Why should the stop valve be low in and high out?
Stop valve, also known as stop valve, belongs to forced sealing valve and is a kind of block valve. There are three connection modes: flange connection, screw connection and welding connection.
The "three modernizations" of valves in China once stipulated that the flow direction of stop valves shall be from top to bottom, so there is directionality during installation.
This type of shut-off valve is very suitable for cut-off or regulation and throttling. Because the opening or closing stroke of the valve stem of this kind of valve is relatively short, and has a very reliable cut-off function, and because the change of the valve seat opening is directly proportional to the stroke of the valve disc, it is very suitable for the regulation of flow.
The stop valve is designed with low inlet and high outlet to reduce the flow resistance and save labor when opening the valve. At the same time, when the valve is closed, the padding between the valve shell and bonnet 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 valves are low in and high out, but they are not. Generally, the stop valve is low in and high out, but there are some special cases where the stop valve is high in and low out:
1. High pressure stop valve with diameter greater than 100mm
Due to the poor sealing performance of the large-diameter valve, the medium pressure acts on the valve disc when the stop valve is closed, so as to increase the sealing performance of the valve.
2. For the two stop valves connected in series on the bypass pipeline, the second stop valve requires "high inlet and low outlet"
In order to ensure the tightness of the valve in a maintenance cycle, the valve with frequent opening and closing operation is required to be equipped with two stop valves in series. For the bypass system, the installation functions of this bypass are:
① Balance the front and rear pressure of the main pipeline valve, make it easy to open and save labor, and reduce the wear of the main pipeline valve;
② Small and medium flow pipe warming during startup;
③ On the main feed water pipeline, control the feed water flow to control the boiler pressure rise speed for boiler hydrostatic test. According to the medium flow direction, the bypass stop valves are primary valve and secondary valve respectively. During normal operation of the unit, the primary valve and secondary valve are closed, and both of them are in direct contact with the medium. In order to prevent the padding between the secondary valve shell and bonnet and the packing around the valve stem from being affected by 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 and low out".
3. Boiler exhaust and vent stop valve
The boiler exhaust and vent stop valves are only used in the process of boiler startup and water supply, and the opening and closing frequency is small, but the working medium loss is often caused by lax sealing. Therefore, some power plants install such stop valves in the direction of "high in and low out" in order to improve the tightness.
4. Solenoid quick disconnect valve
The function of the solenoid quick break valve is to close quickly and cut off the fuel supply quickly. The structure of the electromagnetic quick break valve is similar to that of the stop valve. If the electromagnetic quick break valve is also a working medium entering from the lower part and flowing out from the upper part, the fuel has a great force on the lower part of the electromagnetic quick break valve disc, and the weight of the electromagnetic quick break valve is much smaller than it. Therefore, if the working medium enters the quick stop valve from the lower part, the torque generated by the heavy hammer is less than the torque generated by the fuel pressure. When the quick stop valve acts, the fuel cannot be cut off, so the expected purpose cannot be achieved. If the working medium enters from the upper part of the quick break valve, once the quick break valve acts, the pressure behind the valve decreases rapidly, and the force of the fuel acting on the lower part of the valve disc will soon drop to 0, while the force of the fuel acting on the valve disc and the force formed by the gravity of the heavy hammer and lever.