Calculation and selection of instrument regulating valve diameter

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Calculating the caliber of the regulating valve requires six steps: determining the flow rate, determining the calculated differential pressure, calculating the flow capacity, selecting the flow capacity, checking and determining the caliber of the regulating valve. Today I will share with you the relevant knowledge about selecting the caliber of the regulating valve. Flow capacity is the main basis for selecting the caliber of a regulating valve. In order to correctly calculate the flow capacity, the flow rate and pressure difference of the regulating valve must first be reasonably determined. Usually, the flow rate and pressure difference substituted into the flow capacity calculation formula are called calculated flow rate and calculated pressure difference. Steps for selecting the caliber of the regulating valve: 1. Determination of the calculated flow rate. The calculated flow rate refers to the maximum flow rate through the regulating valve. The flow value should be comprehensively considered and reasonably determined based on factors such as the production capacity of the process equipment, changes in object load, changes in operating conditions, and system control quality. But there are two tendencies that should be avoided: First, too much consideration is given to the margin and the valve diameter is selected too large. This not only causes economic waste, but also causes the valve to often work at a small opening, thereby reducing the controllable ratio, worsening the control performance, and even causing oscillation in severe cases, thus * * Reduces the life of the regulating valve ； The second is to only consider immediate production and one-sided emphasis on quality control, so that when productivity increases slightly, the control valve cannot adapt and is forced to be replaced. The calculated flow rate can also be determined by referring to the capabilities of fluid delivery machinery such as pumps and compressors. Sometimes, multiple methods are combined to determine. 2. Determination of calculated pressure difference Calculated pressure difference refers to the pressure difference on the regulating valve when the regulating valve is fully open and the flow rate is maximum. When determining and calculating the pressure difference, both control performance and power consumption must be taken into consideration. The greater the ratio of the pressure difference on the valve to the pressure difference of the entire system, the smaller the distortion of the flow characteristics of the regulating valve, and the better the control performance can be guaranteed. However, the greater the pressure difference before and after the valve, the more power is consumed. Calculating the pressure difference is mainly based on the size and change of the system pressure difference composed of process pipelines, equipment, etc. The steps are as follows: ①Take the two devices with the closest distance before and after the control valve and basically stable pressure as the calculation range of the system. ②Under the maximum flow condition, calculate the pressure loss △PF caused by various local resistances in the system (except the regulating valve), and then find their sum Σ△PF. ③Select S value. The S value should be the ratio of the pressure difference △PV on the control valve and the total pressure loss of the system when the regulating valve is fully open, that is, S=△PV÷(△PV+Σ△PF). S=0.3-0.5 is often selected. However, some systems can still meet the control performance requirements even when the S value is less than 0.3. For high-pressure systems, in order to reduce power consumption, it can also be reduced to S=0.15. For gas media, because the resistance loss is small and the pressure difference on the regulating valve accounts for a larger component, the S value is generally greater than 0.5. However, in low-pressure and vacuum systems, since the allowable pressure loss is small, S is still 0.3-0.5. ④According to the calculated Σ△PF and the selected S value, the calculated pressure difference of the regulating valve △PV=SΣ△PF÷(1-S) can be obtained by using the formula S=△PV÷(△PV+Σ△PF). Considering that the static pressure in the system equipment often fluctuates, it will affect the change of pressure difference on the valve and further reduce the S value. For example, in the boiler feed water control system, the calculated pressure difference should be increased by 5%-10% of the system equipment static pressure (assuming the boiler rated static pressure is P), that is, △PV=SΣ△PF÷(1-S)+0.0521P. Although an increase in the pressure difference on the regulating valve is beneficial to control, an excessive pressure difference may cause cavitation in the regulating valve. The absence of cavitation should also be considered when determining the calculated pressure difference. 3. Calculation of the regulating opening and controllable ratio of the regulating valve. After calculating the flow rate and calculating the pressure difference, the regulating opening and controllable ratio of the regulating valve should be verified. ①Calculation of the opening of the regulating valve. Generally, the opening of the regulating valve should be about 90% at the maximum flow rate, and not less than 10% at the minimum flow rate. The ideal flow characteristics and working conditions of the regulating valve must be considered when calculating the opening. The opening calculation formulas of two control valves with commonly used flow characteristics under working conditions (series pipelines) are given below. ②Calculation of controllable ratio At present, the ideal controllable ratio R of my country's uniformly designed control valves is generally 30. However, when used, it is limited by the maximum opening and minimum opening, which generally reduces the controllable ratio to about 10. In the case of series pipelines, the actual controllable ratio Rc=R√S. Therefore, the controllable ratio check is calculated according to the following formula Rc=10√S. If Rc>qmax/qmin, the selected regulating valve meets the requirements. Otherwise, the S value of the regulating valve must be changed, and the system pressure can be increased or two control valves (lower S value) can be used for split-range control to meet the controllable ratio requirements. In summary, the steps to determine the caliber of the regulating valve based on the data provided by the process are:: ①Determine calculation flow: According to the production capacity, equipment load and medium conditions, determine the calculated flow rate qmax and qmin. ②Determine the calculated pressure difference: The S value is selected based on the selected flow characteristics and system characteristics, and then the calculated pressure difference is determined. ③Calculate circulation capacity: Based on the determined calculated flow rate and calculated pressure difference, find the flow capacity Cmax at the maximum flow rate. ④Select circulation capacity C: According to the obtained Cmax, select the C value that is greater than Cmax and closest to it in the standard series of the selected product type. ⑤Check calculation: Check the control valve opening and controllable ratio. ⑥Determine control valve size: After passing the calculation, the nominal diameter and valve seat diameter of the control valve are determined based on the flow capacity C value.