As everyone has learned from previous articles, the characteristics of the regulating valve affect the stability and quality of the entire system. In the automatic control of modern chemical plants, regulating valves play a very important role. There are also many types of regulating valves, mainly pneumatic regulating valves, electric regulating valves, self operated regulating valves, etc. The production of these factories depends on the correct distribution and control of the flowing medium. These controls, whether it is energy exchange, pressure reduction, or simple container feeding, all require certain final control components to complete. The five major fluid characteristics are:
1. Resistance of fluid movement
2. By adjusting the medium of the valve
3. Inherent characteristics of regulating valves
4. Flow coefficient of regulating valve and its calculation
5. Static working characteristics of regulating valves
Today's article will talk about the resistance characteristics of fluid motion in regulating valves:
When the fluid medium flows through the pipeline system, energy loss is generated due to friction along the way, and additional energy loss is generated due to local resistance (sudden expansion and contraction of valves, surfaces, elbows, and pipe diameters). The calculation formula for pressure loss Pn in a conduit with a diameter of Dn.
In the formula, PN is the density of the medium, uN is the flow rate of the medium, LN is the effective length of the conduit, Ln is the resistance coefficient of the conduit, and is the energy loss coefficient through the valve.5i is the local loss coefficient of other parts. If the liquid medium flows through a simple pipeline, the pressure loss value can be calculated theoretically based on the experimentally determined coefficient. For local resistance, the theoretical calculation is only carried out in simple cases, as in most cases, the coefficient ζ It is different and must be determined through experiments according to different situations. Actually, the coefficient ζ Represents the actual energy loss of fluid resistance along the path. For two different structures or two different process conditions, except for occasional cases, the hydraulic resistance is unequal, and the coefficient ζ It's also different.
If only local resistance is involved, the calculation formula for pressure drop is as follows: "N is the average velocity of the fluid calculated based on the pipe diameter DN before or after the local resistance component, and in some cases, it is the flow velocity in a specific area.
Note: In most cases, pneumatic control valves are designed for circular cross-section pipelines. For a pipe segment. Local resistance coefficient: Local resistance is divided into valve positive force and other local resistance. The purpose of this division is to facilitate in-depth research on the flow and outflow of the catheter through the valve or by inserting one end of the catheter into the tank. When the container is connected to the catheter, the catheter is installed at the outlet of the container or the population catheter, as shown in Figures 1-36 and 1-37. The values for various scenarios have been given in each diagram.
The change in flow cross-section requires connectors when connecting conduits with different diameters. These connectors cause the cross-sectional area to slowly or suddenly expand or decrease. The following introduces the pressure loss Ap and flow resistance coefficient of various connectors ζ The calculation formula for. Sudden expansion. If the pressure loss is mainly due to the sudden expansion of the flow cross-section A.
The following will only discuss the shut-off valve. From an economic perspective, the smaller the resistance when the valve is fully open, the better. Therefore, the valve's ζ Value is a very important indicator and also the basis for grading.
Further research on the characteristics of valves can lead to the following conclusions: 1) The flow resistance of valves affects system performance.Due to the presence of flow resistance, it is required to increase the starting pressure or reduce the user's usage pressure. 2) The fluid resistance value of a valve is a function of the medium parameters and valve structure. 3) In shorter pipeline systems, the total loss of energy is mainly caused by valve pressure drop. 4) The selection of valve structure should not only consider the valve's function of opening and closing, but also ensure that the local resistance of the valve is as small as possible. According to equation (1-22), the calculation formula for valve pressure loss is ApN=PN 26N (1-30). From equation (1-30), it can be seen that the dimension of 5N is 1. It represents the degree of energy loss when the medium passes through the valve. The reasons for energy loss are diverse, such as changes in direction, expansion or contraction of flow area, and friction of pipe walls. Figures 1-44 show the schematic diagrams of several regulating valves and the situation of medium energy loss. You should know that even for a simple control valve, if you want to calculate it theoretically ζ The value is also impossible. The 6n value needs to be determined through experiments. The following is an introduction to the test results in order to provide convenient and useful data for calculating the energy loss of the valve.
4-Valve energy loss
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1- Pressure loss calculation formula:
2- Calculation formula
2- Calculation formula
