Flame arrestor device with porous membrane

This invention relates to flame arresting devices capable of checking up-stream flashback in a stream of combustible gas while permitting the gas to flow down-stream. In particular, it relates to flame arrestors having such capability when incorporated in a system containing a detonatable gas.

Flame arresting devices that will halt flame propagation in a flow passage containing combustible - but not necessarily detonatable - gas mixtures are known. Such prior-art flame arrestors are normally adapted to function efficiently and safely only at relatively low pressures; for example, at or near ambient pressure. In systems such as those used for detonation of explosives or for gas-flame cutting, that are dependent upon controlled release of chemical energy in a pressurized environment, the known flame arrestors are not adequate.

According to the invention, an efficient flame arrestor device capable of halting upstream flashback in a system containing a potentially detonatable gas under positive pressure, and comprising an arrestor housing having an upstream entry port, a downstream exit port and a flow passage from the entry port to the exit port, is characterized by, in combination, an interior housing well in the said passage, a membrane-holding wall extending from the upstream end of the housing around the entry port and down-stream into the housing well and forming a gas-tight seal at the' upstream end of the housing around the entry port, the membrane-holding wall having an outside diameter less than that of the opposed inside surface of the housing well and defining an enlarged space within the said passage between the membrane-holding wall and the inside surfaces of the housing well; and a gas-permeable flash-resistant porous membrane positioned within said enlarged space and having its periphery secured in gas-tight relationship to the membrane- olding wall at the upstream end of the housing, the membrane being distanced from the wall surfaces of the said space whereby the latter is partitioned into down-stream and up-stream flow gaps that are isolated from each other except through the gas-permeable pores in the said membrane and are respectively connected to the upstream entry port and the downstream exit port by the said flow passage.

The permeable membrane preferably is shaped like an inverted cup with its up-stream-facing open end bonded to the membrane-holding wall. In its preferred application, the arrestor is incorporated in a blasting system, and an igniting device is incorporated within the arrestor housing in firing relationship with the flow passage downstream of the membrane.

When used in non-electric blasting systems using detonatable gases, the flame arrestor device according to the invention will withstand repeated use under rigorous conditions while still retaining the flexibility and safety of chemical activation, and does not require resetting after flashback occurs. Since the enlarged space in the flow passage both exposes a substantial area of the permeable membrane and is of limited cross section or "depth", it permits gas to pass downstream through the porous membrane while retaining the integrity and strength of the membrane barrier to repeatedly check upstream-directed flashback in systems carrying substantial preignition gas pressure.

The up-stream flow gap in the enlarged space around the gas-permeable membrane is connected through the entry port to a source of gas upstream of the arrestor device. A blasting system using such device is conveniently activated by ignition means positioned downstream of the flash resistant gas-permeable membrane without danger from upstream flashback, after initially bleeding the gas under positive pressure through the arrestor device and an attached conduit to the site of utilization.

Figure 1 schematically illustrates one embodiment of the present invention in which the flame arrestor device (10) is incorporated in series in an essentially chemically activated explosives detonator system utilizing ignition means (24) (such as a spark plug incorporated within the housing of the device (10)) in electrical firing relationship with a flow passage that extends downstream of a porous metal membrane (18), and a flow gap (22) downstream of the membrane (18), and then through an exit port (26). A conduit (12A) extends to one or more remote blasting holes (28) containing non-electric blasting caps (30) between the conduit and a main explosive charge (32).

A system as described in Figure 1, without the igniter means (24) and the blasting installation (28), (30), and (32), can be used for other pressurized gas applications that may be subject to flashback problems, such as a gas-flame cutting torch system in which the point of ignition and point of admixture with oxidant gas are located at a nozzle close to the cutting flame.

Figure 2 is a longitudinal section of a flame arrestor of the general type represented schematically in Figure 1.

Figure 3 is a cross section of flame arrestor (10) taken along line 3 -3 of Figure 2.

In further reference to Figure 1, flame arrestor device (10) is used to check upstream flashback in an explosive detonation system utilizing a detonatable gas and ignition means. For such purpose the fuel and oxidant gases conveniently comprise a fuel such as hydrogen and an oxidant such as oxygen obtained from gas feeding means (A) and (B) through as oxygen obtained from gas feeding means (A) and (B) through valving means (14) and interconnecting flow passage as above described.

The speed of a detonation or reaction wave through a conduit (12A) depends upon the character and proportions of fuel and oxidizer in the detonatable gas mixture, and the preignition pressure maintained in the system. A reaction speed of about 8000 ft/sec, however, is found generally acceptable for controlled detonation purposes.

Preparation for detonation in the system of Figure 1 usually includes an initial pressure testing for blockage with an inert gas from a source (not shown) followed by charging of the detonator by metering oxygen from tank (A) and fuel such as hydrogen and/or hydrocarbon mixture from tank (B) through valve (14), and flow passage (12) into the entry port (16) of the flame arrestor device.

The flame arrestor contains a gas-permeable flash resistant porous membrane (18), positioned between up-stream and down-stream flow regions or gaps (22) and (20) . The detonatable gas mixture passes through membrane (18) under pressure and aided by the increased flow area around the membrane provided by gaps (20) and (22).

The detonatable gaseous mixture then proceeds past ignition means (24) and exits the arrestor device by exit port (26). After the mixture leaves the arrestor device it fills conduit (12A) through connections (not shown) to trunklines leading to the desired number of remotely located blasting holes (28) , to blasting caps (30), and main explosive charges (32) as above described.

In further reference to Figure 2 there is shown in greater detail a combined arrestor-ignition device (10) comprising an arrestor housing (34) having an upstream entry port (58) and an interior housing well (40), a downstream exit port (60), a gas flow passage (66) from exit port (60) to an interior housing well (40), an ignitor housing (62) containing ignitor means (not shown) , with an interior threaded port (not shown) opening to the housing gas flow passage for mounting an ignitor means (such as a spark plug) , and spacer means in the form of a "standoff button" (56).

The device further includes a membrane holding means (36) comprising a membrane holding stem or wall (46) and an externally threaded flange (42) secured to the entry port and the interior housing well (40) in gas-tight relationship at the upstream end of the arrestor housing. The membrane-holding wall (46) has an outside diameter less than that of the opposed inside surface of the housing well (40) and defines an enlarged space within the said passage between the membrane-holding wall and the inside surfaces of the housing well.

A gas-permeable flash-resistant porous membrane (38), which may be shaped like an inverted cup as shown, is positioned within the said enlarged space. Its periphery at its open end or lip (50) is secured at (48) in gas-tight relationship to the membrane holding wall (46) at the upstream end of the arrestor housing. It is maintained in position within the enlarged space by said lip and by "stand-off button" (56) so that it is distanced from the inner and outer surfaces of the said space whereby the latter is partitioned into two separate flow gaps, a first or downstream gap (54) downstream of the membrane (38) and in isolation from a second flow gap (52) up-stream of the membrane and isolated from the first gap except through gas-permeable pores in membrane (38). It is connected to the downstream exit port (60) of the arrestor housing by the interconnecting gas flow passage (66) in housing (34) .

The second gap (52), upstream of membrane (38) , is connected by the flow passage (64) formed by inside wall (36) to entry port (58). A conveniently located source of gas (not shown), up-stream of the arrestor housing may be connected to the entry port. The "thicknesses" of the first and second gaps are the distances from the membrane (38) to the inner surface of the housing well and to the outer surface of the wall (36), respectively.

Figure 3 in combination with Figure 2 further demonstrates a convenient (although not mandatory) cylindrical shape of the housing (34) and membrane holding wall or stem (46), and the preferred "cup" shape of porous membrane (38) .

Generally speaking the mass and good conductivity of the flame arrestor housing and membrane holding means serves to protect them from the heat of flashbacks, thereby permitting the utilization of relatively low-melting, easily machined metals such as brass and copper, as well as the usual stainless and carbon steels. Such is not the case, however, with the porous membrane (38) , particularly when comprised of fine particulate metal particles, (e.^g. 3-5 micron) , such as found in a micro filter. In such case the use of stainless steel or similar material is required to avoid heat damage. An example of such material is a stainless steel micro filter comprising a plurality of fine metal particles of the type manufactured by Mott Metallurgical Company (Farmington, Connecticut. U.S.A.) under the identifying code 120-0.678-0.573-0.840-5, in which the melting point of the stainless steel particles is sufficiently high to conpensate for small mass and relatively low heat conductivity.

As previously noted, the thickness or "depth" of each of the first and second gaps (54) and (52) is of functional significance insofar as the heat and flashback resistance and general durability are not necessarily compatible with the gas-permeable properties of the membrane. It has been found, however, that individual gap depths of up to about 20 mil, preferably 5-20 mil, permit the necessary gas flow rate without adversely affecting the flame arrestor properties with a sintered stainless steel membrane having a pore size of about 2.5-25 microns, the smaller size being favored, commensurate with a desired flow rate, when high pre-ignition gas pressures are utilized.