[Popular science stickers] Analysis of the advantages and disadvantages of 5 valves

Analysis of 5 valves Pros and cons

"Know yourselves and know each other, you can't fight forever" The premise of correct selection and use of valves is a comprehensive and specific understanding of the advantages and disadvantages of each valve itself and the physical properties (temperature, pressure) of the medium used; chemical properties (corrosive); also understand The cleanliness of the medium (with or without solid particles or viscosity, etc.), select the appropriate product to obtain its best performance and the most economical service life. Below we will analyze the advantages and disadvantages of the following valves.

NO.1 butterfly valve

A butterfly valve is a valve that uses a disc-type opening and closing member to reciprocate around 90° to open, close, and adjust the fluid channel.

advantage:

     ①Simple structure, small size, light weight, saving consumables, suitable for large diameter valves;

　② Quick opening and closing, small flow resistance;

     ③ It can be used for media with suspended solid particles, and it can also be used for powder and granular media according to the strength of the sealing surface. It can be used for bidirectional opening and closing and adjustment of ventilation and dust removal pipelines, and is widely used in gas pipelines and water channels in metallurgy, light industry, electric power, and petrochemical systems.

Disadvantages:

     ① Due to the limitations of butterfly valve structure and sealing materials, it is not suitable for high temperature and high pressure piping systems.

     ②The sealing performance is poor compared to ball valves and globe valves, so it is used in places where the sealing requirements are not very high.

NO.2 ball valve

The ball valve is evolved from the plug valve. Its opening and closing part is a ball. The ball is rotated 90° around the axis of the valve stem to achieve the purpose of opening and closing. The ball valve is mainly used to cut off, distribute and change the direction of medium flow on the pipeline.

advantage:

     ① Has the lowest flow resistance (actually 0);

　②Because it won't get stuck during work (in the absence of lubricant), it can be reliably used in corrosive media and low-boiling liquids;

　③In a large pressure and temperature range, it can achieve complete sealing;

　④Can realize fast opening and closing, suitable for the automation system of the test bench. When opening and closing the valve quickly, the operation has no impact;

　⑤The spherical closure can be automatically positioned at the boundary position;

　⑥ The working medium is reliable and sealed on both sides;

　⑦ When fully open and fully closed, the sealing surfaces of the ball and the valve seat are isolated from the medium, so the medium passing through the valve at high speed will not cause corrosion of the sealing surface;

　⑧ Compact structure, light weight, the most reasonable valve structure suitable for medium and low temperature medium systems;

　⑨ The valve body is symmetrical, especially the overall structure, can withstand the stress from the pipeline well;

    ⑩The closing piece can withstand the high pressure difference during closing. It is the most ideal valve for petroleum and chemical pipelines.

Disadvantages:

   ①Because the main seat sealing material of ball valve is PTFE, it is inert to almost all chemical substances, and has a small friction coefficient, stable performance, not easy to age, wide temperature range and excellent sealing performance Comprehensive characteristics. However, the physical characteristics of Teflon, including higher coefficient of expansion, sensitivity to cold flow and poor thermal conductivity, require the design of valve seat seals to be based on these characteristics. Therefore, when the sealing material becomes hard, the reliability of the seal is destroyed. Moreover, PTFE has a low temperature resistance level and can only be used at less than 180°C. Above this temperature, the sealing material will age. When considering long-term use, it is generally not used at 120 ℃.

   ② Its regulating performance is worse than that of globe valve, especially pneumatic valve (or electric valve）

NO.3 Globe valve

Shut-off valve refers to a valve in which the closing member (valve) moves along the center line of the valve seat. According to this form of movement of the valve flap, the change of the valve seat port is proportional to the stroke of the valve flap. Because the opening or closing stroke of the valve stem of this type of valve is relatively short, and has a very reliable cut-off function, and because the change of the valve seat port is proportional to the stroke of the valve flap, it is very suitable for adjusting the flow rate. Therefore, this type of valve is very suitable for cut-off or adjustment and throttle.

advantage:

     ① During the opening and closing process, the friction force between the disc and the sealing surface of the valve body is smaller than that of the gate valve, so it is wear-resistant.

　② The opening height is generally only 1/4 of the valve seat channel, so it is much smaller than the gate valve;

　③ Usually there is only one sealing surface on the valve body and valve disc, so the manufacturing process is relatively good and it is easy to maintain;

　④Because the filler is generally polytetrafluoroethylene, the temperature resistance level is higher.

Disadvantages:

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

　② Due to the longer stroke, the opening speed is slower than the ball valve.

NO.4 Diaphragm valve

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

advantage:

     ① The control mechanism is separated from the medium channel, which not only ensures the purity of the working medium, but also prevents the possibility of the medium in the pipeline impacting the working parts of the control mechanism. The valve stem does not need to be sealed in any form unless it is controlling harmful media Used as a safety facility;

     ②Because the working medium is only in contact with 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.

     ③The structure is simple, only composed of three parts: valve body, diaphragm and bonnet assembly. The valve is easy to disassemble and repair quickly, 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 exceeding 150℃;

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

NO.5 Check valve

Check valves are also called reverse flow valves, check valves, back pressure valves, and check valves. These valves are automatically opened and closed by the force generated by the flow of the medium in the pipeline, which belongs to an automatic valve. Check valves are used in pipeline systems. Their main functions are to prevent backflow of the medium, to prevent the pump and drive motor from reversing, and to release the medium in the container. The check valve can also be used to supply the auxiliary system whose pressure may rise above the system pressure. It can be divided into a rotary type (rotating according to the center of gravity) and a lifting type (moving along the axis).

advantage:

     ①The working characteristics of the check valve are that the load changes greatly, and the frequency of opening and closing is small. Once it is turned on or off, the service cycle is very long, and the moving parts are not required to move.

Disadvantages:

     ① As the check valve is qualitatively determined for rapid closing in most practical uses, the medium flows in the opposite direction at the moment the check valve is closed. With the closing of the valve flap, the medium drops rapidly from the maximum reverse flow rate When it reaches zero, the pressure rises rapidly, that is, a "water hammer" phenomenon that may have a destructive effect on the pipeline system. Water hammer is a kind of pressure wave in the transient flow in the pressure pipe. It is the hydraulic shock phenomenon of pressure rise or fall caused by the change of the fluid flow rate in the pressure pipe. The physical cause is the combined effect of fluid incompressibility, fluid motion inertia and pipeline elasticity. In order to prevent the hidden danger of water hammer in pipelines, people have adopted some new structures and new materials in the design of check valves for many years. While ensuring the applicable performance of check valves, the impact force of water hammer is minimized Has made gratifying progress.

Tips:

"Everyone has their own talents, they use them properly, they use them, and they use them when they don't." Choosing the right valve is just like the company's employment. We must know that the valve function itself has no advantages and disadvantages. The important thing is to know The valve makes good use of its strengths, avoids its shortcomings, and makes full use of its functions.