A novel method of using a control valve for measurement and control of flow

Abstract

The control of flow through a pipe requires mainly three devices-a flowmeter for the measurement of flow, a control valve to control the flow, and a controller. The controller gives an appropriate signal to the control valve depending upon the difference between the measured flow rate and the set flow rate. Conventional flowmeters produce a differential pressure in the flow, which is measured and suitably scaled to get the flow rate. Incidentally, a control valve uses the principle of restricting the area for flow which in turn produces a differential pressure. From the above, it is seen that if a relationship is developed between the differential pressure produced by the control valve and the flow through it, the valve itself could function as a flowmeter also, apart from acting as a control valve. A relationship for this was established based on the equation for control valve capacity (C/sub /spl nu//) for different valve positions. The expression requires knowledge of the differential pressure across the valve, the static pressure, temperature, the valve position, and the C/sub /spl nu//V/sub s/ valve position characteristics in order to measure and control the flow rate. Error analysis is performed for the expression and to suggest means to improve the accuracy level. An iterative approach is necessary in order to obtain a particular flow rate using this method. This necessitates the use of a computer or a processor-based system, similar to present day intelligent controllers, to implement the scheme. A prototype was made in the laboratory using a 50-mm conventional control valve with pneumatic actuator, working as the control valve and flowmeter. An HP data acquisition and control system (DACS), used as the controller, was also used to provide the appropriate output signal to achieve the necessary how control. The unit was calibrated at the air flow laboratory to an accuracy of /spl plusmn/1.5%.