What is a gate valve and what are they used for
Gate valves are mostly used for industrial piping, these valves are mostly used as stop valves to turn on and off the flow. Gate valves are general service valves and its operation is based on the open and closing of the disc and this disc is a gate like a disc and that’s how it got its name, this disc would operate at a right angle to the path of the flow. In the gate valve, the flow control is done by raising or lowering the valving element. Gate valves can’t be used for regulation purposes because in the partially open state the disc can be damaged, and when they are fully open they limit the pressure drop across the valve. Gate valves can be used if we need the fluid flow to be a straight line with minimum restriction. All gate valves have bidirectional sealing capabilities and they can be installed in any direction, if the valves are with pressure-relieving feature then they can only be installed in the direction of the arrow.
How does a gate valve work and how are they constructed
Gate valves are multi-turn valves with rising stem, they need a number of turns of operation for fully opening or closing of the valve. Gate valves are mostly used in the fully opened or closed position so flow regulation is not possible. If these valves are used in a half-open position then it could cause damage to the gate. All valves close by rotating the hand-wheel clockwise and opens by rotating the hand-wheel anti-clockwise. The opening element of a gate valve is mostly a disc that would be attached to the end of a threaded stem. The gate fits into a wedge-shaped seat in the valve body to stop the flow through the valve. When the valve is fully opened the gate is positioned above the passage in the valve body and this will allow full flow in either direction. The gate would fill the passage and stops the flow through the valve completely if the gate valve is fully closed. For hand-wheel operated valves the projection of steam above the handwheel indicates whether the valve is in the open or closed position. In gear operated valves the stem top won’t be visible a separate indicator will be provided if needed.
Mostly gate valves have flanged ends, gate valves are constructed by cast iron. Cast carbon, steel, alloy steel, etc. The bonnets of gate valves are made up of cast iron and it would need extra care during overhauling. The valves are mostly gate like disc and they are attached to a threaded spindle working in a nut. According to the requirement and place where the valve is installed bonnets are selected, the bonnet could be screw-in, bolted, or union type. Screw-in bonnets would offer a durable pressure-tight seal. Union bonnets are used for an application that needs suitable inspection and cleaning. For high pressure, purposes bolted bonnets are used, another type of bonnet construction is pressure seal bonnet and this type is used for high-pressure applications.
What are the parts of the gate valve
What are the types of gate valves
Rising and non-rising stem gate valve
Gate valves have either a rising or non-rising stem large gate valves normally have the non-rising stem. When the handwheels are turned the stem rises out of the valve bonnet through the hand-wheel. The position of the disc is shown by the disc it also shows if the valve is opened or closed. In a small gate valve, the wheel and stem could rise together. Some gate valves have a non-rising stem. It is that the stem does not come out of the valve body. The gate moves up on the stem. If space is not enough for the stem to rise out of the valve body then this type of valve is used.
Knife gate valve
Knife gate valve is one of the industrial valves, their design ensures minimum contact between the parts of the valve thus reducing wear and tear. There are certain features for knife gate valves such as they avoid sliding contact between body and gate and thus enable the flush of media to form the valve interior, and they have non-sliding motion. Tight shut off can be done by the bottom edge of the valve even when there is solid particles at the bottom of the body. Knife gate valves are unidirectional valves and they have the ability to cut through flowing media and if any material is in the seating area it would dislodge it and close. These valves can withstand high temperature and abrasive slurries and they are used in mining, power and chemical applications.
Wedge gate valve
In this type, the valve is closed by driving the wedge down to the corresponding taper in the body. Perfect sealing is done by matching the wedge and body seat ring. When the valve is fully open the stem back seats into the bonnet bush, while in case of actuator valves the stem is not allowed to fully backseat to avoid damage. Wedge gate valves are fully closed by torque limit and fully opened by travel limit.
Parallel slide gate valves
Parallel slide gate valves do not require wedging action for proper seating. The valve closing is done by, two independent discs held together by a disc carrier and spring moves down parallel and closes the valve. When the body port is sealed, then the stopper in the stem would prevent further travel. When the valve is opened the disc moves up and the stem backseats. Mostly parallel slide gate valves are fully opened and closed by travel limit.
Double disk gate valve
In this type, the double wedge is of ball and socket construction or of uni-ball type where every single disk has a half socket and a half ball. They adjust themselves into the taper seats ensuring a tight valve and very little maintenance is needed.
What are the advantages of gate valve
- The opening and closing of the gate valve is really slow and this would prevent fluid hammer and piping damage
- The right choice for on-off service
- Full flow, low-pressure drop
What are the disadvantages of gate valves
- Not suitable for throttling application
- Vibration could happen in the partially open state
- It is more subjected to seat and disc wear
- Lapping and grinding repairs are very difficult to accomplish
What are the applications of gate valves
- It can be used for oil, gas, slurries, steam and corrosive liquids
- These valves are suitable for high pressure and temperature applications
- These valves cannot be quickly opened or closed
- It requires a large space for installation, operation, and maintenance
- Large torque is needed to open because of large pressure difference across the seat
- Gate valves are used for steam circuits in ships
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