TYPE AND SIZE – Select the best type of valve for the intended service (gate, globe, or check). Gate and globe valves are available with a selection of wedge or disc and body configurations for different services.Valves may be sized to match existing lines, but they should also be sized according to flow conditions for maximum service life.

RATINGS – Valves must only be used in design conditions within the appropriate ANSI pressure-temperature ratings for valve class rating and body/bonnet material (see Technical Data section).

TEMPERATURE LIMITATIONS – Only use valves within the temperature limitations of their construction. Temperature limitations of body/bonnet materials, trim, bolting and packing and gaskets must all be considered. Special construction for extended service temperatures are available on application.

MATERIAL AND SERVICE – Pressure Seal valves are available in a variety of body/bonnet materials and trims. Specified materials should be compatible with the service.

GATE VALVES

1. Gate valves are normally used for on-off service. They are not recommended for throttling service.
    
2. Gate valves are normally installed in horizontal pipe runs with the valve stem vertically up. They can also be installed in vertical or horizontal pipe runs with the valve stem other than vertical, but special construction may be required depending on valve size, service conditions and material. When purchasing valves for other than the normal installation, valve orientation should be specified when consulting the factory.
    
3. After closing a gate valve with sufficient force to develop shutoff, the stem should be backed off slightly (1/8 to 1/4 turn) to relieve stem load. This will enable the stem to expand slightly without bending or damaging the valve and will not affect valve shutoff.
    
4. Gate valves are offered in two designs: Parallel Disc and Flexible Wedge. Parallel Disc are position seated; Flexible Wedge are torque seated when closing.

Parallel Disc Gate Valve

Wedge Gate Valve

From: NVF valve knowledge