Can not be ignored in the field of construction valve selection large diameter valve why it is difficult to switch? What’s the solution?
In building piping, valves play the role of fluid control. Because of the different structure and material, so the manufactured valves are not the same. In order to ensure that the pipeline system can achieve higher efficiency, lower cost and the longest service life, the correct selection of valves is very important. The valve has four main functions: start and stop the flow of media; Adjust the medium flow; Prevents backflow or reflux and regulates or relieves fluid pressure. The choice of building piping system can be considered according to temperature, medium type, temperature and other factors. , for example, in the high-rise building fire hydrant valve control valve should be used signal, this is related to whether the fire hydrant system is the key to rational use, when the fire hydrant system control valves are set to signal valve, and valve open to display in the center of the fire control, in order to facilitate management inspection, although the cost has increased, however, The ratio of the investment to the overall hydrant system is still very small, and it can make the overall safety of the hydrant system, which is worth the investment.
The type of valve USED in a building piping system must be selected according to the characteristics of the building. If the valve used is not suitable for the design characteristics of the building, a number of potential hazards will constantly arise.
Why is it difficult to switch large caliber valves? What’s the solution? In the daily use of large caliber globe valve users, we often reflect a problem, that is, large caliber globe valve 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, always found there will be leakage phenomenon, it is difficult to close, The reason for this problem is caused by the structural design of the valve and the insufficient output torque of the human limit level.
In the daily use of large caliber globe valve users, we often reflect a problem, that is, large caliber globe valve 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, always found there will be leakage phenomenon, it is difficult to close, 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 reasons for the difficulty in opening and closing large diameter valves
The average adult’s horizontal limit output force is 60-90kg, depending on the different physique.
The general flow direction of the globe valve is designed to be low and high. When the valve is closed, the human body pushes the handwheel horizontally to rotate, so that the valve disc moves downward and closes. At this time, the combination of three aspects of force needs to be overcome, namely:
1) axial jacking force Fa;
2) Friction force between packing and valve stem Fb;
3) The stem contacts the disc core with friction force Fc
The torques sum up to ∑M equals Fa plus Fb plus Fc R
It can be seen that the larger the diameter is, the larger the axial jacking force is. When it is close to the closed state, the axial jacking force is almost close to the actual pressure of the pipe network (because P1-P2≈P1, P2=0 when it is closed).
For example, a DN200 caliber globe valve is used on a 10bar steam pipe, and its **** term closes the axial thrust Fa=10× πR2 =3140kg. The horizontal circumferential force required for closing is close to the limit of the horizontal circumferential force that normal human body can output. Therefore, it is very difficult for a person to completely close the valve under this working condition.
Of course, some factories suggest to install this kind of valve in reverse, which solves the problem of difficult to close, but there is the problem of difficult to open after closing.
Cause analysis of large diameter globe valve prone to internal leakage
Large caliber stop valve is generally used in the boiler outlet, the main cylinder, steam director and other positions, these positions have the following problems:
1) The pressure difference at the outlet of the boiler is generally large, so the steam flow rate is also larger, and the erosion and destruction effect on the sealing surface is also large. In addition, the boiler combustion efficiency can not be 100%, which will cause the boiler outlet steam water content is larger, easy to produce cavitation and cavitation damage to the valve sealing surface.
2) On the outlet of the boiler and the cut-off valve near the cylinder, because the steam just out of the boiler, there is intermittent overheating phenomenon, in the process of saturation, if the boiler water softening treatment is not too good, often precipitate part of the acid and alkali substances, on the sealing surface will cause corrosion and erosion; Some crystallizable substances may also be attached to the valve sealing surface crystallization, resulting in the valve can not be tightly sealed.
3) The inlet and outlet valve of the sub-cylinder, because the steam consumption after the valve is caused by production requirements and other reasons, the steam consumption is large and small, in the case of large change in the flow rate, it is easy to produce flash, cavitation and other phenomena, thus causing erosion, cavitation and other damage to the sealing surface of the valve.
4) Generally, when large pipe diameter pipelines are opened, it is necessary to preheat the pipeline, 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 degree before the cut-off valve can be fully opened, so as to avoid excessive expansion caused by rapid heating 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 use effect, seriously reduce the service life of the valve sealing surface.
Large diameter stop valve switch difficult solution
1) First of all, it is recommended to choose bellows sealing globe valve to avoid the friction resistance influence of plunger valve and packing valve and switch more easily.
2) The spool seat must be made of materials with good erosion resistance and wear performance, such as Siteli carbide.
3) It is recommended to use double disc structure, not due to small opening caused by excessive erosion, affect the service life and sealing effect.
Post time: Aug-06-2022
