Flashback protection apparatus and method for suppressing deflagration in combustion-susceptible gas flows

A flashback protection apparatus and method for suppressing deflagration of a deflagration-susceptible gas in a flow system in which the deflagration-susceptible gas is flowed. The method of the invention comprises monitoring the combustible gas to detect deflagration therein, and upon detection of a deflagration event producing a propagating flame front, opposedly directing at the propagating flame front a pressurized non-flammable gas in a sufficient volume and at a sufficient velocity to provide a non-flammable gas flow having a momentum at least opposedly equal to momentum of the propagating flame front and the associated accelerated combustible gas undergoing deflagration.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a flashback protection apparatus for suppressing 
deflagration in a system including a flow passage through which 
deflagration-susceptible gas is flowed, and to a method of suppressing 
deflagration in a gas stream of a combustible character. 
2. Description of the Related Art 
Combustion systems are utilized in many industrial process systems for such 
applications as generation of heat, combustion disposal of off-gas, and 
operations such as welding, brazing, volatilization of materials such as 
reactants and reagents, etc. 
In such systems, a fuel gas, which may be of a single component or 
multi-component character, is mixed with an oxidant such as high purity 
oxygen or other oxygen-containing gas, e.g., air or nitrous oxide, and the 
resulting fuel-oxidant gas mixture is ignited at a combustion locus. The 
combustion locus may for example be a jet nozzle in the case of welding 
and brazing equipment, a burner in the case of a boiler heater, an 
electrical resistance heater disposed at the effluent end of a vent gas 
stack in the case of combustion treatment of combustible effluent gases, 
etc. 
In semiconductor manufacturing operations, various processing operations 
can produce various premixed combustible gas streams. Hydrogen and a 
variety of hydride gases (e.g., silane, germane, phosphine, arsine, etc.) 
may be present and, if combined with air, oxygen, or other oxidant 
species, such as nitrous oxide, chlorine, fluorine, etc., form such 
combustible multicomponent gas mixtures. Many of such gas streams are 
subjected to scrubbing treatment in the semiconductor manufacturing 
facility, and scrubbers for soluble gas components carried in hydrogen and 
other gases are in common use. If for example an air leak in the process 
piping, joints, etc. occurs, or a cracked scrubber vessel allows air 
ingress, and air is passed to the hydrogen-filled scrubber in such a 
facility, then there is a corresponding danger of explosion of the 
scrubber, due to the accumulated volume of the hydrogen/oxygen combustible 
mixture at such location of the process system. 
Particularly when a gas of a highly volatile and flammable character such 
as hydrogen is used as a significant component of the mixture, there is 
the risk and danger of ignition and of uncontrolled combustion of the 
fuel-oxidant mixture, with resulting propagation of a "flame front" 
through the gas flow channel of the system toward the accumulated source 
of premixed fuel and oxidizer, termed "flashback." The ultimate risk of 
flashback is a catastrophic or otherwise detrimental explosion at such a 
source, which may for example comprise a tank or other vessel holding the 
fuel/oxidant mixture. 
In this respect, it is to be noted that mere burning or deflagration of a 
fuel/oxidant mixture proceeds at a much slower speed than detonation of a 
combustible fuel/oxidant mixture. Detonation proceeds along a shock wave 
having a speed in excess of Mach 1. Accordingly, if the deflagration 
phenomenon proceeds through the transition to detonation, the allowable 
response time "window" for taking remedial action becomes vanishingly 
small. 
A system having as its goal the supression of the risk of detonation 
therefore must be capable of rapid response to the deflagration event, and 
must effectuate the remedial action prior to the onset of detonation. 
Although the art has utilized thermocouples to sense the occurrence of 
deflagration in process systems, commercially available thermocouples, by 
virtue of their operation based on a sensed temperature differential, are 
inherently slow and unable to respond in a very rapid manner to the 
initiation of deflagration in such systems. Pressure sensors have also 
been employed, but pressure sensors respond only after the deflagration is 
well developed and may already be near to the transition to detonation. 
Accordingly, there exists in the art an established and continuing need for 
improved apparatus and method to suppress deflagration of 
deflagration-susceptible gas, in systems in which combustible gas mixtures 
are flowed through or accumulate in the system or a portion thereof. 
It therefore is an object of the present invention to provide a flashback 
suppression apparatus and method for suppressing deflagration in a flow 
system in which deflagration-susceptible gas is flowed, e.g., from a fuel 
source to a combustion site, or accumulated, e.g., in the hydrogen 
scrubber of a semiconductor processing facility. 
It is another object of present invention to provide such a flashback 
protection system of a relatively simple, economic, and easily operated 
character. 
Other objects and advantages of the present invention will be more fully 
apparent from the ensuing disclosure and appended claims. 
SUMMARY OF THE INVENTION 
The present invention broadly relates to flashback protection apparatus and 
a method for suppressing deflagration of a deflagration-susceptible gas in 
a flow system, e.g., a flow system in which the deflagration-susceptible 
gas is flowed from a source to a combustion site, or a flow system in 
which oxidant and fuel components are present in concentrations 
susceptible to deflagration and detonation. 
In a specific apparatus aspect, the present invention relates to a 
flashback protection apparatus for suppressing deflagration in a system 
which includes a flow passage with an inlet end and an outlet end defining 
a gas flow path therethrough, from the inlet end to the outlet end. Such 
apparatus comprises: 
a source of pressurized non-flammable gas joined in latent gas flow 
communication with the gas flow path; 
a flow controller operatively arranged for selectively establishing gas 
flow communication between the source of pressurized nonflammable gas and 
the gas flow path; 
an optical sensor assembly constructed and arranged to detect deflagration 
in the flow passage during flow of combustible gas therethrough, and to 
(i) maintain a non-flow condition of the pressurized non-flammable gas to 
the gas flow path during non-deflagrating gas flow of combustible gas 
through the gas flow path, and (ii) responsively actuate the flow 
controller to selectively establish the gas flow communication between the 
source of pressurized non-flammable gas and the gas flow path upon 
detection of deflagration in the gas flow passage during gas flow of 
combustible gas therethrough; 
wherein the apparatus is constructed and arranged so that the pressure and 
volumetric flow rate of the pressurized non-flammable gas, flowed from the 
source thereof to the gas flow path, upon detection of deflagration in the 
gas flow passage during flow of combustible gas therethrough, provides a 
pressurized non-flammable gas momentum at least opposedly equal to 
momentum of combustible gas undergoing deflagration in the gas flow 
passage. 
In specific aspects of the apparatus broadly described hereinabove, the gas 
flow communication may be effected by a conduit, flow channel, or other 
gas flow conveyance means, for passing the pressurized non-flammable gas 
from the source thereof to the gas flow passage. 
The flow controller may for example include a valve which is selectively 
openable and closable. Such valve typically is held in the closed position 
not allowing gas flow from the pressurized non-combustible gas source to 
the gas flow passage. The valve is selectively openable to initiate sudden 
and substantial bulk flow of the pressurized non-flammable gas from the 
source thereof to the gas flow passage, via valve controller means joined 
in signal communication relationship with the optical sensing assembly. 
Such communication may be established by mans of electrical signal 
transmission wires, wireless signal transmission circuitry, optical 
waveguides (fiberoptic cable), etc. 
The optical sensor assembly which is constructed and arranged to detect 
deflagration in the flow passage, during the flow of combustible gas 
therethrough, may suitably comprise a photodiode and operational amplifier 
circuitry, for detecting the light accompanying deflagration of the 
combustible gas, and generating an amplified electrical signal to the flow 
control means described above. 
Alternatively, other optical sensing means, comprising a fast light meter 
(luminometer), photon counter, or other electromagnetic detection means 
sensitive to the radiation flux, intensity or other optical parameter of 
the deflagration event, may advantageously be employed in the apparatus of 
the present invention. 
In a method aspect, the present invention broadly relates to a method of 
suppressing deflagration of a combustible gas susceptible thereto, 
comprising the steps of: 
monitoring the combustible gas to detect deflagration therein; and 
upon detection of a deflagration event producing a propagating flame front, 
opposedly directing at the propagating flame front a pressurized 
non-flammable gas in a sufficient volume and at a sufficient velocity to 
provide a non-flammable gas flow having a momentum at least opposedly 
equal to the effective momentum associated with the propagating flame 
front in the combustible gas undergoing deflagration. 
The foregoing method, of suppressing deflagration of a combustible gas 
susceptible thereto, may be advantageously carried out in a flashback 
protection system comprising the apparatus according to the present 
invention, as broadly described hereinabove. 
Other aspects and features of the present invention will be more fully 
apparent from the ensuing disclosure and appended claims.

DETAILED DESCRIPTION OF THE INVENTION, AND PREFERRED EMBODIMENTS THEREOF 
The present invention broadly relates to a flashback protection apparatus 
and method for suppressing deflagration of a deflagration-susceptible gas 
in a flow system in which the deflagration-susceptible gas is flowed from 
a source to any potential ignition site. The source of the 
deflagration-susceptible (i.e., combustible) gas may be suitably coupled 
to flow passage means, such as a tube, channel, conduit, or the like, in 
which the combustible gas is flowed. The flow passage may contain fuel, or 
a premixed fuel/oxidant mixture. The fuel may for example be passed in the 
gas flow passage to a scrubber or other gas treatment sub-system, or the 
fuel may be mixed with oxygen or other oxidant, to produce a combustible 
gas mixture for ignition and burning thereof at a desired ignition locus. 
The invention is based on the surprising and unexpected finding that the 
"flashback" phenomenon occurring when a combustible fuel/oxidant mixture 
is undesirably ignited in a flow passage, due to air leakage, spark 
generation, flow anomalies, and/or other hydrodynamic or compositional 
effects, can be efficiently suppressed, without the occurrence of 
potentially catastrophic flame propagation to the accumulated 
fuel/oxidizer source, by opposedly impacting the flame propagation front 
with an amount of a non-combustible gas having a momentum at least equal 
to the momentum of the combusting gas mixture of fuel and oxidant which is 
generating the flame propagation front. 
In such manner, the "bolus" or bulk volume of the moving non-combustible 
gas acts to physically and kinetically "stop" the advance of the flame 
propagation front in the environs in which the fuel/oxidant mixture is 
combusting. 
In contrast to this impulse momentum approach to suppressing the 
propagation of the flame front of the combusting gas, the prior art, in 
instances where a gas susceptible to explosive ignition is being flowed 
from a source vessel to a downstream further processing point, has 
approached the explosion hazard problem by attempts to dilute the gas near 
the point of premature combustion. 
Such approach, however, is generally inadequate to reduce the concentration 
of the combustible component of the gas to below the lower explosive limit 
(LEL) within a suitably short period of time, given that the propagation 
rate of the flame front of highly flammable gases such as hydrogen/air 
mixtures may be very high, e.g., on the order of 3 meters per second. With 
such extremely high rates of advance of the flame front in the combusting 
gas, the dilution effect requires a significant finite time to effect, 
even with highly turbent flows of the dilution gas otherwise favorable to 
mixing and dilution of the contacted fuel/oxidant mixture. Additionally, 
if the deflagration front is sensed by thermocouple or pressure sensing 
means, then the response time can be slow, and the deflagration event can 
progress to detonation before remedial action is taken to prevent 
explosion in the process system. 
Further, relative to the impulse momentum method of the present invention, 
the fundamental rationale of the prior art approach of dilution points the 
skilled artisan away from the methodology of the present invention. This 
stems from the fact that the high volumetric flow rate, high momentum 
flows of deflagration-supressing gas utilized in the practice of the 
present invention are not favorable to mixing and dilution. Contrariwise, 
such flows would be expected to force a high level of flow separation and 
stratification in the streams or jets of fluid being directed at one 
another from opposite directions, with some mixing occurring, but 
accompanied by significant penetration of unmixed portions of the 
respective gas streams into the bulk of the opposingly directed gas flow 
stream. 
Accordingly, the prior art practice of dilution of a combusting gas mixture 
below its LEL of combustible component(s) does not in any way suggest the 
approach of the present invention, but rather emphasizes the novelty and 
unobviousness of the method and apparatus of the present invention. 
Further, the prior art has not contemplated the impulse momentum flashback 
supression approach of the present invention, because there has been a 
failure in the art to recognize the effect on gas velocity (in the 
combusting gas) of gas expansion affects. 
In a spatially restricting passage such as an open pipe carrying the 
combustible gas mixture, burning is accompanied by an expansion wave 
propagated away from the burning area or "flame front" in both directions 
towards the respective ends of the pipe. By way of illustration, suppose 
that the gas in such a system were ignited at one end of the pipe. As the 
flame front moves along (inside) the pipe, the heated and expanded gas 
containing combustion products immediately behind the flame front pushes 
hot reacted gases toward the initially ignited end of the pipe, and the 
cold, as yet unreacted combustible gas is pushed toward the opposite end. 
Suppose now that the combustible gas were initially flowing in the pipe so 
that the flame front is able to be described as moving "upstream" in this 
flow relative to the fixed frame of reference of the pipe. The expansion 
effects can accelerate the cold unreacted gas ahead of the moving flame 
front upstream, thereby slowing or even reversing the flow direction. 
Since the flame front itself is embedded in this moving stream, the speed 
of this flame front through the pipe can greatly exceed the actual burning 
speed relative to the unburned gas alone. 
The effective momentum which must be imparted in the opposing direction to 
this "moving front" at any instant (subsonic flow) is the product of the 
front's velocity relative to the pipe times the mass of the pipe which 
must be brought to the same opposing velocity in order to stop the motion 
of the flame front relative to the pipe. 
An effective injector satisfying this criterion will also subsequently act 
as an ejector pump to keep the gas it accelerates moving in the downstream 
direction at a speed which exceeds the flame front speed. This eliminates 
the possibility of significant upstream propagation of buring elements 
based on velocity considerations alone, i.e., without invoking as yet any 
dilution considerations attributable to the non-combustible gas issuing 
from the injector. 
Against this background, I have determined that in actual deflagration of 
combustible gas in relatively short, open tube experimental systems, the 
velocity of the flame front is typically at least on the order of twice 
the corresponding literature values for maximum flame speeds, for gases as 
disparate in combustibility characteristics as hydrogen and methane. (The 
imparted stream velocity therefore needs to be subtracted from the total 
observed propagation velocity in order to calculate the actual flame 
speed.) In consequence, the actual transit times for the flame of the 
combustion front to traverse the flow passage containing the gas flow 
stream may be significantly shorter than would be expected based solely on 
the published flame speeds alone. 
In the case of strong, e.g., loud, deflagrations, such as hydrogen/air 
deflagrations, the imparted stream velocity effect dominates and the 
actual observed, uncorrected flame speeds can be an order of magnitude 
higher than the published maximum value. In such deflagrations, there is a 
definite pressure rise inside the gas flow passage, and a blast pulse is 
observed, as accompanying the audible report of the deflagration. 
The present invention takes cognizance of the foregoing, and utilizes 
injection of an opposingly directed (relative to the direction of 
propagation of the flame front) non-combustible gas. Such non-combustible 
flashback-supressing gas may be a single component gas species or a gas 
mixture, in an amount and at a sufficient volumetric flow rate and 
velocity to provide injected gas momentum that matches or exceeds the 
momentum of the combusting gas. 
The injection of the non-combustible gas may be effected in the How passage 
in which the combustible gas is flowed, by means of an on-axis injector to 
introduce the non-combustible gas, in the gas flow passage, at a 
sufficient velocity opposing the accelerated stream, to effectively "butt 
heads" with the approaching unreacted combustible gas which carries behind 
it the approaching flame front. 
Advantageously, the point of injection and the amount of the 
non-combustible gas is selected so that there is adequate development of 
the injected gas "front" into plug flow when intercepting the combusting 
gas. The injected gas front of non-combustible gas thus overcomes the 
combustible gas front, and sweeps the combustible gas back downstream. 
Such injection may or may not also reduce combustible gas concentrations 
below their flammability limits. 
The non-combustible gas utilized to suppress the combustion of the 
combustible gas in the gas flow passage may be of any suitable species 
which is inert or otherwise non-combustible in character. Examples of 
potentially useful non-combustible gas species include nitrogen, argon, 
xenon, and krypton, and mixtures thereof. 
The present invention therefore contemplates a flashback protection 
apparatus for suppressing deflagration in a combustible gas flow system 
including a combustible gas flow passage with inlet and outlet ends 
defining a gas flow path therethrough from the inlet (upstream) end to the 
outlet (downstream) end of the passage. As used herein, the term "flow 
passage" in reference to the combustible gas is intended to be broadly 
construed to encompass pipes, conduits, hoses, tubes, channels, ducts, and 
all other flow path-defining structures by and/or in which the combustible 
gas is made to flow in a continuous, bulk, and/or batch manner. 
The source of injection gas (pressurized non-combustible gas) joined in 
latent gas flow communication with the gas flow path in the broad practice 
of the invention may suitably comprise a tank, cylinder, or other storage 
or supply means and associated piping, conduits, etc., by which the 
non-combustible gas is delivered to the gas flow passage. The source may 
by way of specific example comprise a high pressure cylinder of nitrogen 
or other non-combustible gas, joined at a regulator or valve head of the 
cylinder to a line interconnecting the cylinder with the gas flow passage 
containing the flowing combustible gas. 
The supply arrangement for delivering the non-combustible gas to the 
combustible gas flow passage may include a flow controller which is 
operatively arranged for selectively establishing gas flow communication 
of the source of pressurized non-combustible gas with the gas flow path. 
The flow controller may for example include a valve which is selectively 
openable and closable, being normally in the closed position not allowing 
gas flow from the pressurized non-combustible gas source to the gas flow 
passage, and selectively openable to initiate sudden and substantial bulk 
flow of the pressurized (non-combustible) gas from the source thereof to 
the gas flow passage, via valve controller means. Such valve controller 
means may be joined in signal communication relationship with the optical 
sensing assembly, such as by electrical signal transmission wires, 
wireless signal transmission circuitry, optical waveguide (fiber optic 
cable), etc. 
The optical sensing of the deflagration event in the combustible gas is a 
critical aspect of the present invention, due to the rapid response of the 
flashback supression system which thereby is achieved. By contrast, the 
thermocouple-based flashback sensing systems of the prior art have a very 
slow response time, and in many instances may not possess the responsivity 
to arrest deflagration or to prevent the progression of the deflagration 
to detonation in the process system. 
The flow controller in one embodiment of the invention comprises a flow 
control valve and associated valve actuator disposed in the line 
interconnecting the non-combustible gas supply with the combustible gas 
flow passage. 
Alternatively, the flow controller may comprise a regulator assembly 
associated with a high-pressure cylinder of the non-combustible gas, in 
which the regulator assembly includes a selectively openable and closable 
flow control element or subassembly, for selectively initiating or 
terminating the flow of non-combustible gas to the combustible gas flow 
passage. 
An optical sensor assembly is utilized which is constructed and arranged to 
detect deflagration in the flow passage during gas flow of combustible gas 
therethrough, and to (i) maintain a non-flow condition of the pressurized 
non-combustible gas to the gas flow path during non-deflagrating gas flow 
of combustible gas through the gas flow path, and (ii) responsively 
actuate the flow controller to selectively establish the gas flow 
communication of the source of pressurized non-combustible gas with the 
gas flow path upon detection of deflagration in the gas flow passage 
during gas flow of combustible gas therethrough. 
The optical sensor assembly is constructed and arranged to detect 
deflagration in the flow passage during flow of combustible gas 
therethrough, and may suitably comprise one or more photodiode(s) and 
operational amplifier circuitry for detecting the light flash and 
generating an amplified electrical signal to the flow control means 
described above. 
Alternatively, other optical sensing means comprising a light meter, photon 
counter, or other electromagnetic detection means sensitive to the 
radiation flux, intensity or other optical parameter of the deflagration 
event, may advantageously be employed in the optical sensor assembly. 
Such sensing element in the optical sensor assembly is operatively coupled 
with control circuitry or other subassembly component(s) which operates to 
actuate the flow controller to provide contemporaneous, real-time flow of 
the non-combustible gas to the gas flow passage in response to the sensed 
deflagration event. The pressurized non-combustible gas flowed to the gas 
flow passage has a pressure and a volumetric flow rate into the gas flow 
passage providing a the pressurized non-combustible gas momentum that is 
at least opposedly equal to, and preferably significantly greater than 
(e.g., at least 5% higher, more preferably at least 15% higher, and most 
preferably at least 25% higher than) the momentum developed by the 
accelerated combustible gas undergoing deflagration at an upstream 
location in the gas flow passage. 
The system of the invention thus operates to suppress deflagration of the 
combustible gas by (1) monitoring the combustible gas to detect 
deflagration therein; and (2) upon detection of a deflagration event 
producing a propagating flame front, opposedly directing at the 
combustible gas including the propagating flame front, a pressurized 
non-flammable gas in a sufficient volume and at a sufficient velocity to 
provide a non-flammable gas flow having a momentum at least opposedly 
equal to momentum of the propagating flame front and the combustible gas 
undergoing deflagration. 
Referring now to the drawings, FIG. 1 shows a schematic flowsheet of a 
flashback protection system 100 according to an illustrative embodiment of 
the present invention. 
In such flashback protection system 100, a gas flow passage 102 is provided 
in the form of a tube 103 whose wall may be formed of any suitable 
material and thickness. The tube wall of gas flow passage 102 defines 
therewithin an interior volume constituting a gas flow path 104 from an 
inlet end 106 to an outlet end 108 of the tube. 
At the inlet end portion of tube 103 comprising inlet end 106, a fuel from 
a fuel source 118 is flowed by means of line 120 into the inlet end 
portion of the gas flow path 104. Concurrently, an oxidant medium from 
oxidant source 114 is flowed via line 116 into the inlet end portion of 
the gas flow path 104. The fuel may comprise any suitable combustible 
species, such as a hydrocarbonaceous fuel e.g., methane, or other 
combustible gas such as hydrogen. The oxidant in turn may be of any 
suitable composition, e.g., air, oxygen, etc. 
Although FIG. 1 shows a schematic representation of a system in which an 
oxidant medium is added to the fuel component from an oxidant 114 to form 
a fuel/oxidant mixture, it is to be recognized that the fuel/oxidant 
mixture could be formed by air leakage into a fuel gas-containing stream 
flowing in the tube 103, from a leak in the process system. 
At the outlet end portion of gas flow path comprising outlet end 108 of 
tube 103, the fuel/oxidant mixture is flowed via combustible gas mixture 
feed line 144 to burner 146 in furnace 145. The furnace 145 has disposed 
beneath burner 146 a spark ignition device 148 which is provided with 
electrical energy by means of wire 150 from electrical power source 152. 
The spark ignition device 148 is electrically actuated and is selectively 
operated to generate a spark to ignite the fuel/oxidant mixture on burner 
146 in the furnace, thereby producing combusted gases which are discharged 
from the system in line 154. 
At the outlet end portion of the gas flow path 104, within tube 103, is 
provided one or more photosensors. In the specifically illustrated system, 
the photosensor means comprise photodiode sensors 138 and 140, which may 
be deployed behind a heat-resistant, light-transmissive material such as 
quartz or silica glass, so that the sensors can sense light incident to a 
deflagration event at such outlet end portion of the gas flow path. 
The photodiode sensor 138 is joined by signal transmission wire 136 to the 
optoelectronic conversion module 134, in which the optical sensing of 
photodiode sensor 138 is converted to an electronic signal which is passed 
to signal transmission line 132. In like manner, the photodiode sensor 140 
is connected by signal transmission line 142 to the optoelectronic 
conversion module 134, for conversion of an optical sensing by the sensor 
140 to an electrical signal which then is passed to signal transmission 
wire 139. The signal transmission lines 136 and 142 may for example 
comprise a light wave guide, such as a suitably dad optical fiber, or 
other suitable light transmission means, whereby the optoelectronic 
conversion module 134 is able to receive a signal indicative of the 
presence of a deflagration event at the outlet end portion of the gas flow 
path 104. 
Upstream (such direction being identified in reference to the outlet end 
108 of the gas flow passage tube 103) from the outlet end portion of the 
tube 103, a non-combustible gas injection conduit 110 passes through the 
tube wall to an interior discharge open end 112, which is at a central 
region of the gas flow passage tube. The gas injection conduit may be 
formed at its interior end in the gas flow passage with a reduced diameter 
or nozzle structure to provide high-impulse injection of the 
deflagration-supressing gas. 
A source 122 of non-combustible gas is provided, such as a high-pressure 
gas cylinder, a cryogenic air separation plant (producing nitrogen), or 
other suitable gas supply means. The non-combustible gas source 122 is 
joined by feed line 124 to a flow control valve 128, which in turn is 
operatively coupled with valve controller 126. The valve controller 126 in 
turn is connected to signal transmission line 132, and is thereby 
constructed and arranged so that the valve controller 126 is selectively 
actuatable to open or dose valve 128, in response to the signal 
transmitted to controller 126 from signal transmission line 132. 
Valve 128 is connected to flow line 130, which in turn is joined, as for 
example by suitable fitting or other coupling means (not shown), to the 
non-combustible gas injection conduit 110. 
In operation of the above-described system shown in FIG. 1, fuel from fuel 
source 118 is flowed through fuel feed line 120 to the inlet portion of 
the gas flow passage 102, concurrent with flow of oxidant from oxidant 
source 114 through line 116 to such inlet portion of the gas flow passage. 
The resulting fuel/oxygen mixture then is flowed longitudinally through 
the gas flow passage to the outlet end portion of tube 103, being 
discharged in combustible gas mixture feed line 144 and passed to burner 
146 in furnace 145 for combustion. The combustion in furnace 145 is 
originally initiated by spark ignition device 148 which is electrically 
actuated by electrical power source 152 joined to the spark ignition 
device 148 by means of electrical wire 150. The electrical power source 
152 may be joined to computer control or other selectively automated means 
(not shown) whereby the burner may be turned on and ignition begun in an 
appropriate manner, relative to the initiation of flow of fuel and oxidant 
from sources 118 and 114, respectively. 
During operation of such combustible gas mixture burning operation, the 
incidence of any deflagration event will produce a flame front A 
(indicated in dashed outline form in the interior volume of tube 103) 
which will be sensed by photodiode 140. Such event therefore will generate 
an optical sensing signal which is transmitted by signal transmission line 
142 to the optoelectronic conversion module 134, containing circuitry of a 
known conventional type for converting an optic light signal to an 
electronic signal. The resulting output electronic signal may be digital 
or analog in character, and is transmitted by means of signal transmission 
line 132 to the valve controller 126. Prior to such flashback event, valve 
128 is maintained in a dosed condition, so that no flow of non-combustible 
gas from gas source 122 is permitted to flow into gas flow line 130. 
Upon detection of the deflagration event, and corresponding actuation, by 
means of signal transmission wire 132, of valve controller 126, the valve 
controller 126 operates to open valve 128, resulting in the flow of 
high-pressure, non-combustible gas from gas source 122 through line 124 
and valve 128 to gas flow line 130, and injection conduit line 110. The 
high-pressure, non-combustible gas then is discharged at distal end 112 of 
the injection conduit 110, in sufficient volume and velocity to quickly 
produce a plug flow of the non-combustible gas, indicated by flow profile 
line B in FIG. 1. 
The volumetric flow rate and velocity of such non-combustible gas is 
sufficient to provide a momentum which is at least equal to the momentum 
associated with the combusting and approaching flame front A. By such 
opposed flux of non-combustible gas, the flame front is redirected to the 
outlet end 108 of the gas flow passage and the combusting (flashback) 
gases are forced through combustible gas mixture feed line 144 to the 
burner 146 in furnace 145. 
The upstream (relative to photodiode 140) photodiode 138 is provided as a 
fail-safe mechanism, in the event that there is advance of the flame front 
A to a position where the deflagration event is sensed by photodiode 
sensor 138. In such event, the photodiode sensing from sensor 138 is 
passed by optic signal transmission line 136 to the optoelectronic 
conversion module 134, and a correlative electronic signal is passed by 
signal transmission wire 132 to the controller 126, which may for example 
cause the valve 128 to open to a fully open position, beyond the open 
position initiated by the signal generated by sensing from photodiode 
sensor 140, so that the mass flux of non-combustible gas directed through 
injection conduit 110 into tube 103 is significantly increased, to halt 
and reverse the direction of travel of the flame front. Alternatively, in 
such event, the controller 126 may actuate a separate injection of a 
flashback-supression gas from a farther upstream location, or other 
flashback supression, or explosion prevention action. It will be 
recognized that the length of the gas flow passage may be substantial, and 
that the optional backup photodiode, if provided at all, may be spaced 
from the main photodiode 140 by a significant distance. 
FIG. 2 is a schematic representation of a flashback protection system 200 
and associated piping and components of another embodiment of the present 
invention. The overall apparatus assembly comprises flashback protection 
apparatus 200, including non-combustible gas injection line 202 and the 
injector module 204 featuring a pressure switch which may for example be 
adjustable from 3 to 300 psi in pressure level, to correspondingly adjust 
the pressure of the gas injected through non-combustible gas injection 
line 202. The module 204 also includes a solenoid valve and associated 
connectors, whereby the module may be interconnected with a source of 
high-pressure non-combustible gas, such as a cylinder of nitrogen at 
super-atmospheric pressure. The flashback protection apparatus 200 as 
shown in this embodiment is associated with a corresponding length of the 
pipe 206 having a connector 210 at one end thereof and a coupleable 
fixture 212 at its opposite end, such as a flange which is clampable to a 
combustion effluent treatment apparatus (not shown). Since in such system, 
the solenoid valve is the slowing or limiting response element, it is 
desirable to obtain a solenoid valve which is rapidly actuable for opening 
thereof, to initiate the flow of flashback suppressing gas to the gas flow 
passage at a high velocity and volume flow rate. 
Upstream of the fixture 212 is disposed an optical sensing assembly 214, 
including photodiodes, which are interconnected by means of suitable 
signal transmission means (not shown in FIG. 2) to means for converting 
the optical sensing of the light detection means in sensing assembly 214 
to signals for controlling the flow or non-flow conditions of the injector 
module 200. 
In the system shown in FIG. 2, the distance L.sub.1 along the corresponding 
length of pipe shown in the drawing may be on the order of 2.5 meters in 
length, comprising a straight run of pipe of at least 1.3 meters in length 
from the non-combustible gas injection point associated with injector 
module 200, to allow development of plug flow of the injected 
non-combustible gas, upstream of the pipe end 220 at which the fixture 212 
is joined to the effluent combustion or other processing unit in the 
overall system. 
FIG. 3 is a schematic representation of a portion of a semiconductor 
manufacturing facility effluent processing apparatus, including a scrubber 
device 300 which may for example comprise a water scrubber or chemisorbent 
treatment medium with which effluent gases from the semiconductor 
manufacturing operation are contacted, to effect removal of gases such as 
arsine, phosphine, diborane, hydrogen chloride, dichlorosilane, 
trichlorosilane, tetrachlorosilane, ammonia, boron trichloride, aluminum 
trifluoride, aluminum trifluoride, hydrogen fluoride, 
tetraethylorthosilicate, chlorine, fluorine, boron trifluoride, tungsten 
hexafluoride, and/or hydrogen bromide, among others. 
The process effluent from the semiconductor manufacturing facility is 
flowed to the scrubber 300 by feed line 302 and the resulting scrubbed gas 
containing combustible component(s) is discharged from scrubber 300 in 
discharge line 304 and passed to the flashback protection system 306, the 
elements of which are numbered correspondingly with respect to FIG. 2. The 
effluent passing through the flashback protection system 306 then flows 
into the combustor 310, which may for example comprise an active flame 
oxidizer of the type commercially available from ATMI Ecosys Corporation 
(San Jose, Calif.) under the trademark Phoenix. 
The combustor 310 comprises a chamber that mixes the process effluent with 
air and fuel from conduit 206, in a turbulent combustion zone. Such mixing 
results in high destruction rates of the combustible component(s) of the 
effluent, using natural gas or hydrogen fuels. 
The resulting effluent from the combustor 310 is discharged in vent line 
312 and may be passed to the ambient atmosphere or otherwise to further 
disposition and/or processing. 
In the effluent treatment assembly shown in FIG. 3, the settings of the 
flashback suppression gas feed supply associated with the non-combustible 
gas injection line 200 may be set to provide any appropriate flux of 
flashback suppression gas to conduit 206, appropriate to the flow rate of 
the normal gas flow through pipe 206 and the combustibility of the gas in 
such pipe under flashback conditions. 
The specific flux and amount of gas necessary to suppress flashback 
conditions in a given system may readily be determined by those skilled in 
the art without undue experimentation, by the simple expedient of 
simulating process flow and flashback ignition conditions in a gas flow 
passage simulating the actual operating environment in which flashback 
suppression is desired, with sequential injection of varying fluxes of gas 
under flashback conditions. In such determination, the effects of the 
expansion of gas incident to combustion and the acceleration of the flame 
front beyond values tabulated in the literature should appropriately be 
taken into account, to insure the appropriate operation of the flashback 
protection system. 
The features and operation of the present invention are more fully 
illustrated by the following non-limiting example. 
EXAMPLE 
A gas combustion system according to the present invention was constructed 
and operated to effect the combustion of a mixture of hydrogen and air. 
The hydrogen and air were injected for intermixing into a 3.8 centimeter 
diameter stainless steel tube, having a nitrogen injector conduit 1 
centimeter in diameter entering the tube at a slight sloping angle and 
bending near its tip to align along the central axis of the tube. A 
nitrogen solenoid valve was fitted to the end of the 1 centimeter conduit 
outside the tube wall. A photodiode sensor was provided near the tube wall 
at the exit end portion, in spaced relationship to the end extremity of 
the tube. 
The nitrogen pressure of a nitrogen supply cylinder was set by adjustment 
of the regulator head controls to 7.67.times.10.sup.5 Newton/meter.sup.2 
(100 psig) and a large diameter feed hose was used to supply the solenoid 
valve with an unrestricted flow of the nitrogen gas. A 2.5 meter overall 
length of 3.8 centimeter diameter tubing gave excellent results. Care was 
taken to clear residual nitrogen from the 3.8 centimeter diameter tube 
after each run. The flows of hydrogen in the majority of the test runs 
were 0.033 standard liters hydrogen per second into the 3.8 centimeter 
diameter tube for 20 seconds before triggering spark ignition at the 
opposite (exit) end of the flow passage tube. 
A series of ignition flash-back tests was carried out. The transit time of 
the flame front was about 90 milliseconds, and the main peak of the flame 
front passing the photodiode view port was about 10 milliseconds wide at 
half height. When quartz and glass windows were alternatively used in 
front of the photodiodes, similar light intensity results were obtained. 
The nitrogen injector in all instances pushed all of the burning gas out 
of the ignited (exit) end of the tube, and 1 meter long flashes often 
erupted there when the nitrogen injector tube solenoid opened, while no 
flame was observed issuing from the hydrogen inlet end of the 3.8 
centimeter diameter tube, in any of the test runs. 
While the invention has been illustratively described herein with reference 
to various exemplary aspects, features and embodiments, it will be 
appreciated that the utility of the invention is not thus limited, but 
rather embraces numerous other variations, modifications and other 
embodiments, and all such other variations, modifications and other 
embodiments therefore are to be regarded as being within the spirit and 
scope of the invention as claimed.