Pressure test: water before air – why the hydrostatic test is the rule and the gas pressure test the justified exception

Rule of thumb

The strength and tightness test of a piping system is run with liquid – as a rule water. Gas (air, nitrogen) is used only when water is disadvantageous or not feasible, and then only after preceding non-destructive tests and with personal protection measures.

Where does the rule come from?

The rule is in the regulations that govern pressure testing – Pressure Equipment Directive 2014/68/EU (Annex I No. 3.2.2 and 7.4), DIN EN 13480-5, AD 2000 leaflet HP 30, the German Ordinance on Industrial Safety with TRBS 1201-2 and -3 and BG RCI leaflet T 039 – and the manufacturers pass it on in their technical information. Geberit puts it this way: the pressure test is part of the acceptance test and is usually performed hydrostatically; gas pressure tests are permitted only if a hydrostatic test is disadvantageous or not feasible.

What is technically correct?

The physical reason is the stored energy: water is practically incompressible – if a component fails under water pressure, the pressure drops at once. Gas is compressible and stores many times the energy at the same pressure; a failure acts like an explosion. That is why the gas pressure test requires non-destructive tests of sufficient scope beforehand, a clearly visible barrier around the area and access only for the persons in charge below PS or OP. For the test pressure the PED (Annex I No. 7.4) names the higher of 1.25 times the maximum load in service (taking allowable pressure and temperature into account) and 1.43 times the maximum allowable pressure PS – for the recurring test under BetrSichV the test pressure factor applies to the allowable operating pressure OP.

Where does the rule end?

• Processes that tolerate no liquid residues at all (oxygen, high-purity media, certain gas lines): there the gas pressure test is the intended route – with all its conditions.
• Tightness tests of drinking-water installations to DIN EN 806-4 / ZVSHK leaflet with air or inert gas at low pressure are a different test from the strength test under the PED – do not confuse them.
• Other liquids (hydraulic oil, group 1 refrigerants to DIN EN 378-1) are permitted if design or medium exclude water.
• Which piping is subject to monitoring at all is governed by BetrSichV via the CLP classification of the medium (flammable, pyrophoric, acutely toxic, corrosive).

What does the standard say?

DIN EN 13480-5 (testing of metallic industrial piping) and AD 2000 HP 30 fix procedure, test pressure, holding time and assessment; the PED requires the strength test as part of acceptance; BetrSichV orders tests before commissioning and recurring tests; TRBS 1201-2/-3 and BG RCI T 039 describe the procedure with liquid (T 039 item 4.2.2 ff.) and with gas (item 5.1.3 ff.) including personal protection (5.1.4 ff.). We do not hold the standard texts; citation follows the manufacturer information.

DIN/EN versus ASME/ASTM

The ASME world says the same: ASME B31.3 names the hydrostatic test as the rule (1.5 × design pressure, temperature-corrected) and permits the pneumatic test only where the hydrostatic one is impracticable – with a lower factor (1.1 ×) and its own safety clauses. Whoever works between the worlds therefore states test type, test pressure and holding time explicitly in the specification (the rule of thumb “inspection interval comes from the operator’s country” applies by analogy to the test type).

Worked example

A plant builder orders material for a cooling-water line and asks for “testing with air, it is quicker”. The line is a water line, residues do not matter – so hydrostatic, test pressure per specification (PED: the higher of 1.25 × service load and 1.43 × PS), record kept by the installer. Air would not be “quicker” here but an exception requiring justification, with barrier and NDT beforehand.

Zickwolff reference

From daily business: we supply the material and the certificates – the pressure test is done by the installer. What we contribute: the material and inspection certificates 3.1 per item, so that the strength test rests on documented components, and in projects the question of test type and test pressure already in the parts list, because it co-determines material and wall thickness.

Checked today

Verdict: applies (class A). The rule reflects the hierarchy of the regulations (hydrostatic as the rule, gas as the justified exception) and the manufacturer information confirms it literally.

Further reading

• Leak testing and SC-Contur: dry testing, unpressed = leaking, load test – Leak testing and SC-Contur with press systems
• Certificates and documentation: 3.1 certificates, supplier’s declarations, certificates of origin per item – Certificates and documentation
• The inspection interval comes from the operating country, not from the code – why ASME parts have no shorter interval – Inspection interval comes from the operator’s country
• Mixing standards worlds: what fits, what only appears to fit, what does not fit – Mixing standards worlds: EN and ASME in one project
• EN 10204 explained: 2.1, 2.2, 3.1 and 3.2 inspection certificates – Inspection documents to EN 10204