Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
W~92/1772~ PCT/US92/026~
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ELBOW FITTINGS FOR
DOUBLE CONTAINMENT PIPE ASSEMBLIES
BACKGROUND OF THE INVENTION
F.ield of the Invention:
The present invention relates to double containment
pipe fittings and their assembly.
Descri~tion of the Prior Art:
The general concept of providing a double or dual
containment pipe system wherein an inner carrier pipe is
concentrically located within an outer containment pipe
to deliver dangerous or hazardous fluids is well known
and an accepted commercial practice. Historical
applications for such systems have been found in the
nuclear, gas petroleum production and refining and
chemical processing industries. The inner pipe is used
to transport the hazardous or toxic fluid while the outer
pipe is present to confine any leaks. Thus, it is also
known to provide the annulus between the concentric pipes
20~ with various types of detectors and/or drainage apparatus
to handle leakage. Examples of doubIe containment pipe
assemblies are shown in U.S. Patent Nos. 4,786,088;
4,886,305 and 4,930,544.
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With the advent of stricter governmental regulation
concerning the piping of petroleum products and hazardous
chemicals, the anticipated increased use of various types
of pipes in double containment applications is a
certainty. As such, the structural design of these
fittings and method of installing and repair of double
containment pipes containing such fittings is necessary.
The present invention provides novel doublP
containment fittings for forming a double containment
pipe joint. The products that have been developed to
date have not completely addressed some of the unique
problems that arise when a pipe is placed within another
pipe.
One such problem is the ease of maintaining,
modifying or repairing such systems. To date, all above
ground pressure systems have been installed without the
ability to assemble or disassemble the piping or
components in modular sections. Therefore, if a repair
is to be made, the original system manufacturer/
contractor must be called in to facilitate the repair.
In many designs, a repair of a certain section would not
be feasible or even possible due to the location of the
system with respect to adjacent equipment or building
parts. What would be very helpful to most facility
owners is the ability to have a modular system that is
capable of being readily disassembled. A system capable
of being readily disassembled would give the facility
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engineer the ability to have the section requiring
modification or repair sent to a shop type environment
readily suitable and equipped to facilitate the change or
repair.
Another such problem has to do with the ability of a
system to withstand the effects of inner and outer piping
and components that are subjected to different amounts of
thermal expansion and contraction. It is the norm,
rather than the exception, that the inner and outer pipes
of a pressure rated double containment piping system are
sub~ected to different amounts of thermal expansion.
This situation may arise in several different ways. The
most common way involves the situation whereby a hot
fluid is transported through the inner pipe. Under this
circumstance, the external environment (external that is
to the outside diameter of the secondary containment
piping) is normally at a lower temperature than the hot
fluid. Since there is either an illsulating dead air
space between the two pipes, or other insulating
material, the inner piping temperature becomes close to
that of the fluid, while the outside piping remains
closer to that of the external ambient environment.
Therefore, the materials normally grow to different
lengths due to their being at different temperatures.
When this does~occur, there are thermal strains that are
imposed on interconnecting parts and on parts such as
interstitial supports that create a contact point between
the inner and outer piping. The most obvious place where
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there is a problem potential is at the interconnecting
points. However, any place where loads can be
transmitted back and forth between the two pipe systems
can result in a problem. An example would involve inner
and outer elbows where contact is caused between the two
due to differential thermal expansion. When this occurs,
both the inner and outer elbows will impose stresses and
strains upon each other. With the exception of the
development described in this application, all
interconnecting parts designed for this situation are
constructed of a singular material, with such a design
that the residual stresses and subsequent strain on the
materials can lead to a failure of such parts. These
existing parts then become the point in the overall
system at which failure is most likely to occur. This is
compounded by the fact that the components are truly
single containment at that point, without having a
containment area to prevent the hazardous fluids from
reaching the environment. Therefore, exactly where it
would be the least desirable location for failure to
occur is the most likely place for failure to occur.
Additionally, a double containment piping system that is
constructed of an interconnecting part as described
above, actually increases the chance that failure to the
external environment can occur as compared to a singular
containment piping system designed with proper treatment
of the ther~al expansion of its components.
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Another problem with existing systems has to do with
the ability to combine different inner and outer pipe
materials in an efficient manner. Different materials
can mean materials that are of a different class, (such
as metallic-thermoplastic, metallic-reinforced
thermosetting plastic, or thermoplastic-reinforced
thermosetting plastic), or materials that are within the
same class but constitute a different material, (such as
within the thermoplastic family, combining a
fluoropolymer within a polyolefin, or a polyolefin within
another polyolefin). The reason that it is desirable to
combine materials typically has to do with economics. It
is desirable in many situations to combine an expensive
material that is capable of handling a chemical on a full
time basis within a less expensive material capable of
withstanding the corrosive effects of a chemical for a
limited period of time. Another major economic reason
has to do with the use of a material for the outside
piping capable of withstanding the corrosive effects of
atmospheric conditions, thus eliminating the need for
expensive coatings, cathodic protection, etc. A typical
example of this would be in combining a metallic material
within a nonmetallic outside material for the reasons
just described.
~5 Yet another reason involves the selection of a
combination of materials due to structural requirements.
(~.g., the placement of a non-metal within a metal piping
due to heavy burial loads.)
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Piping elbows have been used ever since the first
pipe systems were used, and are considered to be a basic
component of the piping system. Standard elbow patterns
have been created and established that are used in the
majority of all piping applications. For butt-welded
systems, the standard elbow patterns involve short radius
elbows (radius of the elbow is approximately equal to the
nominal diameter of the pipe), long radius elbows (radius
of the elbow is approximately equal to 1-1/2 times the
nominal diameter of the pipe) and for sanitary
applications elbows that have a radius equal to
approximately four times the nominal diameter. While
these elbows have long served a purpose for singular
piping systems, the use of standard size elbows in double
containment piping systems present many potential
problems.
One such potential problem has; to do with certain
combinations where the selection oi standard elbow
patterns result in dissimilar radii. If the difference
is large enough, the elbow may not be capable of being
readily fabricated into an inner and outer arrangement,
while maintaining the concentricity of the pipes leading
into the elbow. Even if the elbows do fit, they will
likely not be positioned in a perfectly concentric
arrangement when placed in the outar elbow. That will
mean that the inner elbow will be positioned in a less
than ideal space relationship with respect to the outer.
The allowable room for movement will therefore be less
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than the ideal relationship, where the inner elbow is
perfectly positioned at any one given pipe within the
outer. If the elbows contact each other due to a
di~ferential movement of the elbows, severe problems can
occur. The inner and outer elbows will be subjected to a
displacement strain and will experience a concentration
in stress. The outer elbow may fail under this strain,
creating a breach in the secondary containment. The
inner elbow may also subsequently occur, resulting in a
breach of the primary containment and a subsequent double
failure. Even if the elbow itself doesn't fail, failure
can result at joints, or even in straight pipe sections
due to excessive bending and torsional loads. These can
also lead to a double failure.
There are also issues of the evenness of heat
transfer and the resistance to potential annular flow.
Each of these engineering aspects are somewhat
compromised by the use of combinations of "standard"
inner and outer elbows as well.
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S~I~ARY OF THE INVENTION
The elbow system employed in the present invention
has many benefits over the current practice of using
existing standard fittings for attachment to their
respective pipes, as shown for example in U.S. Patent
Nos. 4,786,088, 4,886,305 and 4,930,544.
Perhaps the most important is that, because the
matching inner and outer elbows are decigned so that they
maintain their concentricity throughout the bend, there
is maximum allowable space for the elbow in both the
direction of thermal expansion and contraction. In most
piping systems, the pipes are subjected to both thermal
expansion and contraction due to process and ambient
cyclic conditions. Double containment piping is further
complicated because both pipes are subject to different
movements in a simultaneous fashion. Therefore, it is
the differential movement in a double containment piping
system that sets the needed space. The configuration
that has been determined to be the most efficient
maximizes the allowable space in both directions, and it
is e~ual throughout. The likely points of contact are
all at the same optimum distance from the centerline of
the elbows.
Further, due to this configuration, an equal amount
of heat transfer is maintained throughout the elbow
section.
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Finally, the flow of air or other fluid through the
annulus or space between the inner or outer pipes is
maximized and thus, the frictional losses (resistance to
flow) will be minimized. This is vitally important
because flow is poor to begin with through double
containment piping. If resistances to flow is high, it
will require a high pressurP to cause flow. A high
pressure might collapse the primary piping due to
exceeding its collapse pressure. Therefore, the
lo resistances to flow in the annulus should be minimized in
each component in order to cut down on the required
flushing pressure to be used during flushing and cleaning
procedures.
A standard radius pattern can be used as the basis
lS for the primary and secondary containment fittings. This
will allow some conformity to conventional elbows,
although the conventional elbow styles are not intended
to produce adequate flow conditions in a double
containment piping system.
The elbow concept of the present invention also has
benefits beyond its technical advantages. It also allows
the double containment piping assembly to be assembled
and welded (bonded) together in a much more efficient and
easy manner. All elbows could be field assembled and
~5 provide modular sections which can be individually
repaired, replaced and inspected.
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In accordance with the present invention, the angle
of the sector of both the inner or primary pipe and the
outer or containment pipe of both fittings should be
equal. The annular space between the fittings should be
consistent or the same throughout the curvature and
length of the fittings. Either end of the inner and/or
outer fittings or both ends of one of the fittings can be
provided with a straight portion extending beyond the end
of the other fitting. This will save several steps in
the overall fabrication of the double containment pipe
assembly in that it allows the ability to use the same
length of inner and outer pipes between elbows, or
between elbows and other fittings.
Because of the concentric nature of the primary and
outer or containment elbow fittin~s, along with the
consistent annular spacing between the primary and
containment portions of the elbow fittings, potential
problems due to thermal expansion and contraction of the
inner and outer pipes is minimized, as potential contact
of the pipes is minimized; but heat transfer throughout
the elbow section is maximized and the flow through the
annulus or space between the inner and outer pipes is
maximized because the resistance to flow is minimized.
Further, the design permits utilization of different
materials for the inner and outer pipes and allows for
complete field fabrication.
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BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantages of the invention will
become more apparent from the following description and
claims, and from the accompanying drawings, wherein:
FIGURE 1 illustrates the concentric elbow pipe
fittings used to form the double containment pipe
assembly of the present invention, with portions
indicated in phantom lines comprising optional straight
line extensions of the fittings enabling ease of field or
shop fabrication of the elbow fittings in the double
containment pipe assembly;
FIGURE 2 illustrates another form of the concentric
elbow pipe fittings;
FIGURE 3 is a cross-sectional view illustrating the
use of a pair of elbow pipe fittings of PIGURE 1 in a
double containment pipe assembly of the present
invention;
FIGURE 4 is a cross-sectional Vi8W illustrating the
~use of a pair of elbow pipe fittings of FIGURE 2 in a
double containment pipe assembly of the present
invention;
FIGURE 5 is a cross-sectional view illustrating a
concentric elbow pipe fitting wherein the inner pipe has
straight line extensions at either end; and
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FIGURE 6 is a cross-sectional view illustrating the
use of the el~ow fitting of FIGURE 5.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings in detail, wherein
li~e numerals indicate like elements throughout the
several views, the elbow fittings of the present
invention are indicated by the numeral 10 and 12 and are
used as a connection between straight portions of a
double containment pipe assembly (not shown).
The outer or containment pipe elbow lo and the inner
or primary elbow 12 have the same radius of curvature or
occupy the same angle e of the sector of a circle and are
concentrically located with the inner or primary pipe in
the outer or containment pipe 12 so that the annular
space between the outer surface of the inner pipe 12 and
the inner surface o~ the outer pipe 10 is constant, or
the same throughout the radius of curvature or sector.
The end face 14 of the inner pipe 12 and the end
face 16 of the outer pipe 10 are adapted to be connected
to the inner and outer corresponding straight sections of
a double containment pipe assembly, respectively. This
can be accomplished in a number of manners, but, the most
usual way is to butt weld the straight section of each
pipe to the end face 14,1~, respectively, as illustrated
in FIG~RES 3 and 6. If desired, the inner and outer
pipQS 10' ,12' (FIG. 2) may be formed with sockets or
2~ flanges 14',16' along their end face extending in the
direction of the pipe to be joined, for nesting
.
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engagement with the inner and outer pipes, as illustrated
in FIGURE 4. In some instances, this will ease the
fabrication of the double containment pipe assembly,
particularly in the field.
Optionally, either end of the inner or outer pipe
fitting elbow 12,10, respectively, may be formed with a
straight extension portion 18 and 20, respectively.
E.g., in FIGURES 5 and 6, the inner pipe fitting elbow
has straight extensions 18. It is desirable that the
straight portions of the inner section be at one end and
the straight portions of the outer section at the other,
or vice versa, (e.g., see straight sections 18',20' of
the elbow fittings of FIG. 2), or the straight sections
be at both ends o~ the inner or outer pipes (see FIGURES
5 and 6, for example), so that during fabrication of the
double containment pipe assembly, equal lengths of the
straight pipe sections can be used for the inner and
outer pipes between the elbows and other containment
fittings, as illustrated in FIGURE', 3 and 6. This
enables standardization to be achieved for the system.
It may also greatly enhance the ability of the systems to
be inspected and examined prior to and during testing.
However, the inner/outer pipe can have or be provided
with the straight portions at both of its ends, if
desired, and the standardized system could have one
length of inner and one length of outer pipe.
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By providing separate inner and outer fittings for
the elbow, the material can be matched to the inner and
outer pipe material of the straight length sections
between elbow fittings. Further, the portions of the
double containment assembly between elbow fittings can be
modular in that, if a lea~ occurs, it is only necessary
to repair and replace one section of the pipe assembly.
Utilizing concentric elbow fittings enables the maximum
spacing to be achieved between the fittings so as to
provide for thermal expansion and contraction without
potential contact between the fittings which may cause
disruption or cracking, and by utilizing such a system,
improved flow through the system is achieved along with
more uniform heat transfer.
