Flammable vapor control system for devices with standing pilot flame

Abstract

A safety device for shutting off the flow of gas to a gas-fired appliance in the presence of flammable vapors. A flammable vapor sensor is inserted in the circuit supplying a solenoid actuated valve with power from a thermocouple. The sensor responds to the presence of a flammable vapor with an increase in resistance. When a critical concentration of flammable vapors is achieved, the voltage drop across the sensor causes the solenoid actuated valve to close.

What is claimed is:  
     
         1 . A fuel control system for controlling a burner of a gas-fired appliance, employing a standing pilot flame, comprising: 
 a gas valve for controlling the flow of gas to said appliance;    a sensor operative to detect a flammable vapor;    an electrical circuit operative to close the gas valve in the event a preselected concentration of said flammable vapor is sensed by said sensor.    
     
     
         2 . The fuel control system of    claim 1   , wherein said gas control valve is held open by an electromagnet powered by a thermocouple that is exposed to said standing pilot flame, wherein said sensor serves to variably reduce output voltage of said thermocouple.  
     
     
         3 . The fuel control system of    claim 2   , wherein said sensor's resistance is a function of the concentration of said flammable vapors.  
     
     
         4 . The fuel control system of    claim 3   , wherein said sensor's resistance increases as a function of the concentration of said flammable vapors.  
     
     
         5 . The fuel control system of    claim 1   , wherein said appliance comprises a water heater and said sensor is positioned near the base of said water heater.  
     
     
         6 . The fuel control system of    claim 1   , wherein said appliance comprises a water heater having a combustion air intake duct for routing combustion air to a burner used for heating water within said water heater and said sensor is positioned in said duct.  
     
     
         7 . The fuel control system of    claim 1   , wherein said appliance has an air intake for supplying combustion air to its burner and wherein said sensor is positioned in said air intake.  
     
     
         8 . The fuel control system of    claim 1   , wherein said preselected concentration comprises about half of the concentration necessary for combustion.  
     
     
         9 . The fuel control system of    claim 1   , wherein said sensor is capable of detecting hydrocarbons.  
     
     
         10 . The fuel control system of    claim 4   , wherein said electromagnet has a resistance selected such that its resistance in addition to said resistance of said sensor in the absence of flammable vapors allows for a flow of a sufficient current through said elecromagnet so as to hold said valve in its open position.  
     
     
         11 . The fuel control system of    claim 10   , wherein said sensor has a resistance of about 0.2 ohms in the absence of flammable vapors and about 2 ohms in the presence of 50% LEL of gasoline.  
     
     
         12 . The fuel control system of    claim 11   , wherein said resistance of said sensor increases from about 0.2 ohms to about 2 ohms within about 3 seconds after being exposed to 50% LEL of gasoline.  
     
     
         13 . A method of adapting a flammable vapor sensor to a valve control circuit employing a solenoid mechanism including a coil to hold a valve in an open position, and a thermocouple to power said solenoid mechanism when said thermocouple is exposed to a standing pilot flame, comprising the steps of: 
 inserting a variable resistance flammable vapor in series between said thermocouple and said solenoid mechanism;    adjusting the resistance of said coil to provide for a sufficient current flow through the sensor and coil to enable said solenoid mechanism to hold said valve in its open position when flammable vapors below a preselected level are sensed.    
     
     
         14 . The method of    claim 13   , further comprising the step of positioning said sensor so as to detect the presence of flammable vapors in combustion air being drawn into a combustion chamber of a gas-fired appliance controlled by said valve control circuit.  
     
     
         15 . The method of    claim 14   , wherein said preselected level comprises about 50% LFL.  
     
     
         16 . The method of    claim 14   , wherein said gas-fired appliance comprises a water heater.