Five problems often encountered in the use of valves

Update:23-06-2020
Summary:

1. Why does the double-seat valve easily oscillate when […]

1. Why does the double-seat valve easily oscillate when working at a small opening?

 

For a single core, when the medium is a flow-open type, the valve stability is good; when the medium is a flow-close type, the valve stability is poor. The double seat valve has two spools, the lower spool is in flow closure, and the upper spool is in flow opening. In this way, when working in a small opening, the flow-closed spool is likely to cause vibration of the valve. This is the double seat valve The reason why it cannot be used for small opening work.

 

2. Why can't the double seal valve be used as a shut-off valve?

 

The advantage of the double seat valve spool is the force balance structure, which allows a large pressure difference, and its outstanding disadvantage is that the two sealing surfaces cannot be in good contact at the same time, causing large leakage. If it is artificially and compulsorily used in the cut-off occasion, obviously the effect is not good, even if many improvements have been made for it (such as double seal sleeve valve), it is also undesirable.

 

 

3. What anti-blocking performance of straight stroke regulating valve is poor, and anti-blocking performance of angular stroke valve is good?

 

The spool of a straight stroke valve is vertically throttled, and the medium is flowing in and out horizontally. The flow path in the valve cavity must be turned and turned, making the valve's flow path quite complicated (shaped like an inverted "S" shape). such,

 

4. Why is the valve stem of the straight stroke regulating valve thin?

 

It involves a simple mechanical principle: large sliding friction and small rolling friction. The valve stem of the straight stroke valve moves up and down, and the packing is pressed a little, and it will wrap the valve stem very tightly, resulting in a large backlash. For this reason, the valve stem is designed to be very small, and the packing is often used with a small friction coefficient of PTFE packing, in order to reduce the backlash, but the problem is that the valve stem is thin, it is easy to bend, and the packing life is also short. To solve this problem, the best way is to use the rotary valve stem, that is, an angular stroke type regulating valve, its stem is 2 to 3 times thicker than the straight stroke stem, and the long-life graphite packing is used, and the stem rigidity Good, the filler has a long life, but its friction torque is small and the return difference is small.

 

5. Why is the cut-off pressure difference of the angular stroke valve larger?

 

The cutoff pressure difference of the angular stroke valve is large because the resultant force of the medium on the valve core or the valve plate has a very small torque on the rotating shaft, so it can withstand a large pressure difference.

 

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