Advantages of Gate Valves1. They have good shutoff characteristics.
2. They are bidirectional.
3. The pressure loss through the valve is minimal.
Disadvantages of Gate Valves
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 hand wheel or an actuator.
2. Gate valves require large space envelope for installation, operation, and mainte- nance.
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.
Construction of a Gate Valve
|Fig. A: Conduit valve|
The flexible wedge is also one-piece construction like a solid wedge, but areas behind the seating surfaces are hollowed out to provide flexibility. This construction compensates for changes in seat alignment for improved seating while maintaining the strength of a solid wedge in the middle. This design offers better leak tightness and improved performance in situations with potential for thermal binding.
The split wedge consists of two-piece construction which seats between the tapered seats in the valve body. The two pieces of split wedge seat flat against the valve seats as the stem is moved downward, and they move away from the valve seats when the stem is pulled upward.
In the wedge or disc-wedge types either a tapered solid or tapered split wedge is used. In the rising stem valves (Fig. B),
|Fig. B: Rising-stem solid-wedge gate valve for 250-psig steam service|
|Fig. C: Non-rising stem gate valve for 250-psig steam service|
|Fig. D: Split-wedge gate valve|
In the rising-stem type of valve, the upper part of the stem is threaded and a nut is fastened solidly to the hand wheel and held in the yoke by thrust collars. As the hand wheel is turned, the stem moves up or down. In the non-rising stem valve, the lower end of the stem is threaded and screws into the disc, vertical motion of the stem being restrained by a thrust collar. The rising-stem valve requires a greater amount of space when opened.
|Fig. E: Double-disc rising-stem flanged-end gate valve for 150-psig service|
|Fig. F: Double-disc non-rising-stem gate valve|
|Fig. G: Parallel-seat gate valve showing welded construction for high-temperature service with welded-in seat ring|
Unlike the wedge in a wedge-gate valve, which only comes into contact nearly closed, each disc in the parallel seat valve slides against its seat while the valve is being opened or closed. Consequently, these components must be made of metals, which do not gall or tear when in sliding contact with each other. The double-disc parallel-seat gate valve is often favored for high-temperature steam service because it is less likely to stick in the closed position as a result of change in temperature.
Conduit Gate Valve
Conduit gate valves require a large-space envelope because of their longer disc proportions to accommodate both the blank and the spacer halves of the disc assembly. The valve is closed by moving the blank half downward to block the valve port. The spacer is accommodated in the sump part of the valve body. Refer to Fig. A.
Conduit valves with Teflon (PTFE) seats can be used for low to intermediate temperatures (to 450 For 232 C).Metal-seated valves may be used for temperatures up to 1000 F (538 C).
Parallel seated gate valves are most suitable for applications having potential for thermal binding. Split-wedge or flexible-wedge type gate valves are expected to perform better than solid-wedge gate valves when thermal binding is a concern.
in the bonnet cavity when the system returns to operating temperature. This built-up pressure in the bonnet cavity can prevent the valve from opening and may cause damage to valve parts. See Fig. H.
|Fig. H: Pressure binding caused by built-up pressure in bonnet cavity|
|Fig. I: A hole on the upstream side of wedge to release built-up pressure in bonnet cavity|
● Install a small relief valve in the bonnet.
● Edward valves offer a new valve called ACEVE to solve this problem.
Typical Gate Valve Applications
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