Structural Components
A valve typically consists of a valve body, bonnet, seat, closure element, actuation mechanism, seals, and fasteners. The control function of a valve is achieved by utilizing the actuation mechanism-or the fluid itself-to drive the closure element to move vertically, slide, swing, or rotate, thereby altering the cross-sectional area of the flow channel.
Performance Indicators
Valve Strength Performance: This refers to a valve's capacity to withstand the pressure exerted by the medium flowing through it. As a mechanical product subject to internal pressure, a valve must possess sufficient strength and rigidity to ensure long-term operation without rupturing or undergoing deformation.
Valve Sealing Performance: This refers to the ability of the various sealing points within a valve to prevent the leakage of the medium. It is the most critical technical performance indicator for any valve. There are three primary sealing locations within a valve: the contact interface between the sealing surfaces of the closure element and the seat; the interface between the packing, the valve stem, and the stuffing box; and the connection interface between the valve body and the bonnet. Leakage occurring at the first location is termed "internal leakage," while leakage at the latter two locations is termed "external leakage."
Operating Force and Torque: These terms refer to the specific force or torque that must be applied to open or close a valve. When closing a valve, it is necessary to generate a specific sealing contact pressure between the sealing surfaces of the closure element and the seat; simultaneously, one must overcome the frictional forces present between the valve stem and the packing, between the valve stem threads and the nut, at the valve stem's end support, and at other friction-generating points. During the design and manufacturing phases, every effort should be made to minimize the required closing force and torque.
Operating Speed: This is defined as the amount of time required for a valve to complete a single opening or closing cycle. Generally, there are no strict requirements regarding a valve's operating speed; however, certain operating conditions impose specific demands. For instance, some applications require rapid opening or closing to prevent accidents, while others require slow closing to prevent phenomena such as water hammer.
Operational Sensitivity and Reliability: These terms describe the degree to which a valve responds appropriately to changes in the parameters of the flowing medium. For valves designed to regulate medium parameters-such as throttle valves, pressure-reducing valves, and control valves-as well as for valves with specific safety functions-such as safety valves and steam traps-operational sensitivity and reliability constitute critically important technical performance indicators.
Service Life: This indicator reflects the durability of a valve over time; it is a vital performance metric with significant economic implications. It is typically expressed in terms of the number of opening and closing cycles during which sealing requirements can be guaranteed, though it may also be expressed in terms of service life.
Sealing performance and structural strength are the most fundamental and critical characteristics of any valve. Valve sealing is broadly divided into two categories: internal sealing and external sealing. Internal sealing refers to the seal between the valve disc and the valve seat; external sealing refers to the seals at the moving parts of the valve stem relative to the bonnet, between the valve body and the bonnet, and at the connection points between the valve body and the pipeline.
Classification Methods
Valves can be classified in various ways. Based on nominal pressure, they can be categorized into vacuum valves, low-pressure valves, medium-pressure valves, high-pressure valves, and ultra-high-pressure valves; based on operating temperature, they can be classified as normal-temperature valves, medium-temperature valves, high-temperature valves, and low-temperature valves. Valves may also be classified according to the type of actuation device, the method of connection to the pipeline, and the materials used for the valve body.