CA1042942A

CA1042942A - Pipe joint seals

Info

Publication number: CA1042942A
Application number: CA247,617A
Authority: CA
Inventors: Geoffrey Dodd
Original assignee: Marston Excelsior Ltd
Current assignee: Marston Excelsior Ltd
Priority date: 1975-03-05
Filing date: 1976-03-03
Publication date: 1978-11-21
Also published as: NO760515L; BR7601309A; ES445784A1; JPS51111918A; ZA76929B; SE7601946L; DE2609011A1; AU1121276A; GB1534547A; IT1055964B; FR2303226A1; NL7602146A; BE839060A

Abstract

ABSTRACT OF THE DISCLOSURE

The invention provides a novel joint for pipes used in very low temperature service. The pipe joint com-prises a tubular sealing cylinder extending into both pipe ends to be joined and being an interference fit at the distal portions with the inside of the pipe ends. The tubular sealing cylinder is flexible at its ends and is pressed radially outward, in use, by the pressure of fluid within the pipe ends thus effecting a seal.

Description

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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 ` ~ - 3 -l~Z~42 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 _ "~, l - ~I -Z~42 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 ; ~ 5 -4Z~Z
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 ;

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 4Z9~z 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 ~ ` l¢~Z94;~
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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Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A pipe joint comprising a pair of pipe ends and a metal sealing member extending into both pipe ends forming a fluid tight seal there between, characterised in that each of the pipe ends has a cylindrical inner sealing surface and in that the metal sealing member is an integral substantially cylindrical tubular member comprising an integral pair of frusto-conical portions conjoined at their smaller diameter ends so that the external diameter of the distal portions of the cylindrical tubular member engageable with the inner sealing surfaces is slightly greater than the external dia-meter of the central portion of the cylindrical tubular mem-ber, each of the distal portions of the cylindrical tubular member being an interference fit with one of the inner sealing surfaces, the cylindrical 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.

2. A pipe joint comprising a pair of pipe ends and a metal sealing member extending into both pipe ends forming a fluid tight seal there between, characterised in that each of the pipe ends has a cylindrical inner sealing surface and in that the metal sealing member is an integral substantially cylindrical tubular member extending into the pipe ends, the cylindrical tubular member having a central external annular ridge and flexible distal portions, the external diameter of the distal 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.

3. The joint of Claim 2 further characterised in that the cylindrical tubular member includes two frusto-conical portions integral with the annular ridge, the narrower ends of the frusto-conical portions being connected to the annular ridge.

4. A joint as claimed in Claim 1 in which the two pipe ends are fixed relative to each other.

5. A joint as claimed in Claim 1 in which the two pipe ends are movable one towards the other.

6. A joint as claimed in Claim 1 in which the cylindrical tubular member has an external annular ridge in the central portion to fit between the pipe ends to form a positive location therefor and to form a rigid centre portion.

7. A joint as claimed in Claim 6 in which the exter-nal ridge has a groove in the outer surface thereof.

8. A joint as claimed in Claim 1 in which the distal ends of the cylindrical tubular member are of slightly greater external diameter than the internal diameter of the pipe.

9. A joint as claimed in Claim 2 in which the exter-nal surface of the cylindrical tubular member and the cylindri-cal inner sealing surface of the pipe ends are smooth and have a surface finish having a centre-line average not greater than 63 micro-inches.

10. A joint as claimed in Claim 9 in which the centre-line average is in the range 32 to 63 micro-inches.

11. A joint as claimed in Claim 1 in which the mat-erial of the cylindrical tubular member has a coefficient of thermal expansion substantially equal to that of the material of the pipe ends.

12. A joint as claimed in Claim 11 in which the pipe ends and the cylindrical tubular member are formed of the same material.

13. A joint as claimed in Claim 1 in which the pipe ends have external flanges bolted together to maintain the pipe ends in alignment.

14. A joint as claimed in Claim 1 in which the cylindrical tubular member is of circular cross-section.

15. A joint as claimed in Claim 1 in which the cylindrical tubular member and/or the cylindrical inner seal-ing surface are coated with a material having a low coefficient of friction, preferably polytetrafluoroethylene or molybdenum disulphide.

16. The joint of Claim 2 in which the two pipe ends are fixed relative to each other.

17. The joint of Claim 2 in which the two pipe ends are movable one towards the other.

18. The joint of Claim 2 in which the external ridge has a groove in the outer surface thereof.

19. The joint of Claim 2 in which the distal ends of the cylindrical tubular member are of slightly greater external diameter than the internal diameter of the pipe.

20. The joint of Claim 2 in which the external sur-face of the cylindrical tubular member and the cylindrical inner sealing surface of the pipe ends are smooth and have a surface finish having a centre line average not greater than 63 micro-inches

21. The joint of Claim 20 in which the centre line average is in the range 32 to 63 micro-inches.

22. The joint of Claim 2 in which the material of the cylindrical tubular member has a coefficient of thermal expansion substantially equal to that of the material of the pipe ends.

23. The joint of Claim 22 in which the pipe ends and the cylindrical tubular member are formed of the same material.

24. The joint of Claim 2 in which the pipe ends have external flanges bolted together to maintain the pipe ends in alignment.

25. The joint of Claim 2 in which the cylindrical tubular member is of circular cross-section.

26. The joint of Claim 2 in which the cylindrical member and/or the cylindrical inner sealing surfaces are coated with a material having a low coefficient of friction.

27. 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 tubu-lar member sealingly connected to one of the pipe ends and extending into a substantially cylindrical inner sealing sur-face on the other pipe end, the distal portion of the cylindri-cal tubular member being an interference fit with the inner sealing surface, and the cylindrical tubular member being flexible at its distal end so as to be pressed radially outward-ly, in use, under the pressure of fluid within the pipe ends, to effect a seal between the pipe ends.

28. The joint of Claim 27 in which the cylindrical tubular member is frusto-conical and has the larger external diameter at the distal end.

29. A pipe joint as claimed in Claim 27 in which the cylindrical tubular member is fixed to the one pipe end.

30. A pipe joint as claimed in Claim 27 in which the cylindrical tubular is fixed by welding, bolting or by being integral with the pipe end.

31. A pipe joint as claimed in Claim 27 in which the cylindrical tubular member is permitted to slide, in use, within the inner sealing surface to form an expansion joint.

32. A pipe joint as claimed in Claim 31 in which the cylindrical tubular member has an external annular ridge slidable within an outer retainer which forms a stop on both sides of the ridge.

33. A pipe joint as claimed in Claim 1 in which there is an annular external locking member which is screw-threaded to one of the pipe ends and which abuts an external ridge portion on the other pipe end to maintain the two pipe ends in fixed alignment.

34. A joint as claimed in Claim 1 in which the pipe ends are of aluminium and in which the cylindrical tubular member is of aluminium.

35. A pipe joint as claimed in Claim 1 in which the cylindrical tubular member is of steel and in which the pipe ends are of steel.

36. A pipe joint as claimed in Claim 6 in which the external annular ridge has a plurality of holes spaced around its circumference.

37. A joint as claimed in Claim 36 in which the holes are screw-threaded.

38. The joint of Claim 2 in which there is an an-nular external locking member which is screw-threaded to one of the pipe ends and which abuts an external ridge portion on the other pipe end to maintain the two pipes in fixed alignment.

39. The joint of Claim 38 in which there is a lock-ing screw to retain the external member and to restrict its rotary movement after assembly.

40. The joint of Claim 2 in which the pipe ends are chosen from a material selected from aluminium and steel and in which the cylindrical tubular member is selected from a material chosen from the group aluminium and steel.

41. The joint of Claim 2 in which the external annular ridge has a plurality of holes spaced around its circumference.

42. The joint of Claim 41 in which the holes are screw-threaded.

43. The joint of Claim 27 in which there is an annular external locking member which is screw-threaded to one of the pipe ends and which abuts an external ridge portion on the other pipe end to maintain the two pipe ends in fixed alignment.

44. The joint of Claim 43 in which there is a locking screw to retain the external member and to restrict its rotary movement after assembly.

45. The joint of Claim 27 in which the pipe ends are selected from a material chosen from the group aluminium and steel and in which the cylindrical tubular member is manufactured from a material chosen from the group aluminium and steel.