Note: Descriptions are shown in the official language in which they were submitted.
36
FLAME ARRESTOR AND METHOD OF MANVFACTURE
TECHNICAL FIELD
This invention relates to a precisely
manufactured flame arrestor and to a simple,
economical method of manufacture that achieves the
precise tolerances required in an arrestor to provide
a flameproof path.
.
:
'~
- ~L2~9V3~i
BACRGR~VND
Flame arresto rs are used in pneumatic control
devices o provide a flameproof path for the
pneumatics into and out of a control or instrument.
Flame arrectors provide a restricted pneumatic
passage over an extended length such that a flame
cannot propagate through the passage.
Several types of arrestors ~re known in the
art. A sintered metal ~rrestor has been commonly
used; however, recently adopted European standards no
longer permit the use of this type of flame
arrestor. Another type is a Lisk arrestor which is
comprised of layers of crimped and flat sheet metal
that are wrapped together and held in a case by a
rivet. Other types include the parallel plate typæs
that usually consist of flat plates with a slot
ground into the plate to a depth equal to the maximum
gap allowed for the length of the flame path. The
plates are then ~tacked and bonded together at the
un~round Aarea to-form ~ series-of-parallel flame
retardant paths. This type o~ construction is quite
expensive due to the grinding of the slot, and the
arrestors are more bul~y due to the solid area that
supports the gap and forms the bond area. Another
type of flame arrestor would be a gear shaped rod
that is pressed into a tube with the ~lameproof path
provided between the qear teeth. ThLS type of
arrestor would be quite expensive as it requires the
machining of the fine gear teeth, and it would
3~ provide a restricted flow area. ~hese types of flame
arrestors are expensive to make and require fine
machining of the various parts.
Since the allowable gap dimensions in small
flame arrestors are very small, manufacture of a
small flame arrestor is particularly difficult and
expensive. muS~ ~ need exists for an easy,
economical method of manufacturing precision flame
arrestors both small and large that would meet the
~ecessary standards here and abroad.
~Z~036
SUMMARY OF THE INVENTION
The present invention teaches a method of precisely
spacing the elements of a flame arrestor and securing that
spacing within the required tolerances to provide a flameproof
path in pneumatic control devices. The precision ground
elements of a flame arrestor are spaced by using a shim
between the elements such that the shim spaces the elements
within the required flameproof tolerances. The shim is gapped
where necessary to allow weld gases to escape. The assembly
is welded in appropriate locations to hold the elements
securely together. The assembly is then heated so as to burn
out or dissolve the shim leaving a controlled flow path
between the elements for pneumatic flow. This provides a
precision-made flame arrestor that meets the required
standards.
According to one embodiment of the invention, the method
taught by the present invention is used to construct a rod and
tube type flame arrestor. A precision ground rod and tube are
used such that the rod will fit inside the tube, and when held
concentrically by the shim, it provides a radial gap within
the required tolerances for a flameproof path. The rod is
welded to the tube and the assembly is heated to dissolve the
shim. The method taught by the present invention may be used
to produce various types and sizes of flame arrestors.
In accordance with one aspect of the invention there is
provided a method of manufacturing a flame arrestor which
comprises: using a multiplicity of precision ground stock
pieces, spacing the pieces with a shim that is of such a
thickness that the spacing between the pieces provided by the
shim is within the necessary tolerances to provide a barrier
to the propagation of a flame through the arrestor, welding
the pieces together, and removing the shim from between the
pieces to leave a gap between the pieces such that the gap is
within the tolerances necessary to provide a flameproof path
through the arrestor.
C~36
4 a
In accordance with another aspect of the invention there
is provided a flame arrestor construction comprising: a
multiplicity of flame arrestor piecesi a removable shim placed
between adjoining pieces, said shim being of a thickness so as
to create a gap between the pieces that will allow air flow
between the pieces when the shim is removed but such gap is
within the tolerances needed to prevent the propagation of a
flame through the gap; and adjoining pieces being welded
together at at least one point, said weld securely holding the
pieces together.
~Z~ 6
BRIEF DESCRIPTION OF THE DR~WINGS
For a more complete understanding of the present
in~ention and other advantages of it, a Detailed
Description is given in conjunction with the
accompanying Drawings, in which like reference
characters denote like parts in all views and
wherein:
FIGURE 1 is An exploded view of a rod and tube
flame nrrestor 6howin~ the rod with a shim wrapped
~o circumferentially ~round the rod at each end being
inserted into the tube;
FIGURE 2 shows a perspective view of a rod and
tube flame arrestor;
FIGVRE 3 shows a longitudinal cross-sectional
view along line 3-3 showing a rod welded inside a
tube: and
FIGURE 4 shows an end view of a rod and tube
flame arrestor.
:LZ~C~6
DETPlLED DESCRIPTION
The present invention relates to a flame
arrestor ~nd method of manufacturing the same that is
economical and easy to machine and to assemble while
maintaining the precise tolerances necessary ~o make
the arrestor flameproof. The invention relates
flame arrestors that can be used in pneumatic control
devices ~o prevent ~he propagation of a flame.
Particularly, the invention relates ~o a rod and tube
type fl~me arrestor, but the method of manufacturing
can also be applied to other types of arrestors such
~s the parallel plate type.
One embodiment of tbe present invention is a
method for making a rod and tube type flame
arrestor. The dimensions of the rod and tube to be
used in the invention must be ~uch that the diameter
o4 the rod is less than the inner diameter of the
tube, and the difference between the diameter of the
rod and the inner diameter of the tube is within the
required tolerances for a flameproof path. This
difference includes both the diameter difference and
radial dif~erence when the rod is held almost
concentrical within the tube. Precision ground stock
is used for the rod and tube with the rod being
precision ground on the outer surface and the tube
being precision ground on the inner surface. The rod
10 is prepared as shown in FIGURE 1 with a shim 12
wrspped cir~umferentially around rod 10 ~ear both
ends, or alternatively in the middle, leaving gaps 14
in the shim to allow weld gases to e~cape. The shim
is preferably of a material that will decompose when
heated in a nonoxidizing environment or dissolve in a
solvent. Most preferably, the shim i~ a pl~stic
' ~Z9~ 6
tape. me ~him 12 is wrapped around rod 10 in such a
manner as to leave a free weld zone 16 in the center
of the surface of rod 10. Rod 10, with the ~him
~ttached, is then inserted inside tube 20. The shim
12 is of such a thickness in relation to the diameter
difference between rod 10 and t~e inside of tube 20
that when the rod with the ~him a~tached is inserted
into the tube, the rod is held practically c~ncentric
inside the tube by the ~him 12.
As shown in FIGURE 2, the tube 20 is then
welded, pre~erably using ~n electron beam welder 22,
to rod 10 at two spots 32 in the free weld zone 16
that are preferably about 180~ apart. ~Alternatively,
the welding may be done in the center as illustrated
~5 in Figure 2 or at the ends of the rod and tube. A
cross-sectional view along the line 3-3 as shown in
FIGURE 3 shows the two welds 32 in the middle section
of the assembly about 1B0 apart.
After the welding, the shim is removed by one of
several methods. The assembly may be heated in a
nonoxidizing environment such ~s hydrogen, nitrogen,
argon or even a vacuum to decompose the shim.
Another method of removing the shim woul~ be to
immerse the assembly in a solvent includin~ organic
solvents and acids. Preferably the welded assembly
is heated in a hydrogen atmosphere at 1200-F for 20
minutes t9 decompose the shim 12. The assembly then
may be ultrasonically cleAned to remove any
residue. This leaves the rod 10 held ne~rly
concentrically within tube 20 by the welds 32 such
that the spacing between the rod and the tube allows
for pneumatic ~low but is within the critical
di~meter and radial gap tolerances necessary to meet
8 ~2~9C~
prescribed standards to provide a flameproof path.
- FIGUR~ 4 shows an end view of the assembled rvd and
tube flame arrestor with a uniform circumferential
air gap 34 between rod 10 and tube 20. The assembly
is designed such ~ha~ gap 34 is less than the maximum
~llowable radial and diameter differences as ~nown in
the art for providing a flameproof path a~ross the
length of the ~rrestor.
In a flame arrestor, ~he critical fea~ure is the
spacing between ~he pieces such that it ~llows ~or
the flow of gas through the device but prevents the
propagation of a flame. The volume flow o~ the gas
and the length of the flame arrestor determine the
critical spacing requirements. These standards are
well known in the industry and are established by
regulations in some countries. As one~example,
European Standards provide that for a volume of 400
cubic centimeters and a rod and tube arrestor having
~ length grea~er than .374 inches ~9.5 mm), the
diameter difference between the rod and the tu~e can
-be only .00591 inches (.15 ~m) maximum, nd the
radial gap cannot be greater than .00394 in~hes (.10
mm). (European Standards EN50018).
As an example of one construction of t~e
invention that meets these precise requirements, a
rod of length .4 inches is precision ground to a
diameter of .1842 ~ .0001 inchesO The inside
diameter of a tube is reamed and honed to o1898 ~
.0002 inches. m is maintains ~ diameter difference
equal to or le s than .00591 inches ~s required by
EN50018. The tube is cut at a length of .402 inches
to ~nsure a flame path the ~ull length of the rod. A
plastic tape with a thickness of .0025 lnches and
.
03~j
width of .080 inches serves as the ~him and is
wrapped circumferen~ially around the rod near both
ends of the rod leaving a gap of .04 inches in the
wrap to allow weld gases to escape. A shim of this
thickness will hold the rod practically concentric
within the tube as ~ diameter gap of only ~0009
exists between ~he tube, the shim and ~he rod. With
a radial gap standard of .003~4 inches, the tolerance
in the concentricity of the rod with~n the tube is
.001 inches. The selected shim, therefore,
adequately ~paces the rod within the tube. ~olding
the rod to this degree of concentrici~y within the
tube is critical in order to provide the maximum
amount of flow area between the rod and tube while
still meeting the radial gap requirements. In the
construction just mentioned, the radial gap between
the rod and tube is .0028 + .0002 inches if the rod
was held perfectly concentric within the tube. A
diameter gap of .00091 with the shim inserted allows
the rod to be held within the tube such that~the
maximum radial gap that could occur is less than the
.00394 inches maximum radial gap allowed by the
European Standards. To complete the construction,
the tube is electron beam welded to the rod at two
spots in the center of the tube, or al~ernatively at
the ends, approY.imately 180~ apart. The assembly is
then heated to a temperature sufficient to dissolve
the plastic shim.
Since the llowable gap dimensions are very
3~ small, manufacture of such a small flame ~rrestor
usin~ previously known methods is difficult. The
present method of manufacture allows for the precise
manufacture of even small fl~ame arrestors. As the
10 ~Z~
size of the flame ~rrestor increases, a shim with the
appropriate ~hickness ~ay be selected. The inser~ion
of a removable shim provides an inexpensive means to
temporarily fix ~nd hold the pieces of the flame
arrestor to the required precise gap dimensions.
Welding ~he pieces together then penmanently fixes
the precise dimensions, and the weld resul~s in only
a ~mall area that interferes wi~h the flow ~hrough
the arrestor.
1o The u~e of a removable shim in the method of
manufacture just described pro~des an economical
means of holding ~he rod concentrically within the
tube to the required degree of tolerance. The
present invention may be used to construct a
nonconcentric rod and ~ube flame arrestor, but a
concentric construction is preferred as ~he
concentric configuration provides a greater flow area
for the same radial gap requirement. If a
nonconcentric configuration in which the rod
contacted one side of the tube was used, the diameter
of the rod would have to-be increased-from the
concentric rod diameter in order to meet the radial
gap requirementO A larger rod provides less flow
area between the rod and the tube requirin~ a greater
pressure to achieve the same flow rate as a
con~entric configuration allows. Using the same
: dimensions and requirements as illustrated above, the
flow rate for the concentric construction is more
than three times greater than the flow rate for the
nonconcentri~arrestor with the larger rod for the
~ame pressure drop acroRs the arrestors~ The present
~nvention, therefore,~provides ~ means for
positioning the rod within the tube that allows for a
'
.
1 1 12~9~336
smaller di~meter rod to be used thereby allowing a
greater gas flow rate through the flame arrestor.
This method of manufacture can also be used ~o
construct various types and sizes of flame
arrestors. For example, to construct ~ parallel
plate type of flame arrestor, precision ground plates
are alternatively stacked with sheets of the shim
material. Spaces may be lef~ in the shim ~o provide
a weld zone. The pl~tes may be welded together as
e~ch successive plate i8 stac~ed on the underlying
plate, or the plates may be welded nt the ends after
all the plates have been stacked together. ~he
assembly then may be heated to remove the Yhim.
Again, the shim must not have a thlcknes3 greater
1~ than that allowed gap for ~ flameproof path through a
parallel plate arrestor. The method of inserting a
shim, welding the pieces together and removing the
shim allows for precision manufacture of flame
arrestors. The method of manufacturing taught by the
present invention achieves these tolerances in an
-economical~and-simple manner without requiring the
fine machining of many parts.
As will be apparent to persons skilled in the
art, various modifications, adaptations and
variations of the foregoing specific disclosure can
be made without departing ~rom the teachings of the
present invention.
