The flow direction of the regulating valve is the direction of the arrow marked on the body of the regulating valve. Usually, when installing the regulating valve, the direction of the arrow on the valve body should be consistent with the flow direction of the medium to be adjusted.
However, due to the differences in the production site, the requirements for the use of the regulating valve are also different. Practice has proved that the flow direction of the regulating valve will affect the working characteristics of the valve and directly affect the normal operation of the control system.
Regulating valves are divided into two types: "flow open type" and "flow closed type" according to the flow direction of the medium. When the flow direction is different, the force on the spool of the regulating valve is also different. For example, if the pressure in front of the valve acts on the spool, there is a tendency to push the spool open and become a "flow-open type"; on the contrary, the spool is pressed against the seat , So that the valve has a tendency to close, becoming a "flow closed type". From the perspective of the flow direction of the medium to the spool, the "flow-open" medium flows from the small end of the spool to the big end, and the "flow-closed" medium flows from the big end to the small end of the spool, and the flow direction of the regulating valve is Impact on performance.
For small-caliber high-pressure valves, if the flow-open type is changed to the flow-closed type, the service life can be greatly extended. This is because the flow-open type medium flows in the open direction, and cavitation and erosion mainly act on the sealing surface. The root of the valve core and the sealing surface of the valve core and valve seat are quickly damaged; while the flow-closed medium flows in the closed direction, and cavitation and erosion act on the sealing surface of the valve seat after the throttling, which protects the sealing surface and the valve core. The root, thereby extending the service life of the regulating valve.
Sometimes the flow coefficient of the regulating valve is too small due to an increase in output or calculation or selection errors. When the valve is fully opened and cannot meet the production requirements, the method of changing the flow can be used, that is, changing the flow from the open type to the flow closed type. The valve mostly passes 10% to 15% of the flow.
Both flow-open and flow-closed flow directions have their own advantages and disadvantages, so it is necessary to correctly select the flow direction of the regulating valve. The selection should be made according to the actual working conditions. If many equipment require that the regulating valve can completely shut off the working medium, the closing of the valve becomes the main requirement, and the "flow closed type" should be selected. If the selected valve is a single seat valve, the flow direction is just opposite to the arrow marked on the valve body. Angle type high pressure valve has serious cavitation and short life. It should also choose the "flow closed type", which can increase the life of the valve. For occasions with strict requirements on valve stem sealing, to prevent leakage, the "flow open type" should be selected. For suspension, high-viscosity, and solid particles-containing media, in order to prevent leakage and blockage, the "flow closed type" should be selected.
The flow direction of the same valve type is not necessarily the same. For example, a small-caliber high-pressure valve has a higher working pressure difference, severe cavitation, and a short life. You should choose the "flow-closed type", but for larger-caliber valves For high-pressure valves, when the working pressure is large but the pressure difference is not too large, it is better to choose the "flow-open type", because the "flow-open type" has good stability and can not use measures to prevent valve oscillation.
Since the flow capacity of "flow closed type" is 10% to 15% larger than that of "flow open type", when the fully open flow of the regulating valve installed in the direction of "flow open" does not meet the process requirements, it can be changed to "flow closed" Type" temporarily solves difficulties.
Due to the poor stability of the "flow-close type", stability becomes the main problem after the "flow-close type" is selected. Since the valve is easy to oscillate when the opening is small, the minimum opening of the valve should be made as much as possible to be 30% to 40%. To choose an actuator with a large spring stiffness, it should be used with a valve positioner when necessary. The change of valve flow direction will change the flow capacity and flow characteristics, which will inevitably have an impact on the control system. Therefore, when changing the valve flow direction, appropriate consideration should be given to the above problems.