GB1294033A - Method and apparatus for preventing flashback in premix gas burner systems - Google Patents

Abstract

1294033 Preventing flashback in gas burners ELECTRIC FURNACE CO 26 March 1970 [13 May 1969] 14931/70 Heading F4T A method of and a control device for preventing flashback in an atmospheric gas burner A, includes the step of diluting the air/fuel mixture with an incombustible gas at a flow rate to maintain the diluted mixture flowing at a rate greater than its flame propagation velocity, when the flow rate of the undiluted combustible mixture falls below a predetermined value which is sufficiently greater than the flame propagation velocity to prevent flashback. The control device, which may be electrically, pneumatically or hyhydraulically operated preferably comprises measuring means for measuring the flow rate of the undiluted mixture through a burner port 17, diluting means for injecting incombustible gas into the mixture upstream of the port and control means co-operating with and controlling the diluting means and the measuring means. Such a device may be used with a high temperature direct fired furnace for heating continuous metal strip, which furnace requires shutdown when stoppage of the strip occurs. Blower B supplies air through main pipe16 to mixing tee 18 and fuel gas is supplied to the tee 18 through fuel pipe 20. Metering orifices 22 and 24 ensure a constant air/fuel combustion ratio. Differential converter transmitters 26 and 28 monitor the pressures on each side of orifices 22 and 24 and pass the information via relays 30, 32 to ratio computer controller 36 which automatically adjusts fuel valve 42 to provide the required air/fuel ratio. Temperature control device C controls, through line 48, air valve 44 and thus indirectly fuel valve 42 to control the flow of combustion mixture to burners A. When the firing rate of the burners A falls, pressure switch 50 opens and actuates relay 52, powered by a source D of electrical energy, and three-way solenoid operated air valve 60. This allows pressure source E to act to open valve 66 and deliver incombustible gas from source H into the combustion mixture. The incombustible or inert gas may for example be nitrogen or a nitrogen and carbon dioxide mixture. Sufficient incombustible gas may be provided to snuff out the burner flames completely. Another pressure switch 74 opens when the combustion mixture pressure falls further and actuates electric relay 76 and solenoid valves 80 and 82 to close fuel valve 88 and air valve 92. Valves J, each including check valve 96 and throttling valve 98, ensure quick opening and slow closing of fuel valve 88 and quick closing and slow opening of air valve 92 to control the air/gas ratio at start-up and shutdown. Controller K may automatically detect stoppage of the metal strip travelling through the furnace and operate valve 106 to vent line 48 and operate the control device as disclosed. A modulating control valve (103) may be positioned in line 70, as in Fig. 2 (not shown), to permit automatic adjustment of the incombustible gas flow rate with combustion mixture flow rate.