Gate Valve Working Principle
Gate valves are often used when minimum pressure loss and a free bore is needed. When fully open, a typical gate valve has no obstruction in the flow path resulting in a very low pressure loss, and this design makes it possible to use a pipe-cleaning pig. A gate valve is a multiturn valve meaning that the operation of the valve is done by means of a threaded stem. As the valve has to turn multiple times to go from open to closed position, the slow operation also prevents water hammer effects.
Gate valves are generally used for on/off service and are designed to operate fully open or fully closed. Because of excessive vibration and wear created in partially-closed gates, the valves are not intended for throttling or flow regulation. Gate valves are available in solid wedge, flexible wedge, split wedge, and double-disk styles. A typical gate valve and its major parts show in picture Wedge-type gate valves have a tapered wedge that wedges between two tapered seats when the valve is closed. The solid-wedge design is widely used and is suitable for air, gas, oil, steam, and water service. Flexible wedge gate valves are used in services that have a tendency to bind the solid-wedge design due to excessive variations in temperatures.
Gate Valve Parts :
BonnetsA bonnet protects the internal parts of a gate valve . It is screwed in or bolted to the valve body, creating a leak-proof seal. Therefore, it is removable for repair or maintenance purposes. Depending on applications, gate valves can have screw-in, union, bolted, or pressure seal bonnets.
Screw-in BonnetsScrew-in bonnets are the simplest in construction. They are common in small size valves and provide a durable leak-proof seal. Figure 1 shows a gate valve with a screw-in bonnet on the left.
Union Bonnets:Union bonnets are held in place by a union nut. The union nut sits on the lower edge of the bonnet and screws into the valve body’s threads. This type of design ensures that the leak-proof seal created by the nut does not deteriorate by frequent removal of the bonnet. Therefore, union bonnets are common for applications that require regular inspection or maintenance.
Bolted BonnetsBolted bonnets provide sealing in larger valves and higher pressure applications. In this type, the bonnet and valve body are flanged and bolted together. Figure 1 shows a gate valve with a bolted bonnet on the right.
Pressure Seal BonnetsPressure seal gate valves are ideal for high-pressure applications (more than 15 MPa). This type of construction uses internal pressure to create a better seal. Pressure seal bonnets have a downward-facing cup inserted into the valve body. When internal fluid pressure increases, the cup’s forced outward, improving the seal.
Best Suited Control:
- Fully open/closed, non-throttling
- Infrequent operation
- Minimal fluid trapping in line
Applications of gate valve :
- Suitable for oil, gas, air, heavy liquids, steam, non-condensing gases, abrasive and
- Sizes available range from standard cast configurations as small as 2″ to special
fabricated valves exceeding 100″.
- Standard cast configurations have ASME 125/150 bolting patterns and are rated at
Advantages of gate valve :
- High capacity
- Tight shutoff
- Low cost
- Little resistance to flow
- Ability to cut through slurries, scale and surface build-ups
- Provide unobstructed flow paths that not only provide high flow capacity (Cv), but
even allows slurry, large objects, rocks and items routinely found in mining processes
to safely pass through the valve.
Disadvantages of gate valve :
- Poor control
- Cavitate at low pressure drops
- Cannot be used for throttling
- Relatively low pressure limitation – general pressure limitations are 150 psi at maximum.