In the daily use of large-diameter globe valves, people often reflect a problem, that is, large-diameter globe valves are used on mediums with relatively large pressure differences, such as steam, high-pressure water, etc., when working under pressure, it is often difficult to close, no matter Closed forcefully, it is always found that there will be leakage, which 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 person's limit level.
Analysis of the Causes of Difficulty in Opening and Closing of Large Diameter Valves
The general adult's horizontal limit output power is 60-90kg, depending on the physique.
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 flap downward to achieve closing. At this time, a combination of three forces needs to be overcome, namely :
1) Axial thrust Fa;
2) Friction force of packing and stem Fb;
3) The friction force Fc between the valve stem and the valve disc core Fc
The total torque is ∑M = (Fa + Fb + Fc) R
It can be seen that the larger the outlet diameter, the greater the axial thrust. When it is close to the closed state, the axial thrust is almost close to the actual pressure of the pipe network (because P1-P2≈P1, P2 = 0 when closed)
For example, a DN200-caliber globe valve is used on a 10bar steam pipe. Only the first item closes the axial thrust force Fa = 10 × πr2 = 3140kg, and the horizontal circumferential force required for closing is close to the normal human body can output the horizontal circle The limit of force, so it is very difficult for a person to completely close this valve under this working condition.
Of course, some factories suggest to install such valves in reverse to solve the problem of difficulty in closing, but there is also the problem of difficulty in opening after closing.
Analysis of the Causes of Large Leakage Globe Valves Prone to Internal Leakage
Large-diameter globe valves are generally used in boiler outlets, main sub-cylinders, steam main pipes, etc. These positions have the following problems:
1) The boiler outlet generally has a larger pressure difference, so the steam flow rate is also greater, and the erosion damage to the sealing surface is also greater. In addition, the combustion efficiency of the boiler cannot be 100%, which will cause the steam moisture content at the outlet of the boiler to be large, and it is easy to cause cavitation and cavitation damage to the sealing surface of the valve.
2) For the shut-off valve near the outlet of the boiler and the sub-cylinder, because of the intermittent overheating of the steam just coming out of the boiler, during the saturation process, if the boiler water softening treatment is not too good, some acid will often be precipitated Alkali substances will cause corrosion and erosion on the sealing surface; some crystallizable substances may also adhere to the valve sealing surface to crystallize, resulting in the valve being unable to be tightly sealed.
3) The inlet and outlet valves of the sub-cylinder, due to the production requirements and other reasons, the amount of steam used after the valve is large and small. In the case of a large change in flow rate, it is easy to produce flashing, cavitation, etc. The sealing surface of the valve causes erosion and cavitation damage.
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, so that the pipeline can be fully heated slowly and evenly to a certain extent before the shut-off valve can be fully opened to avoid rapid pipeline. Excessive expansion caused by temperature rise, damaging some connecting parts. However, in this process, the valve opening is often very small, resulting in an erosion rate far greater than the normal use effect, seriously reducing the service life of the valve sealing surface.
Solutions to the difficulties in opening and closing large-diameter globe valves
1) First of all, it is recommended to choose the bellows sealed globe valve, to avoid the friction resistance of the plunger valve and the packing valve, and the switch is easier.
2) The valve core and valve seat must choose materials with good erosion resistance and wear resistance, such as Stellite carbide.
3) It is recommended to use a double-valve structure, which will not cause excessive erosion due to small opening, which will affect the service life and sealing effect.
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