Among users who use large-diameter globe valves on a daily basis, they often report a problem that large-diameter globe valves are often difficult to close when they are used in media with a relatively large pressure difference, such as steam, high-pressure water, etc. 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.

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 (because when it is closed, P1-P2≈P1, P2=0)

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 circular force required for closing is close to the horizontal circular force that normal human bodies can output. force limit, so it is very difficult for one person to fully close the valve under 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 of the Causes of Internal Leakage of Large Diameter Globe Valves

Large-diameter globe valves are generally used in boiler outlet, main cylinder, steam main and other positions. These positions 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 boiler outlet 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 acid is often precipitated Alkali substances will cause corrosion and erosion to the sealing surface; some crystallizable substances may also adhere to the valve sealing surface 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. The valve sealing surface causes erosion, cavitation and other damage.

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 heated slowly and evenly to a certain extent before the stop valve can be fully opened to avoid causing the pipeline to rapidly Excessive expansion caused by heating up and damage to some connection parts. 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 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 seat must be made of materials with good erosion resistance and wear resistance, such as Stellite carbide.

3) It is recommended to adopt a double valve disc structure, which will not cause excessive erosion due to the small opening, which will affect the service life and sealing effect.