Q. Air-Gas Cross-Limiting and Air-Gas Ratio Control?

The Steam Generator firing demand signal is directed to two discrete signal selectors. A low signal selector ensures that the lower of the gas demand signal or the gas rate, based upon the actual air flow, becomes the setpoint to the gas flow P.I.D. controller. In a similar manner, a high signal selector determines the setpoint for the air flow controller. This is a standard Steam Generator control strategy, implemented to prevent an excess of combustibles in the firing section. It is completely fail-safe, ensuring that air leads on firing increase, and vice-versa, also ensuring that if the gas flow valve or air flow damper jams, the control scheme cannot drive the Steam Generator into an unsafe condition.

This proposal also includes an air-gas ratio controller to give coarse control for the oxygen level in the flue gas. In addition, this allows the cross limiting to be based upon the real air flow, which can be displayed to the operator. The air-gas ratio controller allows easy enhancement of the firing controls with excess oxygen control.