Dangers associated with mismatching portable (scuba) cylinders and valve fittings

Background

In a recent incident at Mt Gambier a Victorian diver narrowly escaped serious injury when the valves and manifold of a twin SCUBA-set he was filling suddenly separated from its cylinders.

The sudden release of energy propelled the 2kg manifold assembly upward, past the divers face and through the roof of the building. It was later found almost completely buried in a grassed area, approximately 50 metres from the filling station.

The 12.2 litre steel cylinders had an operating pressure of 232 bar. Witnesses claim that the cylinders had been filled slowly to between 215 - 220 bar (approximately 95% of their operating pressure) when the valves were ejected from the cylinders.

The investigation revealed that:

The cylinders, which had 3/4NPSM (American straight pipe thread), were supplied to the diver without the valves.
Due to difficulties in obtaining manifold and valves of the same make as the cylinders subsequently recommended and supplied what he believed to be a suitable alternative manifold and valves assembly.

The manifold assembly and the valves had 25mm (metric) thread, and were imported from Europe by an interstate importer/distributor.

On receipt of the manifold set the diver fitted the 25mm metric valves to the 3/4NPSM cylinders. They were transported to Mt Gambier in this condition.

Once exposed to almost the full working pressure of the cylinders, the mismatched threads on the valves failed allowing the valves and manifold to be ejected from the cylinders.

Warning:

Valve ejections from gas cylinders can cause serious personal injury or death. Investigations of these accidents indicate that the causes of the cylinder valve ejection are usually:

Mismatching straight and taper threads between the cylinder and the valve
Interchanging metric and non-metric threads

Cylinders and valve threads come in a variety of sizes and great care must be taken to ensure that they are properly matched.

Note: In this incident both the cylinders and the valve bodies were clearly marked with their respective thread size.

Precautions

Filled SCUBA and SCBA cylinders contain considerable amounts of energy. The accidental release of extremely large amounts of energy is potentially dangerous.
The filling of cylinders should be considered a hazardous activity and should only be undertaken by competent persons who are fully conversant with all the associated hazards.

Responsibilities

Gas cylinders are regulated under the Workplace Health and Safety Act 1995. The Act places responsibilities on importers, suppliers, employers and owners of plant to ensure that plant supplied or used is safe.

Section 32 of the Act requires that a person who imports or supplies any plant (which includes gas cylinders) shall

ensure as far as is reasonably practicable that the plant is designed and constructed so as to be safe -
1. when properly used and maintained; and
2. when subjected to reasonably foreseeable forms of misuse
ensure that so far as is reasonably practicable that the plant is designed and constructed so that people who might use or maintain the plant are, in doing so, safe from injury and risks to health.

Further reference:

AS 2030.1:1999 The verification, filling, inspection, testing and maintenance of cylinders used for storage and transport of compressed gases. Part 1: Cylinders for compressed gases other than acetylene (non-Queensland Government link)
AS 3848 Filling of portable gas cylinders (non-Queensland Government link)

The information in this alert was provided by the Department for Administrative and Information Services of South Australia.

Last updated July 12, 2005