CH-18-007 -- Valve Flowmeter Enhancement Through Computing Valve Dynamic Behaviors PDF

A virtual water flowmeter was developed that uses thechilled-water control valve on an air-handling unit (AHU) asa measurement device. The flow rate of water through the valvewas calculated using the differential pressure across the valveand its associated coil, the valve command, and an empiricallydetermined, steady state valve characteristic curve. Tocompensate for the loss in accuracy using the valve commandto indicate the valve open position, in this paper, valve dynamicbehavior was empirically described using valve stiction (S)and stiction plus dead band (slip jump: J) to convert valvecommands to actual valve positions. Stiction is defined asstatic friction resistance to valve movement, and dead bandrefers to hysteresis involved in the reversal of valve movementdirections.

To calculate the S and J parameters, extra measurementsof the system, especially valve reaction time, needed to betaken. To determine which values to use for S and J, very smallincremental changes were made manually via valve commandoverrides within the control software and the actual valve positionwas closely monitored for any changes. The stiction valuesmay not have remained constant throughout the period of valvemovement. Therefore, three valve command ranges were alsoexamined. In this study, low, medium, and high ranges of valvecommands were considered. Accuracy improvement of thevirtual flowmeter using converted valve commands was alsodemonstrated by a case study. Uncertainty analyses wereconducted, revealing significant improvements in virtual flowmeteraccuracy. Using the converted valve commands, themean relative error of the virtual flow calculation improvedfrom 13.73% when the valve command was used in the virtualflow rate calculation to 7.26%, when the valve dynamic behaviorr is reflected for low flow rate conditions.