Check Valve
A check valve is a valve that opens or closes automatically depending on the flow and force of the medium to prevent backflow. Check valves are automatic valves, mainly used in pipelines with unidirectional fluid flow, allowing fluid to flow in only one direction to prevent accidents.
Check valves can be divided into three types according to their structure: lift check valves, swing check valves, and butterfly check valves. Lift check valves can be divided into vertical and horizontal types. Swing check valves are divided into single-disc, double-disc, and multi-disc types. Butterfly check valves are straight-through. The above check valves can be divided into three types in terms of connection methods: threaded connection, flange connection, and welding.
The following points should be noted when installing a check valve:
Do not allow the check valve to bear weight in the pipeline. Large check valves should be independently supported so that they are not affected by the pressure generated by the piping system.
During installation, ensure that the direction of fluid flow is consistent with the direction of the arrow marked on the valve body.
Vertical lift check valves should be installed on vertical pipelines.
Horizontal lift check valves should be installed on horizontal pipelines.
 

Gate Valve
The closing member of a gate valve is a plug-shaped valve disc with a flat or conical sealing surface. The valve disc moves linearly along the center line of the fluid. The valve stem can be a rising stem type (the stem rises and falls, but the handwheel does not) or a rising and rotating stem type (the handwheel rotates and rises and falls with the stem, and the nut is located on the valve body). Gate valves are only suitable for fully open and fully closed positions and cannot be used for throttling or regulation.
Gate valves are forced-sealing valves. Therefore, when the valve is closed, pressure must be applied to the valve disc to ensure that the sealing surfaces do not leak. When the medium enters the valve cavity from below the valve disc, the resistance that the operating force needs to overcome is the friction force of the stem and packing and the thrust generated by the pressure of the medium. The force to close the valve is greater than the force to open it, so the diameter of the stem must be large, otherwise, the stem will bend. In recent years, with the emergence of self-sealing valves, the flow direction of the medium in gate valves has been changed to enter the valve cavity from above the valve disc. In this case, the force to close the valve is smaller under the action of the medium pressure, while the force to open the valve is larger, and the diameter of the valve stem can be reduced accordingly. At the same time, this type of valve is also more tightly sealed under the action of the medium. China's "three-modernization" standards for valves once stipulated that the flow direction of gate valves should be from top to bottom.
When a gate valve is opened, when the opening height of the valve disc reaches 25% to 30% of the nominal diameter, the flow rate has reached its maximum, indicating that the valve has reached its fully open position. Therefore, the fully open position of a gate valve should be determined by the stroke of the valve disc.
Gate valves have the following advantages:
Simple structure, easy to manufacture and maintain.
Short stroke, short opening and closing time.
Good sealing, small friction force between sealing surfaces, long service life.
Gate valves have the following disadvantages:
Large fluid resistance, large force required for opening and closing.
Not suitable for media with particles, high viscosity, and easy coking.
Poor regulation performance.
Gate valves are classified according to the position of the stem threads: external thread type and internal thread type. According to the flow direction of the medium, there are straight-through type, straight-flow type, and angle type. According to the sealing form, there are packed gate valves and bellows-sealed gate valves.
The following points should be noted during the installation and maintenance of gate valves:
Gate valves operated by handwheels or handles can be installed at any position on the pipeline.
Handwheels, handles, and actuators cannot be used for lifting.
The flow direction of the medium should be consistent with the direction of the arrow shown on the valve body.