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The average limit output of an average adult 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, three needs to be overcome:
1.Causes of Difficulties in Large-caliber Valve Switching
The average limit output of an adult is 60-90kg, depending on the physique.
Generally, the flow direction of the cut-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, the combination of three aspects of forces needs to be overcome, namely :
1) axial thrust thrust Fa;
2) Friction between packing and valve stem Fb;
3) Contact friction force Fc between valve stem and valve core;
The sum of moments is ∑M = (Fa + Fb + Fc) R
It can be seen that the larger the exit diameter, the greater the axial thrust force. When it is close to the closed state, the axial thrust force is almost close to the actual pressure of the pipe network (because P1-P2≈P1, P2 = 0 when closed).
For example, a DN200 globe valve is used on a 10bar steam pipe, and only the first item closes the axial thrust.
Solutions to difficult opening and closing of large-caliber valves
It is close to the limit of the horizontal circumferential force that can be output by a normal human body, so it is very difficult for a person to completely close the valve in this working condition. Of course, some factories recommend that such valves be installed in the reverse direction, which solves the problem of difficult to close, but it also has the problem of difficult to open after closing.
2.Cause analysis of prone to internal leakage in large-diameter globe valves
Large-diameter globe valves are generally used in boiler outlets, main sub-cylinders, and steam mains, etc. These locations have the following problems:
1) Generally, the pressure difference at the boiler outlet is relatively large, so the steam flow rate is also larger, and the erosion damage to the sealing surface is also larger. In addition, the combustion efficiency of the boiler cannot be 100%, which will cause a large water content in the steam at the boiler outlet, which will easily cause cavitation and cavitation damage to the valve sealing surface.
2) For the shut-off valves near the boiler outlet and the sub-cylinder, intermittent overheating occurs due to the steam just coming out of the boiler. During the saturation process, if the boiler water softening treatment is not too good, some acids will be precipitated. Alkali substances can 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 cannot be tightly sealed.
3) The valve of the inlet and outlet of the split cylinder, due to the amount of steam behind the valve due to production requirements and other reasons, the amount of steam used is large and small, and in the case of large changes in flow rate, it is easy to produce flashing, cavitation and other phenomena, so that The sealing surface of the valve causes damage such as erosion and cavitation.
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 through, so that the shut-off valve can be fully opened after the pipeline is slowly and uniformly heated to a certain degree to avoid rapid pipeline Overheating will cause excessive expansion, which will damage some joints. However, in this process, the valve opening degree is often very small, which causes the erosion rate to be far greater than the normal use effect, which seriously reduces the service life of the valve sealing surface.
3. Difficult solutions for large-diameter globe valve switching
1) First of all, it is recommended to choose a bellows sealed globe valve, which avoids the frictional resistance of the plunger valve and the stuffing valve, and makes it easier to open and close;
2) The valve core and valve seat must choose materials with good erosion resistance and wear resistance, such as Stellite cemented carbide;
3) It is recommended to adopt double disc structure, which will not cause excessive erosion due to small opening, which will affect the service life and sealing effect.