Gate Valve

What is a Gate Valve?

A gate valve, as the name suggests, controls media flow by lifting or dropping a gate. A gate valve is an on/off valve, meaning that it should only be used either fully opened or fully closed, and not to regulate flow, since the relation between the vertical position of the gate and the flow rate is non-linear [1]. In fact, operating in a half-open state could cause erosion of the disc and seating surfaces, as well as vibration and noise. Operation of the valve is done through a clockwise to close (CTC) or clockwise to open (CTO) rotational motion of the stem [2]. 

Advantages:

• Very little pressure loss [2]
• Open bore (open pathway - no obstruction of the flow path) 
• The open bore design also allows for the stem to be removed and valve to be easily cleaned
• Suitable for high pressure and temperature applications
• Bi-directional
• Require little maintenance

Disadvantages:

• No flow-regulating control [2]
• Slow in operation (this is also an advantage as it reduces chances of water hammering effect)
• Vibrations and noise when partially open
• Repairs are difficult due to limited access

Gate Valves Types and Designs:

Many components of the gate valve can be used to sort the valve into different types. The following are some of the most common differentiations:

Parallel vs wedge-shaped gate valves

• • A parallel gate valve contains two parallel seats, and the gate itself is a flat disc. This design leverages line pressure to create a better seal [1]. The center of the gate can also be spring loaded to push each side towards the seat for tighter seals on both ends. Parallel gates are used in low pressure and when the change in pressure is low, as well as when tight shut off is not as important [5].
  • A wedge-shaped gate valve contains two inclined seats with a wedge shaped gate. The design allows for a better seal when the seats are slightly mismatched. A wedge shaped gate also take advantage of a wedging force created by tightening the stem to assist in better sealing [5]. A wedge shaped gate minimizes sticking to the seats when high differential pressure is present, and slows down the wear of the gate and seats from lessening rubbing [1]. Wedge shaped gates comes in many different designs (more details in the next section).

Flexible vs solid vs split wedge gate valves

• • Solid wedges refers to the gate being one solid piece [2]. This design is simple and strong, allows for the valve to be installed in any position, is compatible with most fluids, and can be used for turbulent flow. However, this solidity makes the design unable to deal with misalignment in the seat caused by load or thermal expansion, making it susceptible to leaks. Furthermore, this design is susceptible to thermal locking when used in high temperature applications. 
  • Flexible wedges use a one-piece disc with a cut around its perimeter [2]. This design lets the gate compensate for seat misalignment. The size of the cut affects the flexibility and strength of the disc. Smaller (shallow and narrow) cuts help to maintain strength but results in less flexibility. Larger (deeper and wider) cuts sacrifice strength for greater flexibility.
  • Split wedges use a a two piece construction, that is spring loaded and allows for the gate to adjust and align to both seats individually [2]. This design is suited for use with non-condensing gases, liquids at regular temperatures, and corrosive liquids.

Rising vs non-rising stem gate valves

The stem is a threaded rod that is responsible for raising and lowering the gate. There are two design options for the stem:

• • The rising stem (a.k.a. outside screw and yoke) have the stem fixed in relation to the gate [1]. Therefore, the stem moves up and down with the movement of the gate. The benefit of this design is that it provides a visual indication of the gate position, and it is easy to lubricate the exposed stem. However, the design is not compatible with bevel gears or actuators, and is used only for manual actuation scenarios.
  • The non-rising stem design has the stem fixed to the actuator and threaded into the gate [1]. This design is usually used in applications where space is limited. 

Types of bonnets and bonnet connections

The bonnet refers to the cap, usually located at the top opening of the body. Some of the common ways that the bonnet is connected to the body are:

• • Screwed bonnet: The bonnet is screwed onto the body like one big screw [2]. It is the simplest and most inexpensive.
  • Union bonnet: The bonnet is held onto the body using a union nut, which sits on the bottom edge of the bonnet and is screwed onto the body [1]. This design is commonly used when frequent maintenance of the valve is required. 
  • Bolted bonnet: The bonnet is flanged and bolted to the body [2]. This is the most popular method of attachment, but also requires a gasket to seal the connection area.
  • Welded bonnet: The bonnet is welded to the body [2]. This design is mostly used if disassembly is not required. It is lighter and more secure than the first two options.
  • Pressure seal bonnet: The mechanism shown on the right is implemented so that the pressure in the body cavity of the valve pushes the bonnet tighter against the body. This design is most commonly used in high-pressure high temperature applications [2]. 

References

Contributors:

Faculty Advisor: Rachel Malevich (alumni)