Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to pipe joints.
The conventional design for a pipe joint utilises
flanges at each end oE the pipes to be joined, which flanyes are
bolted together~ A suitable gasket is loaded between the flanges
-to effect the seal. The design requires high strength bolts and
involves high bolting loads and high gasket loads to be effective.
When such conventional bolted flange joints are utilised for low
temperature locations, a particular problem is found during start~
up operations.
When a joint at room temperature is cooled to very low
temperatures such as, for example, at the operating temperatures
of liquefied natural gas vaporisers, the flanges cool down and
shrink more rapidly than the bolts. This means that the bolts
no longer grip the flanges tightly enough, the joint loosens and
le~ks occur. One way of minimising this effect is to utilise
~olts which have an extremely low coefficient oE expansion so as
to maint~in bolt tension at all times. Unfortunately alloys which
h~ve low coefficients of expansion are expensive and this means
that tl~e joints become expensive. After a certain period of
time when the whole joint has cooled down, conventional bolted
flanges stop leaking since the shrinkage of the bolts tightens
the joint to effect a seal.
Although the invention is particularly concerned with
the provision of pipe joints for use in connection with pipes
used at very low temperatures, the joint may be used at other
temperatures.
As an alternative to conventional gasketted joints in
which the gasket is located between the flanges, a joint has bPen
prop~sed (see example British Patent 757336) which utilises a
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cylindrical sealing member which has tapered faces which cooperate with
corresponding tapered faces on the pipe ends. To effect the jointJ the pipe
ends are forced together to distort the sealing member against the tapered
faces on the pipe ends. Again, the joint is effected by bringing together
the pipe ends to produce the seal.
By the present invention there is provided a pipe joint comprising
a pair of pipe ends and a metal sealing member interconnecting the pipe ends
and forming a fluid-tight seal therebetween, characterised in that the metal
sealing member is a substantially cylindrical tubular member sealingly con-
nected to one of the pipe ends and extending into a substantially cylindricalinner sealing suface on the other pipe end, the distal portion of the cylin-
drical tubular member being an interference fit with the inner sealing sur-
'
face, and the cylindrical tubular member being flexible at its distal end so
as to be pressed radially outwardly, in use, wlder the pressure of fluid
within the pipe ends, to effect a seal between the pipe ends.
The present invention further provides a pipe joint comprising a
pair of pipe ends and a metal sealing member extending into both pipe ends
forming a fluid-tight seal therebetween, characterised in that each of the
pipe ends has a cylindrical inner sealing surface and in that the metal
sealing member is an in~egral substantially cylindrical tubular member com-
prising an integral pair of frusto-conical portions conjoined at their smaller
diameter ends so that the external diameter of the distal portions of the cyl-
indrical tubular member engageable with the inner sealing surfaces is slightl~
greater than the external diameter of the central portion of the cylindrical
tubular member, each of the distal portions of the cylindrical tubular member
being an interference fit with one of the inner sealing surfaces, the cylin-
drical tubular member being flexible at its ends so as to be pressed outwardly,
in use, under the pressure of fluid within the pipe ends to effect a seal
between the pipe ends.
The present invention still further provides a pipe joint comprising
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a pair of pipe ends and a metal sealing member extending into both pipe ends
forming a fluid-tight seal therebetween, characterised in that each of the
pipe ends has a cylindrical inner sealing surface and in that the me~al seal-
ing member is an integral substantially cylindrical tubular member extending
into the pipe endsJ the cylindrical tubular member having a central external
annular ridge and flexible distal portions, the external diameter of the dis-
tal portions being slightly greater than the external diameter of the central
portion of the cylindrical tubular member excluding the annular ridge thereon,
each flexible distal portion being telescopically accommodated within one of
the pipe ends, the tip of each distal portion being an interference fit with
: the inner sealing surface, and the distal portions of the cylindrical tubular
member being pressed radially outwardly under pressure, in use, of fluid
. within the pipe ends to effect a seal between the pipe ends.
. By way of example, embodiments of the present invention will now
be described with reference to the accompanying drawings of which:
Figure 1 is a cross-section of one form of joint in accordance
with the invention;
Figure 2A is a cross-section of an alternative form of joint in
accordance with the invention;
Figure 2B is a cross-section of a prior art joint; and
;:~ Pigure 3 is a cross-section of a third form of joint in accordance
with the invention.
The joint `is made between the two ends of pipes 1 and 2~ A small
flange 3 i5 welded to the end of pipe 1 and a corresponding flange ~ is weld-
ed to the end of pipe 2. The internal bores of the ends 1 and 2 are accur-
atel~ machined to form cylindrical recesses 5 and 6 which have an accurate
diameter and which also have a smooth surface~ Located inside the recesses
5 and 6 is a tubular cylinder 7 which forms the actual seal~ The cylinder 7
is sufficiently flexible as to be able to expand radially under ~he action of
internal pressure~ On the outer surface of the cylinder 7 is an annular
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ridge 8 whîch serves to locate the cylinder accurately within the recesses
5 and 6. The ridge 8 also forms a rigid centre portion so that the outer
portions can flex away from the central portion. The ~ips 9, 10 of the cylin-
der at its distal ends are of slightly greater diameter than the portion 11
when the cylinder is outside the pipe ends. The
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ridge 8 has a peripheral groove 12 to facilitate withdrawal of
the cylindrical portion from one end of the pipes when the joint
is dismantled. Since the tips 9 and 10 are an interference fit
within the recesses 6 and 5, the cylinder would be difficult to
remove if the groove were not provided to enable sufficient pur-
chase to be obtained on the ridge 8. The flanges 3 and 4 have
an annular recess 13 to accommodate the annular flange 8. The
joint is simply assembled by locating the cylinder 7 inside the
end of one pipe, either by hand or with the aid of a suitable
10 tool, and then locating the other pipe over the free end of the
cylinder. Bolts such as bolts 10 are then inserted through holes
in the flange to keep the ends of the pipe in engagement one with
the other. The seal then simply operates by being expanded out-
wardly by the internal pressure of the fluid within the pipe.
The material from which the cylinder 7 is manufactured
preferably has the same coefficient of thermal expansion as the
material of the pipes. If this is so, the seal remains effective
irrespective of temperature changes~ For best results, the mat-
erial of the seal is the same as the material of the pipe ends
so that the coefficients of expansion are identical. When the
cylinder is placed into the pipe ends, it immediately forms a
seal because of the interference contact between the outer side
of the tips 9 and 10 and the inner diameters 5 and 6 of the pipe
ends. Increasing the pressure inside the pipe merely increases
the sealing effect. Because of this, the bolts 14 merely have
to retain the structure in place and accommodate the internal
pressures and do not have to provide further sealing pressures.
As a.result, many fewer bolts are needed when compared with a
conventional bolted flange joint and these bolts may themselves
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be smaller, weaker and cheaper.
Because the flexible sealing member is thin, it trans-
mits heat rapidly through its walls and this means that the
joint is unaffected by thermal cycling and by rapid changes of
temperature. To test the resistance of the joint to thermal
cycling, a 2 inch diameter stainless steel unit having a stain-
. .
less steel cylinder with stainless steel pipe ends was testedin a rig which maintained a constant pressure of 30 to 45 psi
inside the joint. The temperature in the joint was varied
through a 120 second cycle as follows. Starting at 75~C, the ~;
temperature was increased to 500C in 10 seconds. The temperature
was maintained at 500C for 80 seconds and was then cooled back
to 75C in 10 seconds. The temperature was maintained at 75C
for 20 seconds and the cycle was then complete. It can be
seen that this is an extreme thermal cycling arrangement and the
joint withstood over 1000 cycles without failure. Examination
o~ the seal after 700 cycles - the rig was stopped, the seal was
disassembled and inspected -showed that there was no fretting
or galling at any of the sealing surfaces and the seal was deemed
to be satisfactory.
To ease the assembly of the seal, it may be impreg-
nated on its surface with polytetrafluoroethylene which can be
applied by simply rubbing the outer surfaces of the cylinder 7
with a block of polytetrafluoroethylene. Since polytetrafluoro-
ethylene has a low coefficient of friction, it eases the assembly
of the seal.
Referring to Figures 2A and 2B (which have been joined
for the sake of ease of understanding of the invention), like
portions of the invention have been given like reference numerals
to Figure 1. considering initially Figure 2B, this shows a
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conventional pipe joint o~ the flange and gasket type. Welded
to the pipe end 1 is a massive flange 15 and a similar flange
16 is welded to the pipe end 2. A sealing gasket 17 is trapped
between the flanges and a series of bolts 18, 19 around the peri-
phery of the flanges are tightened to withstand the pressure
generated within the joint. The pressures applied by the bolts
is made up of two main components, the pressure load plus the
gasket sealing load. The gasket sealing load which is -the neces-
sary axial force required to produce leak-tightness is much
greater than the pressure load and depending on the flange yeo-
~,
metry and gasket material used, can be three or more times
greater. The flanges and bolts have therefore to be very massive
to withstand the applied pressures. The joint shown in Figure 2B
has the same pressure capacity as the joint of the invention
shown in Figure 2A. It can be seen that the joint of the inven-
tion is very much smaller than the prior art joint. In addition,
it is not necessary to use exotic and expensive materials in the
manufacture of the joint.
~ eferring to Figure 2A, it can be seen that the sealing
member 11 is located within the recesses 5 and 6 of the pipe ends
1 and 2 in the same manner as is described above with reference
to Figure 1. Welded to each of the pipe ends is an external
annular ring 20, 21. The ring 20 is screw-threaded as at 22~
It can be seen that the ridge 8 is accommodated within the rings
20 and 21 which are machined after welding so that the outer
faces of the pipe ends 1 and 2 are continuous with the rings 20
and 21. An outer annular tubular retaining member 23 has an
inner flange 24 which abuts the end surface 25 of the ring 21.
The inner surface of the retaining member 23 is screw-threaded
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as at 26 and can be screwed onto the ring 20 from the right-
hand side as shown in Figure 2A, A screw-threaded locking bolt
27 may then be screwed down to prevent the member 23 becoming
unscrewed but not to prevent member 23 rotating relative to ring
21. It can be seen that this annular member holds the pipe ends
, 1 and 2 in axial alignment.
In Figure 3, there is shown a sliding joint which forms
a suitable expansion joint. The sliding member 28 is welded at
one end 29 to a pipe end 2. Partway along the length of the
member 28, there is an external annular ridge 28A and the outer-
most portion of the member 28 is a flexible metal cylinder 30
which is integral with the remainder of the member. The pipe
~, end 1 has a smooth internal surface 5 in which the end 30 can
slide. A retainer 31 has an inwardly directed flange 32 and is
screw-threaded as at 33 to an outwardly directed annular wall 34.
A locking pin similar to pin 27 of Figure 2A may be used if re-
quired. It can be seen that the portion 28 may slide relative
to the end 1 by an amount equal to lengths 35 plus 36. This forms
an effective sliding joint in which the seal is made between the
tip of portion 30 and the inner surface 5. The seal is maintained
by the effect of the pressure inside the joint.
In addition to being much smaller than conventional
joints, the joint of the invention has a very smooth uninter-
i rupted interior and is capable of withstanding very much greater
pressures. A 4" joint manufactured in accordance with Figure 1
was compared with a conventional 4" joint of the type illustrated
in Figure 2B. The flanges and bolts were made from aluminium
~8 and were of the ASA class 150. Using conventional gaskets
of the type illustrated in Figure 2B, the pressure which could
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be contained bv the ioint was about 250 psi. BY comparison, thejoint of Figure 1 was pressurised to 1200 psi and did not leak.
Following this test, the pressure was reduced to zero and the
nuts and bolts 14 were unscrewed so that the bolts had approxi-
mately 1/16" free movement. The unit was again pressurised to
1200 psi and there was no leakage. The pressure was maintained
for one hour and the joint was still found to be leak-tight.
The pressure was then released and the unit was then re-pressur-
ised to 1800 psi. The joint was still leak-tight.
To test the resistance of the joint to bending moment
stresses, a 2" aluminium joint of the type shown in Figure 2A was
connected to a 2" joint of the type shown in Figure 2B and the
pipe 2 was supported in a clamp so that weights could be added
to the end of the pipe to the left of the joint shown in Figure
2A for bending moment loads to be applied. Initially, the as-
sembly was pressurised to 4000 psi at which pressure the joint
of the invention shown in Figure 2A was leak-tight but the prior
art joint of Figure 2B failed. The joints shown in Figures 2~
and 2B are to scale and it can be seen that the massive joint 2B
failed before the smaller joint of the invention. A new gasket 17
was manufactured, the prior art joint of Figure 2B was disassembled
and re-made and the assembly was then pressurised to 1000 psi.
Weights were added to produce a bending moment of 4900 lb. ins.
on the joint of the invention. The joint was leak-tight. The
pressure was removed, the joint was rotated and the pressure was
re-applied and the joint was again found to be leak-tight. The
bending moment was then increased to a value of 2448 lb. ins.
at which bending moment the joint of the invention leaked. On
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disassembly, it was found that the connector 23 had not been
- screwed sufficiently tightly. The sealing cylinder was thenre-stretched slightly so that each of the distal portions 9
and 10 were slightly belled outwardly and the joint re-assembled.
;~ The assembly was again pressurised to 1000 psi and was found
leak-tight. Wei~hts were then added to the pipe to increase
the bending moment to 11,016 lb. ins. and the joint remainded
leak-tight. The test was discontinued at this level because
~` it was impossible to put any greater number of weights into the
scale pan attached to the pipe. It can be seen, therefore,
that the joint o~ the invention is greatly resistant to bending
moment loads and also that it will not transmit torque since
it can be rotated easily.
It can be seen, therefore, that the joint is not only
much smaller than prior art joints but has a much greater capa-
city to wikhstand the internal pressures.
It is an important feature of the invention that the
I tubular sealing cylinder is formed of a material which obeys
Hooke's Law over a long period of time such as three months.
In a material which obeys Hooke's Law, stress is pro-
portional to strain. Thus, if a steel wire is loaded, within
its elastic region, by an amount X gm, there will be an exten-
sion of Ymm. If the load is 2X gm, the extension will be 2Y mm.
If the wire is left for three months with the load X gm suspen-
ded from it, the extension will still be Ymm.
By contrast, rubber and plastics material do not
obey Hooke's Law for a long period of time. Thus, although a
rubber or plastic strand may obey Hooke's Law at room tempera-
ture for loads which are ~uickly applied and removed, the Law
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would not be obeyed over a long period of time. Thus, if a
load A gm produces an immediate extension of B mm on a rubber
strand, the extension after the load has been applied for three
months would be in excess of B mm, because of the viscous flow
of the material under the prolonged effect of the applied load.
; Similarly, if a rubber block were to be positioned underneath
a weight, the block would gradually deform with time by viscous
flow so that the thickness of the block would gradually decrease.
It is quite possible that on removal of the load, the block would
re-assume its original size.
When the metal tubular sealing cylinder of the inven-
tion is inserted into the pipe ends, it is under a compressive
stress which means that the distal portions of the cylinder are
maintained firmly against the inner walls of the pipe ends.
~hen the pressure is applied, therefore, a seal is immediately
available which is enhanced by the action of the internal pres-
sure to increase the sealing load. If the tubular sealing
cylinder were to be made of rubber or plastics material, however,
although it would be under a compressive stress when ~nstalled,
~O it would creep iE not used for several months and the stress
would thereby be reduced or even completely relieved. When pres-
sure was applied, therefore, the fluid would be free to pass
between the cylinder and the wall of the tube, equalising the
pressure on each side of the tubular sealing cylinder. Once this
situation is reached, the seal can never operate and will per-
manently leak
Because of the importance of the initial seal, it is
distinctly advantageous to have a smooth internal surface for the
pipe ends and this is preferably determined by reference to
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centre~line average figures. Preferably, the centre-line average
for the internal surface of the pipe ends and the external sur-
face of the cylinder is less than 63 micro-inches, preferably
within the range 32 to 63 micro-inches. Clearly, the smoother
` the surfaces are, the better but of course it becomes progres-
;~ sively more expensive to produce smoother surfaces because of
the machining time necessary to produce them.
To assist in the removal of the sealing cylinder from
the joint, the flange such as flange 8 may have axial holes
parallel to the central axis of the sealing cylinder through
which bolts may be inserted to force out the sealing cylinder
from the pipe end.
; The pipe joint shown is in fact on circular cross-
section pipes since these pipes are the most commonly found in
practice.
One of the pipe ends may be b~ocked to form a pressure
vessel closure member.
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