The valve is a control component in the fluid conveying system. It has the functions of cut-off, regulation, diversion, prevention of reverse flow, stabilization, diversion or overflow and pressure relief. Valves used in fluid control systems, ranging from the simplest shut-off valves to extremely complex valves used in automatic control systems, have a wide variety and specifications. Valves can be used to control the flow of various types of fluids such as air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media. Valves are also divided into cast iron valves, cast steel valves, stainless steel valves, chromium molybdenum steel valves, chromium molybdenum vanadium steel valves, duplex steel valves, plastic valves, and non-standard customized valve materials. 1. Sealing performance The sealing performance of a valve refers to the ability of each sealing part of the valve to prevent the leakage of the medium. It is the most important technical performance index of the valve. The valve has three sealing positions: the contact between the opening and closing parts and the two sealing surfaces of the valve seat; the matching place between the packing and the valve stem and the stuffing box; the connection between the valve body and the bonnet. The previous leakage is called internal leakage, which is commonly referred to as lax closure, which will affect the ability of the valve to cut off the medium. For shut-off valves, internal leakage is not allowed. The latter two leaks are called external leakage, that is, the medium leaks from the inside of the valve to the outside of the valve. Leakage can cause material loss, pollute the environment, and cause accidents in severe cases. For flammable, explosive, toxic or radioactive media, leakage is not allowed, so the valve must have reliable sealing performance. 2. Flowing medium pressure loss (the pressure difference before and after the valve) will occur after the medium flows through the valve, that is, the valve has a certain resistance to the flow of the medium, and the medium consumes a certain amount of energy to overcome the resistance of the valve. From the perspective of energy conservation, when designing and manufacturing valves, the resistance of the valve to the flowing medium should be reduced as much as possible. 3. Hoisting force and hoisting moment Hoisting force and hoisting moment refer to the force or moment that must be applied to open or close the valve. When closing the valve, it is necessary to form a certain seal specific pressure between the opening and closing parts and the two sealing surfaces of the seat, and at the same time, it must overcome the gap between the valve stem and the packing, the thread between the valve stem and the nut, and the support at the end of the valve stem. The frictional force of other friction parts, therefore, a certain closing force and closing torque must be applied. During the opening and closing of the valve, the required opening and closing force and opening and closing torque are changed, and the maximum value is at the final instant of closing or opening The initial instant. When designing and manufacturing valves, efforts should be made to reduce their closing force and closing torque. 4. Opening and closing speed opening and closing speed is expressed by the time required for the valve to complete one opening or closing action. Generally, there are no strict requirements on the opening and closing speed of the valve, but some working conditions have special requirements for the opening and closing speed. If some require rapid opening or closing to prevent accidents, some require slow closing to prevent water hammer, etc. This should be considered when selecting the valve type. 5. Action sensitivity and reliability This refers to the sensitivity of the valve to changes in media parameters and respond accordingly. For valves with specific functions such as throttle valves, pressure reducing valves, and regulating valves, as well as valves with specific functions such as safety valve
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