Disc Design VALVE disc or gate

Gate valves can have one of two different disc designs: parallel or tapered type. Both operate on the principle of a closure element (disc or gate) sliding into a slot in the pipeline and closing off the fluid path. The tapered disc of the “wedge gate” valve is machined to match a pair of body seats set at the same angle, usually about 10o. If machined correctly, as the tapered disc engages the seats, it locks firmly into place, stopping the flow.

Three types of wedge gates are available: solid disc, one piece flexible type, and two piece split design.

* The solid wedge has been around the longest and at one time virtually all wedge gates were the solid type. The drawback to a solid design is that it does not have any flexibility and if there is any valve body/seat distortion due to extreme temperature fluctuations or pipe stresses, the solid disc can become jammed in the seats. The solid disc is still standard on bronze, cast iron, water service and compact carbon steel valves (API 602 type). Today, solid discs are usually only available as special order items on large diameter gate valves.
* The flexible wedge type is just that- flexible. By the addition of a groove or slot around its periphery, the flexible disc can adapt to temperature changes and adverse piping stresses without binding. The flexible design also is a little easier to manufacture, in that minor imperfections in the seating surface angles can be compensated for by the disc’s flexibility. The “flex-wedge” design is by far the most common type seen on commodity gate valves used in industrial applications.
* The split wedge type consists of a two-piece design with mating surfaces on the back side of each disc half. These mating surfaces allow the downward stem thrust to be uniformly transferred to the disc faces and onto the seats. This flexible design also provides protection against jamming due to thermal expansion. A disadvantage to the split design is that in “dirty” services, residue or debris can cake in between the disc halves, causing the valve to improperly seat or even jam. Split wedge designs are commonly found on stainless steel and high alloy valves, as well as many small bronze valves.

Wedge gates are guided by grooves or ribs cast or welded into the body of the valve. These wedge guides keep the disc in alignment as it opens or closes and also keeps the disc from sliding against the downstream seat during opening and closing.

The second disc design is the parallel type. Unlike the wedge type gate valve, which relies on the stem thrust to “wedge” the disc into the seats to seal, the parallel seat valve needs some assistance to seal properly. The sealing assistance is usually in the form of a spring loaded or mechanically activated spreading action between the two disc halves as the valve closes fully. On most parallel seat designs the friction and sealing force is relieved as the gate disengages from the seats.

The most common use for parallel disc valves today is in the pipeline industry, where elastomer seat seals and ambient operating temperatures make valve virtually leak proof. Parallel gates are also used in some high pressure, high temperature steam applications, to help reduce the possibility of locking the disc in the closed position due to a radical change in temperature.

Regardless of disc design or type, the gate valve closure element must come in perfect contact with seats in the valve body. The body seats may be welded, screwed, pressed or swaged in, or be integral with the valve body. Most industrial steel gate valves utilize seat rings that are welded into the valve body. For most of the 20th century the norm was screwed in seat rings in steel valves. However, advances in welding and valve repair techniques made the screwed-in rings obsolete. Seat rings and valve discs are also often overlaid with corrosion or abrasion resistant alloys to increase their service life.