Process Description of Dissolved Air Flotation
May 15,2023Read More
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.
Figure 1: Gate valves with a screw-in bonnet (left) and a bolted bonnet (right)
A gate valve's main components are body, seat, gate, stem, bonnet, and actuator (Figure 2). 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.
Figure 2: Gate valve components
Gate valves come in a wide variety of designs, each of which uses different technologies to meet various application requirements.
A bonnet protects the internal parts of a gate valve (Figure 2). It is screwed in or bolted to the valve body, creating a leak-proof seal. Therefore, it is removable for repair or maintenance purposes. Depending on applications, gate valves can have screw-in, union, bolted, or pressure seal bonnets.
2.1.1 Screw-in Bonnets
Screw-in bonnets are the simplest in construction. They are common in small size valves and provide a durable leak-proof seal. Figure 1 shows a gate valve with a screw-in bonnet on the left.
2.1.2 Union Bonnets
Union bonnets are held in place by a union nut. The union nut sits on the lower edge of the bonnet and screws into the valve body's threads. This type of design ensures that the leak-proof seal created by the nut does not deteriorate by frequent removal of the bonnet. Therefore, union bonnets are common for applications that require regular inspection or maintenance.
2.1.3 Bolted Bonnets
Bolted bonnets provide sealing in larger valves and higher pressure applications. In this type, the bonnet and valve body are flanged and bolted together. Figure 1 shows a gate valve with a bolted bonnet on the right.
2.1.4 Pressure Seal Bonnets
Pressure seal gate valves are ideal for high-pressure applications (more than 15 MPa). This type of construction uses internal pressure to create a better seal. Pressure seal bonnets have a downward-facing cup inserted into the valve body. When internal fluid pressure increases, the cup's forced outward, improving the seal.
Figure 3: Union bonnet gate valve
The gate comes in a variety of designs and technologies to produce effective sealing for differing applications.
2.2.1 Wedge Gates
In most gate valves, the gate has a wedge form and sits on two inclined seats (Figure 4). In addition to the primary force created by fluid pressure, a high wedging force on the seats created by the stem's tightening assists with the sealing. The wedge-shaped gate does not stick to the seat in case of high fluid differential pressure and has an increased service life due to less "rubbing" on the seats.
2.2.2 Parallel Slide Gates
Gate valves can also come in a parallel form where the gate is flat, and the seats are parallel. Parallel gate valves use line pressure and positioning to make a tight seal. Flat gates consist of two pieces and have a spring in the middle. The spring pushes the pieces towards the seats for enhanced sealing. Due to their inherent design, parallel gate valves have a safety advantage in higher temperature applications. In wedge-shaped gate valves, an additional compression load on the seats may result in thermal binding and restricted opening of the valve due to expansion. Furthermore, since there is no wedging action in parallel gates, closing torques are comparatively smaller, resulting in smaller, less expensive actuators or less manual effort. Due to their sliding into position, parallel gates keep dirt away from the seating surfaces.
Figure 4: Wedge gate valve vs parallel gate valve
2.2.3 Slab Gates
Slab gates, also called through-conduit gate valves, are one-unit gates that include a bore size hole (Figure 5). In the open state, the bore is in line with the two seat rings. This alignment creates a smooth flow with minimal turbulence. This unique design allows for minimal pressure loss on the system and is perfect for the transportation of crude oil and natural gas liquids (NGLs). The valve seats remain clean. However, the disc cavity can capture foreign material. Therefore, the cavity typically has a built-in plug for maintenance purposes of draining the accumulated foreign material.
Figure 5: Slab gate valve
The gate is raised and lowered by the spinning of a threaded stem. A manual wheel or actuator spins the stem. Depending on the design, the stem is either considered rising or non-rising. So, as you spin the stem it either raises or stays in place with the spin as seen in Figure 7.
Outside Screw and Yoke (OS&Y), also referred to as rising stems, are fixed to the gate. Therefore, the threads are on the actuation side. So, as the gate's raised or lowered, the stem moves with it up and down. Consequently, they have built-in visual indicators of the state of the valve and are easily lubricated. Since they have moving components, they cannot be used with bevel gears or actuators. Therefore, rising gate valves are suitable for manual actuation.
On the other hand, a non-rising stem is fixed to the actuator and threaded into the gate. An indicator is often threaded onto the stem to show the open or closed state of the valve. Non-rising gate valves are common in underground installations and applications with limited vertical space. You can easily know the difference between rising and non rising gate valve through the picture below.
Figure 7: Mechanism of rising stem gate valves vs non-rising stem gate valves
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