Gate valve is a linear motion valve used to start and stop the flow of fluid. It’s a full opening/closing valve. Disc or gate used to start or stop the flow which is operated through a combination of stem and handle. Upward and downward linear motion of stem causes the opening and closing of the gate valve.
When the valve is the fully open condition, disc or gate fully removed from the flow path and provide no resistance for flow. This no resistance causes a minimum pressure drop. When the valve fully closed, disc to seal ring contact surface for 360 degrees and provides a tight seal. Therefore, very little or no leakage through disc when the valve at full closed.
On the opening of the gate valve, a large amount of working fluid flow in nonlinear manners (directionless flow) with respect to the gate opening percentage. Partial opening of the gate causes vibration in disc or gate due to fluid flow. Most of the flow change occurs near shutoff with a comparatively high fluid velocity causing disc and seat wear and eventual leakage if used to regulate flow. For all these reasons, the gate valve not used for throttling of flow.
Types of Gate Valve:
Gate valves can be classified as:
1.According to Disc type
Split wedge or parallel disc
2.According to stem design
3.According to seat design
Solid Wedge Gate Valve:
Solid wedge commonly used for different industrial purposes. It is simple in construction and has good strength. Solid wedge valves installed at any position. It is suitable for all types of fluids, especially where turbulent flow involved.
Flexible Wedge Gate Valve:
Flexible wedge gate valve is a one-piece disc with a cut around the edge. This cut around the edge improves the ability to match error or change in the angle between the seats. The cut differs in shape, size, and depth. A narrow, shallow cut provides little flexibility but also keeps strength. On the other hand, a deep and wide cut or cast in recess leaves little material at the center which allows more flexibility but also negotiations on strength. A correct profile of the disc half in the flexible wedge design that provides uniform deflection properties at the disc edge. Therefore, the wedging force applied in seating will force the disc seating surface uniformly and tightly against the seat.
Flexible wedge used in steam systems because it prevents binding of the gate within the valve when the valve is at a closed position. During heating of steam lines, some expansion occurs in steam lines. This expansion causes some distortion of valve bodies. In a cold steam system if a solid gate fits tightly between the valve seat when the steam is heated and pipes elongate. Elongation causes the seats to compress against the gate and clamp the valve shut. This problem overcome by using a flexible wedge gate valve. Because its design allows the gate to flex as the valve seat compresses it.
The major problem related to a flexible wedge is that accumulation of water in the body neck. Under these conditions, entry of steam causes the valve body neck to rupture, seat ring collapse, corrosion or the bonnet to lift off. To avoid this kind of problem, always follow correct warming procedures.
Split wedge Gate Valve:
These are the socket and ball design valves. Split wedge gate valves are self-adjusting and self-aligning gate valves. The disc is free to adjust itself to the seating surface if one-half of the disc is slightly out of alignment due to some foreign matter lodged between the disc and the seat ring. Split wedge mostly uses for corrosive liquids. It is also suitable for handling of condensing gases and liquids at normal temperature. Freely movement of the disc in the carrier prevents binding however the valve has been closed due to expansion and contraction. The split wedge should be installed with a vertical stem position.
Parallel Disc Gate Valve:
Parallel disc gate valve designed to prevent valve binding due to thermal variations/Transients. it uses for both low and high-pressure applications. Wedge surfaces between the parallel face disc split to press together under stem thrust and spread apart the disc to the seal against the disc. Tapered wedges can be part of the disc halves or separate elements. In some designs, wedge contact surfaces curved to keep the point contact close to the finest. In other parallel disc gate valves, the two halves do not move separately under wedge action. Instead, upstream pressure holds the downstream disc against the seat. A carrier ring lifts the discs, and spring or springs hold the discs apart and seated when there is no upstream pressure.
Some parallel disc designs provide sealing only one port. In this type of design, the high-pressure side pushes the disc open on the high-pressure side. But also forces the disc to close on the low-pressure side. With such designs, the amount of seat leakage decreases with differential pressure across the seat increases. These valves have flow direction marking which shows which side is the high-pressure side.
Rising Stem Gate Valve:
In a rising stem gate valve, stem raised from the flow path when the valve is open. During the opening of the rising stem gate valve, stem along with gate moves upward while closing, stem along with gate moves downward. In simple words, the stem is not vertically stationary in stem rising gate valve. Rising stem clearly see when the valve is open. It means the rising stem is the physical indication of valve opening. Rising stem valves are available in two basic designs. Some valves have a stem that rises through the hand wheel and some have a stem that is the thread to the bonnet.
Non-Rising Stem Gate Valve:
In a Non-rising stem gate valve, no upward or downward movement of stem outside the valve body. The stem internally threaded and remains vertically stationary. When the hand wheel on the stem rotates, gate or disc moves upward or downward on the internal threads but stem remains vertically stationary.
In simple words, in non-rising stem gate valve, gate moves vertically up and down while stem remains stationary. These valves provided with pointer-type indicator threaded onto the upper end of the stem to indicate valve position. These valves are used where less space is available for valve operation and the medium is less corrosive for stem material.
Gate Valve Set Design:
Gate valve seats are available either integral with the valve body or in seat ring-type construction. Integral seats have the same construction material as the valve body. A pressed-in or threaded-in seat permits variation. Seat ring construction provides seats, with threaded into position or are pressed into position and seal welded to the valve body.
Small, forged steel, gate valves have hard-faced seats pressed into the body. In some series, this type of valve available in sizes from 1/2 to 2 inches rated for 2500 psig steam service. In large gate valves, discs are often of the solid wedge type with seat rings welded in, pressed in, or threaded in. Screwed in the seat rings are considered replaceable since they may be removed and new seat rings installed.
Advantages of Gate Valve:
Following are the characteristics/advantages of gate valve:
- Straight flow
- Directionless valve
- Provide a minimum pressure drop across the disc
- Valve bore is equal to the pipe opening
- Occupy less space as compared to the globe valve
- Simple in construction, low cost
- Use for full opening/closing (frequent opening/closing not recommended)
- Metallic disc or gate uses to stop the flow of working fluid
Disadvantages of Gate Valve:
- Not suitable for throttling applications
- Create vibration in the partially open position
- It is more subject to seat and disc wear than a globe valve.
- Repairing work such as grinding and lapping, are generally more difficult to achieve