Gate Valve

1 General

Gate valves are the most commonly used valves used valves used in on-off services. They are used where stop valves are required and will normally stop flow in either direction, i.e., they are “bidirectional”. They are often used as isolation valves.

2 Throttling Characteristics

Gate valve should be used in either the fully closed fully open position; never in a throttling mode position, except for :
a. Throttling Gate valves which use a “V-shaped port” in the seat to allow throttling;
b. Throttling Knife Gate valves with a “V-shaped gate” to allow throttling;

3 General Flow Caracteristics

They offer low resistance to flow and, therefore, cause low pressure drop.

4 Stem/Yoke Configurations

Gate valves are manufactured with the following stem/yoke combinations :
a. Outside Screw and Yoke, Rising Stem (OS&Y RS)
b. Inside Screw Nonrising Stem (ISNRS)
c. Inside Screw Rising Stem (ISRS)
The OS&Y RS is most adaptable to power actuators. Also its open-close position is visually indicated by the projection of stem above the handwheel; this is an important operating advantage and is mandatory in services such as fire protection.

5 Disc Variations

There are basically four disc design variations :
a. The double disc, Parallel seat type
b. The through conduit type
c. The knife gate
d. The wedge type. This type valve is manufactured comes in four disc designs :
• Solid wedge
• Hollow wedge
• Flexible wedge
• Split wedge
The solid wedge gate valve is the simplest design. The hollow wedge design is used only for small valves in low temperature applications. The flexible wedge gate valve is less susceptible to thermal binding and seat leakage than the solid wedge; however, it is more susceptible to thermal binding and seat leakage than the split wedge or double disc. The split wedge disc valve performs well in services where thermal binding may occur as with a flexible wedge gate valve, however, this design may be susceptible to “chattering”. The multiple disc, parallel seat type gate valve has a double seated disc. The conduit gate valve is a form of slide gate. Most commonly, flexible wedge gate valves are used for higher temperature services requiring larger size, and solid wedge gate valves of smaller sizes are used in lower temperature applications. Knife gate valves are characterized by a gate-like disc in which the leading edge is tapered to form a knife edge for cutting through suspended solids or scale accumulated against the seat. This design usually uni-directional with a solid seat ring on one side of the gate and widely spaced lugs on the order side to guide the gate; fluid pressure presses the gate against the seat. Most knife gates designs are either wafer style or have flanged ends, and are of the OS&Y design, usually bonnetless. Knife gate valves are used in “dirty” applications for fluids with suspended solids and scalling potential, such as return water service in bottom ash conveying systems.

6 Limitations

Limitations of gate valves are as follows :
a. Metal to metal seated gate valves, typically, do not close buble-tight, therefore, they should not be used where high leak tightness is required, e.g., in hazardous fluid service.
b. Solids or impurities could settle in the seat and make tight closing of the valve difficult or impossible in “dirty” or slurry service.
c. If the fluid is a scaling fluid, the seat groove may scale and make tight closing of the valve difficult.
d. Gate valves in large sizes are relatively expensive, compared to butterfly valves, for example.
e. In high temperature service some gate valves have the potential for developing a problem known as “thermal binding”.
- Wedge type gate valves may have a this problem. In simplified terms, this condition can occur when a valve is closed tightly while the high temperature side is still in operation. Subsequently, when the system is shutdown, and cooldown takes place, seats move inward more than the wedge shrinkage occurring during cooldown.
- Therefore, the differences in thermal contraction can bind the wedge to the seats tightly enough so that the valve cannot be reopened until the reactivated system temperature reheats the valve .
- To perform a quantitative analysis of this effect is quite complicates, therefore, when it appears likely that this phenomenon can occur, the preferred way in which the problem is dealt with is to specify a valve which has a low susceptibility to thermal binding.
f. Seat distortion caused by piping loads can lead to continuos leakage. In addition to thermal binding, wedge gate valves, in systems where severe temperature changes occur, are also subject to excessive seat leakage due to changes in angular relationship between the wedge and the seat faces caused by loads exerted on the valve ends.
g. “Pressure locking”(also known as “hydraulic” or “pressure binding”) is a problem which can occur when trapped liquid in the body-bonnet cavity of a closed double disc or split wedge gate valve is heated and the resulting high pressure loads the two disc pieces to exert a force which is too high for the operator to overcome. This pressure, in some cases, may exceed the rated capacity of the valve and result in “bonnet overpressurization”. The solutions to these problems may include the following :
- Drilling a hole in the upstream disc half;
- Piping the body-bonnet cavity to a lower pressure area, usually the valve downstream end.
h. Some disadvantages of gate valves include :
- The full stem travel requires many turns of a handwheel and many more turns when fitted with gearing.
- Due to the relatively long stem stroke and the outside appurtenances necessary to accommodate this stroke, a large space envelope is required.
- Body seat surfaces for a wedge type date valve may be difficult to machine or refinish due to the wedge angle, which is usually 3-5 degrees.
- Very little or slow movement of the disc near the fully closed position (which caused high velocity flow) may result in scoring of the sliding parts and seating surfaces (this phenomenon is often called “wire drawing”).
- Gate valves subjected to high pressure in the closed position must be equipped with small by-pass valves. These by-pass valves (normally globes) are opened first to equalized the pressure; then the gate valve may be opened.