How To Solve The Problem Of Difficult Closing Of Large Diameter Cast Iron Globe Valve

 

 

Among users who use large-diameter stop valves on a daily basis, we often report a problem, that is, when large-diameter stop valves are used on media with relatively large pressure differences, such as steam, high-pressure water, etc., they are often difficult to close, no matter how When closing with force, 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.

How to solve the problem of difficult closing of large diameter cast iron globe valve

Cast iron stop valve cross section

Generally, the horizontal limit output force of an adult is 60-90kg, depending on different physiques.

Generally, the flow direction of the stop 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, causing the valve disc to move downward to achieve closure. At this time, a combination of three forces needs to be overcome, namely :

1) Axial thrust force Fa;

2) Friction force between packing and valve stem Fb;

3) Contact friction between the valve stem and the valve disc core Fc

The total moment 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 (because P1-P2≈P1, P2=0 when closed), such as a DN200mm cut-off The valve is used on a 10kg/m2 steam pipe. Its only first closing axial thrust force is Fa=10×πr2=3140kg, and the horizontal circumferential force required for closing is = kg, which is close to the horizontal circumference that the normal human body can output. force limit, so it is very difficult for one person to completely close the valve under such conditions. Of course, some factories recommend installing this type of valve in reverse, which solves the problem of difficulty in closing, but also causes the problem of difficulty in opening after closing.

In response to the above problems, the cast iron stop valve can learn from the double-valve structure with a balanced valve disc and a two-way pressure compensation structure to completely solve the above problems, achieve extremely light operation in both directions, and extremely tight sealing. At the same time, it solves the problem of pipeline pre-conditioning during startup. Excessive heating causes deformation and cracking of welding or bending parts.