US2938577A

US2938577A - Method and apparatus for preventing backfire in gas burner systems

Info

Publication number: US2938577A
Application number: US613934A
Authority: US
Inventors: Howard G Hughey
Original assignee: Air Reduction Co Inc
Current assignee: Airco Inc
Priority date: 1956-10-04
Filing date: 1956-10-04
Publication date: 1960-05-31
Anticipated expiration: 1977-05-31

Description

May 31,

1960 H. e. HUGHEY METHOD AND APPARATUS FOR PREVENTING BACKFIRE IN GAS BURNER SYSTEMS Filed Oct. 4, 1956 COM BUST ION SUPPORTING GAS FUEL GAS NON COMBUSTIBLE PURGE GAS.

F I G.

POWER SUPPLY START 25 2O $TOP E 2611 I J22 20b FIG 2 l .L\ ZOc- TIME 24 INVENTOR. DELAY HOWARD G.HUGHEY RELAY ATTORNEY 8. AG ENT METHOD AND APPARATUS FOR PREVENTING BACKFIRE IN GAS BURNER SYSTEMS Filed Oct. 4, 1956, Ser. No. 613,934

2 Claims. (Cl. 158-123) This invention relates to gas burner systems, especially high capacity systems, and particularly to a method and apparatus for preventing the occurrence of backfires, flashbacks, or undesired flame retrogression in such systems.

A problem which continually confronts the gas burner are generally, and the oxyacetylene cutting, welding, and flame treating art particularly, is the prevention of backflres and flashbacks. The terms backfire and flashback are frequently used synonymously. However, in the instant application the term backfire is defined as being the rapid recession of the flame from the discharge face of a burner upstream into the burner, usually back to the mixer. The term flashback as used in the instant application is intended to mean the sustained burning of the gases within the system after retrogression. Backfire and the resultant flashback are generally believed to be caused by a reduced rate of gas flow which may be the result of the burner tip being brought into close proximity with the work surface or by flying particles of molten metal which are ejected from the work area during operation. However, backfiring and flashback may also be the product of improper handling of the burner, overheating the burner, leaks within the system, etc. In general backfire and the resultant flashback can occur Whenever the conditions are such that the velocity of the combustible mixture immediately upstream of an ignition source falls below some maximum rate at which flame retrogression can occur.

'While backfires are normally of only momentary duration, they cause a back pressure to be set up within the system which blocks the flow of both the fuel gas and the combustion supporting gases and extinguishes the flame. Although the burner may reignite spontaneously, such backfiring is nevertheless a nuisance and is undesirable.

In addition to the several causes of backfire and flashback during the operation discussed above, it has now been determined that backflres and flashbacks occur far more frequently on ignition and shut down of the burner than at any other time. This can be attributed to the fact that at these times the velocity of the gases necessarily falls below the rate at which flame retrogression can not occur, thereby establishing a system which is extremely susceptible to flashback and backfire.

.This problem is especially serious for installations having large mixed gas reservoir capacities and/or large gas consumption rates; as, for example, flame hardening machines or billet scarfing machines which employ large quantities of mixed oxygen and acetylene. A flashback in this type apparatus, if not extinguished immediately, generally results in a burned out mixer, burned out gas lines, or in tips obstructed with carbon. In any case, correction necessitates removal of the machine from production while repairs are made. Such repairs are frequently expensive and are always time consuming.

In some instances, where oxyacetylene mixtures are caused, to explode, reactions may develop resulting in local peak pressures of a high order (1800-2000) p.s.i.

nited States Patent "ice (pounds per square inch) which persist for very short periods (of the order 50-100 micro-seconds). This condition constitutes a shock of such character that the impulse may induce activity in acetylene which is classed as a shock sensitive gas. It is therefore obvious that localized pressures of this order are extremely undesirable and should be eliminated wherever possible.

It is a principal object of this invention to provide a method and apparatus for preventing or minimizing the occurrence of flashbacks and backfires in oxyacetylene and other gas burner systems.

It is a further object of this invention to provide a method and apparatus for operating oxyacetylene and other burners in which the gas velocity on shutdown is maintained above the rate at which flame retrogression may occur until substantially all the combustible gas mixture has been eliminated from the system, and in which an ignition the gas attains a velocity above the rate at which retrogression can occur before the combustible mixture issues from the burner.

In general the present invention contemplates a novel mode of operation and apparatus therefore which avoids backfire and flashback on ignition and shutdown, especially where large volumes of premixed gases are employed, and where self-ignition occurs from the workpiece such as in hot scarfing and flame hardening machines. According to the present invention before-the oxygen is turned OK, the acetylene system is purged with nitrogen or other non-combustible gas to fill the system with a noncombustible mixture. Then, when the gases are shut down and the velocity falls to a rate at which flame retrogression might otherwise occur there is no combustible mixture present and no opportunity for the apparatus to backfire. Similarly, during the course of ignition, when the system is filled with a non-combustible mixture, there Figure l is a diagrammatic representation of an oxyacetylene gas burner system embodying the present invention.

Figure 2 is a schematic diagram of an electrical control system for the proper timed sequential operation of the valves of Figure 1, in accordance with the present invention.

Now referring to Figure 1, a gas burner 10 is supplied with a combustible gas mixture from a mixer 11, of conventional design. A fuel gas, such as acetylene, is supplied to the mixer 11 from a source (not's hown) through a pressure regulator 5, a control valve 15, and

conduit 13. A combustion supporting gas, such as oxygen, is supplied to the mixer 11, from a source (not shown) through a pressure regulator 4, a control valve 14, and conduit 12. A non-combustible purge gas, such as nitrogen, is supplied to the system from a source (not shown) through a pressure regulator 7, a control valve 17, and conduit 16 which communicates with conduit 13 at a point 19 immediately downstream from the fuel gas control valve 15. The nitrogen regulator 7 is preferably set to deliver nitrogen at a somewhat lower pressure than the acetylene. while the purge gas referred to. in the description of this specific embodiment of the invention is nitrogen, any non- It should be understood, at this time, that burner through conduit 18 where it issues from the burner orifices and is burned. During. operation the velocity of the gas issuing from the burner orifice or orifices may, for example be of the order of 300-400 feet per second. The term velocity as. used herein means the average velocity computed by dividing the volumetric flowrate by the total orifice area. In this specification no effort will be made to explain factors such as velocity gradient within the stream and the like which contribute to flame stability, but rather the invention will be explained'in terms of the average velocity as above defined;

At velocities up to about 50 feet per second retrogression can occur in ordinarily used oxyacetylene mixtures burned in apparatus having flame orifices of the order of 0.030 to 0.050 inch diameter. In general the larger the orifice the higher the velocity atwhich retrogression can occur. This inventionprovides for the issuance of a non-combustible mixture of gases-from the system both when starting up and shutting down, during the periods when the velocity is within the range in which retrogression can occur.

This. is accomplished by the sequential manipulation of the oxygen and acetylene valves and the introduction of a purge gas downstream of the acetylene inlet valve on shutdown of the burners as follows.

To extinguish the flame and shut down the apparatus according to the present invention, acetylene valve 15, is. closed, while oxygen valve 14 and nitrogen valve 17 are open. As long as the acetylene valve is open no nitrogen flowsinasmuch as the nitrogenregulator is set to deliver nitrogen at a pressure lower than the regu-, lated acetylene pressure. However as soon as the acetylenecontrol valve 15 is closed the pressure in conduit 13 begins to fall and regulator 7 admits nitrogen to the system. Nitrogen is thus admitted tofthe system just downstream of the acetylene valve. 15 and displaces the acetylene in the lines,,the mixer and the burner. After valve 15 is closed, theoxygen-a'cetylene mixtureand the following, oxygen-nitrogen mixture continueto be dis charged from the system at a fiowrate-wellv above the critical velocity of about 50 feet per second. After the oxygen acetylene mixture has all passed from the system, the flame goes out, and the nitrogen valve 17'and oxygen. valve 14 may be closed without any possibility ofbackfire. This leaves the system filled with the-noncombustible oxygen=nitrogen mixture. By introducing niin the system is the small volume above the nitrogen inlet connection 19 and below the'C H valve 15. This volume must be kept small enough thatupon cessationof nitrogen flow, the expanded volume-of this fuel gas will not re-charge the system with a combustible mix tut-e.

tinues to flow and this pressure is imposedrearwardly upon the supply lines; Upon cessation of gas. flow in both lines, the systems act 'as reservoirs, each at the same pressure but each containing its individual gas. Thus, the last increment of gas to drain out is a mixture determined by the volumerelation of the two. reservoirs andis'of potentially explosive character. Purging, as" con,

not ignite at the time the system is shut down this residual explosive atmosphere remains. If exploded, the contents might include CO, H CO H O, C air, C H or 0 Purging as conventionally understood, removes these products but re-establishes the original explosive mixtures of C H and O a To reignite the system after a shut down according to the practice prior t'o'ithe present iflventidn, .it wasneces sary to accelerate the combustible mixture at'rest" in the system through the range of velocities in which flame retrogre'ssi'oni can'occur and up! tothe: normal operating velocity. If there was an ignition source suchas a hot workpiece in front of the burner orifices a backfire could and frequently did occur. According to the present invention, nothing; but. a non-combustible mixture issued from the burner orifices until the system has had time to accelerate through the backfire range. If the volume a of gasin the system downstream of the shut ofi valves is suflicient, as it normally is, it is not necessary to introduce. any nitrogen from the supply upon starting. The

single admission at shutdown serves'both to prevent.

backfire atthat time and to prevent backfire. and flashback on subsequent starting up.

The novel mode of operation and suitable apparatus therefor have been fully described abovefor manual operation. It obviously wouldbe advantageousin machinework such as machinei scarfing or flame hardening to havean automatic system for operating the valves in their proper timed seqnence.- To accomplish this it. is-

obvious that each. of valvesl14,'- 15 and 17 may be remotely controlled valves suchas hydraulically operated valves or electrically operated solenoid valves. In a simple control of conventional design these valves may be made to operate in accordance with the following schedule.

A suitable electrically operated circuit -to producethe desired result is schematically illustrated in Fig. 2; The

circuit comprises essentially a line switch S; a control relay 20; -a time delay relay 21;

solenoids

22, 23 and I 24-which act to open and

close valves

14, 17, and" 15 .55 trogen at or below the fuel gas entry, the only fuel gasventionally employed, doesnot alter' this condition; If

respectively; and the start and stop buttons 25 an'cl 26. The winding of control relay 20 is arranged in series with the normally closed contacts of thestop'button 26 and the normally open contacts of the start button 25 across the power supply line. Control relay 20 has three sets of contacts, 20a normally open (contacts open when coil 20 deenergized) and in parallel with the

start button

25, 20b normally closed (contacts closed when coil 20 deenergized) and in serieswith the time delay relay 21 across the power supply line, and contacts 200 normally open (contacts open when coil 20 deenergized) and in series with the operating coil 24 of valve 15. Time delay relay 21 is of the on delay type which automatically resets andstarts timing each time voltage is applied. Time delay relay 21 has one set of contacts 21a which are normally closed i.e. are closedwhen no power is applied to the'relay, remain closed when power is applied to the relay until'the expiration of the time delay, after which the contacts open and remain open until the power is again interrupted. These contacts 21a- All of the solenoid valves are the" switch S,

valves

14 and 17 will open as the result of operating voltage being applied to the respective operating coils 22, 23. This adnr'ts a noncombustible mixture of combustion-supporting gas and purge gas to the system. Also when the power is first applied to the circuit the timer 21 starts to run. As soon as it times out its contacts 21a open interrupting the circuit to the operating coils of

valves

14 and 17 causing these valves to close, thereby shutting down the system with the noncombustible gas mixture in it. The system will remain in this stand-by condition until such time as the start button 25 is depressed.

When the start button 25 is depressed, control relay 20 is energized simultaneously closing its contacts 20a and 200 and opening its contacts 20b. 20a locks the relay into the circuit so that it remains energized even when the start button is released. The closing of contacts 20c energizes coil 24 of valve 15 initiating the flow of fuel gas. The opening of contacts b deenergizes time delay relay 21 closing its contacts 21:: which energize coils 22 and 23 of

valves

14 and 17 respectively, opening the valves. With all three valves open, fuel gas and combustion supporting gas are admitted to the mixer and the mixture in turn admitted to the burner for normal operation. Even though the purge gas valve 17 is open there is no flow of gas therethrough because back pressure in the line as the result of a higher fuel gas pressure prevents regulator 7 in the purge gas line from delivering any gas.

At the end of the combustion cycle the stop button is depressed breaking the circuit of control relay 20. This, of course, causes the contacts 20a to open so that the circuit will remain interrupted even though the stop button is released. Simultaneously, the contacts 20b close starting the time delay relay to time out, and contacts 200 open causing valve 15 to close. As soon as the fuel gas is interrupted by the closing of valve 15 the back pressure in the fuel gas system downstream of the valve begins to fall and regulator 7 begins to deliver purge gas into the system as described in detail heretofore. At the expiration of the time cycle of time delay relay -21 its contacts 21a open deenergizing the operating coils 22 and 23 of

valves

14 and 17 shutting off the combustion supporting gas and the purge gas. This leaves the system filled with a non-combustible gas mixture. At the time of the next start only this non-combustible mixture issues from the burner for a period long enough to enable the gas flow to reach the necessary velocity.

It will be obvious upon examination that the sequence of operation of the valves corresponds exactly to that described heretofore for the manual operation of the system.

It will be evident that as a result of this invention it is possible to operate a gas burner system, particularly a large capacity system, without the occurrence of backfire and flashback, either at the time of shutting the burner down or starting it up. While essentially only one embodiment of the invention has been shown and described it is to be understood that the invention is not limited to the particular form shown but may be used in other ways without departure from its spirit as defined by the following claims.

I claim:

1. A method of preventing backfire during flame extinction and reignition in a gas burner system of the type in which a fuel gas and a combustion supporting gas are separately supplied to a mixing chamber at a combined flow rate such that the mixed gases are discharged from the burner at a velocity greater than the velocity at which flame retrogression into the burner can occur, and the supply rate of said combustion supporting gas alone is insuflicient to maintain the said discharge velocity above the said velocity at which flame retrogression into the burner can occur, which method comprises extinguish ing the flame by interrupting the supply of fuel gas while maintaining the supply of combustion supporting gas and substituting for the supply of fuel gas a non-combustible purge gas to maintain the burner discharge velocity above the value at which flame retrogression in the burner can occur until the combustible mixture is completely purged from the system, whereby the flame is automatically extinguished and the system is filled with non-combustible gases, said non-combustible gases remaining in the system in sufficient volume to discharge through the burner ahead of the combustible gas mixture upon resumption of the flow of fuel gas and combustion supporting gas to cause only non-combustible gases to issue from the burner until the issuing gas attains a velocity above that at which flame retrogression can occur.

2. In a gas burner system in which a fuel gas and a combustion supporting gas are supplied to a mixer, and the mixed gases are supplied to a burner where they are ignited and burned, the improvement which comprises means for interrupting the supply of fuel gas to said mixer, a source of non-combustible purge gas, means for automatically initiating a flow of purge gas upon the interruption of the supply of fuel gas, means for introducing the said purge gas into the fuel gas system at a point downstream of said fuel gas interrupting means to displace substantially all the fuel gas in said system downstream of said fuel gas interrupting means when the supply of fuel gas is interrupted, and means for automatically interrupting the supply of combustion supporting gas and purge gas a predetermined time after the supply of fuel gas is interrupted.

References Cited in the file of this patent UNITED STATES PATENTS 2,402,826 Lubbock June 25, 1946 2,650,478 Brown Sept. 1, 1953 2,784,959 Carson Mar. 12, 1957 FOREIGN PATENTS 382,751 Germany Oct. 5, 1923 554,664 Germany July 14, 1952 148,201 Great Britain July 9, 1920