The opening and closing member of the bellows stop valve is a plug-shaped valve flap having a flat or tapered surface, and the valve flap moves linearly along the center line of the fluid. The movement form of the valve stem has a lifting rod type (the valve stem is lifted and lowered, the hand wheel does not move up and down), and there is also a lifting rotary rod type (the hand wheel and the valve stem rotate together and the nut is set on the valve body). The shut-off valve is only suitable for full opening and full closing, and adjustment and throttling are not allowed.
The shut-off valve is a forced-sealed valve, so when the valve is closed, pressure must be applied to the valve flap to force the sealing surface to leak. When the medium enters the valve six from below the valve flap, the resistance to be overcome by the operating force is the friction force of the valve stem and the packing and the thrust generated by the pressure of the medium. The force of closing the valve is greater than the force of opening the valve, so the valve The diameter of the rod is large, otherwise the valve stem will be bent. In recent years, after the appearance of the self-sealing valve, the medium flow direction of the shut-off valve is changed into the valve chamber from above the valve flap. At this time, under the action of the medium pressure, the force of closing the valve is small, and the force of opening the valve is large, the valve stem is The diameter can be reduced accordingly. At the same time, this type of valve is also tight under the action of the medium. China's valve "three-way" has stipulated that the flow of the stop valve will be from top to bottom. When the shut-off valve is opened, the opening height of the valve flap is 25% to 30% of the nominal diameter, and the flow rate has reached the maximum, indicating that the valve has reached the fully open position. Therefore, the full open position of the shutoff valve should be determined by the stroke of the valve flap.
Advantages of the stop valve: 1. Simple structure, convenient manufacturing and maintenance. 2. The working stroke is small and the opening and closing time is short. 3. Good sealing performance, small friction between sealing surfaces and long service life.
Disadvantages of the globe valve: 1. The fluid resistance is large, and the force required for opening and closing is large. 2. Not suitable for media with particles, high viscosity and easy to coke. 3. The adjustment performance is poor. The type of the shut-off valve is divided into an external thread type and an internal thread type according to the position of the stem thread. According to the flow direction of the medium, there are straight-through, direct-current and angular. The shut-off valve is divided into seals, with a packing seal shut-off valve and a bellows seal shut-off valve.
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