When purchasing a specialty gas mixture in a high pressure cylinder (HPC), it is important to understand that the valves and valve connections can differ from cylinder to cylinder. This can be the case for the same type of gas or gas mixture.
Although there is an Australian standard for cylinder valves AS2473 – Valves for compressed gas cylinders, many specialty gas mixtures are manufactured overseas resulting in the valve type based upon the country of manufacture.
It is a common occurrence where a customer will order gas mixtures, and get everything on to site only to find that they can’t put them to use because the cylinder valve and regulator connections do not match. This causes delays, and the extra costs of new connectors, freight, and installation.
Worldwide standards include;
- AS2473 (Australia)
- BS341 (UK)
- CGA V-1 (U.S.A.)
- DIN 477 (Germany)
Each of these standards will have a number of different valves, each designed to be used with a specific type of gas or gas mixture. Commonly used pure gases can have their own valve types, whereas mixtures are grouped together based on their properties (flammable, toxic, etc.).
The British Standard BS341 standard is equivalent to the Australia standard AS2473 resulting in common valve/ regulators components.
- Gas mixtures containing only inert, non-flammable gases will typically have a BS 341 No. 3 valve, which is equivalent to an AS2473 Type 10
- Gas mixtures containing inert & flammable gases will typically have a BS 341 No. 4 valve, which is equivalent to an AS2473 Type 20
- Gas mixtures containing any reactive gases will typically have a BS 341 No. 15 valve, which is equivalent to an AS2473 Type 45.
Valves for Liquid Mixtures
Liquefied gas mixtures must be handled differently to totally gaseous mixtures. Some gases can be liquefied when pressure is applied. This can make supply of these gases difficult, as some gases will turn to liquid when very little pressure is applied. This means the volume of such a gas in a cylinder is very limited. However, by supplying these gases in the liquid form, much more product can be filled in a single cylinder.
Because each of the different components in a liquid mixture will liquefy/evaporate at different pressures, as the cylinder empties, different components will remove themselves from the mixture as they turn back into gases.
To combat this we must keep all of the components under pressure, to ensure that all of the components remain liquids. To do this, we use a special valve called a Dual Port / Dip Tube valve.
A pressurised gas head, or overpressure, normally no more than 20BAR, must be maintained to keep all the components as liquid. If this overpressure drops, some of the components may turn back into gases, and therefore remove themselves from the liquid mixture.
Pure liquefied gases can be supplied with the typical valves listed above. However, when a mixture of more than one easily liquefied gas is required, it must be handled differently, but using a special type of valve which has a port both for outlet, and for maintaining the over pressure. These types of valves are called Dual Port / Dip Tube, or DP/DT valves.
A Dual Port / Dip Tube valve & liquefied gas mixture.
Internal vs. External Threads
When speaking about cylinder valves, the terms internal and external thread refer to the threads on the cylinder valve, as opposed to the valve connector’s threads. An external threaded valve’s threads will be on the outside on the mating surface. An internally threaded valve’s threads will be on the inside.
Handedness refers to the direction of the threads. A right-handed thread will tighten when turned clockwise. A left-handed thread will tighten when turned counter-clockwise. The term handedness is used because screw threads follow the right hand rule (see fig. 2).
Type of seal
There are typically two types of seals used with cylinder valves; flat face seals which use a washer, and angled face seals which form a seal when compressed together. If your valve uses a washer, the valve will not seal without it. It is best to be sure which valve type you have before attempting to connect a regulator, as the washers for flat face type valves can often be misplaced.
Here are the specifications of some commonly used valves:
If you are unsure about your valve connection, contact CAC Gas & Instrumentation for help with identification. Valves can typically be identified by photograph.