LPG cylinder steel: technical requirements and incoming inspection

A gas cylinder has to withstand the pressure of LPG over many years in circulation, through countless rounds of filling, transport and knocks. Cylinder quality therefore starts with the material: LPG cylinder steel determines the strength, weldability, formability and service life of the cylinder. This article explains the requirements for the steel and how the manufacturer controls incoming material.

Why is steel the foundation?

A gas cylinder is a pressure vessel. Even with a correct design and good welding, if the steel does not meet requirements the cylinder may deform, crack or fail the pressure test. Steel is also one of the factors with the greatest effect on LPG cylinder price.

Technical requirements for LPG cylinder steel

• Strength under pressure: tensile strength and yield strength high enough that the cylinder body does not suffer permanent deformation under working pressure and test pressure.
• Good weldability: a suitable chemical composition so that welds do not crack or become brittle — details in LPG cylinder welding and weld inspection.
• Ductility for deep drawing: the cylinder heads are drawn from flat sheet, so the steel must be ductile enough not to tear or crack in the drawn area.
• Impact toughness: cylinders are frequently knocked during loading and unloading, and tough steel helps limit cracking and fracture.
• Clean surface, uniform thickness: few surface defects and a consistent thickness across the whole sheet.

For this reason, the manufacturer uses hot-rolled steel plate made specifically for pressure vessels, in a steel grade that meets the applicable standard, rather than ordinary steel plate. Grades commonly used worldwide for welded LPG cylinders include SG295 (JIS G3116), HP295 (GB 6653) and P265NB (EN 10120); the exact grade follows the applicable standard and the certificate of each batch.

How is the wall thickness determined?

The steel thickness is not chosen arbitrarily but calculated from the design: capacity, cylinder diameter, design pressure and the mechanical properties of the chosen steel. Larger cylinders usually need thicker steel. For VietnhatGas 12kg LPG cylinders, the steel used for the cylinder is around 2.0 – 3.0 mm thick. Cutting thickness to lower the cost is a major risk and is unacceptable in pressure vessel manufacturing.

How is incoming steel inspected?

1. CO/CQ documents: the steel supplier’s certificate of origin and certificate of quality for each batch, stating the steel grade, batch number, mechanical properties and chemical composition.
2. Thickness measurement: checks at many points on the coil or sheet, compared against the permitted tolerance.
3. Mechanical testing: tensile test samples are taken to determine tensile strength, yield strength and elongation, compared with the requirements of the standard.
4. Chemical composition check: with attention to elements that affect weldability and ductility.
5. Surface inspection: to detect pitting, laminations, deep rust and rolling marks.

Steel batches that fail must be rejected before they reach the production line.

The risks of using poor-quality steel

• Cracking and tearing when drawing the heads; a high reject rate.
• Brittle welds that crack easily and struggle to pass non-destructive testing.
• Bulging or permanent deformation during LPG cylinder pressure testing.
• Rapid rusting and a short service life, with cylinders rejected early at periodic inspection.
• Safety risks for users, damaging the reputation of the dealer and the gas brand.

The standards VietnhatGas applies

VietnhatGas manufactures LPG cylinders to TCVN 6292:2013, with reference to ISO 22991. Incoming steel comes with full CO/CQ documents and is inspected before production; finished cylinders are pressure tested and leak tested, and delivered with documentation for each batch. Learn about the process on our LPG cylinder manufacturing service page.

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.