Difficult solution for large diameter valve switch

Date: 11/28 2018

1. Analysis of the difficulty of large-diameter valve switch:

The average limit output of adults is 60-90kg, depending on the physical condition.

Generally, the flow direction of the shut-off valve is designed to be low in and out. When the person closes the valve, the human body horizontally pushes the hand wheel to rotate, so that the valve flap moves downward to achieve closing. At this time, it is necessary to overcome the combination of forces in three aspects, that is, :

1) axial thrust Th;

2) filler and valve stem friction force Fb;

3) The valve stem and the disc core contact friction force Fc

The sum of the moments is ∑M=(Fa+Fb+Fc)R

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

For example, a DN200 diameter shut-off valve is used on a 10 bar steam pipe, and only the first item is closed. The axial top thrust is Fa=10×πr2=3140kg.
2. Analysis of the causes of internal leakage of large-diameter globe valves

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

1) The general pressure difference of the boiler outlet is relatively large, so the steam flow rate is also larger, 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 is easy to cause cavitation and cavitation damage to the valve sealing surface.

2) For the shut-off valve near the boiler outlet and the sub-cylinder, intermittent steam overheating occurs from the steam coming out of the boiler. In the process of saturation, if the boiler water softening treatment is not too good, some acid will be precipitated. Alkali substances can cause corrosion and erosion on the sealing surface; some crystallization materials may also adhere to the sealing surface of the valve to crystallize, resulting in the valve not being tightly sealed.

3) The inlet and outlet valves of the cylinders are caused by the steam consumption after the valve due to the production requirements, etc., and the steam consumption is large and small. When the flow rate changes greatly, the phenomenon of flashing and cavitation is easy to occur. The sealing surface of the valve causes erosion, cavitation and other damage.

4) Generally, when the 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 extent, then the shutoff valve can be completely opened to avoid the pipeline rapidity. The temperature rises to cause excessive expansion, which damages part of the joint. However, in this process, the opening degree of the valve is often very small, so that the erosion rate is far greater than the normal use effect, and the service life of the valve sealing surface is seriously reduced.

3. Large-diameter globe valve switch difficult solution

1) It is first recommended to select the bellows seal stop valve to avoid the influence of the frictional resistance of the plunger valve and the packing valve, and the switch is easier.

2) The valve seat must be selected from materials with good erosion resistance and wear resistance, such as stellite cemented carbide;

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