Among users who use large-diameter globe valves in their daily lives, there is often a problem that they are difficult to close when used on media with large pressure differences, such as steam, high-pressure water, etc. When working under pressure, no matter how hard they are closed, leakage is always found and it is difficult to close. The reason for this problem is due to the structural design of the valve and insufficient output torque of the human limit level.

Analysis of Difficulty in Opening and Closing Large Diameter Valves

The average adult's maximum horizontal output is 60-90kg, depending on their physical condition.

The flow direction of a general shut-off valve is designed to be low in and high out. When a person closes the valve, the human body horizontally pushes the handwheel to rotate, causing the valve disc to move downward and achieve closure. At this time, it is necessary to overcome the combination of three forces, namely:

1) Axial jacking force Fa;

2) Friction force Fb between packing and valve stem;

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

The total torque is ∑ M=(Fa+Fb+F      

It can be seen that the larger the diameter, the greater the axial jacking force. When approaching the closed state, the axial jacking force is almost close to the actual pressure of the pipeline network (because P1-P2 ≈ P1, P2=0 when closed)

If a DN200 caliber globe valve is used on a 10bar steam pipe, only the first closing axial thrust Fa=10 × π r2=3140kg, and the horizontal circumferential force required to close 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 this valve under this working condition.

Of course, some factories suggest reverse installation of such valves to solve the problem of difficulty in closing, but there is also a problem of difficulty in opening after closing.

Analysis of the Causes of Internal Leakage in Large Diameter Globe Valves

Large diameter globe valves are generally used at boiler outlets, main distribution cylinders, steam main pipes, and other positions, which have the following problems:

1) The pressure difference at the boiler outlet is generally relatively large, so the steam flow rate is also higher, which has a significant impact on the erosion and damage of the sealing surface. In addition, the combustion efficiency of the boiler cannot be 100%, which will cause a high water content in the steam at the outlet of the boiler and easily cause cavitation and cavitation damage to the valve sealing surface.

2) For the shut-off valves near the boiler outlet and distribution cylinder, intermittent overheating occurs due to the steam just coming out of the boiler. During its saturation process, if the boiler water softening treatment is not good, some acid and alkali substances will often precipitate, causing corrosion and erosion to the sealing surface; Some crystalline substances may also adhere to the valve sealing surface and crystallize, causing the valve to fail to seal tightly.

3) The inlet and outlet valves of the split cylinder have varying steam consumption due to production requirements and other reasons. When the flow rate changes greatly, it is easy to produce phenomena such as flashing and cavitation, which can cause damage to the valve sealing surface such as erosion and cavitation.

4) When opening large diameter pipelines, preheating is usually required, and the preheating process usually requires a small flow of steam to pass through the pipeline slowly and evenly, so that the shut-off valve can be fully opened to avoid excessive expansion caused by rapid heating of the pipeline and damage to some connecting parts. However, during this process, the valve opening is often very small, resulting in a much higher erosion rate than normal use, seriously reducing the use of the valve sealing surface        

Solution to Difficulty in Opening and Closing Large Diameter Stop Valves

1) Firstly, it is recommended to choose 2) The valve core and valve seat must be made of materials with good erosion resistance and wear performance, such as Stellite hard alloy.

3) It is recommended to adopt a double disc structure to avoid excessive erosion due to small opening, which will affect the service life and sealing effect.