Why do nitrox tanks need to be clean for oxygen service?
By Jonathan Cohen
In the following article, I will discuss the reasons why our nitrox tanks need to be compatible with oxygen service, even though some of them won’t be used for containing pure oxygen at all. To understand the reason, we need to know some principals regarding Nitrox blending and handling.
How do we compress EAN mixes?
There are few ways to make Enriched Air mixes for diving. We can divide them into two main methods. The first and most common method is partial pressure mixing- in this method of gas blending, we basically compress pure oxygen to the tanks to a certain pressure and finish it by air. The gas blending is conducted inside the tank and the tank meets oxygen at its pure state. There are other methods where the tank gets the nitrox to mix pre-mixed, whether it was conducted by a gas supplier or by the blending station. In this case, there is less important for the tank to be clean for oxygen service (If the EAN mix used is 40% oxygen or less) although it’s highly recommended.
The Fire Triangle
The second principle we need to know to understand why nitrox tanks need to be clean for oxygen service is The Fire Triangle. Basically, the three elements that support fire. Although oxygen is not flammable by itself, it’s one of the key elements to support combustion. When oxygen is present, it makes everything burn faster and easier, and the higher the percentage the higher are the risks. The three elements needed to create fire are Air (which holds the oxygen), Fuel and Heat. Once you cut one of these elements and the fire will stop immediately. Unfortunately, in our case, it is a little bit different. Oxygen is always present in the system, and so the other elements. This creates a few problems:
- Oxygen is always present. If the fire started, the oxygen within the system will continue to feed it.
- Due to adiabatic compression, the oxygen himself creates heat and can become its own ignition energy.
- Fuel is always present in the system- pipes, hoses, connectors, etc.
Adiabatic Compression refers to the property of gas to create heat when exposed to a sudden increase in pressure. When a sudden increase in pressure occurs, the gas can reach a really high theoretical maximum pressure, the higher the final pressure of the compression will be, the higher the temperature the gas can reach. For example- at a final pressure of 70 bars/1303 psi (which is considered a low-pressure regarding diving tanks) the gas can reach a temperature of 706 °C/ 1303 °F – that’s beyond the ignition point of lots of the components in the system. When using pure oxygen, the ignition point of any material is lowered dramatically and the heat created by adiabatic compression is a big contributor to oxygen fires.
One of the things we do while cleaning a tank for oxygen service is removing all hydrocarbon substances. Hydrocarbons are materials containing hydrogen and carbon (examples of hydrocarbons are oils and oil-based lubes). The presence of hydrocarbon substances in oxygen can create several problems:
- In a pure oxygen environment, the ignition point of any material is reduced drastically
- The flashing of hydrocarbon substances can result in the production of Carbon Monoxide – a highly toxic gas inside the diving cylinder.
Now that we understood these basics in gas blending and oxygen handling, let’s talk about the diving industry’s standards
Those of you have been or still looking to buy any diving equipment probably encountered many terms referring to the equipment’s ability to be used with nitrox or oxygen. Probably you encountered the words “Nitrox Ready”, “Oxygen Clean”, “Oxygen Compatible” and so on. These terms can be really confusing, basically because they pretend to say the same thing – “this piece of equipment is safe for use with pure oxygennitrox”. In fact, its is usually not true, most of the diving equipment, even if it is sold under the tags mentioned above isn’t really made for use with pure oxygen, in fact, only a few specific tanks usually manufactured in Europe are holding the proper term “Clean for Oxygen Service”. The others are just pretending to be- oxygen clean means the tank was cleaned for oxygen in the factory (no hydrocarbon substances should be present in the tank), oxygen compatible means that the system’s structure and components are compatible to be used with oxygen.
Basically, anything that is not sold under the term “Ready for oxygen service” is not compatible with oxygen, therefore, it can’t be used with most of the nitrox blending stations (due to the blending method) and should be used at owner’s own risk.
Even if the tank is within the standard for use with pure oxygen, this state has an expiry date. Cleaning for oxygen service is valid only for one year, and only if the tank wasn’t separated from the valve or contaminated by any source. If a year passed since the last time you cleaned your tank for oxygen service or you have a reason to suspect that your tank was contaminated or opened, you should send it to be cleaned again by a certified VIP technician so your tank will meet the standard again. Using a nitrox cylinder that’s not meeting the standard means you are putting you and your surroundings at significant danger from the hazards that can occur when compressing and handling Oxygen-rich gases.