The temperature of the test fluid should be no less than 40F in piping systems subject to brittle fracture (i.e. carbon steel).

3) If the test fluid temperature produces condensation on the piping exterior surface, the water shall be heated to a temperature above the dew point or the test shall be postponed to a time when the dew point temperature has changed sufficiently such that condensation will not occur on the piping exterior surface.

4) Material and test water temperature shall be approximately equal prior to pressurizing the system.

5) High points in the system shall be vented so that air will be displaced while the system is being filled with the test fluid.

6) The operator shall take adequate measures to ensure that the piping system is not over-pressurized during hydrostatic testing. Adequate measures include a relief valve, or a dedicated operator to monitor pressure, or dual pressure regulators, etc.

TEST PRESSURE :-
1) The minimum hydrostatic test pressure for metallic piping shall be per the following equation.

PT = 1.5 x PD x ST/SD

where: PT = minimum test gage pressure

PD = internal design gage pressure

ST = allowable stress value at test temperature

SD = allowable stress value at design temperature

Note: The maximum allowable value of ST/SD is 6.5.

2) When a maximum test pressure is specified, the test pressure shall not exceed this amount.

3) When no maximum test pressure is specified, the test shall not be greater than 110% of the minimum.

4) When using water, static head due to differences in the elevation of the top of the piping system and the elevation of the test gage shall be accounted for in pressuring the piping system to be tested by the following equations:

SH (psi) = (HE - GE) x 0.433

PST = PT + SH

where: HE = high point elevation (ft)

GE = gage point elevation (ft)

SH = static head (psi)

PST = minimum test gage pressure corrected for static head

0.433 = conversion factor (ft of waster to psi)

5) Pressure gages should be connected directly to the piping. Calibrated pressure gages shall be used in all Code testing. Pressure gage range should exceed the intended test pressure by approximately double but in no case should the range be less than one and one-half (1 ½) times the test pressure.

HYDROSTATIC TESTING OF PIPING WITH VESSELS AS A SYSTEM :-

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 test pressure of the piping.
Where the test pressure of the piping exceeds the vessel test pressure and isolation is not considered practicable, the piping and the vessel may be tested together at the test pressure of the vessel, if approved by the design authority. The vessel test pressure must not be less then 77% of the piping test pressure.

EXAMINATION FOR LEAKS:-

Test personnel shall ensure the hydrostatic pressure is maintained for sufficient time to determine if there are any leaks. A minimum time of 10 minutes is required by Code. After the hydrostatic pressure time has been satisfied, all joints shall examined visually for leaks.
Examination shall be made of all welds and mechanical joints. There shall be no visible evidence of leakage. Welds and joints previously tested need not be examined for leaks.
Leakage detected in welded joints shall be repaired by draining, repair welding, non-destructively examining in accordance with original requirements, and re-tested to the original test pressure.
Mechanical joint leakage at permanent joints shall be repaired, examined in accordance with original requirements, and re-tested to the original test pressure.