Scuba cylinder valve
A scuba cylinder valve or pillar valve is a high pressure manually operated screw-down shut off valve fitted to the neck of a scuba cylinder to control breathing gas flow to and from the pressure vessel and to provide a connection with the scuba regulator or filling whip. Cylinder valves are usually machined from brass and finished with a protective and decorative layer of chrome plating. A metal or plastic dip tube or valve snorkel screwed into the bottom of the valve extends into the cylinder to reduce the risk of liquid or particulate contaminants in the cylinder getting into the gas passages when the cylinder is inverted, and blocking or jamming the regulator.
Cylinder valves are classified by four basic aspects: the thread specification for attachment to the cylinder, the connection to the regulator, pressure rating, and some functional distinguishing features. Standards relating to the specifications and manufacture of cylinder valves include ISO 10297 and CGA V-9 Standard for Gas Cylinder Valves.Structure of the valve
The valve body is usually machined from a solid brass casting or forging, which is screwed into the cylinder neck thread, and sealed by o-ring or thread tape. The outlet is machined to fit one of the standard scuba regulator connection systems, and a gas passage is provided from the interior of the cylinder to the regulator connection. Control of gas flow through the gas passage is by opening and closing a valve orifice machined into the valve body, by turning the valve knob to drive the valve spindle which moves the valve seat towards or away from the orifice. The spindle engages with the valve seat by a flat and slot or a square socket on the inner end of the spindle, which passes through the spindle seal in the valve bonnet. Rotation of the seat drives it along its axis on a screw thread concentric with the orifice. The spindle is usually sealed by an O-ring where it passes through the bonnet, and axial loads on the spindle are usually carried by a teflon or similar low friction coefficient washer. Other arrangements have been used, but the one described is very common and is known as a balanced valve because the pressure of the gas in the cylinder is exerted on both sides of the valve seat when it is not sealed, because the gas can leak past the threads of the seat. Historically, two other spindle arrangements were also used, the unbalanced valve where the periphery of the seat is sealed, and the glandless valve, where the valve seat does not rotate, but is sealed into the valve body behind a diaphragm. The valve outlet is connected to a regulator for diving, or a filling whip for charging. The valve must be open for these operations, and closed to keep the gas inside the cylinder for storage.Cylinder neck threads
The neck of the cylinder is the part of the end which is shaped as a narrow concentric cylinder, and internally threaded to fit a cylinder valve. Cylinder threads may be in two basic configurations: Taper thread and parallel thread. The valve thread specification must exactly match the neck thread specification of the cylinder. Improperly matched neck threads can fail under pressure which can have fatal consequences.
There are several standards for neck threads, these include:
- Taper thread, with a 12% taper right hand thread, standard Whitworth 55° form with a pitch of 14 threads per inch and pitch diameter at the top thread of the cylinder of. These connections are sealed using thread tape and torqued to between on steel cylinders, and between on aluminium cylinders.
Parallel threads are made to several standards:
- M25x2 ISO parallel thread, which is sealed by an O-ring and torqued to on steel, and on aluminium cylinders;
- M18x1.5 parallel thread, which is sealed by an O-ring, and torqued to on steel cylinders, and on aluminium cylinders;
- 3/4"x14 BSP parallel thread, which has a 55° Whitworth thread form, a pitch diameter of and a pitch of 14 threads per inch ;
- 3/4"x14 NGS parallel thread, sealed by an O-ring, torqued to on aluminium cylinders, which has a 60° thread form, a pitch diameter of, and a pitch of 14 threads per inch ;
- 3/4"x16 UNF, sealed by an O-ring, torqued to on aluminium cylinders.
- 7/8"x14 UNF, sealed by an O-ring.
The 3/4"NGS and 3/4"BSP are very similar, having the same pitch and a pitch diameter that only differs by about, but they are not compatible, as the thread forms are different.
All parallel thread valves are sealed using an O-ring at the top of the neck thread which seals in a chamfer or step in the cylinder neck and against the flange of the valve.Connection to the regulator
A rubber O-ring forms a seal between the metal of the cylinder valve and the metal of the diving regulator. Fluoroelastomer O-rings may be used with cylinders filled with oxygen-rich breathing gas mixtures to reduce the risk of fire. There are two basic types of cylinder valve to regulator connection in general use for scuba cylinders. They are both very widely used for cylinders containing air and in many countries also for other breathing gases for diving:Yoke connectors
The yoke connector, also known as an A-clamp or international connector, is a component of the regulator that fits around the valve body at the outlet and presses the outlet O-ring of the valve against the inlet seat of the regulator. The connection is officially described as connection CGA 850 yoke. The yoke clamping screw is screwed down snug by hand to ensure metal to metal contact between the valve and regulator to sufficiently constrain the O-ring against extrusion. Overtightening can make the yoke impossible to remove later without tools. The seal is created by clamping the O-ring mounted in a groove on the face of the valve between the surfaces of the regulator and valve. When the valve is opened, cylinder pressure expands the O-ring against the outer surface of the O-ring groove in the valve and the face of the regulator inlet. This type of connection is simple, cheap and very widely used worldwide. Several O-ring sizes are in use, and both overall and section diameters may vary, but the correct size for the valve is necessary for a reliable seal and so that the O-ring does not easily fall out during handling and storage. It has a maximum pressure rating of 240 bar, and is not well protected against overpressurisation. Insufficient clamping force may allow the pressure to slightly stretch the yoke structure, opening a gap between the sealing faces of the valve and the regulator sufficient to extrude the O-ring through the gap, resulting in a potentially catastrophic leak. A similar effect can occur if the first stage is bumped against the environment, flexing the yoke enough to open a gap. When underwater this is most likely in an overhead environment where the diver cannot make an immediate emergency ascent. The risk of this cause for O-ring extrusion is roughly proportional to the pressure in the cylinder, and is less for a more rigid yoke structure. Older regulators may have a yoke rated at 200 bar, and these may not fit over more recent 240 bar valves.DIN connectors
In the DIN screw thread connectors, the regulator screws into the cylinder valve, trapping the O-ring securely between the sealing face of the valve and the O-ring groove in the regulator. These are more reliable than A-clamps because the O-ring is well protected and the assembly is considerably more rigid, and has a lower profile, making O-ring extrusion under impact less likely, but operators in many countries do not widely use DIN filling connectors on compressors, or cylinder valves which have DIN fittings, so a diver traveling abroad with a DIN system may need to take an adaptor, either for connecting the DIN regulator to a rented cylinder, or for connecting an A-clamp filler hose to a DIN cylinder valve. The DIN connection is slightly more complex to manufacture, but if the seal is good when the valve is opened it is likely to remain good throughout a dive, even if banged against a solid overhead, and is consequently preferred by technical divers even where the yoke fitting is more generally popular. DIN connections are available in two specifications; for working pressures up to 232 bar, and for 300 bar. The original design 200 bar regulator fitting with five threads will not seal in a 300 bar valve, preventing potential overload, particularly of the high pressure hose and submersible pressure gauge, but the DIN 300 bar regulator inlet fitting with seven threads available on almost all recent regulators is compatible with 200 and 232 bar valves as well as the 300 bar valves. The thread form is G5/8" x 14 tpi. The O-ring is carried in a groove on the regulator. Two sizes of O-ring are in common use.Adaptors
Adaptors are available to allow connection of DIN regulators to yoke cylinder valves, and to connect yoke regulators to DIN cylinder valves. There are two types of adaptors for DIN valves: plug adaptors and block adaptors. Plug adaptors are screwed into a 5-thread DIN valve socket, are rated for 232/240 bar, and can only be used with valves which are designed to accept them. These can be recognised by a dimple recess opposite to the outlet opening, used to locate the screw of an A-clamp. Block adaptors are generally rated for 200 bar, and can be used with almost any 200 bar 5-thread DIN valve. A-clamp or yoke adaptors comprise a yoke clamp with a DIN socket in line. They are slightly more vulnerable to O-ring extrusion than integral yoke clamps, due to greater leverage on the first stage regulator.Conversion kits
Several manufacturers market an otherwise identical first stage varying only in the choice of cylinder valve connection. In these cases it may be possible to buy original components to convert yoke to DIN and vice versa. The complexity of the conversion may vary, and parts are not usually interchangeable between manufacturers. The conversion of Apeks regulators is particularly simple and only requires an Allen key and a ring spanner.