Valves are control components in pipeline fluid transport systems. They are used to alter the cross-sectional area of the flow path and change the direction of medium flow, performing functions such as guiding flow, shutting off flow, regulating flow, throttling, preventing backflow, diverting flow, or relieving pressure by overflow. Valves used for fluid control span a wide variety of types and specifications—from the simplest shut-off valves to highly sophisticated valves employed in automated control systems. The nominal diameters of these valves range from extremely small instrument valves to industrial pipeline valves with diameters as large as 10 meters.

　　Valves can be used to control the flow of various types of fluids, including water, steam, oil products, gases, slurries, a wide range of corrosive media, liquid metals, and radioactive fluids. The operating pressure of valves can span an ultra-high-pressure range from 0.0013 MPa to 1000 MPa, while their operating temperatures cover an ultra-low range from -269°C down to an ultra-high range of 1430°C. Valve actuation can be achieved through a variety of drive mechanisms, such as manual, electric, hydraulic, pneumatic, worm gear, electromagnetic, electro-hydraulic, electro-pneumatic, gear-driven, bevel-gear driven, and others. Under the influence of pressure, temperature, or other sensing signals, valves can perform actions according to pre-set requirements; alternatively, they can operate simply by opening or closing without relying on any sensing signals. Valves rely on actuators or automatic mechanisms to cause the valve members to move in lifting, sliding, oscillating, or rotating motions, thereby changing the size of the flow passage area and achieving their control functions.

　　1. The strength performance of a valve refers to its ability to withstand the pressure exerted by the medium. As a mechanical product that endures internal pressure, a valve must possess sufficient strength and rigidity to ensure long-term operation without cracking or deformation.

　　2. The sealing performance of a valve refers to its ability to prevent medium leakage at all sealing points. It is one of the most critical technical performance indicators for valves. There are three primary sealing areas in a valve: the contact surface between the closing member and the valve seat; the interface between the packing and the valve stem as well as the stuffing box; and the joint between the valve body and the valve bonnet. Leakage at the first of these locations is known as internal leakage, or commonly referred to as "leakage when closed," and it affects the valve's ability to shut off the medium flow. For shut-off valves, internal leakage is absolutely unacceptable. Leakage at the latter two locations is called external leakage—the medium leaks from inside the valve to the outside. External leakage can lead to material loss, environmental contamination, and in severe cases, even accidents. For media that are flammable, explosive, toxic, or radioactive, external leakage is especially unacceptable. Therefore, valves must possess highly reliable sealing performance.

　　3. After a fluid medium flows through a valve, a pressure loss occurs (i.e., the pressure difference between the valve’s upstream and downstream sides). In other words, the valve imposes a certain resistance to the flow of the medium, and the medium must expend a certain amount of energy to overcome this resistance. From an energy-saving perspective, when designing and manufacturing valves, every effort should be made to minimize the resistance that the valve offers to the flowing medium.

　　4. Motion Performance

　　1) Motion sensitivity and reliability

　　This refers to the degree of sensitivity with which a valve responds to changes in the parameters of the medium. For valves used to regulate medium parameters—such as throttling valves, pressure-reducing valves, and control valves—as well as for valves with specific functions—such as safety valves and steam traps—their functional sensitivity and reliability are critically important technical performance indicators.

　　2) Opening and closing force and opening and closing torque

　　The opening and closing force and torque refer to the force or torque that must be applied to open or close a valve. When closing a valve, it is necessary to generate a certain sealing specific pressure between the sealing surfaces of the closure member and the valve seat. At the same time, one must also overcome the frictional forces arising from the interaction between the valve stem and packing, between the valve stem and the nut’s threads, at the support points at the end of the valve stem, and in other friction-prone areas. Consequently, a certain closing force and closing torque must be applied. During the valve’s opening and closing process, the required opening and closing force and torque vary; their maximum values occur at the final instant of closing or the initial instant of opening. When designing and manufacturing valves, efforts should be made to minimize both the closing force and the closing torque.