what is a valve

As a mechanical device, a valve should be selected to do the job expected of it and should be properly installed. It will then give long service before it starts to leak or wear out. After installation, and periodically during service, a valve should be checked to ensure that it has the necessary seat tightness

When wear or leakage shows up, it will require some maintenance to restore the valve to its original efficiency. General maintenance methods for typical valves are described in this chapter, but the valve manufacturer’s literature should be consulted for specific procedures. Another helpful reference is MSS-SP-92, “MSS Valve Users Guide.” Wear occurs more frequently in globe or check valves, and features are built into these valves to facilitate maintenance or renewal of parts. The seat of all globe valves is directly opposite the top opening of the body, making it easy to get at the seat for inspection or repair. Gate valves are installed where they are not operated very often, and hence do not wear out quickly, and they do not as a rule have the maintenance features of globe and check valves.

Mechanical devices should be operated occasionally. Valves which are placed in lines and then forgotten may become hard to operate. This is especially so in hot-water lines, hard-water lines, or any other lines in which there is a tendency to deposit scale or solids. Valves actually have been known to scale up or coke up so badly over a period of years that they had to be disassembled and cleaned before they were usable.

The statement that a valve is used in connection with a pressure-containing vessel deserves consideration because the pressures and temperatures at which a valve of a given size may be used depend upon wall thickness and material of the pressure-containing parts. For the purposes of indicating the pressure temperature ratings of valves, the American National Standards Institute (ANSI) has established pressure class numbers (or classes for short). The pressure class number corresponds to the former steam pressure (SP) or primary pressure rating of the valve. Since the tensile and yield strength of valve materials is higher at room temperature than at the temperature of steam, the rating of a given class of value is higher at room temperature than at the temperature of steam. The pressure rating of the valve at ambient temperature (0 to 150″F for bronze and iron and 0 to 100″F for steel) is called the cold working pressure (CWP) rating of the valve. The CWP rating of the valve corresponds to the former water-oil-gas (WOG) or secondary pressure rating of the valve. The CWP is about two times the pressure class number for Classes 300 and below, and about 2.4 times the pressure class number for Classes 350 and higher, depending upon material and size.

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