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.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention generally relates to safety devices for mitigating the danger posed by the presence of flammable vapors in proximity to a gas-burning appliance and more particularly pertains to a control system that shuts down the operation of the appliance when the presence of flammable vapors is detected.  
           [0002]    A potential hazard inherent in the operation of a gas-burning appliance is that it may cause flammable vapors to ignite that happen to collect in and around the appliance. Ignition may either be caused by the burner or pilot flame. This is especially problematic in for example water heaters or furnaces that are located in garages in which cars are parked where the leakage or spillage of gasoline may occur. Once a combustible mixture reaches the appliance, a fire or explosion may result.  
           [0003]    Efforts to address this potential safety hazard have been previously directed to ensuring adequate ventilation in and around the appliance, the elevation of the appliance in an effort to distance it from flammable vapors that are denser than air, and more recently, the isolation or sealing of the burner from its surroundings. Attempts have also previously been made to adapt specialized sensors for the purpose of sounding an alarm in an event the presence of certain flammable vapors is detected.  
           [0004]    A flammable vapor control system is needed that is able to mitigate the danger of a gas-burning appliance that employs a standing pilot flame so as to automatically and positively prevent such appliance from igniting flammable vapors. Such system must be capable of reliably removing all potential ignition sources once a certain concentration of flammable vapors has been detected.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention overcomes shortcomings of previously known approaches for mitigating the dangers associated with a gas-burning appliance. By employing a sensor that is capable of detecting the presence of flammable gases and associating it directly with the function of the gas valve that controls the flow of gas to the appliance, a fully automatic and reliable safety system is provided. Once the presence of a threshold concentration of flammable gas is detected, the system causes the valve to shut off the flow of all gas to thereby immediately extinguish the burner flame, should it be lit, as well as the standing pilot flame. Removal of such ignition sources will preclude the appliance from igniting the flammable vapors. When the absence of hot water is detected, a person undertaking to check on the operation of the water heater would presumably notice the presence of flammable vapors in the vicinity of the water heater and remedy the situation.  
           [0006]    The safety system of the present invention is easily adapted to the type of gas control valves that are currently being produced with only minimal modification. Such valves employ an electromagnet to hold the gas valve in its open position by the voltage that is generated by a thermopile or thermocouple as it is subjected to the heat of the pilot flame. Such configuration is favored as it automatically causes the supply of gas to be curtailed in the event the pilot flame becomes extinguished as the removal of the heat source will cause the temperature of the thermopile and hence its output voltage to decrease to a level below that which is required to hold the valve in its open position. The present invention calls for a variable resistance flammable vapor sensor to be inserted in the circuit between the thermopile and the electromagnet. Its resistance increases as a function of the flammable vapor concentration and as a result, it controls the voltage that is supplied to the electromagnet. Once the resistance reaches a preselected level in response to a preselected concentration of flammable vapor, the voltage supplied to the electromagnet falls below the threshold level required to maintain the gas valve in its open position and the valve closes to shut off the flow of gas to the appliance. Accommodation of such device in existing valve configurations requires the modification of the resistance of electromagnet. The resistance of the coil must be modified by an amount commensurate with the resistance of the sensor at sub-critical flammable vapor concentrations such that the power supplied to the electromagnet remains sufficient to allow the electromagnet to maintain the valve in its open position.  
           [0007]    These and other features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment which, taken in conjunction with the accompanying drawings, illustrates by way of example the principles of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic illustration of a water heater employing the safety system of the present invention; and  
         [0009]    [0009]FIG. 2 is a semi-schematic circuit diagram of the safety device of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0010]    The drawings depict a preferred embodiment of the present invention wherein a standing pilot flame is employed in the gas-burning appliance. When a concentration of flammable vapors that exceeds a preselected level is detected, the gas controller valve is caused to shut off the supply of gas to the appliance. The appliance&#39;s burner and its standing pilot flame are thereby removed as potential ignition sources for the flammable vapors.  
         [0011]    [0011]FIG. 1 is an illustration of a water heater employing the safety system of the present invention. A water heater  12  employs a gas controller valve  14  which serves to control the flow of gas from an inlet conduit  16  to the feedline  18  that supplies the burner (not shown) and the feedline  20  the supplies the standing pilot flame (not shown). A flammable vapor sensor  22  is situated near the base of the water heater as many flammable vapors of concern, such as gasoline, are heavier than air. Alternatively, the sensor may be positioned directly in an intake duct through which combustion air is routed to the water heater&#39;s burner.  
         [0012]    [0012]FIG. 2 is a semi-schematic illustration of the safety system of the present invention. A valve  24 , controls the flow of gas from the inlet conduit  16  to feedlines  18  and  20 . The valve is normally held in its closed position by the action of spring  26 . Energization of solenoid  28  overcomes the force of the spring to open the valve. The subsequent flow and distribution of the gas is regulated as is well known in the art. The flow to the burner  30  is controlled so as to maintain the water temperature contained within the water heater near a preselected set temperature. The pilot  32  is constantly supplied with gas so as to maintain a standing pilot flame  34  for the purpose of igniting the burner whenever the flow of gas is directed thereto.  
         [0013]    The solenoid  28  is energized by the voltage supplied by a thermocouple  36  or thermopile that is positioned so as to be heated by the standing pilot flame  34 . In the event the pilot flame is extinguished, the output voltage of the thermocouple quickly drops below the voltage that is necessary for the solenoid  28  to overcome the bias of the spring  24  which causes the valve  24  to close. The risk of a build-up of gas that would otherwise continue to issue from pilot and possibly the burner is thereby obviated. Reignition of the pilot flame requires the valve to first be manually opened and then held open for a period of time sufficient to allow the temperature and hence the output voltage of the thermopile to rise to a level at which the solenoid is able to overcome the force of the spring.  
         [0014]    The flammable vapor sensor  22  is inserted in line with one of the thermocouple wires. The sensor introduces a variable resistance to the circuit as a function of the concentration of flammable vapors. Resistance increases as a function of vapor concentration to reduce the voltage supplied to the solenoid. Once the resistance increases to a level sufficient to reduce the voltage to below the threshold voltage necessary to maintain the valve in its open position, the valve automatically closes. The threshold voltage is preferably selected such that the valve closes well before a combustible mixture of the flammable vapor is present. It is especially preferred that the such valve closes at about 50% LFL (low flammable level) or 50% LEL (low explosive level).  
         [0015]    Various different sensor devices can be employed to provide for the desired resistance variation as a function of flammable vapor concentration. A preferred device takes the form of a solid state sensor such as an ADSISTOR Vapor Sensor supplied by ADSISTOR Technology, Inc of Seattle Wash. One of such devices that has been found to be particularly well suited for the present application has a resistance of about 0.2 ohm at 70 degrees Fahrenheit in the absence of flammable vapors. Upon exposure to 50% of the LFL concentration of gasoline, the sensor&#39;s resistance will rise to exceed 2 ohms within about 3 seconds. While the initial 0.2 ohm resistance of the device allows a sufficient voltage to be applied to the solenoid  28  in order to hold the valve  24  in its open position, the increase to 2 ohms provides for a sufficiently large voltage drop to cause the valve to close by action of spring  26 . An additional benefit of such device is that once saturated with the ADSISTOR flammable vapor sensor becomes saturated with the vapor to which it is sensitive, it takes on the order of a few hours for the device to degas sufficiently to again reduce its resistance to below the threshold level. This serves as a built-in time delay to deter a user from attempting to relight the pilot flame prematurely. The device is particularly sensitive to hydrocarbons that are in a liquid state at normal atmospheric pressures.  
         [0016]    In order to adapt such sensing device to a particular thermocouple/solenoid combination, it is necessary for the resistance of the coil or coils within the solenoid to be adjusted such that the total resistance for the circuit allows for a sufficient current flow through the solenoid so as to be energized when the thermocouple is heated by the standing pilot flame. An example of a system that has been found to provide satisfactory result requires the use of a thermocouple that has an output voltage of 28 mV, a sensor device that has a resistance of 0.2 ohm in the absence of flammable vapors and a solenoid coil that has an internal resistance of 1 ohm. Such circuit generates a current of 23 milliamps which is sufficient to hold valve  24  hold open against spring  26 . The resistance increase to 2 ohms upon being subjected to a 50% vapor concentration reduces the current to 12 milliamps which causes the valve to close.  
         [0017]    In adapting an existing gas controller configuration to accommodate the use of a variable resistance flammable vapor sensor, it is necessary to increase the internal resistance of the solenoid coil. This can be accomplished by for example the use of higher gauge wire (smaller diameter) and an increase in number of coils in the electromagnet.  
         [0018]    While a particular form of the invention has been illustrated and described, it will also be apparent to those skilled in the art that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited except by the appended claims.