Gas Cylinder Valves
Gas cylinder shells are open-ended. To allow a gas to be contained within the gas cylinder shell a valve is fitted into the open end, at the neck. Once a valve is fitted the assembly is referred to as a cylinder. The valve may have several functions.
- It provides a closure to contain the contents within the cylinder. In legal texts a valve is sometimes referred to as a 'closure'.
- It provides an outlet port to allow gas to be released at a controlled rate.
- It provides an inlet port to allow a cylinder to be filled at a controlled rate.
- it can accommodate safety devices, such as non-return valves, pressure relief valves and residual pressure devices.
- It can incorporate an integrated pressure regulator, often referred to as a valve with an integrated pressure regulator (VIPR).
- It can incorporate an outlet flow regulating device.
- It can incorporate a contents gauge.
- It can provide a connection for external components, such as regulators and pipelines.
- It can provide a means of attaching a valve guard.
Gas cylinders and their associated valves that are, or are intended to be, used to transport dangerous goods have to comply with the Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations.
Valves are designed, constructed, inspected, tested and approved against authorised standards. Due to the many properties of the various gases, valves have to be manufactured from compatible materials. Valves should not be used if their working pressure is below that of the cylinder.
Some common valve standards include:
- BS 341, Transportable gas container valves.
- BS EN ISO 10297, Gas cylinders. Cylinder valves. Specification and type testing.
- BS EN ISO 11118, Gas cylinders. Non-refillable metallic gas cylinders. Specification and test methods, which has a section devoted to valves used in non-refillable cylinders.
- BS EN ISO 15996, Gas cylinders. Residual pressure valves. Specification and type testing of cylinder valves incorporating residual pressure devices.
- BS EN ISO 22434, Transportable gas cylinders. Inspection and maintenance of cylinder valves.
- BS EN ISO 22435, Gas cylinders. Cylinder valves with integrated pressure regulators. Specification and type testing.
Gas cylinder valves (particularly VIPRs) are usually allocated a service life by the manufacturer and are always marked with a date of manufacture. It is helpful to fit valves which have same length of usable service life as the inspection and test period of the gas cylinders to which they are assembled. This will allow the cylinder periodic inspection and test date and the valve maintenance dates to align. Historically, valves have had typical service lives of 10 years, however, where cylinders can now have an extended periodic inspection and test life of up to 15 years (if fitted with a residual pressure device), an extended valve service life may need to be taken into consideration. For information on the service life of a valve, including the requirements for extending the valve service life, refer to BCGA TIS 46, Transportable gas cylinders. Valve life.
Certain valves, are designed to withstand mechanical impact (for example, from a cylinder falling over), without needing additional protection. However, the majority of valves are protected from impact and general damage in a variety of ways. The following methods are common:
A valve guard - a device that is securely fixed around the valve, but stands taller than the highest point of the valve. Guards can be of cast or welded metallic construction or made from non-metallic moulded plastics. They are often attached to the cylinder shell via a neck ring with a screw mounting, a clamp design or a circlip. In some cases, particularly in small medical cylinder service, the guard may be attached to the valve rather than to the cylinder shell although this imposes extra stress on the valve in an impact situation. Guards do not need to be removed for access to connect to the cylinder or to operate the valve.
A valve shroud - a type of valve guard but which is an integral part of a welded cylinder or pressure drum, most commonly seen on liquefied petroleum gas (LPG) or refrigerant gas service.
A valve protection cap - a cover that is securely fixed to the cylinder neck ring and which covers the valve during handling, transport and storage. Caps have to be removed for access to the valve to allow connection and operation. They are designed not to contact the valve or the hand wheel, even in an impact situation (which offers maximum protection to the valve).
The valve outlet port may be protected from tampering and contaminant ingress during transport and storage by:
- A temporary covering, such as a plastic sleeve, often heat shrunk over the outlet port.
- A specialist security cap, typically manufactured from plastic, which requires a small amount of effort to break open. Once open it cannot be refitted.
- A removable (plastic) plug, that screws onto the thread in the outlet port.
- A re-useable fill port plug, (which may be gas-tight) particularly for very high value, high risk or high purity gases.
Globally, there are many standards for cylinder valve outlets. Modern ISO standards have attempted to create a small number of internationally recognised standards for valve outlets to prevent the interconnection of non-compatible gases and pressures, but there remain many cylinder valve outlets in use which are based on legacy national standards. Care should therefore be taken to ensure that only appropriate compatible connections are made.
A valve outlet is designed and designated for a specific pressure and gas, or specific hazard group. For example, valves for non-flammable gases are fitted with right-hand threaded valve outlets, whereas valves used for flammable gases are normally fitted with left-hand threaded valve outlets (although diameter indexed valves for ultra-high purity gases are all right hand threaded).
Connectors for valve outlets with a left-hand thread are often identified by having a notch cut into the points of the hexagonal connecting nut.
Diagram: Component with a left-hand thread.
Some common valve outlet standards include:
- BS 341, Transportable gas container valves.
- BS EN ISO 407, Small medical gas cylinders. Pin-index yoke-type valve connections.
- BS ISO 5145, Cylinder valve outlets for gases and gas mixtures. Selection and dimensioning.
It is notable that the legacy UK valve standard BS 341 has fewer outlet connections when compared with most legacy European national standards. Specifically, UK valves for inert gases ≤ 300 bar (nitrogen, helium, argon, etc.) share a BS 341-3, 5/8" right hand female outlet which is also used for oxygen. Most, but not all, other national outlet standards have a separate oxygen outlet. BS ISO 5145 addresses this for pressures above 300 bar, refer to BCGA TIS 10.
Medical pin index valves use a special system of outlets where separate pin arrangements exist for oxygen valves as well as many other medical gases. It is used throughout the UK and Europe through the adoption and use of BS EN ISO 407 an equivalent system exists in USA.
Valve outlets may be configured as a top outlet or as a side outlet. More complex valves may have more than one type of valve outlet.
Do not use an adaptor to connect equipment to the valve outlet if it is not compatible. Never use sealant or PTFE tape to create a pressure seal. A correctly specified and manufactured connection will not require it.
Valves with a dip tube
Some gas cylinders that are used to contain liquefied gases incorporate valves fitted with dip tubes. These may be identified by a white line painted on the side of the cylinder, and / or an ‘indicator' ring under the valve.
Dip tubes can be of various lengths and designs and are fitted to the inlet of the valve (inside the cylinder). They can have several functions in use, such as allowing liquid to be withdrawn, enabling a homogenous gas mixture during filling, and maintaining an adequate ullage space within the cylinder. Some cylinders in medical and breathing gas services have very short dip tubes fitted called anti-dust tubes. These are intended to prevent debris in the cylinder entering the valve during customer use, particularly when the cylinder may be being used inverted or in a non-upright orientation.
Some valves may have twin outlets, allowing a choice between gas or liquid delivery.
Remove any valve outlet protection which may be fitted. Carry out a visual inspection of the gas outlet. Ensure it is clear of any contaminants. Any visible material or moisture should be removed by cleaning with a clean, dry, lint-free cotton cloth. Do not apply any oils or greases. For advice on connecting gas cylinders refer to BCGA TIS 22.
It is rarely necessary ever to open a valve without a regulator connected or other connection being made. Such connections should be initially made by hand and then an appropriately size tool used to make the connection leak-tight.
Valves on the majority of gas cylinders are opened by turning the operating device (normally a handwheel or a spindle key drive) anti-clockwise and closed by turning the device clockwise. Spindle keys are available from your gas supplier.
Before opening a valve, point the outlet away from personnel. Valves are only to be opened slowly. When in use an opened spindle should NEVER be left against the backstop (i.e. the fully open position), but should be turned back at least half a turn to avoid seizure in an open position.
NOTE: Whilst this applies to the majority of valves, there are valves with a specific operating requirements. It is therefore important that the correct operating procedures are fully understood before attempting to operate a valve.
When the cylinder is in use and the valve is open, keep the spindle key inserted into the valve. In-use a cylinder should always be secured to prevent it toppling over. Be aware that a loose cylinder falling over with a spindle key inserted can cause severe damage to the valve as the key can act as a lever, multiplying the force.
Valves should be checked for leaks using an appropriate leak detection fluid. Care needs to be taken regarding the choice of leak detection fluids. Many valves are manufactured from non-ferrous material such as brass or bronze, however, as copper-based materials, they are susceptible to stress corrosion cracking if exposed to amines or ammonia, yet many commonly available leak detection fluids intended for domestic gas applications contain these chemicals. For further information refer to EIGA 78, LDF use with Gas Cylinders.
Always connect a gas cylinder to the item being charged via a regulator and any appropriate pressure system components. Note that some valves are designed complete with an integrated regulator, such as a VIPR, which include pressure and sometimes flow regulation, making them suitable for connection directly to user devices such as welding torches or breathing masks.
Where the gas cylinder valve assembly has a residual pressure device and/or a non-return valve fitted the user shall not interfere with or remove these devices. A residual pressure device retains a positive gas pressure inside the cylinder. This retained pressure prevents the possible ingress of contaminants into the cylinder should the valve not be closed after disconnecting a regulator. The non-return feature prevents back flow from a customer's process whenever the cylinder is at a lower pressure than the application (involving a fluid) to which it is connected. The use of these devices will have been assessed by the cylinder owner (usually the gas supplier) and will form part of the construction requirements of the cylinder and valve assembly.
Connecting a valve to a cylinder
There are a variety of threaded connections available for connecting a valve to a gas cylinder. The choice of valve will be determined by the gas supplier. The inlet thread on the valve must be compatible (to the same dimensional standard) with the neck thread on the gas cylinder. Before fitting a valve, it should be inspected to ensure it is serviceable, it is suitable for the gas contained within the cylinder and it has sufficient life remaining, for example, to allow its use until the gas cylinder is due its next inspection and test.
The type of threaded connection used is important in terms of achieving a seal without causing valve material strain. There are two common types:
Tapered thread - these seal through the use of a sealant on the threads, such as PTFE tape, and the application of a specified torque. ISO examples include 17E, 18T and 25E (which replaced the BS 341, 1").
Parallel thread - these seal against a captured ‘O' ring seal under the shoulder of the valve body, as the valve is clamped down on to the flat face of the cylinder. For this type of seal no additional sealant shall be used. ISO examples include M18, M25 and M30.
NOTE: Any materials, including seals and sealants, that are in contact with the gas shall be compatible with the gas.
Of particular importance is the torque that is applied to secure the valve in the cylinder. Torque settings are detailed in standards, such as BS EN ISO 13341, Fitting of valves to cylinders, which also provides information on the use of PTFE tape. Advice on torque settings can also be obtained from the valve or cylinder manufacturers.
Disconnecting a valve from a cylinder
Cylinders contain gases under pressure. Valves should only be removed under the authority of the cylinder owner taking all necessary precautions to do so safely, refer to ISO 25760. Gas cylinders. Operational procedures for the safe removal of valves from gas cylinders, or EIGA Safety Information 18, Devalving gas cylinders
In-use, valves shall never be removed or exchanged by customers.
Valves in operation
There are two main types of valve mechanism for controlling the flow of gas.
The simplest design makes use of an ‘O' ring in the gland area to prevent escape of gas to atmosphere via the spindle. This operates in a similar manner to a domestic water tap. This type of valve is acceptable for general use and is used in very great numbers throughout the world.
A more sophisticated design utilises a diaphragm to seal the gland. The spindle is connected to a flexible diaphragm, which it moves up and down to open and close the flow. A diaphragm design prevents any possible gas flow past the spindle. This type of valve is used to control high value, high purity or high risk gases, where even small leaks are undesirable, for example in laboratory situations.
Both types of valve use a spindle to close off the gas passage to / from the cylinder. The gas seal is normally achieved by using a soft non-metallic material as this provides a tight seal using relatively low forces by the user. On some aggressive gas duties, a hard seal (metal to metal) is used, but these designs, in general, require a greater force to achieve a gas seal.
Storage and transport
When a gas cylinder is not in use, for example when in storage or being transported, the valve should always be closed. This will not only stop gas escaping, but on a nominally empty cylinder it will also prevent atmospheric contamination entering the cylinder and help to maintain its integrity.
Where provided, valve protection caps should always be fitted.
If you have a gas cylinder with a damaged valve, do not use the cylinder. Seek the advice of your gas supplier. If safe to do so, move the gas cylinder to a secure place and quarantine it until appropriate action is taken.
If you are not sure who your gas supplier is, refer to the Cylinder Recovery & Disposal page.
Within BCGA gas cylinder valves for industrial gases are the responsibility of Technical Sub-Committee (TSC) 2. Gas cylinder valves for medical gases are the responsibility of TSC7. Members can access information on TSC2 and TSC7 via the ‘Members' area.
BCGA publish many publications providing advice and guidance on how to use, store, transport and handle gas cylinders safely. All BCGA publications are accessible via the ‘Publications' page. The following are of interest:
Gas cylinder valve services
BCGA member companies can provide gas cylinder valves and/or a range of services and associated equipment.