Figure 1: Gate valves with a screw-in bonnet (left) and a bolted bonnet (right)
Gate valves open when a gate is lifted from the path of the flow and close when the gate returns to its position. The distinct feature of a gate valve is the straight-through unobstructed passage way, which induces minimal pressure loss over the valve. The unobstructed bore of a gate valve also allows for passage of a pig 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 generally slower in actuation than quarter-turn valves and are most commonly used in applications where valve operation is infrequentsuch as isolating valves. Gate valves are primarily designed for fully open and fully closed service functions and are not typically used as control or regulating valves. Although electrically actuated gate valves exist, the infrequent functioning of these valves make manual actuation a cost-effective option.
Figure 2: Gate Valve Components
Main components of a gate valve are: body, seat, gate, stem, bonnet and actuator (Figure 2). The main operation mechanism is very simple. When the handwheel is turned, it rotates the stem, which is translated into the vertical movement of a gate via threads. They are considered multi-turn valves as it takes more than one 360° turn to fully open/close the valve. When the gate is lifted from the path of the flow, the valve opens and when it returns to its closed position, it seals the bore resulting in a full closure of the valve.
Conventional gate valves are typically not used to throttle flow. In a gate valve, the relationship between vertical travel of the gate and the flow rate is nonlinear, with the highest changes occurring near shutoff. If used to regulate flow, the relative high velocity of the flow at partial opening results in gate and seat wear, which along with possible vibrations of the gate shortens service life of the valve.
Gate valves come in a wide variety of designs, each of which use different technologies to meet various application requirements.
A bonnet provides coverage of the internal parts of a gate valve (Figure 2). It is often screwed in or bolted onto the valve body and creates a leak-proof seal. This piece can often be removed for repair or maintenance purposes. Depending on applications, gate valves can have screw-in, union, bolted or pressure seal 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.
Figure 3: Union Bonnet Gate Valve
Union bonnets are held in place by a union nut that sits on the lower edge of the bonnet and engages with threads on the valve body. 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 used in applications where regular inspection and maintenance is required.
Bolted bonnets are generally used to 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.
Pressure seal gate valves are designed for high pressure (more than 15 MPa) applications. This type of construction uses the internal pressure to create a better seal. A pressure seal bonnet features a downward-facing cup which is inserted into the valve body. When internal fluid pressure increases, the cup is forced outward improving the seal.
The gate comes in a variety of designs and technologies to produce effective sealing for differing applications.
In most gate valves, the gate has a wedge form and sits on two inclined seats (Figure 4). In this type of valve, in addition to the primary force created by fluid pressure, a high wedging force on the seats created by the tightening of the stem assists with the sealing. The wedge-shaped gate does not stick to the seat in case of high fluid differential pressure and has a high service life due to less “rubbing” on the seats.
Figure 4: Wedge gate valve vs. parallel gate valve
Gate valves can also come in 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 are made of two pieces and 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, due to expansion, an additional compression load on the seats may result in thermal binding and restricted opening of the valve. 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 5: Slab Gate Valve
Slab gates, also called through-conduit gate valves, are one-unit gates that include a bore size hole (Figure 5). In open state, the bore size hole is aligned with the two seat rings to create a smooth flow with minimal turbulence. This special design allows for minimal pressure loss on the system and is perfect for 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.
Expanding gate valves are comprised of two slab gates matched together that provide sealing through mechanical expansion of the gate (Figure 6). When lifted, both of the slab gate’s cavity allow the the media to flow. The upward force on one slab and the stoppage of the second slab, by a step in the valve body, allows for outward mechanical expansion for a proper seal. When closed, the slab gates block the media flow and the downward force (stem) on one slab and upward force (step in valve body) allows for outward mechanical expansion for a proper seal.
These valves are used to provide effective seal simultaneously for both upstream and downstream seats. They are used in critical applications such as isolation valves in power plants, block valves in process systems and high temperature valves in refineries.
Figure 6: Expanding gate functioning
Knife gate valves are used for very thick fluids and dry bulk solids and are only comprised of a single piece of metal, typically pointed, as the gate. These valves are self-cleaning as they pass the seat rings every time they open and close.
In gate valves, the gate is raised and lowered by the spinning of a threaded stem which is either driven manually or controlled by an actuator. Depending on which end of the stem is threaded, stems can be rising or non-rising.
Outside Screw and Yoke (OS&Y), also referred to as rising stems, are fixed to the gate and the threads are on the actuation side. This causes the stem to raise and lower with the gate as it spins. Therefore, they have built-in visual indicators of the state of the valve and are easier to be lubricated. However, these gate valves often cannot be used with bevel gears or electric actuators as they have moving components. Therefore, rising gate valves are suitable for manual actuation.
On the other hand, a non-rising stem design is fixed to the actuator and threaded into the gate. As the gate movement in concealed, often an indicator is threaded onto the stem to show the state of the valve. Non-rising gate valves are common in underground installations and in applications with limited vertical space.
Figure 7: Mechanism of rising stem gate valves vs. non-rising stem gate valves
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