Explosion Proof Lighting Fixture

Abstract

An explosion suppressing vent for an electrical fixture which includes continuous flame-arresting channel of air space passing through the fixture wall and connecting air-space within the fixture with exterior air. Electrical fixtures partially enclosed by glass, or other relatively fragile membranes, benefit greatly from the invention since, through its use, maximum internal pressures during combustion of enclosed potentially explosive gas mixture is suprisingly low.

Description

BACKGROUND OF THE INVENTION

Industrial electrical fixtures, such as lighting fixtures, particularly those intended for use in areas classified as hazardous because of presence of combustible gas, vapor, etc. are commonly completely enclosed. Part of the enclosure commonly includes glass or other translucent, relatively fragile material, and consequently the latter fixtures cannot be subjected to substantial interior pressures with safety. Explosion suppression devices heretofore available have been generally expensive to manufacture. For example, one device which was found to be satisfactory from a performance viewpoint was a structure which included a relatively expensive gas-porous metal wall.

Rupture of a glass-containing lighting fixture during the time of an internal "explosion" would completely nullify the advantage of providing such a fixture with explosion-resistant or explosion-suppressing elements. One solution heretofore suggested has been the utilization of increasingly heavy glass members which are capable of withstanding elevated internal explosion pressures.

This invention provides an explosion suppressing structure which is particularly advantageous in fixtures containing glass or other relatively fragile materials, as part of the fixture's enclosure wall. Internal pressures generated by an "explosion" within a fixture improved in accordance with this invention are rapidly and effectively dissipated. The result is that during internal combustion of gas phase within the fixture, the internal pressures do not reach the level at which the fixture's enclosure wall members, including glass members, are likely to rupture. In addition, the internal gas space is vented into the surrounding atmosphere in such a way that, during an internal combustion of most explosive mixtures, flame propagation is arrested as the exit gases safely pass through the enclosure into the air space surrounding the fixture.

SUMMARY OF THE INVENTION

This invention relates to electrical fixtures for use in areas classified as hazardous because of the presence of combustible vapor, gas, or other material. The improved fixtures include means for venting gases within the fixture through continuous, open, flame-arresting channels of air space passing through the fixture wall, said open channels being defined by closely adjacent heat conducting metal surfaces. The improved fixture includes channels of such size and number as to eliminate substantial pressure build-up within the vented fixture during internal combustion therein of explosive gas mixtures.

DESIGNATION OF THE FIGURES

FIG. 1 is an elevational view, partially in cross-section, of a light fixture showing portions of the covering elements removed therefrom, and showing a portion of an explosion-suppressing venting structure of this invention in cross-section.

FIG. 2 is a fragmentary cross-sectional view taken approximately along the line 2--2 in FIG. 1.

FIG. 3 is an enlarged cross-sectional view showing the mating of some of the channel forming components in accordance with this invention.

FIG. 4 is a fragmentary cross-sectional view taken approximately along the line 4--4 of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT

Although the following disclosure offered for public dissemination in return for the grant of a patent, is detailed to ensure adequacy and aid understanding, this is not intended to prejudice that purpose of a patent which is to cover each new inventive concept therein no matter how others may later disguise it by variations in form or additions or further improvements. The claims at the end hereof are intended as the chief aid toward this purpose; as it is these that meet the requirement of pointing out the parts, improvements, or combinations in which the inventive concepts are found.

The improvement of this invention is particularly useful in connection with electrical fixtures such as a light fixture generally 10 having a base portion 11 enclosing electrical connectors 12, and other electrical components such as socket 13 and bulb 14. Socket 13 is mounted on a plug 17 threaded into end component 16. Fixture 10 also includes a glass cover 15. Glass cover 15 is affixed to base section 11 by suitable attachment means generally 20 for hermetically sealing interface 21 between glass cover 15 and base portion 11. Mouth 22 in base 11 includes conventional means for sealing base 11 onto a conduit, pipe, etc., (not shown because not part of the invention). Vent section generally 23 provides the only venting of fixture 10. Since the electrical elements, glass cover means and guard bars 24 are conventional they will not be discussed in further detail herein. Cone shaped cover 25 protects vent section 23 from occlusion, e.g., due to maintenance painting, etc. Fixture 10 is attached to mounting 27 by means of threads 39.

Sidewall, generally 26 of base section 11, mounting 27, and glass cover 15 preferably enclose air spaces 28, 28' in which all the internal elements of the fixture reside. Plug 17 divides the interior of the fixture into two pneumatically separated spaces 28 and 28'.

In the illustrated embodiment, threaded end-component 16 of base portion 11 is cast to include a first flange 30 having concentric spaced-apart flat bottomed grooves 31 in face 32 of flange 30. Circular disc-like element 35 has flat topped ribs 37 extending from face 36 which is shown in FIGS. 1 and 3 as the upper face. The surfaces of flat topped ribs 37 are dimensioned and positioned to reside in, but be spaced-apart from the surfaces of flat bottomed grooves 31 in flange 30. All ribs, e.g. 37, and grooves, e.g. 31, are tapered in the illustrated embodiment but are not necessarily so. It is essential however, that opposing surfaces be maintained spaced-apart to define a plurality of zig-zag passageways 38 between the interior space 28 and the exterior of the fixture. Means for maintaining the surfaces in spaced-apart condition in the illustrated embodiment are discussed hereinafter. Grooves 41 are positioned in bottom surface 40 of disc-like insert 35 in a configuration which is substantially identical to that of grooves 31 in flange 30, and, moreover the entire lower surface of discs 35, 35' are identical to the lower surface of face 32 of flange 30. Flange 30 includes interiorly extending planar element 43.

Second disc-like element 35', identical to element 35, may be mated with and maintained spaced-apart from element 35 as illustrated.

The other end component 45 of base portion 11 also includes a medial flange 46 having face 47. From face 47 of flange 46 extend concentric flat topped ribs 48 having a configuration substantially identical to ribs 37. Hence, ribs 48 mate with, i.e. reside in, and are spaced-apart from, the walls of grooves 31 or grooves 41.

It will be appreciated that in those preferred embodiments of this invention in which disc elements 35, 35' are substantially identical, any number of such elements may be stacked one on the other as illustrated to provide greater or smaller amount of venting of interior space 28 in accordance with this invention. The amount of venting required will depend, among other things, on the volume of enclosed gas space 28. It is preferred however that at least two disc-like elements or inserts 35, 35' be used in a lighting fixture.

Interior planar element 43 of flange 30 is drilled through boss 50 to receive bolts 51. Interior planar elements 53 and 53' of discs 35, 35' include bosses 49 and 49' which are also drilled to receive bolts 51. Flange 46 has an interior planar element 32 with bosses 50 which also surround bolts 51. When the end flanges and inter-positioned disc-like members are assembled or stacked as illustrated, dependent spacer shoulders 50, 50' bear against bosses 49' thereby defining the thickness of passageway 38.

When ring 35 is seated as illustrated on flange 30 so that ribs 37 reside somewhat within grooves 31, all opposing faces of disc-like element 35 and flange 30 except the butting surfaces of the bosses defining spacers 50 and bosses 49' are maintained in a spaced-apart condition thereby forming a continuous channel or passageway 38 of air-space connecting enclosed space 28 with the air external to the fixture. Placement of a second disc 35' on the other flange 46 likewise brings about an abutment of spacer elements 50' with bosses 49'. Thus, between each of the components is maintained an air channel 38, 38', etc., which forms a continuous, zig-zag passageway between interior space 28 and air external to the fixture. The means, described above to maintain discs 35, 35' spaced-apart from flanges 30, 46 are used to maintain all discs spaced-apart from each other as well, discs 35, 35' being identical. Flanges 46 and 30 and circular disc-like elements 35, 35', etc. are maintained in a fixed position relative to one another by means of bolts 51 passing through holes 57, and by abutment of spacers 50 and shoulders 49'. The fact that these passageways 38 are circuitous (e.g., zig-zag) provides a large cooling surface to cool the gas therein and prevent the propagation of flame from the interior space 28 to the exterior.

PERFORMANCE AND TESTS

In tests actually performed on fixtures in accordance with this invention, ethyl ether vapor explosions deliberately detonated within chamber 28 were found to be suppressed in the sense that gas pressure in air space 28 did not exceed 45 pounds per square inch and in preferred embodiments, did not exceed 20 PSIG. Equally important, the same explosive vapors which were maintained external to the fixture during the test were not ignited by gases passing through channels 50 into the surrounding explosive atmosphere during the internal "explosion" or flash.

In these tests it was found that adding two inserts 35, 35' to a fixture of this invention reduced the average peak internal pressure and the maximum peak internal pressure to values which are approximately half of that encountered in the embodiment with no insert 35, 35'. Reducing the number of concentric grooves and tongues from five to four was found to have a much smaller effect, if any, on the peak pressures encountered inside the apparatus during a flash therein.

For the following tests a fixture described in the attached drawing was drilled and tapped for connection to (1) supply lines carrying air and an explosive mixture into the fixture interior, (2) spark plugs, (3) pressure recording device, and (4) gas sampling line. The test fixture was installed in a test chamber having another inlet and an outlet connection to the supply line leading from a carburetor system which produced the explosive test gas mixture. A window was provided in the test chamber for observing the fixture during the test. The explosion tests were conducted in the following manner. Explosive ethyl ether vapor-air mixtures were introduced into the fixtures device, and the device was flushed with the explosive mixture until the original air had been displaced. Samples of the explosive vapor-air mixtures were withdrawn for analysis from (1) the pipe carrying the explosive mixture, (2) the test chamber, and (3) the interior of the fixture. The inlet and outlet connection to the device and to the test chamber were then closed and the mixture within the fixture ignited by means of a spark plug. The device was observed during each test for the appearance of sparks or flame, and to determine whether the explosive mixture in the test chamber ignited. Internal pressures were recorded. At the conclusion of each test the products of the combustion were removed from the device by means of a stream of air.

The following series of tests were conducted using the above described equipment.

TEST SERIES NUMBER 1

In these series the vent structure tested was the same as that employed in the fixture illustrated in the attached drawings except that no inserts 35, 35' were present therein. Five concentric grooves and tongues were provided in the assembly, the total clearance between the walls of each opposing tongue and groove being 0.040 inch. A plurality of spot spacer discs or shims 0.020 inch thick were provided at several points between the crest of the tongue and its opposing surface to provide a 0.020 inch spacing at this section of the path. A 200 watt 28 amp lamp, not lighted, was installed in the fixture. Ignition of the explosive mixtures was obtained by means of the spark plug inside the fixture.

TEST SERIES NUMBER 2

The conditions and spacing were the same as Test Series Number 1 except that one insert 35 was added to increase the internal venting area to twice that provided in Test Series Number 1. The clearances were as described in Test Series Number 1.

TEST SERIES NUMBER 3

Same as Test Series Number 2 except that one additional disc insert 35 was added to increase internal venting area to three times that provided in Test Series Number 1, i.e. a total of two inserts were employed in Series Number 3.

TEST SERIES NUMBER 4

Same as Test Series Number 3 except that the spot spacer shims 0.025 inch thick were provided at several points between the crest of the tongues and the bottom of the grooves to provide 0.025 inch spacings at this section of the path, i.e. between crest and opposing bottom.

TEST SERIES NUMBER 5

Same as Test Series Number 3 except that one full tongue and groove was removed from the assembly of each insert. Four concentric tongues and grooves were provided in this test series.

TEST SERIES NUMBER 6

Same as Test Series Number 5 except that 0.025 inch thick spot shims as provided in Test Series Number 4 were used.

TEST SERIES NUMBER 7

Same as Test Series Number 6 except that the total clearance between each tongue and groove was increased to 0.070 inch and U-shaped spacers 0.035 inch thick were provided in each groove to maintain clearances.

The data results of the above tests are summarized in Table I in which all spacings are expressed in inches and all pressures are pounds per square inch gage. --------------------------------------------------------------------------- TABLE I

number test of number spacings highest* average** series discs of wall-crest- explosion explosion number 35 grooves wall bottom pressure pressure __________________________________________________________________________ 1(6)*** none 5 0.020 0.020 41 36 2(6) one 5 0.020 0.020 32 24 3(10) two 5 0.020 0.020 23 18 4(11) two 5 0.020 0.025 23 27 5(3) two 4 0.020 0.020 28 26 6(6) two 4 0.020 0.025 29 25 7(26) two 4 0.035 0.035 16 13 __________________________________________________________________________

total (68) *Maximum maximum pressure **Average maximum pressure ***Number in parenthesis is number of tests in the series.

In not one single instance in the tests of any of the above series did the explosion within the fixture propagate to the vapor in the test chamber surrounding the fixture. Thus, the above seven series of tests which included 68 individual tests showed 100 percent effectiveness with respect to arresting explosion occurring within the fixture, preventing its propagation to an explosive air mixture surrounding the fixture.

In an additional series of tests the test apparatus and conditions of Test Series Number 7 were repeated except that, instead of an ethyl ether explosive mixture, an explosive mixture of hydrogen and air was used, and 7 concentric tongues and grooves were present in the assembly. It was found that maximum internal explosion pressures ranged from 74 PSIG to 100 PSIG, and that in every instance, the internal explosion propagated through the vent assembly causing an explosion in the atmosphere surrounding the fixture.

Thus, even though the various embodiments of the apparatus of this invention were found to be 100 percent effective in explosive ethyl ether vapor-containing atmospheres, in the above tests, the fixture tested in the hydrogen-air mixture was not effective with respect to preventing propagation of internal explosions to the surrounding atmosphere when those atmospheres were a mixture of hydrogen and air. Consequently, the described fixture of this invention is of considerable utility, even though it is not effective in what are considered to be the most severe conditions, e.g., conditions falling within Underwriter Laboratory "A" and "B" classification, namely acetylene-air, and hydrogen-air mixtures, respectively. Most fixtures are directed for use in conditions falling within, or below, Underwriters' "C" or "D" groupings, e.g., ethyl ether-air, or gasoline-air mixtures.

DISCUSSION

It is clear from the experimental data disclosed hereinbefore that an extremely large number of variations are possible in the spacings, the number of tongues and grooves, the number of discs, and the types and explosivity of gas mixtures. Also, much variation in the volume of gas enclosed in fixtures is possible. Having been taught the general principles of manufacturing a safe article in accordance with this invention, one with ordinary skill in the art can now perform routine tests and exercise ordinary judgement in utilizing variables in order to settle on specifics involved in particular embodiments which are safe for use with a particular gas mixture. For example we have found that, using explosive mixtures of ethyl-ether and air, an embodiment of this invention utilizing five rings of tongues and grooves, with 0.015 inch spacing between opposing faces, generates pressures which are too high in the sense that the numerical values were higher than desired, even though no mechanical failure of the fixture occurred. We have found that in those lighting fixtures of the type described hereinbefore in which 25 PSIG maximum internal pressure is considered tolerable, the use of five tongues and grooves at spacings of 0.025 and 0.035 are entirely satisfactory, and the use of four tongues and grooves with spacings at 0.035 inch are also satisfactory. Broadly speaking however spacings in the range 0.015 to 0.040 inch inclusive, are useful in accordance with this invention. However, generally speaking, it is preferred that the number of tongues and grooves utilized be not less than four. Again, generally speaking, the spacings must be large enough, and the channels must be in sufficient number to prevent excessive internal pressure build-up, e.g. much above 25 PSIG, and the length of the path, and the number of changes of direction in the path of a gas in transversing the path must be sufficient to cool and quench the flame front as it passes through the channel in order to prevent propagation of the flame or explosion into the air space surrounding the fixture.

As seen in FIG. 2, for a sector about the vertical axis of the fixture the volume of the passageway is the smallest at the inner end of the passageway and becomes increasingly greater toward the outer end of the passageway. Thus, with a build-up of pressure in the fixture (as by reason of an explosion therein) that pressure will exert its greatest force tending to separate the parts of the passageway at the inner end of the passageway, with progressively less force (on the walls about the passageway) toward the outer end of the passageway. The fastening means (bolts 51) are immediately adjacent the inner end of the passageway.

Claims

We claim:

1. In an electric light fixture for use in a potentially combustible atmosphere wherein the fixture comprises a base, a glass globe below the base, an attachment device connected to the base and supporting the globe with respect thereto whereby the base and globe define a light receiving enclosure, said device including means for venting said enclosure while preventing the propagation of flame from the enclosure to the exterior of the fixture in the event of combustion within the enclosure, the improvement wherein said device comprises:

a first annular generally horizontal flange integral with the bottom of the base, said flange having a bottom face with a plurality of annular ridges extending downwardly therefrom and defining grooves between the ridges;

a second annular generally horizontal flange concentric with and below said first flange, said second flange having an upper face with a plurality of annular ridges extending upwardly therefrom and defining grooves between the ridges thereof;

at least one annular horizontal disc concentric with and between said flanges, said disc having a top face with annular ridges extending upwardly therefrom and defining grooves therebetween, and a bottom face with annular ridges extending downwardly therefrom and defining grooves therebetween, said ridges and grooves on the upper and lower faces of the disc interdigitating with the ridges and grooves of said first and second flanges to define circuitous passageways radiating in all directions about said fixture and increasing in volume from the interior end to the exterior end of the passageways;

said flanges and disc having vertically aligned bosses at spaced intervals adjacent the interior ends of the passageways, said bosses being in vertical contact with each other to define the thickness of said passageways, and

fastening means at each of said bosses, connecting the flanges and holding said bosses in contact.

2. In an electric fixture as set forth in claim 1, wherein there are at least two of said discs and the thickness of the passageways is between 0.25 and 0.35 of an inch, and that said discs and flanges have at least four ridges.