Valve Flowmeter Enhancement Through Computing Valve Dynamic Behaviors

摘要

A virtual water flowmeter was developed that uses the chilled-water control valve on an air-handling unit (AHU) as a measurement device. The flow rate of water through the valve was calculated using the differential pressure across the valve and its associated coil, the valve command, and an empirically determined, steady state valve characteristic curve. To compensate for the loss in accuracy using the valve command to indicate the valve open position, in this paper, valve dynamic behavior was empirically described using valve stiction (S) and stiction plus dead band (slip jump: J) to convert valve commands to actual valve positions. Stiction is defined as static friction resistance to valve movement, and dead band refers to hysteresis involved in the reversal of valve movement directions. To calculate the S and J parameters, extra measurements of the system, especially valve reaction time, needed to be taken. To determine which values to use for S and J, very small incremental changes were made manually via valve command overrides within the control software and theactual valve position was closely monitored for any changes. The stiction values may not have remained constant throughout the period of valve movement. Therefore, three valve command ranges were also examined. In this study, low, medium, and high ranges of valve commands were considered. Accuracy improvement of the virtual flowmeter using converted valve commands was also demonstrated by a case study. Uncertainty analyses were conducted, revealing significant improvements in virtual flowmeter accuracy. Using the converted valve commands, the mean relative error of the virtual flow calculation improved from 13.73% when the valve command was used in the virtual flow rate calculation to 7.26%, when the valve dynamic behavior r is reflected for low flow rate conditions.