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　　The valve opening and closing parts and the shape of the valve flow channel so that the valve has a certain flow characteristics. In selecting the valve must be taken into account.
1 cut off and switch on media valve
Usually choose less flow resistance, flow channel for the straight valve. This kind of valve has gate valve, globe valve, plunger valve. Downward closed valve, due to the flow of twists and turns, flow resistance is higher than other valves, it is less choice. However, in allowing a higher flow resistance of the occasion, but also the use of closed valve.
2 valve for flow control
Usually choose to adjust the flow of the valve. Such as regulating valve, throttle valve, plunger valve, because of its valve seat size and open and close parts of the trip into a positive proportion between. Rotary type (such as plug valve, ball valve, butterfly valve) and flexible body type (pinch valve, diaphragm valve) valve can also be used for throttle control, but usually only in the limited range of the valve diameter. In most cases, people usually used to change the valve shape after the throttle stop valve for. It should be noted that the use of a change in the gate valve or the opening height of the valve to achieve the throttle function is extremely unreasonable. Because the media in the throttle state, the flow rate is very high, the sealing surface is easy to wear and tear, lose the sealing effect, in the same way, with the throttle valve as a cutting device is not reasonable.
3 valve for directional shunt
According to the need for the diversion of diversion, this valve can be three or more channels, suitable for the selection of plug valves and ball valves. Most of the valves are used in the direction of the shunt valve. In some cases, other types of valves are connected to each other by two or more, and can also be used as medium for commutation.
4 valve for medium with suspended particles
If the medium with suspended particles, the most suitable for the use of its open and close parts along the sealing surface of the valve with a wipe effect. Such as flat gate valve.
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