The solution to the difficulty of large diameter valve switch
Release time：2018-09-11Browsing volume：506
The average horizontal output power of adults is 60-90kg, depending on physical fitness. In general, the flow direction of the globe valve is designed to be low in and high out. When a person closes the valve, the human body moves the hand wheel horizontally to make the valve disc move downward and close. Three problems need to be overcome:
Analysis of difficult reasons for 1. large diameter valve switches
The average horizontal output power of adults is 60-90kg, depending on physical fitness.
In general, the flow direction of the globe valve is designed to be low in and high out. When a person closes the valve, the human body pushes the hand wheel horizontally to make the valve disc move downward and close. This requires overcoming the combination of three forces, namely:
1) axial thrust Fa;
2) friction force between filler and stem Fb;
3) the contact force between the stem and the disc core is Fc;
The total torque is Sigma M= (Fa+Fb+Fc) R
It can be seen that the larger the caliber, the greater the axial thrust. When approaching the closed state, the axial thrust is almost close to the actual pressure of the pipe network (because P1-P2_P1, P2 = 0).
If a DN200 caliper is used on the steam pipe of 10bar, it only closes the thrust of axial top only.
Fa=10 * PI r2=3140kg, and the horizontal circumferential force required for closing is
The solution to the difficulty of large diameter valve switch
It is close to the limit of the horizontal circumferential force that a normal human body can output, so it is very difficult for a person to completely close the valve under this condition. Of course, some factories suggest that such valve reverse installation, to solve the problem of difficult to close, but there has been a closed difficult to open the problem.
Analysis of the causes of internal leakage of 2. large caliber globe valves
Large caliber globe valves are generally used in boiler outlet, main cylinder, steam main pipe, etc. These locations have the following problems:
1) Boiler outlet pressure difference is generally larger, so the steam flow rate is also greater, the erosion of the sealing surface damage is also greater. In addition, the boiler combustion efficiency can not be 100%, which will cause the boiler outlet steam moisture content is large, easy to valve sealing surface cavitation and cavitation damage.
2) For the cut-off valve near the boiler outlet and distributor cylinder, because the steam just out of the boiler has intermittent overheating phenomenon, in its saturation process, if the boiler water softening treatment is not too good, often will precipitate part of the acid and alkali substances, will cause corrosion and erosion of the sealing surface; there are also some crystalline substances; May also adhere to the valve seal surface crystallization, resulting in valve can not be tightly sealed.
3) For the inlet and outlet valves of the sub-cylinder, the amount of steam used after the valve is large or small due to the production requirements and other reasons. In the case of large flow rate changes, it is easy to produce flashing, cavitation and other phenomena, thus causing erosion, cavitation and other damage to the sealing surface of the valve.
4) When a large diameter pipeline is opened, it is necessary to preheat the pipeline, and the preheating process generally requires a small flow of steam to pass through, so that the pipeline can be slowly and evenly heated to a certain extent, then the globe valve can be fully opened to avoid excessive expansion caused by rapid heating of the pipeline and damage part of the connection. But in this process, the valve opening is often very small, resulting in erosion rate is far greater than the normal effect of use, seriously reducing the service life of the valve sealing surface.
3. big caliber stop valve switch difficult solution
1) First of all, it is suggested to choose bellows sealing globe valve, which avoids the influence of friction resistance of plunger valve and packing valve and makes the switch easier.
2) the spool valve seat must select materials with good erosion and wear properties, such as Stellite cemented carbide.
3) It is suggested to adopt double disc structure without excessive erosion due to small opening, which will affect the service life and sealing effect.
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