In the daily use of large-diameter cut-off valve users, we often reflect a problem, that is, when the large-diameter cut-off valve is used in the medium with large pressure difference, such as steam and high-pressure water, it is often difficult to close. No matter how hard it is closed, it is always found that there will be leakage, which is very difficult. The reason for this problem is caused by the structural design of the valve and the insufficient output torque of people at the limit level. Cause analysis of difficulty in opening and closing large-diameter valves; The average adult's horizontal ultimate output force is 60-90kg, depending on different constitutions. Generally, the flow direction of cut-off valve is designed to be low in and high in. When a person closes the valve, the human body horizontally pushes the handwheel to rotate, so that the disc moves downward to realize closing. At this time, it is necessary to overcome the combination of three forces, namely: 1) axial jacking force fa; 2) Friction FB between packing and valve stem; 3) Contact friction Fc between valve stem and disc core The total moment is ∑ m = (FA+FB+FC) r. It can be seen that the larger the diameter, the greater the axial jacking force. 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, if a DN200 caliber globe valve is used on a 10bar steam pipe, its first closing axial thrust force Fa=10×πr2=3140kg, and the horizontal circumferential force required for closing is close to the limit of the horizontal circumferential force that normal human body can output, so it is very difficult for a person to completely close the valve under this working condition. Of course, some factories suggest installing this kind of valve in reverse, which solves the problem of being difficult to close, but it is difficult to open after closing. Cause analysis of internal leakage of large diameter globe valve; Large-diameter globe valves are generally used at boiler outlet, main cylinder, main steam pipe, etc. These positions have the following problems: 1) Generally, the pressure difference at the boiler outlet is relatively large, so the steam velocity is also greater, and the erosion damage to the sealing surface is also greater. In addition, the combustion efficiency of the boiler can't be 100%, which will cause the steam water content at the outlet of the boiler to be large, and it is easy to cause cavitation and cavitation damage to the valve sealing surface. 2) For the cut-off valve near the boiler outlet and sub-cylinder, because the steam just coming out of the boiler has intermittent overheating, in the process of saturation, if the softening treatment of boiler water is not very good, some acid and alkali substances will often precipitate, which will cause corrosion and erosion to the sealing surface; There are also some crystallizable substances that may adhere to the sealing surface of the valve and crystallize, resulting in the valve not being tightly sealed. 3) For the inlet and outlet valves of the sub-cylinder, the steam consumption after the valve varies from time to time due to production requirements and other reasons, and it is easy to cause flash evaporation, cavitation and other phenomena when the flow rate changes greatly, thus causing erosion, cavitation and other destructive effects on the valve sealing surface. 4) Generally, when large-diameter pipelines are opened, they need to be preheated, and the preheating process generally requires a small flow of steam to pass through, so that the stop valve can be completely opened after the pipeline is slowly and evenly heated to a certain extent, so as to avoid excessive expansion and damage to some connection parts caused by rapid temperature rise of the pipeline. However, in this process, the valve opening is often very small, resulting in erosion rate far greater than the normal use effect, seriously reducing the service life of the valve sealing surface. Solutions to difficulties in opening and closing large-diameter globe valves; 1) firstly, it is suggested to choose the bellows sealing globe valve, which avoids the influence of friction resistance of plunger valve and packing valve and makes the switch easier. 2) The valve core and valve seat must be made of materials with good erosion resistance and wear resistance, such as Stellite cemented carbide; 3) It is recommended to adopt double disc structure, so as not to cause excessive erosion due to small opening, which will affect the service life and sealing effect.