At a glance
- For vessels: the higher of 1.25 × the maximum in-service loading corrected by the allowable stress ratio, and 1.43 × PS (Annex I point 7.4).
- At ambient temperature 1.43 × PS almost always governs; the temperature term takes over once the stress ratio exceeds 1.144 (= 1.43 / 1.25).
- The static liquid column, the test condition as its own load case, and the option of another recognised test (3.2.2) all come on top.
The two terms of point 7.4
Annex I point 7.4 requires, for pressure vessels, that the hydrostatic test pressure of point 3.2.2 is no less than the higher of two values: the pressure corresponding to the maximum loading the equipment may be subjected to in service, given its maximum allowable pressure and temperature, multiplied by 1.25 — and the maximum allowable pressure multiplied by 1.43.
The first is the temperature term: at design temperature the allowable stress is lower than at test temperature, and multiplying by that ratio loads the vessel at the test as heavily as in service. The allowable general membrane stress is not free to choose — point 7.1.2 caps it per material type.
The crossover is exact: once the stress ratio ftest/fdesign exceeds 1.144 (that is 1.43 divided by 1.25), the temperature term wins. Below it, 1.43 × PS always governs. Practically: a vessel designed at ambient temperature nearly always tests at 1.43 × PS; a hot vessel tips over.
Does this also cover piping, boilers and accessories?
Not directly, and the answer differs per type. Point 7.4 opens with "for pressure vessels"; point 3.2.2 extends the value to all pressure equipment "where appropriate". What the harmonised standards then make of it:
- Pressure vessels — point 7.4 applies directly; the vessel standard confirms the same two terms but calculates the first with the design pressure, takes the smallest stress ratio over the main pressure-bearing parts, and relaxes the 1.25 for certain single-side-welded joints and testing groups.
- Industrial piping — the piping standard prescribes exactly the same two terms, so the outcome is the normative value; it adds the smallest stress ratio across all main parts, a stress ceiling at test temperature, and the rule that the static liquid column counts once it adds more than a few per cent.
- Boilers — the water-tube and shell boiler standards use the same two terms but with the bare yield ratio as temperature term, without the caps of point 7.1.2 — at higher temperatures the boiler value comes out higher than the vessel formula.
- Accessories — the product standard and the manufacturer's specification.
The failure mode is always the same: taking the number without walking through the accompanying normative conditions, and a test protocol ends up too low or impermissibly high.
The standard-specific conditions above are referenced by clause in the full report; standard texts themselves are copyrighted and are not reproduced here.
What else counts at the test
- The static liquid column of a tall or standing item adds to the test pressure at the lowest point.
- The test condition is its own load case in the strength calculation; the structure must carry the test pressure, which is not self-evident if you raise it.
- Point 3.2.2 permits another recognised test where the hydrostatic test is harmful or impracticable, provided additional measures such as non-destructive testing are applied first.
The calculator
The free calculator on ped-check.nl works out the minimum test pressure per equipment type, including the liquid column and the crossover between the two terms. Its interface is currently Dutch; an English version follows. The numbers and the article references are language-independent — a Dutch-reading colleague can verify the outcome in a minute.
Frequently asked questions
What is the minimum test pressure under the PED?
For pressure vessels: the higher of 1.25 times the maximum in-service loading — corrected by the ratio of allowable stress at test and at design temperature — and 1.43 times the maximum allowable pressure PS (Annex I point 7.4). Point 3.2.2 extends that value to other equipment where appropriate.
When does the temperature term govern instead of 1.43 × PS?
Once the ratio of the allowable stress at test temperature to that at design temperature exceeds 1.144, which is 1.43 divided by 1.25. Below that ratio, 1.43 × PS always gives the higher value.
Does the liquid column count in the test pressure?
Yes, at the lowest point of a standing or tall item the static column adds to the pressure. Point 7.4 does not regulate it; the product standard and your procedure do — and ignoring it is a common finding at witnessed tests.
May I test pneumatically instead of hydrostatically?
Point 3.2.2 permits another recognised test where the hydrostatic test is harmful or impracticable, provided additional measures such as non-destructive testing are applied beforehand. It is the exception, not the default, and it needs agreement with the body involved.
Is a higher test pressure always safer?
No. The test condition is a load case of its own: the structure must be shown to carry the test pressure. Raising the test pressure without checking that case can damage the equipment you are trying to prove sound.
Want the number for your equipment?
The full report gives the test pressure for your equipment with its legal or normative basis, the test setup and a fill-in test protocol.
Run the free check or see the full report