Why is the large diameter valve difficult to open? Whats the solution?

Why is the large diameter valve difficult to open? What's the solution?

In the daily use of large-caliber stop valve users, we often reflect a problem, that is, the large-caliber stop valve is used in the medium with large pressure difference, such as steam, high pressure water and other pressure work, it is often difficult to close, no matter how hard to close, there will always be leakage, it is difficult to close, This problem is caused by the structural design of the valve and inadequate horizontal output torque.

Analysis of the reasons for the difficulty of large diameter valve switching

The average adult's horizontal limit output force is 60-90kg, depending on different physical conditions.

The general flow direction is designed to be low and high. When the valve is closed, the human body pushes the handwheel horizontally to rotate and make the valve disc move downward to realize closing. At this time, the combination of three forces needs to be overcome, namely:

1) Axial thrust Fa;

2) Friction between packing and stem Fb;

3) Stem and disc core contact friction Fc

The total torque is ∑M=(Fa+Fb+Fc)R

It can be seen that the larger the diameter is, the greater the axial thrust is. When it is close to the closed state, the axial thrust is almost close to the actual pressure of the pipe network (p1-P2 ≈P1, P2=0 when it is closed).

For example, if a DN200 caliber globe valve is used on a steam pipe of 10bar, only one closing axial thrust Fa=10×π R2 =3140kg, and the horizontal circumferential force required for closing is close to the limit of the horizontal circumferential force that can be output by normal human body, so it is very difficult for a person to completely close the valve under such working conditions.

Of course, some factories suggest reverse installation of such valves, which solves the problem that it is difficult to close, but it is difficult to open after closing.

Analysis of the causes of internal leakage of large diameter stop Valve

Large diameter globe valve is generally used in boiler outlet, main sub-cylinder, steam supervisor and other positions, where there are the following problems:

1) The pressure difference at the boiler outlet is generally large, so the steam flow rate is also larger, and the erosion damage to the sealing surface is also larger. In addition, the boiler combustion efficiency can not be 100%, which will cause the boiler outlet steam water content is large, easy to produce cavitation and cavitation damage to the sealing surface of the valve.

2) for the boiler outlet and the cut-off valve near the cylinder, because the steam just out of the boiler, intermittent overheating occurs, in the process of saturation, if the boiler water softening treatment is not too good, will often precipitate some acid and alkali substances, will cause corrosion and erosion of the sealing surface; There are also some crystalline substances may also adhere to the valve sealing surface crystallization, leading to the valve can not be tightly sealed.

3) cylinder inlet and outlet valves, due to the amount of steam after the valve due to production requirements and other reasons caused by large amount of steam, in the case of large flow rate changes, it is easy to flash, cavitation and other phenomena, resulting in erosion, cavitation and other damage to the sealing surface of the valve.

4) Generally, when the pipeline with large diameter is opened, it is necessary to preheat the pipeline, and the preheating process generally requires a small flow of steam to pass through, and the pipeline is slowly and evenly heated to a certain extent before the globe valve can be fully opened, so as to avoid excessive expansion caused by rapid temperature rise of the pipeline and damage to part of the connection. But in this process, the valve opening is often very small, resulting in erosion rate is far greater than the normal effect, seriously reduce the service life of the valve sealing surface.

Large diameter globe valve switch difficult solution

1) First of all, it is recommended to choose bellows sealed globe valve, which avoids the influence of friction resistance of plunger valve and packing valve and makes switching easier.

2) The valve seat must choose good material with erosion resistance and wear performance, such as Stali hard alloy.

3) It is recommended to use double disc structure to avoid excessive erosion due to small opening, affecting service life and sealing effect.