What speed should you turn on your oxygen cylinder? -

What speed should you turn on your oxygen cylinder?

Article by Mark Powell

When using pure oxygen, either for decompression or for rebreather diving, what speed should you open the cylinder? If you answered slowly then you are only partially correct as that is only addressing part of the problem.

At the Diving Talks event in Portugal last year, Martin Parker gave a fascinating talk about diving risks. I would suggest you watch the whole presentation which is available here.

In this article, I want to reinforce the key aspect of Martin’s talk. Specifically, the safest way to turn on an oxygen cylinder.

In addition to the in-water risks, we need to manage when diving a CCR there are also a number of surface risks. One of the biggest of these is the risk of an oxygen fire. As technical divers, we should all know about the fire triangle.

The Fire Triangle

In order to have a fire we need fuel, oxygen and an ignition source. If we remove one of these, we cannot have a fire. If there is no oxygen we cannot have a fire. Smothering a flame removes the air and so removes the oxygen. Unfortunately, if we are using pure oxygen then the presence of oxygen is a given and we cannot remove it.

However, with increased percentages of oxygen, things get even worse. Materials that will not be a fuel source and will not burn in air, will become a fuel source and will burn in pure oxygen. The ignition temperature of a material in pure oxygen is lower than the ignition temperature in air. As we cannot remove the oxygen or potential fuels, this means that removing or reducing the ignition source or heat source is the most effective way to control the risk of a fire.

New Valves are Usually Not a Problem

Oxygen fires rarely occur in newly purchased equipment. CE testing is very rigorous and ensures that the valve when manufactured is fully oxygen clean and compatible and so reduces the risk of an oxygen fire. A new valve can be opened very quickly without causing an oxygen fire because the original design and the way it was manufactured means there is very little chance of contamination or damage.

This can build bad habits as you can get away with opening your oxygen valve quickly when it is new. However, older valves can get contamination from corrosion, incorrect lubricants, water entry or other foreign objects. They can also get damaged leading to rough edges or burrs that can cause localised heating of gas flows.

The bad habits you developed when the valves were new and low risk now come back to bite you. The rate you open your valves did not cause a problem with new valves, but as they get older and become contaminated and damaged, those same rates start to become extremely dangerous and this is when oxygen fires occur.

Opening the Valve is the Critical Moment

Oxygen fires almost always happen when a valve is first opened and the system is pressurised. Once the valve is open it is extremely unlikely that an oxygen fire will start after that. This means those few seconds while you are opening the valve are the critical moments.

The Result Can Be Devastating

Oxygen fires are more common than you might imagine. Some of them are high profile and you may well have seen the images of a boat being burnt to the waterline as an example in a number of blending and other courses. Of course, the combination of fire and boats is particularly worrying.

If you do encounter an oxygen fire the best solution is to throw the unit overboard. If you jump overboard and leave the burning unit on the boat it will cause the whole boat to burn to the waterline and result in additional explosions.

A number of divers have also suffered from serious burns to the hands and arms. Of course, this is a key reason why you should always turn on your oxygen cylinder before you get into your CCR. Having it catch fire is bad enough but if the CCR unit is strapped to your back it could be fatal.

Oxygen fires occur mostly in the 1^st stage and in the High-Pressure Hoses, but that doesn’t mean you can ignore other components in the system. Oxygen fires in cylinder valves are extremely rare but can be devastating.

How can we reduce the risk?

Everyone says open the cylinder slowly to reduce the risk and you may well have been taught this on your CCR courses. However, this is an overly simplistic answer. When you open a valve the rate of flow is not uniform across the whole range of movement. There is a key part of the movement where most of the flow occurs. You can often open a valve part way, maybe a quarter of a turn, before any gas starts to flow. Between a quarter and one full turn the majority of the flow will have occurred. By the time you have opened the valve one full turn there is not much scope left for further gas flow so any subsequent turns are not having any impact. Of course, this means that the speed you open those subsequent turns also has very little impact.

Pressurise Slowly

The key point is that we should be focusing on the speed of pressurisation rather than the speed of opening. As the pressurisation takes place over a very small range of motion it is this range that is critical. The process for opening a valve with minimal risk of causing an oxygen fire is;

Start to open the valve very slowly and just a little.
Watch the pressure gauge.
As soon as the pressure gauge shows any movement, pause and do not open the valve any further.
Watch the pressure gauge for the slow increase in pressure until it reaches the target pressure.
It should take 1-2 minutes for the gauge to show full pressurisation
If you think the gauge is pressuring too quickly close the valve a tiny bit.
Once the system is fully pressurised you can open up the remaining range of movement as quickly as you like as there is no change in pressure.

Summary

The key rules for reducing the risk of an oxygen fire are;

Ensure valves and system are 02 cleaned regularly
Ensure no hydrocarbons or silicone grease is introduced into the system
Pressurise slowly – open valve slightly, and then WAIT
Allow 1-2 minutes to pressurise the system
Turn on your oxygen cylinder before getting into your CCR
If your unit catches fire throw it overboard rather than jumping overboard yourself.

Thank you to Martin Parker from AP Valves for the fantastic presentation on which this article is based and also for reviewing the text to ensure it accurately reflected the presentation.

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