Nov.07.2024
Gate valve is a type of valve that uses a gate or wedge-shaped disc. The disc moves perpendicular to the flow to either allow or stop the flow of fluid in a pipeline. Gate valves are the most commonly used type of valve in process equipment and belong to the linear motion valve category. Gate valves are widely used; generally, any shut-off device with a diameter (DN) of ≥50mm uses them, and sometimes even smaller shut-off devices use gate valves.
During operation, gate valves are either fully open or fully closed. When fully open, the gate is completely removed from the fluid, resulting in minimal flow resistance. Therefore, pressure hardly drops when fluid passes through a gate valve.
To achieve proper sealing, there needs to be a 360° surface contact between the gate and the seat when the valve is fully closed. Gate valves are not used for throttling or regulating flow as they cannot provide precise control. High-velocity flow in partially open gate valves can cause erosion to the gate and seat surfaces, leading to vibration and noise.
I. Main Components and Functions of Gate Valve
● Valve body is the main part of the valve, holding all components together. It is usually connected to the pipeline through flanges and gaskets and serves as the pressure boundary of the valve.
● Bonnet serves as the second pressure boundary and is connected to the valve body via bolts or flanges.
● Stem, seat, and disc assembly are collectively called the trim. The performance of the valve depends on the arrangement of the seat and disc.
● Seat and disc are the most critical parts of a gate valve, containing a gate and a seat that snugly fits the gate.
● Stem connects the actuator inside the gate valve to the disc, enabling the opening and closing of the gate. There are two types of stems: rising and non-rising. Rising stems move vertically when opening the valve, while non-rising stems do not.
● Actuator is a manual wheel used to operate the stem and disc.
II. Types of Gate Valves
1) Solid wedge gate valve
The sealing surfaces form an angle with the vertical centerline, typically 2°52′, 3°30′, 5°, 8°, or 10°, depending on the medium's temperature. Generally, the larger the working temperature, the larger the angle to minimize the chance of wedging due to temperature changes. There are single-disc, double-disc, and flexible wedge types within solid wedge gate valves.
This is a very popular type of gate valve that can be used with almost all materials. It is suitable for medium-pressure conditions but not for high-pressure conditions as it can get wedged and cause leaks. Sometimes, due to thermal expansion, the seat cannot open and gets wedged, a phenomenon known as thermal binding.
2) Parallel disk or split gate valve
This type overcomes the problem of thermal binding as the disks and seats are free to move and are held together by springs. It is suitable for low and medium-pressure applications.
3) Flexible wedge gate valve
It features a small opening on the disc that matches with the seat, preventing leaks between the seat and disc. It is used for steam applications.
4) Non-rising and rising stem gate valve
In rising stem gate valves, the stem rises when opening and descends when closing. The stem nut is located on the bonnet or bracket, and the stem's up and down movement is achieved by rotating the stem nut. This structure benefits stem lubrication and has clear opening and closing, making it widely adopted.
In non-rising stem gate valves, the stem nut is inside the valve body and directly contacts the medium. Opening and closing the gate is achieved by rotating the stem. The advantage of this structure is that the gate valve's height remains constant, making it suitable for large-diameter gate valves or those with limited installation space. This type should be equipped with an open/close indicator to show the degree of opening. The downside is that the stem thread is not lubricated and directly exposed to medium erosion, making it prone to damage.
III. Precautions
1) Handwheels, handles, and drive mechanisms should not be used for lifting and must avoid collisions.
2) Double-disc gate valves should be installed vertically, with the stem in the vertical position and the handwheel at the top.
3) Gate valves with bypasses should open the bypass first to balance the pressure difference and reduce the opening force.
4) Gate valves with actuators should be installed according to the product's instructions.
5) If the valve is frequently opened and closed, it should be lubricated at least once a month.
6) Gate valves are only for fully open or fully closed operations in pipelines or equipment, not for throttling.
7) Gate valves with handwheels or handles should not be operated with additional levers. If there is a sealing issue, check and repair the sealing surfaces or other parts. Turn the handwheel or handle clockwise to close and counterclockwise to open. Gate valves with actuators should be used according to the product's instructions.
IV. Installation Points
1) The installation position, height, and inlet/outlet direction must meet design requirements, and connections should be firm and tight.
2) Manual valves installed on insulated pipes should not have handles pointing downwards.
3) Valves must undergo an appearance check before installation. For valves with a working pressure greater than 1.0 MPa and serving as shut-off valves on main pipes, strength and tightness tests should be conducted before installation. During the strength test, the test pressure is 1.5 times the nominal pressure, lasting no less than 5 minutes, with no leakage from the valve body and packing as qualified. During the tightness test, the test pressure is 1.1 times the nominal pressure; the test pressure and duration should meet specified standards, with no leakage from the valve disc sealing surface as qualified.
4) Handwheels, handles, and drive mechanisms should not be used for lifting and must avoid collisions.
5) Gate valves with actuators should be installed according to the product's instructions.
