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

Control of flow through a pipe requires mainly three devices, a flow meter for the measurement of flow, a control valve to control the flow and a controller. The controller gives appropriate signal to the control valve, depending upon the difference between the actual flow rate and the set flow rate. Conventional flow meters produce a differential pressure in the flow, which is measured and suitably scaled to get the flow rate. A relationship was established based on the equation for control valve capacity (C_v) for different valve positions. An iterative approach is necessary in order to obtain a particular flow rate. 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 flow meter. An HP data acquisition and control system (DACS) was used as the controller. The signal for valve movement was also provided from the DACS. The iterative computation of the flow rate and appropriate setting of the same was done by the DACS. The appropriate output signal is given by the DACS to the valve to achieve the necessary now control. The unit was calibrated at the air flow laboratory and an accuracy of ±1.5% was achieved for the same