[Science Posts] Analysis of the advantages and disadvantages of the five major valves, you may not know them all (below)!

Monday

May 24, 2020​

The shut-off valve refers to the valve whose closing part (disc) moves along the center line of the valve seat. According to this movement form of the valve disc, the change of the valve seat port is proportional to the valve disc stroke. Since the opening or closing stroke of the valve stem of this type of valve is relatively short, and it has a very reliable cut-off function, and because the change of the valve seat port is in direct proportion to the stroke of the valve disc, it is very suitable for flow adjustment. Therefore, this type of valve is very suitable for cutting off or regulating and throttling.

advantage:

　　①During the opening and closing process, the friction between the valve clack and the sealing surface of the valve body is smaller than that of the gate valve, so it is wear-resistant.
　　②The opening height is generally only 1/4 of the valve seat passage, so it is much smaller than the gate valve;
　　③Usually there is only one sealing surface on the valve body and the valve disc, so the manufacturing process is better and easy to maintain;
　　④Because the packing is general It is polytetrafluoroethylene, so the temperature resistance level is higher.

Disadvantages:

     ① Since the flow direction of the medium through the valve has changed, the minimum flow resistance of the stop valve is also higher than that of most other types of valves;

　　②Due to the long stroke, the opening speed is slower than that of the ball valve.

Diaphragm valve refers to a flexible membrane or combined diaphragm installed in the valve body and bonnet, and its closing part is a compression device connected with the diaphragm. The valve seat is weir-shaped.

advantage:

     ①The control mechanism is separated from the medium passage, which not only ensures the purity of the working medium, but also prevents the possibility of the medium in the pipeline from impacting the working parts of the control mechanism. There is no need to use any form of separate seal at the valve stem, unless it is controlling harmful media Used as a safety facility;

     ②Since the working medium is in contact with only the diaphragm and the valve body, both of which can use a variety of different materials, the valve can ideally control a variety of working media, especially suitable for media with chemical corrosion or suspended particles.

     ③Simple structure, only composed of three parts: valve body, diaphragm and valve cover assembly. The valve is easy to quickly disassemble and repair, and the replacement of the diaphragm can be completed on site and in a short time.

Disadvantages:

     ① Due to the limitation of the diaphragm material, the diaphragm valve is suitable for low pressure and low temperature occasions. Generally not more than 150℃;

     ②The adjustment performance is relatively poor, and it can only be adjusted in a small range (usually it can be used for flow adjustment when it is closed to 2/3 opening).

Check valves are also known as reverse flow valves, check valves, back pressure valves and one-way valves. These valves are automatically opened and closed by the force generated by the flow of the medium in the pipeline, and belong to an automatic valve. The check valve is used in the piping system, and its main function is to prevent the medium from flowing back, prevent the pump and the drive motor from reversing, and to discharge the container medium. Check valves can also be used to supply pipelines for auxiliary systems where the pressure may rise above the system pressure. It can be divided into swing type (rotating by the center of gravity) and lifting type (moving along the axis).

advantage:

     ①The working characteristics of the check valve are large load changes and low opening and closing frequency. Once it is put into the closed or open state, the service period is very long, and the movement of moving parts is not required.

Disadvantages:

     ①Since the check valve is qualitatively determined to be used for fast closing in most practical applications, the medium flows in the opposite direction at the moment when the check valve is closed. As the valve flap is closed, the medium decreases rapidly from the maximum backflow speed. To zero, and the pressure rises rapidly, that is, the phenomenon of "water hammer" that may have a destructive effect on the piping system is generated. Water hammer is a kind of pressure wave in the transient flow in the pressure pipeline. It is the hydraulic shock phenomenon of the pressure rise or fall caused by the change of the fluid velocity in the pressure pipeline. The physical reason is the combined effect of fluid incompressibility, fluid motion inertia and pipeline elasticity. In order to prevent the hidden danger of water hammer in the pipeline, for many years, people have adopted some new structures and new materials in the design of check valves to ensure the applicable performance of the check valve while minimizing the impact of water hammer. Gratifying progress has been made.

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