Gate valves consist of a solid wedge mounted within the valve body, which is connected to the internal end of the valve stem. A handwheel actuator is connected to the external end of the valve stem for manual operation. When the valve stem is turned completely in the counterclockwise direction, the wedge is raised to the open position. When the valve stem is turned completely in the clockwise direction, the wedge is lowered to the closed position. Gate valves have a straight-through body configuration; that is, the flow exits the valve in the same direction that it enters the valve.
Gate valves are intended to be fully opened or fully closed; they are not suitable for throttling (i.e., functioning in a partially open position). When in the closed position, the metal-to-metal contact of the wedge within the seat does not provide absolute (bubble-tight) shutoff. Bubble-tight shutoff can be achieved by adding a resilient coating to the wedge. However, resilient wedge gate valves are normally rated for a maximum of 160°F, so their applications for HVAC piping systems are limited.
Gate valves are available in both rising-stem and nonrising-stem configurations. The rising-stem configuration is preferred because the stem threads are protected from the fluid flow. However, clearance must be provided external to the valve in order to allow for the raised stem when the valve is in the open position.
Rising-stem gate valves provide an indication of the valve’s position. If the stem is in the raised position, the valve is open; if the stem is in the lowered position, the valve is closed.
Nonrising-stem gate valves are used when clearance for the raised stem is not available. However, the threads of the valve stem on which the wedge travels are always exposed to the fluid flow. As a result, these threads can become eroded14 or corroded15 over time, which can prevent the valve from functioning properly