Why are large-diameter valves difficult to open and close?

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Among users who routinely use large-diameter globe valves, a common issue is reported: when such valves are used with media under high pressure differences, such as steam or high-pressure water, it is often difficult to close them. No matter how much force is applied to close them, leakage still occurs, and it’s hard to achieve a tight seal. The reason for this problem lies in the valve’s structural design as well as the insufficient torque that can be generated by human effort. Analysis of the reasons for difficult operation of large-diameter valves: The maximum force that an average adult can generate is 60–90 kg, depending on individual physical conditions. Generally, the flow direction of a globe valve is designed such that fluid enters from below and exits from above. When the valve is closed, a person rotates the handwheel horizontally, causing the valve disc to move downward and thus achieving closure. At this time, it is necessary to overcome a combination of three types of forces: 1) the axial thrust Fa ； 2) Friction force Fb between the packing and the valve stem ； 3) The combined torque resulting from the frictional force Fc between the valve stem and the core of the valve disc is given by ∑M = (Fa + Fb + Fc)R. It can be seen that the larger the diameter, the greater the axial thrust; when the valve is near its closed position, this axial thrust is almost equal to the actual pressure in the pipeline system (since P1 – P2 ≈ P1 and P2 = 0 when the valve is closed). For example, if a DN200 globe valve is used on a steam pipeline operating at 10 bar, the axial thrust required to close it, represented by Fa, is 10 × πr2 = 3140 kg. The horizontal circumferential force needed to close the valve approaches the limit of the horizontal force that an average human can generate; therefore, it is very difficult for one person to fully close such a valve under these conditions. Of course, some factories recommend installing such valves in reverse, which solves the problem of difficulty in closing them; however, it introduces the issue of difficulty in opening them once they are closed. Analysis of the reasons for internal leakage in large-diameter globe valves. Large-diameter globe valves are typically used at locations such as the boiler outlet, main steam cylinders, and steam main pipes. These locations present the following issues: 1) The pressure difference at the boiler outlet is usually high, which results in a higher steam flow rate and thus greater erosion and damage to the sealing surfaces. Furthermore, the combustion efficiency of a boiler cannot be 100%; this results in a 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 distribution cylinders, since the steam emerging from the boiler experiences intermittent superheating, during its saturation process, if the water in the boiler is not properly softened, certain acidic and alkaline substances may be precipitated, which can cause corrosion and erosion of the sealing surfaces ； There are also some crystallizable substances that may adhere to the valve sealing surfaces and crystallize there, preventing the valve from sealing properly. 3) The inlet and outlet valves of the steam distribution cylinders: since the amount of steam used after these valves varies depending on production requirements and other factors, large fluctuations in flow velocity can easily lead to phenomena such as flashing and cavitation, which in turn cause erosion and cavitation damage to the valve sealing surfaces. 4) When opening large-diameter pipelines in general, it is necessary to preheat the pipelines. During this preheating process, steam at a very low flow rate is used to heat the pipelines slowly and evenly to a certain temperature before the stop valves can be fully opened, thereby preventing rapid temperature rise that could lead to excessive expansion and damage to some of the connection points. However, during this process, the valve opening is often very small, resulting in an erosion rate that is much higher than that under normal operating conditions, which significantly reduces the service life of the valve’s sealing surface. Solutions for difficult operation of large-diameter globe valves: 1) It is recommended to use globe valves with diaphragm sealing, as this eliminates the frictional resistance associated with plug valves and packing valves, making operation easier. 2) The valve core and seat must be made of materials with good resistance to erosion and wear, such as Stellite cemented carbide ； 3) It is recommended to adopt a double-valve-leaf structure to prevent excessive erosion at small opening angles, which could affect the service life and sealing performance. ‍