GAS CYLINDER VALVES

Navy standard gas cylinder valves are of two basic designs: packed valves and diaphragm-type packless valves.

Packed valves require a packing material around the valve stem to prevent leakage. The valve stem is packed to prevent gas from leaking out around the stem when the valve is open. MIL-V-2 covers the authorized packing material for gas cylinder valves.

Packless valves are sealed against leakage around the valve stem by flexible metallic diaphragms securely clamped to the valve bonnets. The basic packless design may be classified into two types: nonbackseating and backseating. The nonbackseating type is designed so that the metallic sealing diaphragms may not be replaced under pressure. In the backseating type the metallic diaphragms maybe replaced without undue hazard or loss of contained gases if the outlet cap is in place and secure. Diaphragms in packless valves should be replaced only by activities carrying spare diaphragms specifically designed for the valves in need of reconditioning. These diaphragms are made from materials selected for service at varying high pressures. In addition, they are often designed only for use with valves built by a given manufacturer and for a specific gas.

The Navy gas valve program (and concerned civilian agencies) provides noninterchangeable valve outlets and connections for different gases to prevent use of the wrong gas at any time.

Construction and Identification of Valves

Valves designed to control the flow of compressed gases are forged of brass, bronze, or steel, and are made in various shapes and sizes.

Figures 5-52 and 5-53 show typical gas cylinder valves. The valves are opened and closed with either hand-operated or wrench-operated spindles. When the valves are open, gas flows through a threaded male cylinder connection into the valve body and past the valve outlet connection into the pressure regulator. (Pressure regulators reduce the pressure of compressed gases from the cylinder pressure to the desired working pressure. All regulators are marked with the name of the gas for which they are intended.)

To prevent leakage of gas above the valve stem when the valve is opened, each valve is equipped with asbestos, leather, or rubber packing or metal diaphragms. Most valves have safety devices. (The safety devices for acetylene and ammonia are in the cylinders rather than in the valves.) These safety devices consist of fusible metal plugs, rupture disks, or both. Spring-loaded safety devices are used for some gases. If heat causes too much pressure, the fuse plugs melt and the disks burst, releasing the contents of the cylinder. Acetylene valves have screens in the cylinder connections; other valves do not.

Valves manufactured according to the latest military specifications have the name of the gas, or service for which they are designed, indented

Figure 5-52.-Oxygen cylinder gas valves: (A) external view of one type of valve, with related safety and outlet-connection caps; (B) cutaway view of same valve, showing diaphragms to prevent gas leakage when valve is opened; (C) cutaway view of another type of valve, with asbestos packing to prevent gas leakage when valve is opened.

Figure 5-53.-Cutaway view of an acetylenecylinder valve, showing asbestos packing.

on at least one of the flats on the sides of the valves. Valves must be used only for the gases or fluids indicated. Otherwise, personnel may be injured or the equipment may be damaged.

Safety Devices for Valves

Military specifications and ICC regulations require that valves designed for certain services be fitted with safety devices. These devices guard against a buildup of hazardous pressures caused by heat. This can easily happen with CO2 cylinders used for fire fighting in fire-rooms and enginerooms. Pressure can also build up from overcharging or similar causes. These safety devices may be divided into four general categories based on functional design as follows:

1. FUSIBLE PLUGS. A fusible plug may be described as a threaded hex-head plug with a center filled with fusible metal. When the cylinder is overheated, the fusible metal melts and permits the gas to escape. This type of device is used on chlorine, freon, acetylene, and such gases.

2. SPRING-LOADED SAFETY DEVICE. These devices usually function as "pop" valves that open to release excess pressure when pressure in the cylinder overcomes spring tension. Devices of this sort are used on liquefied petroleum gas valves. They operate generally at about 150 percent of the cylinder's ICC-approved pressure.

3. UNBACKED SAFETY CAP WITH RUPTURE DISK. This safety device is essentially a safety cap that covers a safety port in the valve. The cap retains a breakable disk firmly over the safety port. Pressures ranging from 2,600 to 3,000 psi will rupture the safety disk and allow the gas in the cylinder to vent to the atmosphere. This type of safety device is used in carbon dioxide service,

4. BACKED SAFETY CAP WITH RUP-TURE DISK. Backed safety caps with rupture disks are essentially the same as those described in paragraph 3 above. However, the breakable disk is supported by fusible metal contained in the safety cap thus blocking off escape ports. This cap works when the cylinder, valve, and therefore the fusible metal, are heated above the melting temperature. When the pressure within the cylinder reaches 2,600 to 3,000 psi, the breakable disk ruptures and reduces the pressure. This type of device is used commonly on air, argon, helium, hydrogen, nitrogen, and oxygen valves.