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

Method and application for preventing flashback in premix gas burner systems Download PDF

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Publication number
US3558249A
US3558249A US824214A US3558249DA US3558249A US 3558249 A US3558249 A US 3558249A US 824214 A US824214 A US 824214A US 3558249D A US3558249D A US 3558249DA US 3558249 A US3558249 A US 3558249A
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Prior art keywords
fuel
mixture
air
valve
burner
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US824214A
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English (en)
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David Cope
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Electric Furnace Co
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Electric Furnace Co
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Assigned to MELLON BANK N.A.MELLON SQIUARE, PITTSBURGH, PA. 15230, A NATIONAL BANKING ASSOCIATION reassignment MELLON BANK N.A.MELLON SQIUARE, PITTSBURGH, PA. 15230, A NATIONAL BANKING ASSOCIATION SECURITY INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ELECTRIC FURNACE COMPANY, THE
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23D—BURNERS
    • F23D14/00—Burners for combustion of a gas, e.g. of a gas stored under pressure as a liquid
    • F23D14/46—Details, e.g. noise reduction means
    • F23D14/72—Safety devices, e.g. operative in case of failure of gas supply
    • F23D14/82—Preventing flashback or blowback
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
    • F23D—BURNERS
    • F23D2209/00—Safety arrangements
    • F23D2209/30—Purging

Definitions

  • 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.
  • premix gas burner systems air and fuel are mixed to form a combustible mixture before delivery through a conduit to a gas burner port.
  • 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.
  • a premix gas burner system includes premixing means for mixing air and fuel to a combustible mixture before delivery through a gas burner port.
  • 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.
  • 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.
  • FIG. 1 is a schematic diagram illustrating the improved control apparatus of the present invention installed in a premix gas burner system
  • 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.
  • 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.
  • 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.
  • the furnace chamber is flooded with an emergency atmosphere when the burners are shutdown.
  • 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.
  • burners A 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.
  • 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.
  • hydraulic or electrical controls, or other ratio maintaining devices may be used in place of the pneumatic controls described.
  • the pressure at which the air-fuel mixture is delivered to burners A may be of the order of 12 inches water gauge.
  • the mixture pressure must be more than some minimum; 0.4 inch water gauge will be termed here a safe minimum.
  • the pressure difference varies as the square of flow rate.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • nitrogen valve 68 requires a positive pressure signal to open, while fuel valve 88 and air valve 92 require positive pressure signals for closing.
  • either positive or zero pressure signals may be used to cause either opening or closing operations of the valves.
  • 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.
  • 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.
  • a controller K 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Feeding And Controlling Fuel (AREA)
  • Gas Burners (AREA)
  • Regulation And Control Of Combustion (AREA)
US824214A 1969-05-13 1969-05-13 Method and application for preventing flashback in premix gas burner systems Expired - Lifetime US3558249A (en)

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US82421469A 1969-05-13 1969-05-13

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US (1) US3558249A (de)
JP (1) JPS503017B1 (de)
DE (1) DE2018187B2 (de)
ES (1) ES379950A1 (de)
FR (1) FR2047659A5 (de)
GB (1) GB1294033A (de)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20070006596A1 (en) * 2005-07-08 2007-01-11 Mitsubishi Heavy Industries, Ltd. Flashback-detecting equipment, flashback-detecting method and gas turbine
US20090214991A1 (en) * 2008-02-18 2009-08-27 Applied Materials, Inc. Apparatus and methods for supplying fuel employed by abatement systems to effectively abate effluents
CN102618302A (zh) * 2012-03-18 2012-08-01 莱芜市泰山焦化有限公司 防止焦炉煤气主管形成负压的装置及其使用方法
US20150107217A1 (en) * 2011-12-21 2015-04-23 General Electric Company Vent system for use in a gas turbine and method of operating thereof

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4144313A (en) * 1976-06-04 1979-03-13 Bayer Aktiengesellschaft Method of purifying gases by combustion
EP0024444A1 (de) * 1979-08-24 1981-03-11 G. Kromschröder Aktiengesellschaft Regelvorrichtung zur Regelung des Mischungsverhältnisses von Brenngas und Luft bei Gasbrennern
DE4127883A1 (de) * 1991-08-22 1993-02-25 Abb Patent Gmbh Einrichtung zur waermeerzeugung durch katalytische verbrennung
DE19635576A1 (de) * 1996-09-02 1998-03-05 Linde Ag Verfahren und Vorrichtung zum Abschalten eines Brenners

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20070006596A1 (en) * 2005-07-08 2007-01-11 Mitsubishi Heavy Industries, Ltd. Flashback-detecting equipment, flashback-detecting method and gas turbine
US7788895B2 (en) 2005-07-08 2010-09-07 Mitsubishi Heavy Industries, Ltd. Flashback-detecting equipment, flashback-detecting method and gas turbine
US20090214991A1 (en) * 2008-02-18 2009-08-27 Applied Materials, Inc. Apparatus and methods for supplying fuel employed by abatement systems to effectively abate effluents
US20150107217A1 (en) * 2011-12-21 2015-04-23 General Electric Company Vent system for use in a gas turbine and method of operating thereof
US9822707B2 (en) * 2011-12-21 2017-11-21 General Electric Company Vent system for use in a gas turbine and method of operating thereof
CN102618302A (zh) * 2012-03-18 2012-08-01 莱芜市泰山焦化有限公司 防止焦炉煤气主管形成负压的装置及其使用方法

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DE2018187B2 (de) 1971-11-11
FR2047659A5 (de) 1971-03-12
JPS503017B1 (de) 1975-01-30
GB1294033A (en) 1972-10-25
DE2018187A1 (de) 1971-01-21
ES379950A1 (es) 1973-04-01

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AS Assignment

Owner name: MELLON BANK N.A.MELLON SQIUARE, PITTSBURGH, PA. 15

Free format text: SECURITY INTEREST;ASSIGNOR:ELECTRIC FURNACE COMPANY, THE;REEL/FRAME:004007/0517

Effective date: 19820614