Boiler water liquid level control

Claim 1

In a boiling water boiler having a boiler tank which is externally fired by a burner under control of a fuel supply valve, and is supplied with make-up water through a make-up water valve, both of said valves being under the control of a control system, said control system including an upper control level sensor disposed at an elevation in the tank at the lowermost water level at which firing is to be enabled, and below which firing is not to be enabled, and a lower control level sensor disposed at an elevation below the upper sensor, the improvement comprising: said circuitry including said sensors and means for applying electrical current to them which will flow through the respective sensor when said sensor is submerged, but not when it is not submerged; level electronics means responsive to flow or absence of flow through said sensors; switching means connected to said fuel valve and to said make-up water valve, enabling flow to fuel through said fuel valve when both sensors are submerged, and preventing it when either sensor is not submerged, which enables flow of water through said make=up water valve when either of said sensors is not submerged, except in the anomalous condition when said upper sensor indicated it is submerged and said lower sensor indicates it is not submerged, in which event flow of both fuel and water through their respective valves are prevented; said circuitry further including lock-out means which prevent automatic resumption of fuel flow and of make-up water flow by said switching means in the event that said anomalous condition has existed, resumption of either flow being enabled only after the switching means respective to said upper control level sensor has been initiated to enable make-up water to be provided sufficient to submerge said lower control level sensor and provided that said anomalous condition is not re-asserted.