Note: Descriptions are shown in the official language in which they were submitted.
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Description
An Improved Threaded Pipe Connection
Technical Field
Tapered pipe threads have been used for many years for
connecting and sealing the ends of pipes in many
industries, such seals being primarily dependent on the
use of a paste like sealing compound to ~eal leak paths
05 inherent between conventional matin~ pipe threadsO
However, in far too many cases~ fluid pressures, thermal
changes and mechanical forces causP the p~ste to gradually
flow out or dry and shrink so as to cause a leak. Recent
use of plastic tape wrapped around the male thread before
assembly has in some cases, produced better sealing hut in
general, the same end result occurs over a period of time.
Su~h leaks occurring in refineries, factories, gas wells
and other industries all too frequently oause injuries,
deaths~ plant shutdowns and economic losses to workers and
companies~
So as to minimize such losses, industry construction and
operating codes such as A.S.M.E. have severely limited the
use of thread~d pipe connections to small sizes and ~o low
pressures, specifying the mandatory use of welded or
flanged connections. HoweverO there are needs for high
pressure threaded pipe connections where welded or flanged
connections cannot be used. Such a need may best be shown
for use in oil and gas wells where the hole size and
tension load requirements prohibit the use of a flange and
the danger of fire prohibits the welding of pipe ends
together.
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Background Art
Several types of pipe joints have been developed over the
years for the purpose of solving the leakage problem.
Perhaps the most successful type, depicted in US Patent
No. 4,085,951, a special premium cost connection that
05 requires very special care and handling. Said connector
has been shown to leak when slight damage occurs on
surfaces as at 12 or 14, hence the need for adding the
plastic ring 24. However~ no provision has been made for
thermal expansion of the "compressible ring" and upon an
increase in temperature~ differential thermal expansion of
the plastic ring causes an undesireable increase in the
connection hoop stress,
US Pat~nt 2,907,589 explains the susceptability of the
Hydril connector to damage, in columns 1 and 2, which may
leak due to a variety of reasons. The same patent
discloses another plastic seal ring and although "end
clearance may he provided to allow for dirt and sand or
for machining tolerances" no definite room is provided for
~0 thermal expansion of the plastic ring. Such a condition
may provide an erratic tendency to seal, giving a false
assurance in a dangerous situation.
US Patent 2,980,451 discloses a plastic ring deposed
intermediate the engaged threads and confined so as to
~extrude" the plastic~ clearly allowing for no thermal
expansion of the plastic and allowing for a progressive
relaxation of the plastic seal which in the presence of
changing t~mperatures and pressures, in time allows
leakage. US Patent 3,047,316 also allows for extrusion of
a plastic ring but makes no provision for sealing of the
plastic ring after thermal bulk contraction occurs.
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US Patent 3,572,777 and 3,100,Ç56 show soft seals trapped
by the end of the male connector but make no provision to
protect the soft seals against: blowout by external
pressure; erosion and wear from internal abrasive fluid
05 flow.
Conventional pipe threads utilizing tapered threads
wherein the pitch diameter of the mating threads lie along
a conical path,~s~L~g~e-~7 are still by far the most
iO commonly used, and necessarily have machining tolerance
for that taper. After extensive study of experimental and
field results over the years~ the American Petroleum
Institute has recognized that should the male threaa have
a faster taper than the female thread~ even though within
tolerance, ~Y_J~R~_Z~ that the seal diameter will be
toward the largest diameter of engaged threads which in
turn, produces a greater hoop stress on the coupling than
would occur had the male thread been machinea with a
slower taper than the female thread. Therefore, such
threaded connections will be derated pressurewise, due to
the lesser thread contact pressures that may be generatea
between the male and female threads; using a given wall
- thickness of the coupling.
A paper presented by Thomas L. Blose on July 21, 1970 to
modify API 8 Round Thread Casing Couplings proposed the
use of a second taper within the coupling 9 toward the
small end oE the e~gaged threads, having a steeper taper
than the remaining threads so as to ensure the maximum
~0 sealing pressure being at the minimum possihle seal
diameter, thus preserving a higher pressure rating for the
connection. However, no such coupling has been made
available to the industry to applicants best knowledge.
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API paper 83-PET presented by the applicants discloses
informa-tion relative to the instant invention as does their
publication in the "Oil and Gas Journall' on April 4, 1983.
Disclosure of Invention
The problems of the prior art are overcome by the presen-t
invention which provides a threaded pipe coupling Eor
sealing connection with a threaded pipe thread formed on -the
periphery of a pipe adjacent the end thereof. The coupling
comprises an internal mating pipe thread having a firs-t
section of maximum diameter, a second section of lesser
diameter and a third section of least diameter. The firs-t
section has a smaller taper angle than does the pipe thread
with which it mates. The second section has a taper angle
substantially the same as the pipe thread and -the third
section has a greater taper angle than the pipe -thread.
Upon assembly of the pipe and coupling a greater radial
interference, and thence a greater sealing force, is exerted
along the first section against external pressure and along the
third section against internal pressure than is exerted along
the second section.
The present invention therefore provides a threaded pipe connec-
tion that ensures greater contac-t pressure between the mating
threads at each end of the engaged thread length than -the
average con-tact pressure between the entire length of -the
engaged -threads. Thus, a seal area is accomplished near the
least diameter end of the engaged -threads to thereby gain a
higher rating against internal fluid pressure. Also, a seal
area against external fluid pressure is accomplished near the
greatest diameter of the engaged threads to -thereby prevent
external fluid pressure from ac-ting against a lesser thickness
of the male thread, to thereby which in turn gain a higher
rating against external fluid pressure.
To ensure a seal against high internal pressure even in the
absence of sufficient sealing compound and under conditions
of changing temperatures, a plastic seal ring may be provided
near the small diameter end of the engaged threads within an
annular space sufficiently larger in volume than the plastic
seal ring so as to allow for thermal expansion without
extruding the seal ring by thermal expansion.
Brief Description of Drawings
Figure 1 is a fragmentary section of a conventional threaded
pipe connection wherein tolerances allow for perfect matching
of the tapers.
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Figure 2 is ~he same as Figure 1 except tolerances allow
the male thread to have a steeper taper than the female
thread~
.
05 Figure 3 illustrates a conventiona~ male pipe thread made
up hand tight into a female thread formed according to the
present invention.
Figure 4 is the same as Figure 3 except the connection has
beel~ made up power tight, to the operating position.
Figure 5 is the same as Figure 4, but with an annular
space and plas~ic seal ring added in accord with the
present invention.
Figure 6 is an alternative embodiment of the present
invention shown in the hand tight position.
Figure 7 is a third embodiment of the present invention
chown in the hand tight position,
Figure 8 is the same as Figur~ 6 except the connection has
been made up power tightO
~5 Figure 9 is an enlarged fragmentary section of Figure 5 to
better detail the plastic seal one turn before full power
tight makeup~
Figure 10 depicts the plastic seal of Figure 9 after full
power tight ma~eup.
Best Mode for Carrying Out the Invention
For cooperation with a conven~ional male pipe thread shown
as 10 in Figure 3, a coupling formed in accord with the
present invention is shown generally at 12, the two
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members being made up to the hand tight position.
Coupling 12 comprises a continuous thread as at 14 which
may inclu~e three different sections 16, 18 and 20, each
having a diferent taper angle. Section 18 may have a
05 taper angle substantially the same as does the male pipe
thread 10, section 16 may have a slower taper than does
the male pi~e thread and sec~ion 20 may have a faster
taper than does the male pipe thread.
When in the hand tight position as depicted in Figure 3,
threads of sections 18 or 16 do not engage the male pipe
threads fully but only the smallest male thread will fully
engage a female threadO After approximately one turn of
power tigh~ ~akeup, the first thread contact of section 16
occurs as ~ 21, the largest female thread. After two but
before three turns of power tight makeup, all threads
between t~e smallest contacting male thread and the
largest contacting female thread are in contact. A~ter
three turns, the connection is power tight~ When made up
power tigh~ as depicted in Figure 4, all threads between
the smallest of the male pipe threads and the largest of
the coupling threads comprising sections 16, 18 and 20,
are engage~ in tight sealing contact. Of course be~ore
assembly, a conventional pipe thread sealing compound is
applied to the threads so as to seal minute gaps that
usually occur between the engaged threads. Thus, it will
be appreciated that more thread contact pressure exists at
the largest female thread as at 24 and at the smallest
male threa~ as at 22 than exists in section 18 because a
greater ra~ial elastic deformation of the pipe and
coupling ha~ occurred at those two positions than has
occurred wit~in section 18o Dotted lines as at 17 and 71
shown within sections 16 and 20, respectively, illustrate
the position of thread crests that would be the case in a
conventional eoupling per Figure 1~
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It is now apparent that a maximum sealing effect again~t
internal fluid pressure may bP formed at the smallest
engaged thread diameter as at 22 and that a maximum
sealing ef~ect against external fluia pressure may be
05 formed at the largest engaged thread diameter as at 24~ by
practice of the present invention. The seal as at 22
reduces the outward fluid pressure load on section 18 of
the coupling to a minimum value and increases the outward
fluid pressure load on the male thread to increase thread
contact pressure to thereby allow a higher pressure rating
for the connection and also provide for an enhancea axial
tensile strength of the connection because all threads are
in full intimate contact and the full engaged thread
height is insured as in 16. The seal as at 24 prevents
external fluid pressure from moving to a smaller diameter
of the male thread, as to section 18, to thereby force
inwardly on a thinner section o~ pipe and thereby reduce
the pressure rating against external fluid pressure.
By way of example, a coupling to connect with a 5-1~2~
O.D. long thread 8 round API oil field casing thread may
have the following dimensions: .625~+.022 length of
section 16; 1.696n+.002 length of section 18; ~975~.030 or
-oO02 length of section 20; a 5~40237 pitch dia at the
left end of section 16; a 5~3649 pitch diameter at the
left end of section 18 and a 5.2589 pitch diameter at the
le~t end of section 20. Thus: a taper equal to the API
pipe taper of .0625~/~ would be formed in section 18; a
slower taper, . 0600n/~ would be formed in section 16 and a
faster taper, .0938~/" would be formed in section 20.
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In some cases due to a poorly machined conventi~nal pipe
thread, the conventional sealing compound may not be
sufficient to seal off imperfections between the engaged
threads. In that event, the pipe coupling may be formed
D5 with an annular space as at 26 of Figure 5 by removing a
portion of the female thread in section 20. A suitable
seal ring of plastic or the like 27 may then be placed in
the annular space before assembly of the connection so as
to be deformed by and to seal against the male member
during make up of the connection. So that the plastic
seal ring 27 will not be extruded when the connection
experiences an increase in operating temperature, the
annular space should be made sufficiently large so as to
~ allow for thermal expansion of the ring.
Figure 9 illustrates plastic ring 27 within space 26
before full power tight makeup. Conical surface 42 formed
on the male member adjacent the end thereof, is shown
contacting but not moving, preformed cooperating conical
29 surface 44 of ring 27.
Annular space 26 is partially defined by end surfaces 46
and 48 and conical surfaces 50 and 52~ all formed within
coupling 1~. Surfaces 50 and 52 have a common circle of
intersection as at 54 which contacts outer conical surface
56 of ring 27 before power tight makeup, thereby defining
annular space 58 between portions of surface 46~ 50 and 56
and defining annular space 60 between portions o~ surfaces
48~ 52 and 56~ Therefore, before power tight makeup, ring
27 substantially fills annular space 26 except for annular
spaces 58 and 60. Ring 27 also pro]ects radially inwardly
from space ~6, to conical surfaces 44 and 60 so as to
provide sufficient ring volume to cooperate w;th surface
42 50 as to provide proper compression of ring 26~
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In Figure 1~, surface 42 is shown advanced to the position
of power tight makeup, thereby forcing ring 27 farther
into space 26, reducing but not filling spaces 5B and 60.
Ring 27 is also show~ deformed to fill the space between
05 thread flank 62 and surface ~2, which form a minute but a
controlled annular relief passage 64 for escape of excess
material of rin~ 27, over and above that required to cause
ring 27 to seal against surface ~2 and circle 54 to
thereby prevent leakage of internal fluid pressure. A
most important feature of the present invention is that
the pressure on ring 27 to cause extrusion through passage
64 is not sufficient to cause ring 26 to fill spaces 58
and 60, the portions of surfaces 50 and 52 between 66 and
68 being sufficient to generate en~ugh force on ring 27
required to move it into sealing position depicted in
Figure 10. Ring 27 may be made of any suitable material
such as Teflon impregnated with glass to achieve the
strength, chemical resistance and other characteristics
re~-{uired for the service intended.
It is now ob~ious that when the assembled connection is
placed in service and is subjected to an increas~ of
temperature, that differential bulk expansion of ring 27
will be accomodated by void spaces 58 and 60 such that no
further extrusion occurs through passage 64, such that
upon return of the temperature to normal, ring 27 will
return to the position of Figure 10. Therefore, a plastic
seal that will withstand thermal cycling without leaking
is provided by the present invention.
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Another embodiment of the present invention is illustrated
in ~igure b wherein coupling 30 is formed with female
threads 32 which have a curvea instead of a conical taper,
the instantaneous taper being a minimum at the female
05 thread of largest diameter 34 and increasing to a maximum
taper angle as at 36. Taper angles at 34 and 36 may be
the same respectively as tapers in sections 16 and 20 of
Figure 3.
~ variation of the embodiment of Figure 6 is shown in
Figure 7 wherein the coupling has a sin~le conical taper
and the pipe has a curved taper 40 to accomplish the same
end result as described above. A similar variationf not
shown, may be made of Figure 4 wherein the coupling was
formed wi~h a single conical taper and the male thread was
formed with three sections having a taper near its end
slower th~n the coupling taper, an adjacent taper the same
as the coupling taper and a third taper faster than the
coupling ~aper.
While the variations of the present invention may have
certain us~s 7 the preferred embod;men~ is depicted in
Figures 3~ 4 and 5 for which applicants have immediate
use~
