Why are LPG cylinders tested with water instead of air?

Water is effectively incompressible, so it stores very little energy under pressure. If a cylinder fails during a hydrostatic test it releases that energy harmlessly. The same test with compressed gas would store enough energy to cause a dangerous rupture, which is why proof testing is done hydrostatically.

Is every cylinder burst tested?

No — burst testing destroys the cylinder, so it is performed on samples drawn from a batch. Every cylinder receives a non-destructive hydrostatic proof test and a leak test; the burst test proves the design margin for the batch as a whole. Ask a supplier how frequently they sample.

What does a hydrostatic test actually prove?

That the cylinder withstands a pressure substantially above normal service pressure without leaking and without permanent deformation. Expansion beyond the allowed limit is a failure even if no leak appears, because it indicates the steel has yielded rather than flexed elastically.

Why is the weld inspected separately?

The circumferential weld is the most safety-critical feature of a welded cylinder, and internal defects such as porosity or lack of fusion are invisible from the outside. Construction standards therefore require volumetric examination and mechanical testing of weld samples in addition to visual inspection of every weld.

Standards explained

How LPG Cylinders Are Safety Tested Before They Leave the Factory

A cylinder that fails in service is a safety incident, so the testing regime behind it is not a formality. This guide walks through the main tests applied to welded steel LPG cylinders, what each is designed to catch, and how to read a test report you have been handed.

5 min read

Hydrostatic proof test: every cylinder, every batch

The proof test fills the cylinder with water and raises the pressure to the stamped test pressure, holding it while the cylinder is examined.

Water is used rather than air deliberately. A liquid is effectively incompressible, so if the shell does fail under test it releases almost no stored energy. The same test performed with compressed gas would be dangerous.

What the test proves is that the cylinder withstands a pressure well above anything it will see in normal service, without leaking and without permanent deformation. Permanent expansion beyond the allowed limit is a rejection even if nothing leaks — it means the steel yielded.

Burst test: destructive, on samples

The burst test takes a cylinder from the batch and pressurises it until it fails, recording the pressure at which that happens and how the failure propagated.

This is the test that proves the design margin. A compliant cylinder must not only exceed a minimum burst pressure, it must fail in a controlled way — tearing open along the shell rather than fragmenting, and without the failure starting at a weld.

Because it destroys the cylinder, it is done on samples. The sampling frequency is as important as the result, which is why it is worth asking a supplier how often they draw one.

Weld inspection

The circumferential weld joining the two halves of the shell is the most safety-critical feature on a welded cylinder, and most of the inspection effort concentrates there.

Inspection typically combines visual examination of every weld with radiographic or equivalent volumetric examination of samples, plus mechanical testing of weld coupons to confirm the joint is as strong as the parent material.

A weld that looks sound externally can still contain porosity or lack of fusion internally, which is precisely why visual inspection alone is not accepted as sufficient by the construction standards.

Leak test and valve integrity

After the valve is fitted, the assembled cylinder is leak-tested — commonly by pressurising with air and immersing it, or with an equivalent method sensitive enough to find a slow leak.

This catches a different class of defect from the proof test: a cylinder can be structurally perfect and still leak at the valve thread. Because the valve is fitted after the shell is tested, leak testing has to come last in the sequence.

Reading a test report

If you have been given test documentation, four things make it useful rather than decorative:

It identifies the batch, and that batch identifier appears on the cylinders you received.
It states the actual measured values, not just a pass mark.
It names who performed the test and when, with a traceable signature or stamp.
It records the sample size the destructive tests were drawn from, so you can judge how representative they are.

Frequently asked questions

Why are LPG cylinders tested with water instead of air?: Water is effectively incompressible, so it stores very little energy under pressure. If a cylinder fails during a hydrostatic test it releases that energy harmlessly. The same test with compressed gas would store enough energy to cause a dangerous rupture, which is why proof testing is done hydrostatically.
Is every cylinder burst tested?: No — burst testing destroys the cylinder, so it is performed on samples drawn from a batch. Every cylinder receives a non-destructive hydrostatic proof test and a leak test; the burst test proves the design margin for the batch as a whole. Ask a supplier how frequently they sample.
What does a hydrostatic test actually prove?: That the cylinder withstands a pressure substantially above normal service pressure without leaking and without permanent deformation. Expansion beyond the allowed limit is a failure even if no leak appears, because it indicates the steel has yielded rather than flexed elastically.
Why is the weld inspected separately?: The circumferential weld is the most safety-critical feature of a welded cylinder, and internal defects such as porosity or lack of fusion are invisible from the outside. Construction standards therefore require volumetric examination and mechanical testing of weld samples in addition to visual inspection of every weld.

See the lab behind the tests

Our Quality page sets out the testing and inspection procedures applied across our production lines.

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