LPG cylinder pressure testing: why every cylinder must be tested

An LPG cylinder holds liquefied gas under pressure for many years, through hundreds of fillings and journeys. A small weld defect or an abnormally thin area of steel may be invisible to the eye, but it shows up once the cylinder is under pressure. That is why LPG cylinder pressure testing is a mandatory step for every single cylinder before it leaves the factory — not just a few representative ones.

What does pressure testing involve?

At the factory, cylinders usually go through two pressure tests:

• Hydrostatic (strength) test: the cylinder is filled with water and the pressure is raised to the test pressure set by the standard — well above the working pressure — and held for the specified time. The cylinder passes if it neither leaks nor deforms.
• Leak test: the cylinder is filled with compressed air and checked for leaks at the welds, collar and foot ring, usually by immersion in water or by applying a bubble-forming solution.

Water is used for the strength test because it is almost incompressible: if a cylinder fails, the pressure drops quickly without storing large amounts of energy as a gas would, which is safer for the operators.

The testing process at the factory

1. Visual inspection after welding: continuous welds with no pores or cracks.
2. Hydrostatic test of each cylinder on the test rig, recording the pressure and holding time.
3. Draining and drying the inside of the cylinder before the next stage.
4. Leak test with compressed air, checking each weld and the collar.
5. Separating any failed cylinders so they never reach painting and valve fitting.
6. Recording the results on the batch inspection sheet, which supports the inspection records.

Pressure testing is step four of the 7-step control process at VietnhatGas — see the full process on our LPG cylinder manufacturing service page or in our manufacturing process video.

Which defects does pressure testing reveal?

• Incomplete fusion, porosity or cracks in welds — the most common cause of leaks.
• Thin areas or material defects that make the cylinder bulge under pressure.
• Leaks at the collar, where the valve is fitted.
• Forming deformation that has not reached the correct shape.

Besides testing every cylinder, depending on the standard applied, each batch also undergoes tests on representative samples (such as destructive tests and material property tests) to verify the quality of the whole batch. VietnhatGas cylinders are manufactured to TCVN 6292:1997 and refer to ISO 22991.

Pressure testing and inspection records

Pressure test results are an important part of the inspection report supplied with each new batch. Throughout their service life, cylinders are also periodically re-inspected under the regulation, including a repeat pressure test. Dealers should file records by batch and check serial numbers — see LPG cylinder inspection records.

Why not test just a few cylinders?

Weld and material defects occur randomly, not by batch. If only a few representative cylinders are tested, defective ones can still reach customers. Testing every cylinder ensures each one leaving the factory has withstood the test pressure — this is the “100% pressure tested” commitment dealers should ask manufacturers for. On delivery, combine it with the 7 checks for a compliant cylinder to reject any cylinders damaged in transport.

Frequently asked questions

Written by the VietnhatGas technical and sales team — a batch manufacturer of LPG cylinders. This content is for reference; regulations and standards may change, so please check the current texts.