Valve Types

Gate Valves

Gate valves are primarily designed to serve as isolation valves. In service, these valves generally are either fully open or fully closed. When fully open, the fluid or gas flows through the valve in a straight line with very little resistance. Gate valves should not be used in the regulation or throttling of flow because accurate control is not possible. Furthermore, high-flow velocity in partially opened valves may cause erosion of the discs and seating surfaces. Vibration may also result in chattering of the partially opened valve disc. An exception to the above are specially designed gate valves that are used for low-velocity throttling; for example, guillotine gate valves for pulp stock.

Advantages of Gate Valves

1. They have good shutoff characteristics.

2. They are bidirectional.

3. The pressure loss through the valve is minimal.

Disadvantages of Gate Valves

The following are some of the disadvantages of gate valves that must be considered when selecting a gate valve for an application:

1. Gate valves are not quick opening or closing valves. Full-stem travel to open or close a gate valve requires many turns of its handwheel or an actuator.

2. Gate valves require large space envelope for installation, operation, and maintenance.

3. The slow movement of the disc near the full-closed position results in high-fluid velocities, causing scoring of seating surfaces, referred to as wire drawing. It also causes galling of sliding parts.

4. Some designs of gate valves are susceptible to thermal or pressure binding, depending upon the application.

5. In systems experiencing high-temperature fluctuations, wedge-gate valves may have excessive leakage past the seats due to changes in the angular relationship between the wedge and the valve seats caused by piping loads on the valve ends.

6. Repair or machining of valve seats in place is difficult.