Wednesday, July 22, 2015

Valve Selection - Gate Valves

Fluid flow control in industrial processes is most often accomplished through the use of valves installed at specific locations in the piping system to start, stop, throttle, or divert the fluid flow. The application characteristics are broad and varied, resulting in an almost unmanageable menu of possible valve choices. Since we have limited time to spend on any task, it’s useful to employ a simple filter in your valve selection procedure. This generally allows for the elimination of a large group of candidates and allows your efforts to be quickly focused on potentially useful products. One filtering criteria that can be applied to industrial control valves is the valve type.

Valve type generally refers to the means employed to control fluid flow. The closure means and the valve characteristics associated with it can be used to determine whether a valve type may be suitable for your process application. Some common valve types include ball, gate, plug, and butterfly, with numerous variations and customization occurring within each type. Let’s consider the gate valve.

industrial gate valve with hand wheel actuator
Gate Valve With Handwheel
Courtesy WEY Valve
Gate valves have a movable wedge, often rectangular or round, that is positioned in the fluid flow path. This wedge, the “gate” or “disc”, is connected to a stem which extends to the exterior of the valve body. Motion of the stem will position the gate within the valve body to provide some degree of obstruction to the fluid flow path. Fully inserting the gate into the media path will shut off the flow, while successively withdrawing the gate will increase the opening through which fluid can pass and allows for increasing flow rates. Seals are provided along the planar surfaces where the gate contacts the valve body.

What are potential positive attributes of gate valves?

  • When fully open, these valves provide little or no restriction to fluid flow. In addition to having a fluid path cross section generally similar to the connected piping, the valve construction does not impose any change in the fluid flow direction as media passes through the valve. This valve type provides a straight through passage.
  • Gate valves can be employed bi-directionally, with flow control effectively available in both directions.
  • Installation space dimensions along the flow path are generally low, when compared to some other valve types.
  • The energy requirements to change valve position are generally low, since the gate movement is perpendicular to the flow direction.
  • The rate of closure is comparatively slow, inhibiting physical shock (hammering) in the connected piping system.

Here are some other attributes of gate valves.
gate valve with actuator
Gate Valve With Actuator
Courtesy DeZurik

  • As the valve opens, the seals are exposed to the fluid flow. The effects of media velocity and chemical aggressiveness on the seal material may cause deterioration at an unacceptable pace, especially in throttling applications.
  • Gate valve stems generally extend substantially from the valve body, as they must accommodate the mounting of any required actuator and the entire range of motion of the gate. Some installations may not be able to provide the needed space for this arrangement.
  • The slow opening and closing activity of gate valves may not be suitable for some applications.
  • Gate valves are not exceptional at throttling flow. Additionally, attempts at throttling flow may result in gate vibration. There is also concern about premature seal wear due to exposure to higher media velocities while throttling.

Your selection focus may be shifted by these general criteria. Combine your knowledge and understanding of your process and media with the experience and knowledge of valve and control experts to specify a valve configuration that will provide the performance you need.