Why is the stop valve low in and high out?
Stop valve, also known as shut-off valve, belongs to the forced sealing valve, is a kind of block valve. According to the connection mode, it can be divided into three types: flange connection, screw thread connection and welding connection.
China’s “three modernizations to” once stipulated that the flow direction of the stop valve should be from top to bottom, so the installation has directionality.
This type of shut-off valve is very suitable for shut-off or regulation and throttling. Because the opening or closing stroke of the valve stem is relatively short, and has a very reliable cut-off function, and because the change of the valve seat opening is proportional to the stroke of the valve disc, it is very suitable for the flow regulation.
The cut-off valve is designed as low inlet and high outlet to reduce flow resistance and save labor when opening the valve. At the same time, when the valve is closed, the gasket between the valve shell and the valve cover and the packing around the valve stem are not stressed, which will not be affected by the medium pressure and temperature for a long time, which can prolong the service life and reduce the probability of leakage. In addition, the packing can be replaced or added when the valve is closed, which is convenient for maintenance.
Many people think that the stop valve is low in and high out, but it is not. Generally, the globe valve is low in and high out, but there are some special cases where the stop valve is high in and low out.
High pressure stop valve with diameter greater than 100 mm
Due to the poor sealing performance of large-diameter valves, the medium pressure acts on the upper part of the valve disc to increase the sealing performance of the valve.
Two stop valves are connected in series on the bypass pipeline, and the second one requires “high inlet and low outlet”
In order to ensure the tightness of the valve in a maintenance cycle, the valve which is often opened and closed requires to be equipped with two stop valves in series. For the bypass system, the installation functions of the bypass system are as follows: 1) balance the pressure before and after the main pipeline valve to make the opening convenient and labor-saving, and reduce the wear of the main pipe valve; 2) warm the pipe with small and medium flow during the start-up process; 3) on the main water supply pipe, control the feed water flow to control the boiler pressure rise speed for boiler hydraulic test. According to the flow direction of the medium, the bypass stop valves are primary valve and secondary valve. During normal operation of the unit, the primary valve and secondary valve are closed, both of which are in direct contact with the medium. In order to prevent the padding between the shell and cover of the secondary valve and the packing around the valve stem from being affected by the medium and temperature for a long time, and the packing of the valve can be replaced during operation, the installation direction of the secondary valve is required to be “high in, low out”.
Boiler exhaust and vent stop valve
Boiler exhaust and vent stop valves are only used in the process of boiler start-up and water filling, and the opening and closing frequency is small, but the working fluid loss is often caused by loose sealing. Therefore, in order to improve the tightness of some power plants, the installation direction of such stop valves is “high inlet and low outlet”.
Electromagnetic quick break valve
The function of the solenoid quick stop valve is to quickly close and cut off the fuel supply. The structure of the electromagnetic quick break valve is similar to that of the stop valve. If the working fluid enters from the bottom and flows out from the upper part, the force of the fuel acting on the lower part of the electromagnetic quick stop valve disc is very large, and the weight of the electromagnetic quick stop valve is far less than it. Therefore, if the working fluid enters the quick stop valve from the bottom, the torque generated by the heavy hammer is less than the torque generated by the fuel pressure, and the fuel can not be cut off when the quick stop valve acts, so the expected purpose can not be achieved. If the working fluid enters from the upper part of the quick stop valve, the pressure behind the valve will decrease rapidly once the quick stop valve acts. The force of fuel acting on the lower part of the valve disc will soon drop to zero, and the force formed by the force of the fuel acting on the valve disc and the gravity of the heavy hammer and lever.
Generally, it is difficult to close the valve with low inlet and high outlet under large diameter and high pressure. If low inlet and high outlet are adopted under high pressure and large diameter, the valve stem is easy to deform and bend under water pressure for a long time, which will affect the safety and sealing performance of the valve. If the high inlet and low outlet is selected, the valve rod diameter can be smaller, and the cost can be saved for manufacturers and users.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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