Piping hydrotest pressure calculator: B31.3 hydrostatic and pneumatic leak test

How to use the piping hydrotest calculator

1. Pick the test: hydrostatic (water or another liquid) or pneumatic (gas).
2. Enter the internal design gage pressure in psi, kPa, bar or MPa.
3. For a hydrostatic test, enter the allowable stress at test temperature (S_T) and at design temperature (S) from your code book, in any one unit, or a ratio you already have.
4. Optionally open the ceiling check and enter the outside diameter, the minimum wall, the yield strength at test temperature, or a flange or fitting rating at test temperature.
5. Read the minimum test pressure and the steps under it.

Formulas used

• Hydrostatic leak test, minimum: P_T = 1.5 × P × R_r, with R_r = S_T ÷ S. The ratio is capped at 6.5, and the minimum is never below 1.5 × P, so a ratio under 1 is taken as 1. ASME B31.3 345.4.2.
• Pneumatic leak test, minimum: P_T = 1.1 × P. ASME B31.3 345.5.
• Pressure that brings the hoop stress to yield: P = 2 × t × S_y ÷ D, from the hoop stress S = PD ÷ 2t, with t the minimum wall and D the outside diameter.
• Component limit: 1.5 × the component's rating at test temperature. Where the minimum is above the lower of these two, B31.3 345.4.2(c) lets the test pressure come down to it.

The hydrostatic and pneumatic forms and the 6.5 cap are printed by What Is Piping and The Piping Talk, and Wikipedia's hydrostatic test article describes the same stress-ratio rule and yield limit. The Piping Talk also works the yield check with S = PD ÷ 2t. Paragraph numbers are from the code; this page holds no code text and no stress or rating tables. Read B31.3 345.4.2 and B31.3 345.5 in your own copy for the full rules and exceptions.

Worked example 1: hydrostatic test with a stress ratio

Invented numbers, not from any stress table or real line. Design pressure 300 psi. The allowable stress is entered as 20,000 psi at test temperature and 16,000 psi at design temperature.

1. Stress ratio: 20,000 ÷ 16,000 = 1.25, which is between 1 and 6.5.
2. Minimum hydrostatic test pressure: 1.5 × 300 × 1.25 = 562.5 psi.

A pneumatic test of the same line uses 1.1 × 300 = 330 psi.

Worked example 2: a thin wall meets the yield ceiling

Invented numbers. The same 562.5 psi minimum on a pipe with a 24 in outside diameter and a 0.2 in minimum wall, with a yield strength of 30,000 psi entered at test temperature.

1. Pressure at yield: 2 × 0.2 × 30,000 ÷ 24 = 500 psi.
2. The minimum, 562.5 psi, is 62.5 psi above that ceiling, so B31.3 345.4.2(c) allows the test pressure to come down to 500 psi.

These are original arithmetic examples, not exam questions.

Common mistakes

• Using the vessel multiplier. Pressure vessels built to Section VIII, Division 1 use a different factor. For a vessel, use the vessel hydrotest pressure calculator.
• Turning the ratio upside down. R_r is the stress at test temperature over the stress at design temperature. A hot line usually has the lower allowable at design temperature, so R_r is 1 or more.
• Forgetting the cap. A ratio above 6.5 is used as 6.5.
• Applying the stress ratio to a pneumatic test. The pneumatic form is the design pressure times 1.1, with no ratio.
• Ignoring elevation. The minimum applies at every point, so the low points of a tall system see more by the static head of the liquid above them. Work that with the static head calculator.

What this calculator does not do

It does not look up allowable stresses, yield strengths or component ratings, check longitudinal stress, add static head, set hold times or examination pressure, plan a pneumatic test's safety measures, or cover other piping codes or editions. It does not decide whether a pressure test can be replaced by other examination after a repair. The pressure design thickness of the pipe itself is in the pipe minimum thickness calculator. Test temperature matters for more than the stress ratio: brittle fracture explains why cold metal under test pressure is a risk.

Piping test pressure questions

Leak test pressure on the API 570 exam

The API 570 Body of Knowledge lists pressure testing as its own section: hydrostatic and pneumatic leak testing under B31.3 345.4 and 345.5, and calculating a test pressure for either.