Friday, October 11, 2013

Gate Valve Types, Construction, Applications and Advantages

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 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

Gate valves consist of three major components: body, bonnet, and trim. The body is generally connected to the piping by means of flanged, screwed, or welded connections. The bonnet, containing the moving parts, is joined to the body, generally with bolts, to permit cleaning and maintenance. The valve trim consists of the stem, the gate, the wedge, or disc, and the seat rings.
Conduit valve
Fig. A: Conduit valve
Two basic types of gate valves are the manufactured-wedge type and the double-disc type, and there are several variations within each of these types. A third type of gate valve, called conduit valve, is shown in Fig. A.

Wedge Type

There are four types of wedges: solid, hollow, split, and flexible wedge. The solid wedge is a single-piece solid construction. It does not compensate for changes in seat alignment due to pipe end loads or thermal fluctuations. As such it is most susceptible to leakage. Except for NPS 2 (DN 50) and smaller, solid-wedge discs are generally not recommended for use in applications having temperatures in excess of 250 deg F (121 deg C). Solid-wedge gate valves are considered the most economical. Almost all small, NPS 2 (DN 50) and smaller, gate valves are solid-wedge gate valves. Solid-wedge gate valves are generally used in moderate to lower pressure-temperature applications. It is common practice to use cast iron or ductile iron solid-wedge gate valves in cold or ambient water lines.

A hollow wedge is a variation of solid wedge with the exception of a hole in the center. The hollow wedge travels along the stem when the threaded stem is rotated, thus opening or closing the valve port.

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), 
Rising-stem solid-wedge gate valve for 250-psig steam service
Fig. B: Rising-stem solid-wedge gate valve for 250-psig steam service
the operating threads are out of direct contact with the fluid or gas. The non-rising stem type (Fig. C) 
Non-rising stem gate valve for 250-psig steam service
Fig. C: Non-rising stem gate valve for 250-psig steam service
is preferred where space is limited and where the fluid passing through the valve will not corrode or erode the threads or leave deposits on the threads. Also, the non-rising stem valve is preferred for buried service. When the valve is closed, the gate disc is wedged on both sides against the seat. In split-wedge gate valves (Fig. D), 
Split-wedge gate valve
Fig. D: Split-wedge gate valve
the two-piece wedge disc is seated between matching tapered seats in the body. This type is preferred where the body seats might be distorted due to pipeline strain.

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.

However, it is generally preferred because the position of the stem indicates at once whether the valve is open or closed. Non-rising stem valves are sometimes provided with an indicator for this purpose.

Double-Disc Type

In the double-disc parallel-seat valves (Figs. E,F and G), 
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
the discs are forced against the valve seats by a wedging mechanism as the stem is tightened. Some double-disc parallel seat valves employ a design which depends mainly upon the fluid pressure exerted against one side of the disc or the other for its tightness. The major advantage of this type is that the disc cannot be jammed into the body, an action that might make it difficult to open the valve. This is particularly important where motors are used for opening and closing the valve.

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

It is also referred to as a slide valve or parallel slide. The disc surfaces are always in contact with the body seats. Like the double-disc or parallel-seated gate valve, its disc seats against the downstream seat, depending on the flow direction. The inside diameter of a conduit gate valve is equal to the inside diameter of the connecting pipe. These valves are used in pipelines where pigs are run through the piping to perform cleaning of builtup deposits or debris. The typical applications of conduit valves include dirty river water with suspended solids or water with sludge or debris.

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).

Thermal Binding

Thermal binding occurs when a valve is tightly shut off while the high temperature system is in operation. Later when the system is shut down and allowed to cool, thermal contraction of the valve seats move inward more than the wedge shrinkage. This can bind the wedge and seats tight enough to not allow the wedge to unseat or move when the hand-wheel or the valve actuator is activated to open the valve.

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.

Pressure Binding

Sometimes in high-temperature applications, the flow medium, such as water or steam, is trapped in the valve bonnet area when the valve is closed for system shutdown. The valves that do not permit this trapped liquid or the condensate to reenter the piping either upstream or downstream may experience excessive pressures
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
Pressure binding may not occur if the leakage past the upstream seat is adequate to prevent over pressurization of the valve bonnet cavity. The following options offer solutions to this problem:
● Drill a small hole on the upstream side of the disc. See Fig. I.
Fig. I: A hole on the upstream side of wedge to release built-up pressure in bonnet cavity
● Install a small manual stop valve between the valve bonnet-neck and the upstream end of the valve. This valve shall be opened during start up.

● Install a small relief valve in the bonnet.

● Edward valves offer a new valve called ACEVE to solve this problem.

Typical Gate Valve Applications

Socket or butt-welding end-gate valves in air, fuel gas, feedwater, steam, lube oil, and other systems are typical applications. Threaded-end gate valves may be used in air, gaseous, or liquid systems. Concern for leakage from threaded connection can be addressed by seal welding the threaded connection or by using thread sealants, as appropriate. In low-pressure and low-temperature systems such as fire protection systems’ water piping or water distribution pipelines, flanged gate valves are commonly used.
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