OPUS

Valve Gate


The term gate valve refers to the wedge shaped disc that is lifted out of the main flow chamber to open the valve. A gate valve is only used fully open or fully closed. It should not be used partially open as a throttling device, since the wedge will vibrate and quickly become damaged. Gate valve designs are determined by three features: the disc, the stem, and the bonnet/body connection.

Gate Valve Disc Construction
There are three types of disc features which can be found in gate valve construction.

Solid Wedge Discs
Solid wedge discs of gate valves are the most prevalently used due to their simple and usually less expensive design.

Split Wedge Discs
Split wedge discs of gate valves, also called double discs, have somewhat better sealing characteristics than solid discs. Two disc halves are forced outward against the body seats by a spreader, after the disc has been fully lowered into its seated position. When the valve is opened, pressure on the disc is relieved before it is raised, eliminating friction and scoring of the body seats and the disc itself.

Flexible Wedge Discs
Flexible wedge discs of gate valves are solid only at the center and are flexible at the outer edge and seating surface. This design enables the disc face to overcome the tendency to stick in high temperature services where wide swings in temperature occur. Flexible wedge discs are generally found only in steel valves.

Gate Valve Stem Construction
There are three types of stem features which can be found in gate valve construction: (1) rising stem/outside screw and yoke, (2) rising stem/inside screw, and (3) nonrising stem/inside screw.

Rising Stem/Outside Screw and Yoke
Rising stem/outside screw and yoke construction retains stem threads outside the valve. Rising stem/outside screw and yoke construction is recommended where high temperatures, corrosives, and solids in the line might damage stem threads inside the valve. When the handwheel is turned, the stem rises as the yoke bushing engages the stem threads. The external threads enable easy lubrication; however, care must be taken to protect the exposed stem threads from damage. And advantage of rising stem valves is the ability to determine valve position by observing the position of the stem.

Rising Stem/Inside Screw
The rising stem/inside screw is the most common stem design in bronze gate valve construction. Because the hand wheel and stem both rise, adequate clearance must be provided for operation. The stem and handwheel positions indicate the position of the disc inside the valve. In the open position, the backseat helps protect the stem threads; but, care must be taken to protect the stem externally.

Nonrising Stem/Inside Screw
Nonrising stem/inside screw design has the chief advantage of requiring minimum headroom for operation. Since the stem does not travel vertically, packing wear is reduced. Heat, corrosion, erosion, and solids may damage the stem threads inside the valve and cause excessive wear. In addition, it is impossible to determine the disc position since the handwheel and stem do not rise.

Gate Valve Bonnet Construction
There are four types of gate valve bonnet construction: bolted, union, threaded, and welded designs.

Bolted and Union Bonnets
The bolted and union bonnet designs, featuring three piece construction, are preferred for rugged service. Bolted construction and union construction are stronger and safer than threaded body bonnet design. Industrial valve users generally select union bonnet bronze or bolted bonnet, iron body gate valves.

Two Piece Threaded Bonnet
The two piece threaded bonnet is the least expensive design and should be used for lower pressures or where shock and vibration are not encountered. A union bonnet valve would be more suitable than a threaded bonnet valve, if frequent disassembly of the valve is required.

Welded Bonnet
The welded bonnet construction provides the most leak free, body to bonnet joint. This design is usually found in 2 inch and smaller forged steel valves. The disadvantage of the welded bonnet is that it provides no access to the trim parts if repairs are necessary.



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