Among the users who use large-diameter globe valves on a daily basis, they often report a problem, that is, when large-diameter globe valves are used in media with a relatively large pressure difference, such as steam, high-pressure water, etc., it is often difficult to close, no matter how hard they work. When closed, it is always found that there will be leakage, and it is difficult to close tightly. The reason for this problem is caused by the structural design of the valve and the insufficient output torque of the human limit level.
Analysis of the Difficulty in Switching Large Diameter Valves
The average adult's horizontal limit output force is 60-90kg, depending on different physiques.
Generally, 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 handwheel to rotate horizontally, so that the valve flap moves downward to realize the closing. At this time, it is necessary to overcome the combination of three forces, namely :
(1) Axial thrust force Fa;
(2) Friction force Fb between packing and valve stem;
(3) The friction force Fc between the valve stem and the valve disc core
The sum of moments is ∑M=(Fa+Fb+Fc)R
It can be seen that the larger the diameter, the greater the axial thrust force. When it is close to the closed state, the axial thrust force is almost close to the actual pressure of the pipe network (due to P1-P2≈P1, P2=0) when it is closed.
For example, a DN200 caliber globe valve is used on a 10bar steam pipe, only the first closing axial thrust Fa=10×πr2=3140kg, and the horizontal circumferential force required for closing is close to the horizontal circumferential force that normal human bodies can output. force limit, so it is very difficult for one person to fully close the valve in this condition.
Of course, some factories recommend installing such valves in reverse, which solves the problem of being difficult to close, but there is also the problem that it is difficult to open after closing.
Analysis on the Causes of Internal Leakage of Large Diameter Globe Valves
Large-diameter globe valves are generally used in boiler outlets, main cylinders, steam mains and other locations. These locations have the following problems:
(1) Generally, the pressure difference at the boiler outlet is relatively large, so the steam flow rate is also larger, and the erosion damage to the sealing surface is also larger. In addition, the combustion efficiency of the boiler cannot be 100%, which will cause the steam at the outlet of the boiler to have a large water content, which will easily cause cavitation and cavitation damage to the valve sealing surface.
(2) For the shut-off valve near the outlet of the boiler and the sub-cylinder, because the steam that has just come out of the boiler has intermittent superheating phenomenon, in the process of its saturation, if the softening treatment of the boiler water is not very good, part of the water tends to be precipitated. Acid and alkali substances will cause corrosion and erosion to the sealing surface; some crystallizable substances may also adhere to the sealing surface of the valve and crystallize, resulting in the valve not being able to be tightly sealed.
(3) For the inlet and outlet valves of the sub-cylinders, the steam consumption after the valve is large and sometimes small due to production requirements and other reasons. Cause erosion, cavitation and other damage to the valve sealing surface.
(4) Generally, when a large-diameter pipeline is opened, the pipeline needs to be preheated, 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 before the stop valve can be fully opened to avoid causing pipeline damage. Rapidly heating up, resulting in excessive expansion and damage to part of the connection. However, in this process, the valve opening is often very small, which causes the erosion rate to be far greater than the normal use effect, and seriously reduces the service life of the valve sealing surface.
Solutions to Difficulties in Switching Large Diameter of Globe Valves：
(1) First of all, it is recommended to choose a bellows-sealed globe valve, which avoids the influence of the frictional resistance of the plunger valve and the packing valve, and makes the switch easier.
(2) The valve core and valve seat must be made of materials with good erosion resistance and wear performance, such as Stellite carbide;
(3) It is recommended to adopt a double valve disc structure, which will not cause excessive erosion due to small opening, which will affect the service life and sealing effect.