# What is hydrotest pressure and how to calculate the hydrotest pressure

Hydro Test Pressure

Hydro test is the technique in which pipes, pressure vessels, gas cylinders, tanks are tested  at required design pressure of the system or pipes or vessel to check the leak and strength.

How to calculate Hydro test pressure

Hydro-Test pressure

1  Hydro test pressure is 1.5 Times of Design Pressure.

2. For a design temperature above the test temperature, minimum test pressure can be calculated as:
Pt = (1.5 X P X St) / S

Pt : Minimum Test Pressure.
P : Internal design pressure.
St: Allowable stress at test temperature.
S : Allowable stress as design temperature.

3. If a test pressure as per above would produce a stress in excess of the yield strength at test temp. the test pressure may be reduced to maximum pressure that will not exceed the yield strength at test temp.

4. If the test pressure of piping exceeds the vessel pressure and it is not considered practicable to isolate piping from vessel, the piping and vessel may be tested together at test pressure of the vessel when approved by owner and provided the test pressure for vessel is not less than 115% of piping design pressure adjusted for temperature as per point no.2.

Thumb rule for Pipe spacing

Pipe Spacing (mm) = (Do + Dt) / 2 + 25mm + Thickness of Insulation (mm).

Where: D0 : OD of Small size Pipe (mm)

Dt : OD of Flange of Large size Pipe (mm)