Valve Gate

Gate valves are used to shut off the flow by inserting a “gate” into the path of a flowing fluid. The gate is generally a disc with parallel sides or is wedge shaped. A gate valve is only used as a shut off valve; either fully open or fully closed. It should not be used partially open as a throttling device, since the disc 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 two types of gate valve discs: parallel sides or wedge shaped. Each has various configurations.

Parallel side discs can have single discs or two discs with a spring in between them. In parallel side discs, the sealing force is provided by the upstream pressure, which forces the disc against the downstream seat.

Conventional Parallel Gate valves have twin discs with a spring in between. The spring keeps both upstream and downstream surfaces seated during opening and closing. The spring also aids the valve to remain seated in low pressure situations.

Through-Conduit Gate valves have a single gate made of a plate with a hole bored in the bottom to allow fluid to flow when the valve is open. When closed, the portion with the hole is pushed below the flow stream and replaced with the solid part of the gate. These valves are typically not used in HVAC or plumbing applications, but are common in crude oil and liquid natural gas pipelines.

Knife Gate valves have a gate made of a plate material that is sharpened on the bottom so that fibrous material in the flow can be severed. Like the trough-conduit valve, knife gate valves are not used in the HVAC industry. They are typically used in heavy industry applications in which suspended material are found in the pipeline, such as the paper and pulp industry.

Wedge shaped discs have a tapered disc that, when closed, is wedged tight against the seats to provide positive flow stoppage at either high or low pressure.

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) non-rising 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.

Non-rising Stem/Inside Screw: non-rising 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. The welded design is the strongest, but it cannot be taken apart to repair valve trim components. The other three can all be disassembled.

Bolted Bonnet: The bolted bonnet valve uses flanges and a series of nuts and bolts to connect the bonnet to the valve body, much like a flange is used to connect two pipes. It is easily disassembled by removing the bolts for access to the valve trim component. It is used on high pressure valves and on valves larger than 2”. 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.

Union Bonnets: The union bonnet valve uses a single union-style nut to connect the bonnet to the valve body external threads on the valve body. This style is common on bronze valves up to 2”.

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. In this design, the bonnet is machined with external threads which thread into the valve body.

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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