What Are the Top Factors to Consider When Selecting Industrial Gate Valves?

Working Principle

The gate valve’s opening and closing parts are the gate discs, and the movement direction of the gate discs is perpendicular to the direction of the fluid. The gate valve can only be fully opened or closed, and cannot be adjusted or throttled. The gate valve is sealed by the contact between the valve seat and the gate disc. Usually, the sealing surface is welded with metal materials to increase wear resistance. There are rigid gate discs and elastic gate discs. According to the different gate discs, the gate valves are divided into rigid gate valves and elastic gate valves. The gate disc has two sealing surfaces. The most commonly used mode is that the two sealing surfaces of the gate valve form a wedge. The wedge angle varies with the valve parameters, usually 5°, and 2°52′ when the medium temperature is not high.

Movement mode

When the gate valve is closed, the sealing surface can be sealed only by the medium pressure, that is, the sealing surface of the gate plate is pressed against the valve seat on the other side by the medium pressure to ensure the sealing of the sealing surface. This is self-sealing. Most gate valves use forced sealing, that is, when the valve is closed, the gate plate is forced to be pressed against the valve seat by external force to ensure the sealing of the sealing surface.

The disc of the gate valve moves linearly with the valve stem, also known as a outstanding rising stem gate valve. Usually there is a trapezoidal thread on the lifting rod, which converts the rotary motion into linear motion through the nut at the top of the valve and the guide groove on the valve body, that is, the operating torque into operating thrust.

Gate Valve Structure

The gate valve is generally composed of several parts such as valve body, seat, stem, disc, bonnet, sealing ring, etc. The valve’s opening and closing parts are discs, and the movement direction of the disc is perpendicular to the direction of the fluid. The valve can only be fully opened and fully closed, and cannot be adjusted or throttled.

Advantages

• Small flow resistance. The medium channel inside the valve body is straight, the medium flows in a straight line, so the flow resistance is small.
• It is more labor-saving when opening and closing. Compared with the stop valve, because whether it is open or closed, the movement direction of the gate is perpendicular to the flow direction of the medium.
• The height is large and the opening and closing time is long. The opening and closing stroke of the gate is large, and the lifting is carried out by the screw. Water hammer phenomenon is not easy to occur as the long closing time.
• The medium can flow in any direction on both sides, which is easy to install. The two sides of the gate valve channel are symmetrical.
• The structural length (the distance between the two connecting end faces of the shell) is small.
• Simple shape, short structural length, precision manufacturing enhance wide application in industry.
• Compact structure, good rigidity, smooth passage, small flow resistance and hard sealing surface greatly prolongs the service life. The PTFE packing makes the sealing reliable and the operation light and flexible.

Disadvantages

• Generally, the gate valve has two sealing surfaces, and the processing and manufacturing are more complicated than the globe valve.
• There is friction between the sealing surfaces, which causes wear and tear. It is inconvenient to repair the worn sealing surfaces.
• The large-diameter industrial gate valve cavity structure results in a long structure length, large external dimensions, large installation space, which is particularly bulky.
• The valve opening and closing time is long.
• A closed valve cavity can be formed, so a pressure relief structure should be set to prevent abnormal pressure rise in the cavity when necessary.

Installation points and precautions

1. The installation position, height, inlet and outlet directions must meet the design requirements, and the connection should be firm and tight.
2. Handwheels, handles, and transmission mechanisms are not allowed to be used for lifting, and collision is strictly prohibited.
3. For all types of manual valves installed on insulated pipes, the handles must not be downward.
4. Double-disc gate valves should be installed vertically (that is, the valve stem is in a vertical position and the handwheel is at the top).
5. Before opening the gate valve with a bypass valve, the bypass valve should be opened first to balance the pressure difference between the inlet and outlet so that to reduce the opening force.
6. For industrial gate valves with transmission mechanisms, must install and use them according to the product instructions.
7. The appearance of the valve must be inspected before installation. For valves with a working pressure greater than 1.0 MPa/150LB and that serve as a cut-off on the main pipeline, shell and sealing pressure tests should be carried out before installation, and they can only be used after passing the test. The test pressure of the shell is 1.5 times the nominal pressure, and the duration is not less than 5min. The test pressure of sealing is 1.1 times the nominal pressure. Both tests should be no leakage to be qualified. The test pressure and time should comply with the requirements API 598.
8. The gate valve is only used for fully opening and closing, not allowed to be used for throttling.
9. For gate valves with handwheels or handles, no auxiliary levers shall be added during operation (if the seal is not tight, the sealing surface or other parts shall be checked and repaired). The handwheel and handle are closed when turned clockwise, and opened when turned counterclockwise.
10. If the valve is frequently opened and closed, lubricate it at least once a month.