Monday, September 28, 2015

Gate Valve

A Gate Valve is also know as Sluice Valve, is a valve that opens by lifting a round or rectangular gate/ wedge out of the path of the fluid.

Gate valves are primarily designed to start or stop flow, and when a straight-line flow of fluid and minimum flow restriction are needed. In service, these valves generally are either fully open or fully closed.

Construction of a Gate Valve

Gate valves consists of three main parts: body, bonnet, and trim. The body is generally connected to other equipment by means of flanged, screwed or welded connections. The bonnet, which containing the moving parts, is attached to the body, usually with bolts, to permit maintenance. The valve trim consists of the stem, the gate, the disc or wedge and the seat rings.

Discs of Gate Valve

Gate valves are available with different disks or wedges.

The most common types of Discs are :

Solid Wedges
Solid wedge is the most commonly used disk by its simplicity and strength.
A valve with this type of wedge can be installed in each position and it is suitable for almost all liquids. The solid wedge is a single-piece solid construction, and is practically for turbulent flow.

Flexible Wedges

Flexible wedges are featured with mechanical flexibility to adjust it's own shape following the shape of body seats for a tightly secured mutual contact. This is particularly important when larger Gate Valves service at extremely high pressure and temperature, where temporary deformation of the seats always occur. With Flexible Wedges operational torque is smaller, seat wear is less and valve closure is tighter.

Flexible Wedge Front View

Stem to Wedge Connection
Flexible Wedge Side View
Flexible wedge is a one-piece disc with a cut around the perimeter to improve the ability to correct mistakes or changes in the angle between the seats.
    The reduction will vary in size, shape and depth. A shallow, narrow cut gives little flexibility but retains strength.

      A deeper and wider cut, or cast-in recess, leaves little material in the middle, which allows more flexibility, but compromises strength.

      Split Wedges
      Split wedge is self-adjusting and self-aligning to both seats sides. This wedge type consists of two-piece construction which seats between the tapered seats in the valve body. This type of wedge is suitable for the treatment of non-condensing gases and liquids at normal temperatures, particularly corrosive liquids.

      Stem of Gate Valve

      The stem, which connects the handwheel and disk with each other, is responsible for the proper positioning of the disk. Stems are usually forged, and connected to the disk by threaded or other techniques. To prevent leakage, in the area of the seal, a fine surface finish of the stem is necessary.

      Gate valves are classified as either based on stems :

      Rising Stem :
      For a valve of the Rising Stem type, the stem will rise above the hand wheel if the valve is opened. This happens, because the stem is threaded and mated with the bushing threads of a Yoke. A Yoke is an integral part from a Rising Stem valve and is mounted to the Bonnet.

      Non Rising Stem :
      For a valve of the non Rising Stem type, there is no upward stem movement if the valve is opened. The stem is threaded into the disk. As the handwheel on the stem is rotated, the disk travels up or down the stem on the threads while the stem remains vertically stationary.