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.

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.

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. Parallel threads are more tolerant of repeated removal and refitting of the valve for inspection and testing.

There are several standards for neck threads, these include:

Parallel threads are made to several standards:

The 3/4"NGS and 3/4"BSP are very similar, having the same pitch and a pitch diameter that only differs by about 0.2 mm (0.008 in), 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.