“1.5 times design pressure” is the figure most people remember for a hydrotest. It is a good starting point, but the real test pressure depends on the code, the material, the temperature and some upper limits. This guide explains how the main codes set the pressure and the hold time.
This is an overview to help with planning. The code edition and owner standards named in your contract always govern.
ASME B31.3 (process piping)
The minimum hydrostatic test pressure is:
P_T = 1.5 × P × (S_T ÷ S)
- P = design pressure
- S_T = allowable stress at test temperature
- S = allowable stress at design temperature
- The ratio S_T ÷ S is capped at 6.5.
For piping designed at ambient temperature, S_T ÷ S is 1, so the test pressure is simply 1.5 × P. For hot piping, the ratio raises the test pressure to make up for the lower strength of the metal at design temperature.
There is also an upper limit: the test pressure should not produce a stress above the material’s yield strength at test temperature. If it would, the test pressure is reduced.
Hold time: the test pressure must be held for at least 10 minutes, after which all joints are examined for leaks. Many owners require longer holds.
Worked example: a carbon steel line with a design pressure of 20 barg at a temperature where S_T ÷ S = 1.1:
P_T = 1.5 × 20 × 1.1 = 33 barg (subject to the yield check and any owner rules).
ASME B31.4 (liquid pipelines)
For pipelines operating at higher stress levels, the hydrotest is typically at least 1.25 × design pressure, held for at least 4 hours. If the line cannot be visually inspected for leaks during the test (for example, buried), an additional leak test period is required.
ASME B31.8 (gas pipelines)
Test pressure depends on the location class, which reflects how many people live near the line. Lines in remote areas may be tested at a lower multiple of design pressure than lines near towns. The code sets both test pressure ratios and hold times for each location class.
ASME Section VIII Division 1 (pressure vessels)
The standard hydrostatic test pressure is:
1.3 × MAWP × (stress ratio)
where the stress ratio is the allowable stress at test temperature divided by the allowable stress at design temperature. After reaching the test pressure, the pressure is reduced for a close visual inspection of all joints.
Summary table
| Code | Typical minimum test pressure | Hold time |
|---|---|---|
| ASME B31.3 | 1.5 × P × stress ratio | At least 10 minutes |
| ASME B31.4 | 1.25 × design pressure | Typically at least 4 hours |
| ASME B31.8 | Depends on location class | Per location class |
| ASME VIII Div. 1 | 1.3 × MAWP × stress ratio | Long enough to inspect |
Practical points
- Gauge position: pressure is usually specified at the highest point. At low points, add the head of water: about 0.1 bar per metre of height.
- Temperature: in a full system, a small temperature change causes a noticeable pressure change. In Saudi conditions, sun on exposed pipe can raise pressure during the day and the night can lower it. Record temperatures alongside pressure.
- Weakest component: the test pressure must not overstress valves, flanges or equipment in the test boundary. Isolate or remove items rated lower.
- Gauges: use calibrated gauges whose full-scale range is about twice the test pressure (ASME Section VIII, for example, allows 1.5 to 4 times), so the reading sits near the middle of the dial.
Frequently asked questions
Why is the B31.3 hold time only 10 minutes?
The code sets a minimum to allow a full examination of joints. Owners often set longer holds, especially with chart recorders.
Do I test at the top or bottom of the system?
Your procedure will say. Most specify the test pressure at the high point and check that the low point stays within limits.
Can the test pressure be higher than calculated?
Within limits. It must not exceed yield or the rating of any component in the test.
For the rules owners add on top of the codes, see hydrotest requirements in Saudi Arabia. Our hydrotest services prepare test packages and run tests in Al Khobar, Rabigh, Riyadh and elsewhere. Contact us.
