Override process control system

An override process control system for regulating a process from a single final control element from at least two process variables that are interdependent and must not exceed safe minimum and maximum limits, the first process variable being directly influenced by the setting of the final control element, said system comprising:

• A. a primary electronic process controller responsive to the first process variable to compare this variable with a first set point to produce in a non-override mode a first output signal that reflects the deviation of the first process variable from the set point and is applied to said final control element to govern this element accordingly;
• B. a standby electronic process controller responsive to the second process variable to compare this variable with a second set point to produce in the non-override mode a second output signal that reflects the deviation of the second process variable from the set point, which second signal under certain conditions may assume an unsafe value to produce an unsafe signal;
• and
• C. an override control selector station coupled by individual lines to said controllers to monitor the operations thereof, and in the event an unsafe signal is received from the standby controller, to cause said controllers to operate in an override mode in which the primary controller responds to the unsafe signal from the standby controller to govern the final control element accordingly.