Among the users who use large-diameter stop valves on a daily basis, everyone often reflects a problem, that is, large-diameter stop valves are often difficult to close when they are used in media with a relatively large pressure difference, such as steam and high-pressure water. If you close it forcefully, you will always find that there will be leakage and it is difficult to close. The reason for this problem is the lack of the valve's structural design and the human limit level output torque.

Analysis of the Causes of Difficulties in Opening and Closing Large Caliber Valve

The ultimate horizontal output power of a general adult is 60-90kg, depending on different physiques.

Generally, the flow direction of the shut-off valve is designed to be low in and high out. When a person closes the valve, the human body pushes the handwheel horizontally to move the valve disc downwards to achieve closing. At this time, a combination of three forces needs to be overcome, namely :

    1) Axial thrust Fa;

    2) Friction between packing and stem Fb;

    3) The friction force Fc between the valve stem and the valve core

    The total torque is ∑M=(Fa+Fb+Fc)R

It can be seen that the larger the outlet diameter, the greater the axial pushing force. When it is close to the closed state, the axial pushing force is almost close to the actual pressure of the pipe network (due to P1-P2≈P1, P2=0)

For example, if a DN200 stop valve is used on a 10bar steam pipe, only the first item closes the axial pushing force Fa=10×πr2=3140kg, and the horizontal circumferential force required for closing is close to the horizontal circumferential output of a normal human body. The limit of force, so it is very difficult for one person to completely close the valve under such conditions.

Of course, some factories suggest that this kind of valve be installed in the reverse direction, which solves the problem of difficulty in closing, but there is also the problem of difficulty in opening after closing.

Analysis of Causes of Internal Leakage of Large Diameter Globe Valves

Large-diameter stop valves are generally used in boiler outlets, main sub-cylinders, steam mains, etc. These positions have the following problems:

1) Generally, the pressure difference at the outlet of the boiler is relatively large, so the steam flow rate is also greater, and the erosion damage to the sealing surface is also greater. In addition, the boiler combustion efficiency 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 just coming out of the boiler has intermittent overheating phenomenon, during the process of saturation, if the boiler water softening treatment is not good, part of the acid will often be 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, causing the valve to fail to seal tightly.

3) For the inlet and outlet valves of the cylinders, the amount of steam used after the valve is sometimes large or small due to production requirements and other reasons. When the flow rate changes greatly, it is easy to cause flashing, cavitation and other phenomena. The valve sealing surface causes damaging effects such as erosion and cavitation.

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 is slowly and evenly heated to a certain level before the shut-off valve can be fully opened to avoid rapid pipeline Excessive expansion caused by heating up, damage part of the connection part. However, in this process, the valve opening is often very small, causing the erosion rate to be far greater than the normal use effect, and seriously reducing the service life of the valve sealing surface.

Solution to the difficulty of opening and closing large-diameter valves

1) First, it is recommended to choose a bellows-sealed globe valve to avoid the frictional resistance of the plunger valve and the packing valve and make the switch easier.

2) The valve core and seat must be made of materials with good erosion resistance and wear properties, such as Stellite cemented 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.