US3558249A - Method and application for preventing flashback in premix gas burner systems - Google Patents

Abstract

A GAS BURNER SYSTEM INCLUDES PREMIXING MEANS FOR MIXING AIR AND FUEL TO A COMBUSTIBLE MIXTURE BEFORE DELIVERY THROUGH A CONDUIT TO A GAS BURNER PORT. THE SYSTEM INCLUDES A MEASURING MEANS FOR MEASURING THE FLOW RATE OF THE COMBUSTIBLE MIXTURE THROUGH THE BURNER PORT AND FURTHER INCLUDES DILUTING MEANS WHICH INJECTS INERT GAS INTO THE FLOW LINE TO THE BURNER AND MAINTAINS

FLOW RATE OF THE MIXTURE THROUGH THE BURNER PORT ABOVE ITS FLAME PROPAGTION VELOCITY. THE MEASURING MEANS AND DILUTING MEANS COOPERATE WITH CONTROL MEANS WHICH ACTUATES THE DILUTING MEANS WHEN THE FLOW RATE OF THE MIXTURE THROUGH THE BURNER PORTS IS NOT SUBSTANTIALLY GREATER THAN ITS FLAME PROPAGATION VELOCITY.

Description

* J in. 25', 1911 0. COPE I 3,558,249 METHOD AND APPLICATION FOR PREVENTING FLASHBACK IN PREMIX GAS BURNER SYSTEMS 2 Sheets-Sheet 1 Filed Maylli, 1969 INVENTOR. DAVID COPE ATTORNEYS.

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United States Patent Oflfice 3,558,249 METHOD AND APPLICATION FOR PREVENTING FLASHBACK IN PREMIX GAS BURNER SYSTEMS David Cope, Salem, Ohio, assignor to The Electric Furnace Company, Salem, Ohio, a corporation of Ohio Filed May 13, 1969, Ser. No. 824,214 Int. Cl. F23n 5/24 US. Cl. 431-12 Claims ABSTRACT OF THE DISCLOSURE A gas burner system includes premixing means for mixing air and fuel to a combustible mixture before delivery through a conduit to a gas burner port. The system includes a measuring means for measuring the flow rate of the combustible mixture through the burner port and further includes diluting means which injects inert gas into the flow line to the burner and maintains flow rate of the mixture through the burner port above its flame propagation velocity. The measuring means and diluting means cooperate with control means which actuates the diluting means when the flow rate of the mixture through the burner ports is not substantially greater than its flame propagation velocity.

BACKGROUND OF THE INVENTION This pertains to the art of gas burner control and more particularly to prevention of flashback in premix gas burner systems.

In premix gas burner systems, air and fuel are mixed to form a combustible mixture before delivery through a conduit to a gas burner port. In such systems, the combustible mixture flows through the burner port at a very low velocity when firing of the burner is cutting off or starting, During such periods, the flow rate of the combustible mixture becomes less than the flame propagation velocity thereof. When this occurs, the flame at the burner propagates itself back through the burner port and may even enter the conduit through which the combustible mixture is flowing. This usually produces an explosion upstream of the burner port. Such explosions are dangerous and annoying, and may cause damage to delicate control elements connected with the piping through which air and fuel are supplied. It would be desirable to eliminate flashback in premix burner systems and prevent explosions in system piping.

' SUMMARY OF THE INVENTION In accordance with the present invention, a premix gas burner system includes premixing means for mixing air and fuel to a combustible mixture before delivery through a gas burner port. In a preferred arrangement, measuring means are provided for measuring the flow rate of the combustible mixture through the burner port. A control apparatus is provided for preventing flashback which would usually occur when the combustible mixture is flowing at a very low velocity through the burner port. More specifically, a diluting means is provided in the form of an inert gas which is injected into the flow stream upstream of the burner port to maintain gas flow at a rate greater than the flame propagation velocity of the combustible mixture.

It is a principle object of the present invention to provide a device for preventing flashback in premix type gas burner systems.

It is an additional object of the present invention to provide an improved method for preventing flashback in premix type gas burner systems.

3,558,249 Patented Jan. 26, 1971 BRIEF DESCRIPTION OF THE DRAWING The invention may take physical form in certain parts and arrangements of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof.

FIG. 1 is a schematic diagram illustrating the improved control apparatus of the present invention installed in a premix gas burner system; and

FIG. 2 is a schematic diagram illustrating another form of the improved control apparatus of the present invention installed in a premix gas burner system.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, wherein the showings are for purposes of illustrating a preferred embodiment of the invention only and not for purposes of limiting same, FIG. 1 shows a bank of gas burners A which are supplied with combustible fuel through burner feed pipes or feedlines 12 connected with supply conduit 14 through manifold 15. Each burner A includes a burner port 17 through which fuel flows from feedlines 12 for combustion in burners A.

In one arrangement, the apparatus of FIG. 1 may be used with a furnace of the type disclosed in US. Pat. 3,396,951. This patent discloses a high temperature direct fired furnace for rapidly preheating continuous metal strip prior to a final heating step. Such furnaces may be used for various purposes such as heating ferrous metal strip prior to galvanizing. The metal strip travels continuously through the preheating furnace and is subject to sudden stoppage due to strip breakage, or mechanical or electrical failures. When stoppage occurs, the metal strip may be destroyed due to the high temperature. Therefore, it is common to shutdown the burners when stoppage occurs. In one arrangement, the furnace chamber is flooded with an emergency atmosphere when the burners are shutdown.

In accordance with the present invention, a blower B supplies combustion air through main pipe 16 to mixing T 18 while fuel gas is supplied from a suitable source through fuel pipe 20 to mixing T 18. The air and fuel are mixed to a combustible mixture at T 18 and the mixture then flows through conduit 14 to burners A. It will be recognized that any number of burners, or banks of bumers', may be supplied with the combustible mixture through conduit 14. Commercially available devices are used for maintaining a constant air to fuel ratio for delivery as a combustible mixture through conduit 14. Metering orifices 22 and 24, are provided in air line 16 and fuel line 20 respectively. Differential converter transmitters 26 and 28 receive upstream and downstream pressures from metering orifices 22 and 24, and deliver pressure signals proportional to the pressure difference to square- root extracting relays 30 and 32. Relays 30 and 32 send out pressure signals which are proportional to the square-roots of the pressure signals received from differential converter transmitters 26 and 28. It will be recognized that this pressure signal sent by square- root extracting relays 30 and 32 is also proportional to the pressure differentials at metering orifices 22 and 24. The pressure signal sent from relays 30 and 32 are received by a ratio computer-controller 36. The incoming signals from relays 30 and 32 are balanced against one another by ratio controller 36. If the air to fuel ratio agrees with the setting of controller 36, the output signal transmitted by controller 36 through tube 38 remains unchanged. However, if the air to fuel ratio does not agree with the setting on controller 36, controller 36 sends a pressure signal through tube 38 to operator 40 of fuel valve 42.

Fuel valve 42 is thereby automatically adjusted until the pressure signals from relays 30 and 32 are once again balanced in accordance with the setting on controller 36. With this arrangement, the ratio of fuel to air flowing through ports 17 of burners A is controlled by operation of only a single valve 42.

An air control valve 44 is positioned in air pipe 16 and is controlled by pneumatic operator 46. A temperature control device C sends pneumatic signals through line 48 to controller 46 for varying air valve 44. It will be recognized that an adjustment in air valve 44 will vary the pressure differential across metering orifice 22 and controller 36 -will then operate automatically to adjust fuel valve 42 and maintain the air to fuel ratio constant at mixing T 18. Those skilled in the art will recognize that hydraulic or electrical controls, or other ratio maintaining devices, may be used in place of the pneumatic controls described.

At the maximum firing rate the pressure at which the air-fuel mixture is delivered to burners A may be of the order of 12 inches water gauge. To avoid danger of flashback at the burners the mixture pressure must be more than some minimum; 0.4 inch water gauge will be termed here a safe minimum. For turbulent flow through a restriction such as a burner port the pressure difference varies as the square of flow rate. Then the safe turn down range is 12 to 0.4 which equals a 30 to 1 pressure range, or the square-root of 30. This is approximately a 5.5 to 1 flow range. When a pressure approaching the safe minimum is reached, it is desirable to cut off the flow completely and quickly. If air valve 44 and fuel gas valve 42 are simply closed, even almost instantaneously, from nearest-to-closed flashback-safe position, a flashback is almost certain to occur. A principle purpose of the invention is to prevent this by delivering a sufiicient flow of inert or incombustible gas into the air-fuel mixture, before cutting it off, to maintain the flow rate of the mixture above its flame propagation velocity until it becomes incombustible.

In operation, temperature controller C sends pressure signals through line 48 to operator 46 of air valve 44 to vary the pressure differential across metering orifice 22. Controller 36 then operates fuel valve 42 to mainatin a constant mixture ratio at T 18 and yields a burning rate at burners A in accordance with the temperature setting of control C. Burners A may be shutdown by an auto matic device when a mechanical failure occurs in feeding of metal strip through a furnace, or they may be shutdown by adjusting temperature control C downward. In this regard, it will be recognized that temperature control C, in effect, is monitoring the flow rate of the air and fuel mixture through conduit 14 to burners A. Obviously, the flow rate of the combustible mixture through conduit 14 could be measured in other ways such as a direct measurement of flow rate or pressure in conduit 14. As the pressure signal sent through line 48 by temperature controller C is reduced, this also reduces the firing rate of burners A. When the firing rate of burners A falls to a level corresponding to around 0.75 inch water gauge pressure of the air and fuel mixture flowing through burner feed pipe 12, pressure switch 50 breaks contact or opens. A pressure of around 0.75 inch water gauge is conservatively above the assumed safe minimum of around 0.40 inch water gauge to prevent flashback upstream of burner ports 17. Pressure switch 50 is connected to a source D of electrical energy through line 54, and is electrically connected with an electrical relay 52 through line 56. Opening of pressure switch 50 actuates electrical relay 52 which is connected to the other side of electrical source D through line 58. Actuation of electrical relay 52 energizes a three-way solenoid operated air valve 60 through line 62. Solenoid operated air valve 60 is connected with a source of pressure E through line 64 and sends a pressure signal through line 66 to valve 68 upon energization by relay 52, The pressure signal received by valve 68 through line 66 opens valve 68 and delivers an inert gas from source H into the flow system as by connection through line 70 to conduit 14 with a suitable T 72. The gas from source H may be nitrogen or any other suitable inert gas. For example, a suitable inert mixture of carbon dioxide and nitrogen may be produced for use from source H by removing condensed water and cooling the products formed by complete combustion of fuel gas with air. A throttling valve 73 or a fixed orifice may be positioned in line 70 to regulate the flow of nitrogen to conduit from source H. Main shut-off valve 75 may be provided in line 70. With the pressure of the combustible air and fuel mixture in conduit 14 flowing at a rate corresponding to a pressure of around 0.75 inch water gauge, the flow of nitrogen from source H through line 70 into conduit 14 is sufiicient to maintain a flow rate greater than the flame propagation velocity of the mixture flowing through conduit 14. In one example, the fuel delivered through fuel line 20 may be natural gas which usually contains 90% or more of methane, A fuel to air ratio of 9.052 to 1 represents 95% perfect combustion. Dilution of the fuel with about 5.5 parts nitrogen, while the air/fuel ratio is maintained constant, will result in a noninflammable mixture. Then by injecting 5.5 parts, or about 6 parts for any air to fuel ratio, the flame will be snuffed out. This will determine the amount of nitrogen needed in one form of the invention where it is desired to completely snuff out the flame at burners A. A typical 4 zone preheating furnace may have a design capacity for heating 25,000 pounds per hour of steel strip with a 2,200" F. chamber temperature. The total fuel burning capacity will be about 10,000 cubic feet per hour of natural gas. This is 2,500 cubic feet per hour for each zone if the zones are of identical capacity. If the maximum mixture pressure at the burners is 12 inches water gauge, and a zone is to be cut off at a conservatively safe mixture pressure of 0.75 inch water gauge, the maximum to minimum pressure ratio is 12/ 0.75 which equals 16.0 and the flow ratio will be the square-root of 16.0 which is 4.0. This indicates that the zone should be cut off at a fuel flow rate of 2,500/4 or 625 cubic feet per hour of gas. For 6 to 1 dilution with nitrogen, the necessary volume of nitrogen is 625 times 6 or 3,750 cubic feet per hour. This is the amount to be delivered at point 72 at the time of cutoff in order to insure against flashback. In addition, some safety factor obviously should be used.

Another pressure switch 74 is set to open at a signal in line 48 corresponding to a fuel and air mixture pressure in conduit 14 of about 0.50 inch water gauge. Opening of pressure switch 74 actuates electric relay 76. Pressure switch 74 is connected with source D of electrical energy through line 54 and to electrical relay 76 through line 78. Electrical relay 76 is connected to the other side of source D of electrical energy through line 58. Actuation of electrical relay 76 energizes solenoid valves 80 and 82 which are connected with relay 76 by electrical line 84. Solenoid valve 80 delivers a pressure signal through tube 86 to tight closing fuel valve 88 in fuel line 20. This causes tight closing fuel valve 88 to close. Solenoid valve 82 delivers a pressure signal through tube 90 to tight closing air valve 92 in air line 16 and causes valve 92 to close. In the arrangement described, it will be recognized that nitrogen valve 68 requires a positive pressure signal to open, while fuel valve 88 and air valve 92 require positive pressure signals for closing. In designing for fail-safe operation, it will be recognized that either positive or zero pressure signals may be used to cause either opening or closing operations of the valves.

When signal pressure in line 48 corresponds to a fuel and air mixture pressure in conduit 14 of around 0.50 inch water gauge or less, nitrogen valve 68 is open while fuel and air valves 88 and 92 are closed. The nitrogen delivered at point 72 continues to flow through that part of mixture conduit 14 which is downstream from T 72, then through lines 12 to burners A.

In one arrangement of the present invention, valves I may be provided in pressure signal line 86 from solenoid valve 80 to fuel valve 88, and in pressure signal line 90 from solenoid valve 82 to air valve 92. Each valve J includes a check valve 96 and a throttling valve 98. Valves J are adjusted to cause quick opening and relatively slow closing of gas valve 88, and to cause quick closing and relatively slow opening of air valve 92. In this manner, the air-gas ratio is never in the excess air range during rapid valve movement, for example, during times of startup and shutdown.

In accordance with another aspect of the present invention, a controller K is provided for automatically detecting stoppage of movement of a metal strip traveling through a heating furnace. Stoppage of strip movement causes control K to send an electrical signal through line 102 to electrical relay 104. Actuation of relay 104 energizes solenoid operated three-way valve 106 through electrical line 108. Energization of solenoid 106 closes oif pressure signal line 48 at its point of connection to solenoid valve .106 and vents that portion of line 48 which communicates with air valve -44, and pressure switches 50 and 74. With this venting of that portion of line 48 described, the apparatus functions as previously described to shutdown burners A. The only difference in operation, is that the time interval between opening of pressure switches 50 and 74 will be very short. Therefore, fuel valve 88 and air valve 92 will be closed rather rapidly. However, proper adjustment of valves J to provide relatively slow closing of fuel valve 88 will still provide suflicient pressure in conduit 14 to avoid flashback before valve 68 opens to admit nitrogen from line 70. In the arrangement described, it will be recognized that the mixture flowing through burner valve port 17 is maintained at a rate above its flame propagation velocity by injection of inert gas into the flow stream before the flow rate of the combustible mixture is reduced to a point not substantially above its flame propagation velocity.

Those skilled in the art will recognize that safety devices may be provided such as a safety shutoff valve 112 and manual shutoff valve 114 in fuel line 20, as well as pilot and flame safety devices for burners A.

In accordance with another arrangement, as shown in FIG. 2, a modulating control valve 103 is provided in line 70 for modulating control of inert gas flow from source H. Modulating control valve 103 includes a controller 105 connected with pressure signal line 107. Pressure signal line .107 communicates with pressure signal line 48 from temperature controller C through line 109 and inverter or signal reversing device 110. With this arrangement, modulating fuel and airfvalves 42 and 44 are modulated by pressure signal sent through line 48 from temperature controller C through operators 40 and 45. The pressure signal in line 48 which is sent to controllers 40 and 46 is also sent through line 109 and is inverted in reversing device 110 for transmission through line 107 to controller 105 of modulating valve. Therefore, a pressure signal sent through line 48 which causes operators 40 and 46 to close valves 42 and 44 will cause controller 105 to proportionally open valve 103 and cause inert gas to be injected through line 70 into conduit 14. In the preferred arrangement, controller 105 of valve 103 is adjusted so that it does not open valve 103 and begin modulation thereof until valves 42 and 44 have been closed to such an extent so that the flow rate of the combustible mixture through burner port 17 approaches the flame propagation velocity of the combustible mixture. Once valves 42 and 44 have been closed to such an extent that the flow rate of the combustible mixture through burner port 17 approaches the flame propagation velocity thereof, controller 105 opens valve 103 and injects inert gas into the flow system at a modulated rate which simply maintains a flow rate of inert gas, fuel gas and air through burner port 17 at a rate which exceeds the flame propagation velocity of the combustible mixture of fuel gas and air. Valves 42 and 44 may be subsequently closed further so that the flow rate increment contributed by the fuel gas and air through burner port 17 is substantially below the flame propagation velocity thereof and controller will operate valve 103 to inject suflicient inert gas to maintain a flow rate through burner port 17 which is greater than the flame propagation velocity of the combustible mixture. In some situations, a furnace is operated at an extremely low temperature for a long period of time so that fuel gas and air are flowing through burner port 17 at a very low rate which is still suflicient to support combustion and is below the flame propagation velocity of the combustible mixture. With the arrangement of FIG. 2, burners A may be operated at such a very reduced temperature without danger of flashback because modulating valve 103 maintains a flow rate through burner port 17 which is greater than the flame propagation velocity of the combustible mixture. With this arrangement, the flow of inert gas into the system is modulated to maintain a flow rate which is greater than the flame propagation velocity of the combustible mixture although combustion is still maintained. When the flow rate of fuel gas and air approaches the cutoff point, the flow rate of inert gas will be suflicient to render the mixture incombustible. With this arrangement, burners A may be operated at an extremely reduced temperature for a long period of time. Temperature controllers C will maintain a low combustion temperature in burners A by modulating the flow of fuel gas, air and inert gas. Setting of temperature controller C at a substantially zero temperature seting will still operate to completel cut off flow of fuel gas and air by operating pressure switch 74 to close valves 98. In addition, pressure loss in line 108 downstream of solenoid valve 106 causes full opening of valve 103 when the pressure in line 48 drops to zero upon receipt of a signal by solenoid 106 from controller K.

While the invention has been described with reference to a preferred embodiment, it is obvious that modification and alterations will occur to others upon the reading and understanding of this specification.

Having thus described my invention, I claim:

1. In a control device for preventing flashback in a burner system wherein air and fuel are premixed to a combustible mixture before delivery through conduct means to burner port means, the improvement comprising; measuring means for measuring the flow rate of said mixture through. said burner port means, diluting means for injecting inert gas into said mixture upstream of said burner port means, and control means cooperating with said measuring means and said diluting means for activating said diluting means when the flow rate of said mixture through said conduit means is not substantially greater than the flame propagation velocity of said combustible mixture, said diluting means operating to maintain a flow rate through said burner port means greater than the flame propagation velocity of said combustible mixture.

2. The device of claim 1 and including premixing means for premixing said air and fuel before delivery through conduit means to said burner port means, said diluting means communicating with said conduit means downstream of said premixing means.

3. The device of claim 1 and further including modulating means for modulating the flow rate of said air and fuel and blocking means for cutting off flow of said air and fuel, said control means cooperating with said modulating means to reduce the flow rate of said air and fuel, said control means cooperating with said blocking means and actuating said blocking means to cut off flow of said air and fuel subsequent to activation of said diluting means.

4. In a control device for preventing flashback in a burner system wherein air and fuel are premixed to a combustible mixture before delivery through burner port means, said mixture having predetermined flame propagation velocity flow rates the improvement comprising; measuring means for measuring the flow rate of said mixture through said burner port means, diluting means for injecting inert gas into said mixture, said diluting means being operative to inject inert gas into said mixture and maintain a flow rate through said burner port means greater than the flame propagation velocity of said mixture when the flow rate of said mixture is not substantially greater than the flame propagation velocity thereof, and control means cooperating wtih said measuring means and said diluting means for rendering said diluting means inoperative when the flow rate of said mixture is greater than its flame propagation velocity.

5. A method of preventing flashback in a burner system wherein air and fuel are premixed to a combustible mixture before delivery through burner port means, including the step of; diluting said mixture with a substantially inert gas at a suflicient flow rate to maintain said mixture flowing at a rate greater than its flame propagation velocity when the flow rate of said mixture is less or not substantilly greater than the flame propagation velocity thereof.

6. The method of claim 5 and including the step of maintaining the air and fuel in said mixture at a substantially constant ratio, modulating the flow rates of said air and fuel downward until the flow rate of said mixture is not substantially greater than the flame propagation velocity of said mixture, and then performing said diluting step.

7. The method of claim 6 and further including the step of completely discontinuing the flow of air and fuel subsequent to said diluting steps.

8. The method of claim 7 and further including the step of reestablishing combustibility of said mixture by the steps of reestablishing flow of air and fuel while continuing flow of said inert gas, and discontinuing flow of said inert gas when the flow rate of said mixture is above the flame propagation velocity thereof.

9. The method of claim 5 and further including the step of modulating the flow of air and fuel downward until the diluted mixture delivered through said burner port means is incombustible.

10. The method of claim 6 and further including the step of reestablishing combustibility of said mixture by modulating the flow of air and fuel upward, modulating the flow rate of said inert gas downward to maintain a flow rate through said burner port means greater than the flame propagation velocity of said mixture and discontinuing flow of said inert gas when the flow rate of said combustible mixture is above the flame propagation velocity thereof.

References Cited UNITED STATES PATENTS 2,785,960 3/1957 Ribble et al. 43129X 3,183,864 5/1965 Stengel 431-3 3,202,139 8/1965 Livingston et al 431-3X 3,335,782 8/1967 De Livois 431-3 FREDERICK L. MATTESON, Primary Examiner R. A. DUA, Assistant Examiner

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