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3. The flow resistance is different. When the gate valve is fully open, the entire flow passage is straight through. At this time, the pressure loss of the medium operation is the smallest. Compared with the shut-off valve, its main advantage is that the fluid flow resistance is small, and the flow resistance coefficient of the ordinary gate valve is about 0.08~0.12. The drag coefficient of the globe valve is approximately 3.5 to 4.5. The opening and closing force is small. Gate valves are generally suitable for use without the need to open and close frequently, and to keep the ram fully open or fully closed, not suitable for adjustment and throttling. The flow resistance of the shut-off valve is large throughout the stroke, and the unbalanced force is large, and the required driving force or torque is correspondingly much larger. However, it is well suited for use as a conditioning and throttling fluid.
For high-speed flowing media, the ram can cause vibration of the valve under partial opening conditions, and vibration may damage the sealing surface of the ram and the valve seat, and throttling will cause the ram to be eroded by the medium.
4. The flow direction is different. When the shut-off valve is installed, the medium can enter from the bottom of the spool and enter from above. The advantage of the medium entering from below the valve plug is that the packing is not under pressure when the valve is closed, the life of the packing can be extended, and the packing can be replaced with the pressure in front of the valve. The disadvantage of the medium entering from the bottom of the valve core is that the driving torque of the valve is relatively large, about 1.05~1.08 times of the upper entry, the axial force of the valve stem is large, and the valve stem is easy to bend. For this reason, the medium enters from below, generally only for small-diameter shut-off valves (below DN50), and the shut-off valves above DN200 use the medium to flow in from above. Electric shut-off valves generally use a medium to enter from above. The disadvantage of the way the medium enters from above is exactly the opposite of the way it enters below. The flow direction of the gate valve is the same from both sides.
Compared with the gate valve, the cut-off valve has the advantages of simple structure, good sealing performance and convenient manufacturing and maintenance; the disadvantage is that the liquid resistance is large, and the opening and closing forces are large.
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