Hazardous location expansion fitting

A fitting which couples two conduit members in a manner to permit relative longitudinal motion therebetween and which prevents the gases within the fitting from escaping at a temperature which might ignite an external ambient atmosphere. One form of the fitting includes a bushing affixed to an end of one of the conduit members and positioned and configured to mate with the interior of a body member affixed to an end of the other conduit member to permit sliding motion therebetween. The bushing has sufficient surface area in close proximity to the interior surface of the body member as to cause any gases passed between the bushing and body member to approach a temperature equilibrium with the body member.

BACKGROUND OF THE INVENTION 
As is well known, it is customary, in many situations to place electrical 
conductors in a metallic conduit. Lengths of conduit may be connected 
together and/or formed, as may be required, to provide the effect of 
continuous conduit over extended lengths. A wide variety of coupling and 
angle changing fittings are provided, as may be required, to satisfy a 
wide variety of circumstances. In some construction situations, it is 
possible for two pieces of conduit to experience relative motion in a 
longitudinal direction. Unless special provisions are made, and unless the 
relative motions are very small, such motion may damage the conduit and/or 
fittings to which they are attached. Some of the more common and/or 
obvious situations wherein relative longitudinal motion may occur are on 
bridges which may rise and fall with temperature and load variations; 
interconnections between two buildings which may sway in response to wind 
loading; and situations wherein the ambient temperature of the conduit may 
be exposed to wide variations. Other situations will readily occur to 
those experienced in the art. 
The industry has provided a variety of expansion fittings for use in these 
and other circumstances. For example, reference may be had to U.S. Pat. 
No. 3,154,632 issued Oct. 27, 1964 to F. L. Browne; U.S. Pat. No. 
3,783,178 issued Jan. 1, 1974 to Robert A. Philibert, et al. and U.S. Pat. 
No. 4,250,350 issued Feb. 10, 1981 to Robert F. Polimine, all of which are 
assigned to the present assignee. Typically, the expansion fittings of the 
type described in the aforementioned patents will accommodate relative 
conduit movement of at least plus or minus four inches. The various cited 
patents, among other things, deal with means for providing superior 
grounding continuity between the two sections of conduit. 
Another situation with which those familiar with the art are all too 
familiar are the dangers presented when the atmosphere within the conduit, 
and/or external to the conduit, may contain an explosive atmosphere. The 
explosive atmosphere may be unavoidable or unintentional and may result 
from various gases, vapor, dust, lint or other substances. As those 
familiar with the installation of electrical conduit are well aware, it is 
quite possible for the conduit to serve as a transmission path of an 
explosive atmosphere from one location to another. To prevent such 
transmission of explosive atmospheres, and/or to limit the propagation of 
products of combustion resulting from combustion and/or an explosion 
devices of the type indicated in U.S. Pat. No. 4,216,349 issued Aug. 5, 
1980 to E. Wium have been designed. U.S. Pat. No. 3,974,933 issued Aug. 
17, 1976 to Maria Toth, et al discloses another structure designed to 
contain and limit the effects of explosions ignited by electrical sparks. 
Prior art expansion fittings provided the expansion feature but did not 
include means for preventing the escape of hot gases resulting from a 
explosion within the conduit. 
There is at least one expansion fitting available in the market place that 
is designed for use in hazardous locations. This fitting is intended to 
prevent any exploded gases within the fitting from escaping at a 
tempearature which would ignite the atmosphere external to the fitting. 
The commercially available fittings, known to applicant, are relatively 
costly and permit a maximum expansion of only approximately 1.25 inches, 
an amount which is not comparable with the eight or more inches provided 
by the expansion fittings of the cited prior art patents. U.S. Pat. No. 
2,900,436 issued Aug. 18, 1959 to A. I. Appleton describes this fitting. 
The patented fitting is not liquid tight and as a result, the critical 
surfaces may be exposed to corrosive atmospheres. In addition, the 
grounding technique described could damage the interior surface and permit 
rapid escape of hot gases. 
Prior art techniques for confining hot gases and/or providing for 
relatively large longitudinal movement includes formation of a large "U" 
bend in the conduit or perhaps the use of a full circle. These techniques 
require costly flexible couplings. 
Another technique was to use an expansion fitting of the type disclosed in 
one of the cited patents and near each end thereof provided a seal similar 
to that of the cited Wium patent. This combination is fraught with danger 
as longitudinal movement of the conduits could result in one or both of 
fractured seals and rupture of the conductor due to tension. 
SUMMARY OF THE INVENTION 
The structure of the present invention uses some elements similar to those 
of the prior art expansion fittings. However, the body member is modified 
by having the inside diameter reamed to a carefully controlled dimension. 
The conduit which fits within the body member, and moves longitudinally 
with respect thereto, has affixed to its end, a metallic bushing having a 
smooth outer diameter machined to a value which is only slightly less than 
the reamed diameter of the body member. Typically, the bushing is also 
provided with a portion containing flats for the purpose of wrench 
tightening the bushing onto threaded conduit. 
Alternately, the conduit could be of a predetermined diameter to mate with 
the passage through the head. This is generally inferior as the conduit 
would be exposed to external atmosphere and/or mechanical damage. 
The body member and all associated parts are designed to withstand an 
accidental internal explosion and if such explosion should occur, the 
pressure within the fitting will exceed the external atmospheric pressure 
and there will be a tendency for the hot gases within the fitting to 
attempt to escape by passing between the smooth surface of the bushing and 
that of the body member. Because the body member will be at a temperature 
substantially equal to the external ambient temprature and because of the 
small space between the bushing and the reamed interior of the body member 
and because of the surface area of the bushing proximate to the body 
member, any gas which is transferred through the space between the bushing 
and the body member will be cooled to a temperature insufficient to ignite 
an explosive atmosphere. Gases from within the conduit and fitting may 
also escape through the threaded joint between the conduit and the body 
member. This is acknowledged as being acceptable for the application as 
the gases will be adequately cooled prior to emergence to the atmosphere. 
It is an object of this invention to provide an expansion fitting with all 
of the characteristics and features of prior art expansion fittings and to 
provide a structure which will be satisfactory for use in hazardous 
locations. 
It is a more specific object of the invention to provide an expansion 
fitting wherein gases escaping from the interior, due to an explosion, 
will be cooled to such an extent that they will be incapable of igniting 
an explosive atmosphere, when and if they escape from the fitting. 
It is another object of the invention to cause any escaping gases to be 
cooled by the close proximity of metallic surfaces which comprise the 
escape route. 
It is another object of the invention to provide a fitting having the 
described characteristics and which is liquid tight and provides ground 
continuity. 
It is another object of the invention to provide a structure of the 
character recited which is reliable in performance, economical to 
manufacture, and convenient to assembly and use.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The cited patents relating to expansion connectors disclose a variety of 
techniques for providing ground continuity between the two pieces of 
conduit coupled to the expansion joint. The expansion joint used in 
connection with the present invention may incorporate any one of these 
ground continuity techniques and/or other techniques. For convenience, the 
grounding technique indicated in FIG. 1 of cited U.S. Pat. No. 3,783,178 
is illustrated herein. However, it should be understood that any other 
expedient form of ground continuity may be employed. 
It is believed that the invention, together with its structural parts and 
their interaction, one with the other, can best be understood from a 
consideration of the drawing taken together with the following 
specification. For convenience, elements of this drawing which have 
corresponding elements in the drawing of FIG. 1 of U.S. Pat. No. 3,783,178 
have been given corresponding element numbers. 100 represents the 
expansion fitting generally which serves to couple together two sections 
of rigid conduit 101 and 105. It should be understood that a modified 
version of the expansion fitting 100 could be employed for coupling a 
section of rigid conduit 101 to some other member, such as a wall member 
(not shown) instead of to a second section of rigid conduit 105. The 
expansion fitting 100 includes a generally cylindrical body section 110 
which, at one end 111, is suitably formed to facilitate connection to 
rigid conduit 105 or some other member as may be expedient. The method of 
joining the conduit 105 or other member to the end 111 of body section 110 
will normally comprise a technique which does not admit of relative 
longitudinal motion between the two elements after the joining has been 
completed. Such technique may involve an external thread; an internal 
thread, as illustrated, or might comprise any suitable form of fitting, 
such as welding, soldering or brazing or joined flanges, etc. The joint 
will typically comprise a technique through which gases interior of the 
fitting 100 and subjected to greater than atmospheric pressure may be able 
to escape in a slow and controlled manner. 
Although such would not normally be the case, the coupling at end 111 of 
the body section 110 could be of the same general character as the type to 
be described with respect to the end 112 of the body section 110. 
The end 112 of the body section 110 is threaded with threads 113 to the 
facilitate the coupling of the head 120 to the body section 110. As may be 
seen, the head member 120 has a first recessed area 121 which is adapted 
to retain an O-ring 122 which may be coated with a silicon grease. The 
dimension of the recess 121 and the O-ring 122 inserted therein are such 
that when a section of rigid conduit 101 is inserted through the opening 
of the head 120, there will be a sealing connection between the O-ring 122 
and the conduit 101 for excluding water and/or atmospheric contaminants 
from entering the interior of the body section 110. The recess 121 has a 
dimension parallel to the longitudinal direction of the rigid conduit 101 
which is greater than the diameter of the material of the O-ring 122 in 
order to facilitate compression and/or rolling motion of the O-ring 122 in 
response to relative longitudinal motion between the rigid conduit 101 and 
the head 120. 
There is another recess 123 in the head 120 which serves a function to be 
described herein below. The interior of the head 120 is threaded to mate 
with the threads 113 of the body section 110. It should be observed that 
the recess 123 has a lesser diameter than the threaded section of the head 
120 thereby forming a wall, or step, 124 between the recess 123 and the 
threaded section of the head 120. Accordingly, it will be evident that it 
will be impossible to thread the body section 110 and the end 112 thereof, 
into the head 120 any further than the wall 124. Accordingly, the recess 
123 cannot be reduced below a predetermined volume which is a function of 
the various dimensions. Contained within the recess 123 is a grounding 
ring 130 which may be of the type more fully described in the cited 
patent. Surrounding a portion of the grounding ring 130 is a compression 
grommet 131 which fits into the recess 123. Between the compression 
grommet 131 and the end 112 of the body section 110 is a pressure ring 
132. As the body section 110 is screwed into the head section 120, the end 
112 of the body section 110 will push the pressure ring 132 against the 
compression grommet 131 thereby squeezing the compression grommet 131 into 
the recess 123 and increasing the pressure of the grounding ring 130 
against the rigid conduit 101 and against the wall 125 of the recess 123. 
By controlling the dimension of the recess 123 and the dimensions and 
material of the compression grommet 131, it is possible to control the 
maximum pressure that will be applied between the grounding ring 130 and 
the conduit 101. 
The body member 110 is shown with a discontinuity in order to illustrate 
the fact that the interior chamber may have any desired length between 
ends 111 and 112. 
The interior chamber 115 includes surface 116 which is normally cylindrical 
and reamed to a relatively smooth and controlled diameter. 
The conduit 101 has threads 102 at the end thereof which is inserted 
through a passage in the head 120 into the body member 110. After the 
conduit is passed through the head 120, a bushing 140, which has threads 
for mating with threads 102 of the conduit 101, is coupled to the conduit 
101. Thereafter the head 120 is coupled to the body member 110 by threads 
113. The bushing 140 may be coupled to the conduit 101 by means of a 
wrench placed on the flats 141 which comprise a portion of the bushing 
140. The major portion 142 of the bushing 140 is configured to fit within 
the body 110 with relatively little clearance therebetween. The difference 
in the diameters may be of the order of 0.002 inches. The major portion 
142 of the bushing 140 will have a width W which will control a 
characteristic of the fitting 100 to be described more fully hereinbelow. 
From the description, as thus far described, it will be seen that the 
bushing 140 may move back and forth within the interior 115 of the body 
member 110 in a manner somewhat analagous to that with which a piston 
moves within its chamber. The bushing 140 divides the chamber in the body 
member 110 into two spaced subchambers and provides a through passage 160 
therebetween. 
It will be appreciated that the conduits 101 and 105 are hollow and that 
the bushing 140 has a central passageway therethrough in order to 
accommodate conductors (not shown) which may be fed through the conduits 
101 and 105. The conduits 101 and/or 105 may couple to other conduit 
members and ultimately be open to atmospheres which may differ materially 
from that surrounding the expansion fitting 100. Thus, there may be an 
atmosphere 151 transmitted through the conduit 101 to the chamber 115 
and/or an atmosphere 152 transmitted through conduit 105 to the chamber 
115. Either of these atmospheres 151 and/or 152 may or may not differ 
materially from the atmosphere 153 surrounding the fitting 100. If there 
is a possibility that the atmosphere 153 could comprise an explosive 
atmosphere, it could be extremely dangerous to have an atmosphere within 
the chamber 115 ignited by any set of normal or abnormal circumstances if 
the hot gases resulting from such explosion within the chamber 115 could 
escape to the atmosphere 153 at a temperature which could ignite the 
atomsphere 153. The present structure is provided to protect against this 
untoward circumstance and to render any escaping gases incapable of 
igniting the atmosphere 153. First and foremost, the expansion fitting 100 
and the conduits 101 and 105, and all associated parts, must be 
constructed of material which is sufficiently strong to prevent rupture in 
response to an internal explosion. Furthermore, the hazardous location 
expansion fitting 100 must be constructed in a manner to prevent the 
escape of gases from the chamber 115 at a temperature sufficiently high to 
ignite the atmosphere 153. 
There are two general escape routes for gases in the chamber 115 to the 
atmosphere 153. These are through the threads joining conduit 105 with end 
111 of body member 110 and between the bushing 140 and the body 110 and 
subsequently through any of a variety of escape routes to the atmosphere 
153. The escape through he threaded joint is recognized as being capable 
of permitting such a slow escape as to be able to cool the escaping gases 
below a temperature which would be capable of igniting the atmosphere 153. 
This is because the end 111 of the body member 110 has sufficient mass 
that it will be close to the ambient temperature of the atmospheres 153 
and therefore, because of the relatively slow progress of the escaping 
gases through the threaded connection, such escaping gases will be cooled 
to near that of the ambient atmosphere 153 by the time they escape to the 
atmosphere 153. With the temperature reduced, as described, the escaping 
gases cannot ignite an explosive atmosphere 153. It will be recalled that 
the bushing 140 has a diameter which is only slightly less than the 
internal diameter of the chamber 115 and therefore, any hot gases within 
the chamber 115 which escape through the space (the leakage control 
passage) separating the bushing 140 and the body member 110 will pass 
therebetween relatively slowly and have ample time to be cooled by the 
ambient temperature of the body member 110 provided the longitudinal 
length W of the bushing 140 is sufficient. That is, the longitudinal 
length W of the bushing 140 must have a value which is sufficient to 
assure reduction of the temperature of escaping gases. The width which may 
be required will be a function of several factors including, but not 
limited to, the possible nature and temperature of the atmosphere 153, the 
maximum pressure differential between the chamber 115 and the atmosphere 
153, the possible temperature of gases within the chamber 115 and the 
maximum spacing which may exist between the bushing 140 and the wall 116 
of the body member 110. The maximum temperature and pressure differential 
are most likely to exist subsequent to ignition of gases in chamber 115. 
The relationship of the bushing 140 with the interior surface 116 of the 
body member 110 was previously alluded to as being analogous to that of a 
piston in its chamber. Indeed, the fitting 110 could be fabricated with 
the bushing 140 having piston rings somewhat in the manner of the pistons 
of an automobile. The piston rings could comprise a set of closed rings 
and/or a spiral ring. In either event, the rings must be formed and shaped 
to prevent the rapid transmission of gases from the chamber 115 towards 
the exterior atmosphere 153. Piston rings, if used, could float in radial 
grooves on the bushing 140 and be biased radially outward. 
As thus far described, the bushing 140 and the interior of the body member 
110 are cylindrical. With this shape, it is evident that the conduit 101 
could have rotational motion relative to the conduit 105. If such motion 
is not to be encountered and/or may not be tolerated, the bushing 140 (or 
the piston rings, if used) could have a mating relationship with the 
interior surface 116 of the body member which is other than cylindrical. 
As previously stated, the separation between the outer surface 142 of the 
bushing 140 and the interior surface 116 of the body member 110 should be 
of the order of 0.002 inches. The surfaces should be smooth and would 
normally be lightly lubricated to facilitate the required sliding motion 
and to inhibit rust. If desired, or required by other circumstances, the 
bushing 140 and/or surface 116 may have a special finish including 
plating. 
Reference has already been made to U.S. Pat. No. 3,783,178. The cited 
patent teaches the use of a stop collar 242 as shown in FIG. 2 of that 
patent. The fitting 100 could include a similar stop collar if 
circumstances and the operating conditions thereof warranted. 
It will be appreciated that an alternate structure could be fabricated 
wherein the conduit 101 has a carefully controlled diameter as would a 
mating surface of the head 120. This might be used in addition to, or in 
place of, the mating relationship of surfaces 142 and 116. However, the 
mating relationship of the conduit and head exposes the conduit to 
possibly corrosive atmospheres and/or mechanical damage. Accordingly, use 
of the bushing 140 are reamed surface 116 is a preferred embodiment. 
While there has been shown and described what is considered at present to 
be the preferred embodiment of the invention, modifications thereto will 
readily occur to those skilled in the related arts. For example, in 
another structure, relative proportions could be modified, and other 
grounding techniques might be employed. It is believed that no further 
analysis or description is required and that the foregoing so fully 
reveals the gist of the present invention that those skilled in the 
applicable arts can adapt it to meet the exigencies of their specific 
requirements. It is not desired, therefore, that the invention be limited 
to the embodiments shown and described, and it is intended to cover in the 
appended claims all such modification as fall within the true spirit and 
scope of the invention.