What is Gate Valve?
A gate valve, also known as a sluice valve, is a valve that opens by lifting a barrier (gate) out of the path of the fluid. Gate valves require very little space along the pipe axis and hardly restrict the flow of fluid when the gate is fully opened. The gate faces can be parallel but are most commonly wedge-shaped in order to be able to apply pressure on the sealing surface.
A gate valve controls the media’s flow by lifting the gate (open) and lowering the gate (closed). A gate valve’s distinct feature is the straight-through unobstructed passageway, which induces minimal pressure loss over the valve.
The unobstructed bore of a gate valve also allows for a pig’s passage in cleaning pipe procedures, unlike butterfly valves. Gate valves are available in many options, including various sizes, materials, temperature and pressure ratings, and gate and bonnet designs.
Gate valves tend to be slightly cheaper than ball valves of the same size and quality. They are slower in actuation than quarter-turn valves and are for applications where valve operation is infrequent, such as isolating valves.
Gate valves should be used either fully open or fully closed, not to regulate flow. Automated gate valves exist with either an electric or pneumatic actuator, but a manual gate valve is cost-effective since they have infrequent usage.
Principle Of Operation
A gate valve’s main components are the handwheel, spindle, gasket, bonnet, valve body, flange, and gate. The primary operation mechanism is straightforward. Turning the handwheel rotates the stem, which moves the gate up or down via the threads.
They require more than one 360° turn to fully open/close the valve. Lifting the gate from the path of the flow, the valve opens. Lowering the gate to its closed position seals the bore resulting in a full closure of the valve.
For a gate valve, the relationship between the gate’s vertical travel and the flow rate is nonlinear, with the highest changes occurring near shutoff.
When used to regulate flow, the relatively high velocity of the flow at partial opening results in gate and seat wear, which along with possible vibrations of the gate, shortens the valve’s service life.
Valve construction
Common gate valves are actuated by a threaded stem that connects the actuator (e.g. handwheel or motor) to the gate. They are characterized as having either a rising or a non-rising stem, depending on which end of the stem is threaded.
Rising stems are fixed to the gate and rise and lower together as the valve is operated, providing a visual indication of valve position. The actuator is attached to a nut that is rotated around the threaded stem to move it.
Non-rising stem valves are fixed to, and rotate with, the actuator, and are threaded into the gate. They may have a pointer threaded onto the stem to indicate valve position since the gate’s motion is concealed inside the valve. Non-rising stems are used where vertical space is limited.
Gate valves may have flanged ends drilled according to pipeline-compatible flange dimensional standards. Gate valves are typically constructed from cast iron, cast carbon steel, ductile iron, gunmetal, stainless steel, alloy steels, and forged steel.
All-metal gate valves are used in ultra-high vacuum chambers to isolate regions of the chamber.
Bonnet
Bonnets provide leakproof closure for the valve body. Gate valves may have a screw-in, union, or bolted bonnet. A screw-in bonnet is the simplest, offering a durable, pressure-tight seal. A union bonnet is suitable for applications requiring frequent inspection and cleaning. It also gives the body added strength. A bolted bonnet is used for larger valves and higher-pressure applications.
Pressure seal bonnet
Another type of bonnet construction in a gate valve is a pressure seal bonnet. This construction is adopted for valves for high-pressure service, typically in excess of 2250 psi (15 MPa). The unique feature of the pressure seal bonnet is that the bonnet ends in a downward-facing cup that fits inside the body of the valve.
As the internal pressure in the valve increases, the sides of the cup are forced outward. improving the body-bonnet seal. Other constructions where the seal is provided by external clamping pressure tend to create leaks in the body-bonnet joint.
Knife gate valve
For plastic solids and high-viscosity slurries such as paper pulp, a specialty valve known as a knife gate valve is used to cut through the material to stop the flow. A knife gate valve is usually not wedge-shaped and has a tapered knife-like edge on its lower surface. 1928, Introduction of the first knife gate valve, the “Massaventil” (Pulp stock valve / MV), engineered by Mr. Canell. In sweden, Stafsjö Valves AB.
Types of Gate Valves
There are three types of Gate valves:
- Solid taper wedge.
- Flexible wedge.
- Split wedge or Parallel disks Valve.
1. Solid Wedge Gate Valve
A solid wedge is the most common & widely used disk type because of its simplicity and strength. A valve with a solid wedge may be installed in any position, and it is suitable for almost all fluids. It can be used in turbulent flow also.
However, it does not compensate for changes in seat alignment due to pipe loads or thermal expansion. So, this type of disk design is most susceptible to leakage. A solid wedge is subjected to thermal locking if used in high-temperature service.
Thermal locking is a phenomenon in which a wedge is stuck between the seats due to the expansion of the metal. Solid-wedge gate valves are generally used in moderate to lower pressure-temperature applications.
2. Flexible Wedge Gate Valve
The flexible wedge is a one-piece solid disk with a cut around the perimeter. These cuts vary in size, shape, and depth. A shallow, narrow cut on the wedge perimeter gives less flexibility but retains strength. A cast-in recess or deeper and wider cut on the wedge perimeter gives more flexibility but compromises the strength.
This design improves seat alignment and offers better leak tightness. It also improved performance in situations where thermal binding possible. Flexible wedges Gate valves are used in steam systems.
Thermal expansion of the steam line sometimes causes distortion of valve bodies which may lead to thermal blinding. The flexible gate allows the gate to flex as the valve seat compresses due to the thermal expansion of the steam pipeline and prevents thermal blinding.
The disadvantage of flexible gates is that line fluid tends to collect in the disk. These may result in corrosion and ultimately weaken the disk.
3. Split wedge or Parallel disks Gate Valve
Split wedge Disk consists of two solid pieces and holds together with the help of a special mechanism. In case, one-half of the disk is out of alignment; the disk is free to adjust itself to the seating surface. The split disk can be in a wedge shape or a parallel disk type.
Parallel disks are spring-loaded, so they are always in contact with seats and give bi-directional sealing. The split wedge is suitable for handling noncondensing gasses and liquids at normal and high temperatures.
Freedom of movement of the disk prevents thermal binding even though the valve may have been closed when a line is cold. This means when a line is get heated by fluid and expand it does not create thermal blinding.
Uses of Gate Valve
A gate valve is generally used to completely shut off fluid flow or, in the fully open position, provide full flow in a pipeline. Thus, it is used either in fully closed or fully open positions.
Gate valves are used to shut off the flow of liquids rather than for flow regulation. When fully open, the typical gate valve has no obstruction in the flow path, resulting in very low flow resistance. The size of the open flow path generally varies in a nonlinear manner as the gate is moved. This means that the flow rate does not change evenly with stem travel. Depending on the construction, a partially open gate can vibrate from the fluid flow
Gate valves are mostly used with larger pipe diameters (from 2″ to the largest pipelines) since they are less complex to construct than other types of valves in large sizes.
Advantages of Gate Valves
- Gate Valves Have Low Fluid Resistance. The gate valve’s body is a straight-through shape that does not alter the flow direction, allowing fluid to pass through the valve (when in the fully open position) with minimal resistance compared to other types of valves.
- Gate Valves Provide Superior Sealing Performance. Gate valves provide better sealing performance than shut-off valves. The gate valve’s opening and closing mechanism are faster and more convenient than those of the shut-off valve.
- Gate Valves Have a Wide Application Range. Gate valves are extremely versatile and can be used in applications with mediums like steam, oil, and other media. Plus, gate valves can be used in mediums containing granular solids and with high viscosity. It can also be used as a venting valve and a low vacuum system valve.
- Gate Valves are Bidirectional. Gate valves have dual flow directions. They’re not limited to one-way flow, nor are they subject to the flow directions of the medium. Therefore, gate valves are suitable for use in pipelines where the flow direction may change. Plus, they are easier to install than other more sophisticated types of valves.
Disadvantages of Gate Valves
The Main Disadvantages of Gate Valves:
- Gate Valves Open & Close Slowly. During the opening process, the valve plate needs to be lifted to the upper part of the valve chamber. During the closing process, the valve plate needs to fall into the valve seat. The distance that the valve plate must move in order to open or close is relatively large, leading to a long opening and closing time. This disadvantage makes gate valves impractical for applications that require rapid actuation.
- Gate Valves can be Prone to Scratches. The opening and closing processes between the valve plate and the two sealing surfaces of the valve seat can create excess friction that may lead to scratches on the sealing surface. These scratches might seem innocuous, but eventually, they have an impact on the sealing performance and the service life. These damages are not always easy to repair, but gate valves are easy to install and replace.