CTS 87.146: NPC Journal 4(1), Jan 1987, pp 73-74

Diving Cylinder Testing

Paul Atkinson

It is a statutory requirement under the Health and Safety at Work Act, that all cylinders designed to contain pressurised gas shall be inspected on a regular basis according to the manufacturing specification. Diving cylinders fall under the same regulations. Under the Health and Safety regulations it is the owner's responsibility to ensure that the correct inspections are carried out at the prescribed intervals and that the inspections themselves are carried out correctly to BS 5430.

Since the majority of cylinders used in caving are of steel construction, it is intended to discuss only the testing of these, according to Part 1 of BS 5430 as prescribed in the following manufacturing specifications:

HOS - Home Office Specification for diving cylinders manufactured prior to 1976. Periodic inspection and hydraulic testing required at 2 yearly intervals.

BS 5045 - Equivalent specification to HOS subsequent to 1976. Periodic inspection and hydraulic testing required at 2 yearly intervals with initial test 3 years from manufacture.

HOT - Home Office Specification for special portable containers for emergency use only, ie. industrial breathing apparatus - not diving. However, after consideration by the HSE, a concession was agreed that cylinders manufactured to this specification could be utilised for diving provided that a periodic inspection was carried out annually and a hydraulic test every 2 years.

Periodic Inspection

Detailed visual inspection, both internal and external with the following failure criteria :
Bulge - all fail
Dent - fail if any greater than 2 mm
Wear - reduction of wall thickness greater than 25%
General Corrosion - reduction of wall thickness greater than 20% OR where original surface is not visible.
Area Corrosion - (ie. where corrosion is limited to less than 20% of surface) a reduction of wall thickness greater than 25%
Isolated pitting - Reduction of wall thickness greater than 40%
Thread damage - all physical damage or imperfections ( recutting of threads can be achieved up to a maximum tolerance )
Weight Check - Weight of cylinder ( minus valve ) less than 95% of the tare weight stamped on the cylinder
If in the opinion of the inspector, any of the above criteria are borderline, then hydraulic testing may also be required.

Hydraulic testing

This can take the form of either a proof pressure test or a volumetric expansion test. Normally the proof pressure test will be carried out; however, if the cylinder suffers from significant corrosion or other imperfections, the volumetric expansion method should be used.

Proof Pressure test :- cylinder is pressurised with water to the test pressure stamped on cylinder. This is held for at least 2 minutes during which there shall be no detectable drop in pressure.

Volumetric expansion test :- cylinder is pressurised with water to test pressure and the volumetric expansion of the cylinder measured by either monitoring the quantity of water required to pressurise the cylinder, subsequent to filling, or by utilising a water jacket around the cylinder. On depressurising, any permanent expansion can be determined by either monitoring the amount released from the cylinder or the amount required to fill up the water jacket. Failure criterion is that the permanent expansion should not exceed 5% of the total expansion at test pressure.

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