Hydraulic valve position control system for regulating smokestack

Abstract

A control system for regulating exhaust gas pressure in a furnace smokestack by means of three power cylinder actuated butterfly valves in the smokestack opening. An electronic control circuit provides digital output signals, which operate spool valve devices via which a hydraulic fluid circuit is established to control the power cylinders. In a trim mode control circuit, a first comparator senses a difference between the actual stack pressure and a desired pressure. The comparator output is converted by an analog to digital section to provide the digital output signals for control of a single butterfly valve. A rangefinder mode control circuit includes a second comparator that senses an output from the first comparator in excess of a predetermined value. The second comparator output is also converted by an analog to digital section similar to that of the aforementioned trim mode control circuit to provide digital output signals for controlling a second butterfly valve concurrently with the first butterfly valve to obtain a fast pressure adjustment. A manual mode control circuit similar to the trim and rangefinder mode circuits controls the position of a third butterfly valve irrespective of the stack pressure in order to force a desired change in the stack pressure and thereby cause the first and second butterfly valves to be operated to maintain a desired pressure, as a means of maintaining their operating positions in the median of their operating range.