Why is it difficult to open and close large-diameter valves?

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Among users who regularly employ large-diameter globe valves, a common issue is frequently reported: when these valves are used with media having a significant pressure difference—such as steam or high-pressure water—it becomes very difficult to close them. No matter how much force is applied during closure, leaks still occur, making it hard to achieve a proper seal. This problem arises due to both the valve’s structural design and the fact that human strength has its limits in terms of the torque that can be exerted. Analysis of the reasons for difficulty in operating large-diameter valves: The maximum horizontal output force that an average adult can exert is 60–90 kg, depending on individual physical conditions. Generally, the flow direction of a globe valve is designed such that fluid enters from the lower side and exits from the upper side. When closing the valve, a person rotates the handwheel horizontally, which causes the valve disc to move downward and thus close the valve. In this process, it is necessary to overcome a combination of three types of forces: 1) the axial thrust Fa ； 2) Friction force between packing and valve stem Fb ； 3) The total moment resulting from the frictional forces Fc between the valve stem and the core of the valve disc is ∑M = (Fa + Fb + Fc)R. It can be seen that the larger the diameter, the greater the axial thrust. When the valve is nearly closed, this axial thrust almost equals the actual pressure in the pipeline network (since when closed, P1 – P2 ≈ P1 and P2 = 0). For instance, for a globe valve with a DN200 diameter used in a steam pipeline operating at 10 bar, the first component of the axial closing thrust, Fa, amounts to 10 × πr² = 3,140 kg. The horizontal circumferential force required to close such a valve is already close to the maximum limit that a human can exert. Therefore, it is extremely difficult for one person to fully close this valve under such conditions. Of course, some factories recommend installing such valves in reverse, which solves the problem of them being difficult to close; however, this then leads to the problem of them being difficult to open after being closed. Analysis of the reasons for internal leakage in large-diameter globe valves. Large-diameter globe valves are generally used at locations such as boiler outlets, main steam distribution drums, and main steam pipelines. The following problems exist at these locations: 1) At boiler outlets, the pressure difference is usually quite large; consequently, the steam velocity is also higher, resulting in greater erosive damage to the sealing surfaces. Additionally, the combustion efficiency of a boiler cannot be 100%. This results in a relatively high moisture content in the steam at the boiler outlet, which can easily cause cavitation and erosion damage to the valve sealing surfaces. 2) For the stop valves located at the boiler outlet and near the steam manifold, since the steam just emerging from the boiler experiences intermittent superheating, during its saturation process, if the water softening treatment in the boiler is not performed adequately, certain acidic and alkaline substances tend to precipitate out. This can cause corrosion and erosion of the sealing surfaces ； There are also some crystallizable substances that may crystallize on the valve sealing surfaces, preventing the valve from sealing properly. 3) For the inlet and outlet valves of steam separators, since the steam consumption after the valves varies significantly due to production requirements and other factors, phenomena such as flashing and cavitation are likely to occur under conditions of large fluctuations in flow velocity. This, in turn, causes erosion and cavitation damage to the valve sealing surfaces. 4) Generally, when opening pipelines with large diameters, it is necessary to preheat them. The preheating process typically requires a very low flow rate of steam to be passed through the pipeline, allowing it to be gradually and uniformly heated to a certain temperature before the stop valve can be fully opened. This prevents rapid temperature increases in the pipeline, which could lead to excessive expansion and damage to certain joints. However, during this process, the valve opening is often very small, resulting in an erosion rate that is much higher than under normal operating conditions; this significantly reduces the service life of the valve sealing surfaces. Solutions for difficulty in operating large-diameter globe valves: 1) It is recommended to choose a bellows-sealed globe valve first; this eliminates the impact of frictional resistance associated with piston valves and packed valves, making operation much easier. 2) The valve stem and seat must be made of materials with good erosion and wear resistance, such as Stellite hard alloy ； 3) It is recommended to adopt a double-disc structure to prevent excessive erosion caused by small valve openings, which could affect the service life and sealing performance.