Note: Descriptions are shown in the official language in which they were submitted.
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Corrosion Protection System For A Pipeline Crossing
Back~round Of The Invention
Filed Of_The Invention
This invention relates to a corrosion protection system
to be employed on a pipeline crossing and more particularly to a
method and apparatus for sealing a cased carrier pipe and
introducing an inert gas into a casing annulus about the carrier
pipe,
Description Of The Prior Art
lo It has been for many years the common practice in the
pipeline industry to protect a pipeline crossing under an obstacle
such as a road or a riverbed by providing a casing about the
carrier pipe where it passes under the crossing. In such a cased
crossing a variety of techniques are used to avoid the corrosion
problems associated with electrical current passing between
conductive members in the pipeline system. These would include
coatings, pipeline wraps, impressed current cathodic protection
systems, sacrificial anode systems, to name a few. If in the
crossing system the casing and the carrier pipe come into direct
contact, a corrosion cell may be set up and at th~ same time lower
the cathodic potential of the carrier pipe near the short,
potentially allowing further damage to the pipe.
A typical prior art pipeline crossing system and solution
to the problems associated therewith are shown in U.S. Patent
2,816,575 which describes an apparatus for effecting a water tight
seal for the casing pipe ends to prevent ground water carrying
alkalines or other harmful salts from entering the casing and
setting up direct corrosion or electrolytic action. This system
employs a vent open to the atmosphere. One draw back to such an
open vent is that moisture in the alr comes into the casing
annulus though an open vent and may condense on the pipe. This
moisture then may become an electrolyte through which external
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cathodic current can pass from the soil through the electrolyte on
to the pipe. The protective cathodic current i5 dissipated back
on to the pipe through this electrolyte resulting in the lowering
of protective potentials on the pipe at either end of the casing.
If a pipe ls bare or if there is damage to a protective
pipe coating that exposes the pipe surface, these bare or
otherwise exposed surfaces may be sub;ect to atmospheric corrosion
inside the casing. Attempts to eliminate such corrosion have
involved the use of dielectric filler or an inhibitor liquid in
o the annulus, such as a gel. One problem associated with therepair of a road crossing system as in the present application is
that of disrupting the use of the carrier pipeline. If removal
and replacement of the carrier pipe or cutting of the carrier pipe
is required, a great expense is incurred in treating the problem.
Therefore, it is important that a remedial system be capable of
being retrofitted to the existing cased pipellne crossing if at
all possible.
It is, therefore, an ob~ect of the present invention to
provide a simple and inexpensive pipeline crossing system which
can be retrofitted onto a presently installed pipeline crossing
and wherein the casing armulus surrounding a carrier pipe is
maintained free oi electrical leaks between the casing and carrier
and further which eliminates the intrusion of moisture into the
casing annulus, and still further which permits the escape to the
surface through the vent of any product which may leak into other
casing annulus.
Summary Of Th~ Tnv~ntion
With this and other ob~ects in view, the present
invention is directed to a pipeline crossing system having a
casing surrounding a carrier pipe with the casing annulus at each
end of the crossing being positively sealed to hold a protective
fluid under pressure within the annulus at a low pressure. Casing
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annulus vents are provided on each side of the crossing to provide,
a vent of the annulus at each end to the surface. Each of the
vents is provided with a valve for opening and closing the vent
lines to the atmosphere.
An installation of the protective system includes
opening both of the valves on the vent lines and then introducing
a protective fluid into one of the vent lines and continuing such
introduction of the protective fluid until substantially all
ambient fluids in the crossing such as water and air are purged
from the annulus and replaced by the protective fluid. The oxygen
content of purged fluids may be monitored at the other vent until
a casing annulus is substantially oxygen free. At this point the
other valved vent is closed off so that a low pressure is
developed on the protective fluid within the casing annulus. The
one vent is then closed to hold such low pressure on the system.
A non-resetting relief valve is provided on at least one of the
vent lines which relief valve will open at a precise predetermined
pressure level on the vent line which is greater than the low
pressure maintained on the protective fluid.
In one embodiment of this system argon gas is used as
the protective fluid and it is maintained at a pressure of 5 psi
or less. The relief valve on the vent is then set to open at say
7 psi so that if product fluids begin to leak into the casing
annulus and the annulus pressure reaches the precise predetermined
level, the valve will open and will remain open regardless of
whether the casing annulus pressure recedes below the precise
predetermined pressure.
Another feature of the present invention is that the
protective fluid is introduced into the vent line which intersects
3~ the casing annulus at the highest elevation to aid in the purging
of ambient fluids from the system.
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Brief Descriptlon Of The Drawings
F~gure 1 is 8 fragmentary vertical cross sectional view
of a pipeline road crossing;
Figure 2 is an elevational view of the vent closing
valves with the valve rotated 90 about it s longitudinal axis
from the position shown by Fig. l;
Figure 3 is a right end elevational view of Fig. 2 with
the valve further rotated 90 about its longitudinal axis;
Figure 4 is a left end elevational view of Fig. 3; and,
Figure 5 is a longitudinal sectional view of the
pressure relief vent valve taken substantially along the line
5---5 of Fig. 3 and rotated from the position of Fig. 3 to dispose
its longitudinal axis vertically.
Desçription Of The P~e~E~ mbodiments
Like characters of references designate like parts in
those figures of the drawings in ~hich they occur.
Referring first to Fig. 1., the reference numeral 10
indicates a typical highway crossing, shown in vertical section,
and a typical installation of a carrier pipeline crossing 12 in
underlying relation with respect to the highway roadbed 14. The
crossing 12 includes an enlarged casing 16 annularly surrounding
and sealed, as at 17, at its end portions with the outer
peripheral wall of a pipeline carrier pipe 18 to form a gas tight
annulus 20 between the casing and pipeline. The seal means 17 is
comprised of a pair of segmented plates or discs 17a and 17b which
have a central circular opening (not shown) for fitting over the
carrier pipe 18. The plates 17a and 17b are constructed of an
electrically insulative material such as Nylon~ or other such
tough plastic material. An elastomer packer 17c is likewise
provided with a circular opening (not shown) and is segmented or
otherwise broken at some point so that it may be applied about the
pipe 18 without cutting or otherwise disrupting the integrity of
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the pipe. The packer 17c is positioned with its central
openiny about the carrier pipe 18 and sandwiched between
the segmented plates 17a and 17b to form the seal assembly
shown at 17. The horizontal lines on the plates and
packer represent cuts in the plates and elastomer packer
respectively to show that the members 17a, 17b, and 17c of
the assembly are segmented. Typically the segments would
be semi-circular. The elastomer packer may be opened at
only one place in the annular circle of the packer. Bolts
or the like are positioned through plate 17a and threaded
into plate 17b to permit the plates to be forced together
wherein the annular elastomer packer 17c i squeezed to
e~pand radially outwardly into sealing contact with the
outer peripheral surface of carrier pipe 18 and the inner
wall of casing 16 to thereby seal the casin~ annulus 20.
Such a segmented sealing device although not appropriate
for this applicati~n, is shown in U.SO Patent 2,237,680.
The seal arran~ement for use in this application can be an
expandable packer type seal such as shown in U.S. Patent
3,649,034, entitled "Modular Interwall Seal Unit" to Bruce
G. Barton. This modular arrangement of the Barton seal
would permit its being retrofitted about a carrier pipe
and casing already in place in a pipeline crossing so that
it is not necessary to interrupt the flow of product by
interrup~ing the integrity of the carrier pipe 18 as it
extends continuously from within and away from the
protective casing 16. Although the specific seal shown
and described by the Barton patent has been tried in the
present application and failed to hold a positive casing
annulus pressure, another simpler and more integral seal
system has been developed specifically for the present
system and is set forth in U.S. patent no. 4,930,536
issued on June 5, l99a. This later seal can be
retrofitted to the cased crossing without disturbing the
use of the carrier pipe and requires no welding or
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the like. The casing annulus is vented to the atmosphere bX
conventional tubular vents 22 on opposing sides of the roadbed.
The reference numerals 30 and 30' indicate a valve means
for closing the road crossing vents 22. Since the valve means 30
and 30' ~re identical, only the valve means 30 is described in
detail, in the interest of brevity.
Next referring to Figs. 2 to 5, the valve means 30
comprises a hexagonal head portion 32, a sleeve body portion 34,
and an access valve means 36, (Figs. 1, 3 and 5). The head 32 is
centrally bored as at 33 (Pig. 5) and threaded as at 38 for
connection with the exhaust end portion of the respective road
crossing vent 22. The other end portion of the head 32 is
provided with a threadedly connected ring-like valve seat 40,
having opposing spanner wrench sock~ts 44, sealed at its inner
limit with the head bore 33 by an 0-ring 42.
The sleeve-like body means 34 comprises a sleeve 46
which loosely surrounds the flats 48 of the hexagonal head 32.
This end of the sleeve 46 is internally threaded as at 50 for
threaded engagement with the points Sl (Fig. 3) of the hexagonal
head to maintain a rigid coaxial connection between the sleeve 46
and the head 32. The opposite end of the sleeve 46 is provided
with a foraminated disc 52 which may be a section of expanded
metal.
A cup-shaped piston forms a valve 54 with one end
portion closely received in sliding relation by the valve seat 40
and is sealed therewith by longitudinal spaced 0-rings 56 and 58.
The piston valve seating end portion is circumferentially
recessed, as at 60, between the position of the seals 56 and 58 to
form a pressure relief space between the seals 56 and 58 in the
event of fluid seepage cross the seal 56.
The other or outward end portion of the piston valve is
provided with a step diameter flange forming one annular shoulder
62 abutting the outward end of the valve seat 40 and a second
shoulder 64 normally disposed in spaced relation with respect to
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the outward end of the head 32. The piston valve means 54 when
seated with the seat 40 thus closes the exhaust end of the road
crossing vent 22 for the purposes presently explained.
The sleeve 46 contains a piston valve guide for
maintaining the piston valve axially aligned with the head 32 and
valve seat 40 when unseated by excess pr~ssure in the road
crossing vent 22. This valve guide comprises a plurality (4),
only three being shown, tubular guides 68 connected at one end in
90 spaced relationship with the larger diameter of the flanged
end of the piston valve 54. These tubular guldes 68 slidably
surround a like plurality of sleeves 70 in turn closely
surrounding a like plurality t4) of bolts 74 extending between the
outwardly disposed end of the head 32 and a guide plate 72 axially
disposed downstream from the piston valve 54.
One end of each bolt 74 is threadedly connected as at 76
with the head 32 and its other end portion is provided with
threads 78 slidably received by the guide plate 72. Nuts 80 on
the bolts 74 abut the guide plate 72 against the adjacent ends of
the sleeves 70.
As shown by dotted lines (Fig. 5) the piston valve 54
may be moved axially outward, by fluid pressure, against its
inward end, tGward the foraminated plate 52. During ~his movement
the valve guide is maintained concentric wlth the valve seat 40 by
the guides 68 sliding relative to the sleeves 70.
The piston valve 54 is normally maintained seated with
the valve seat 40 by an elongated rod herein called a pin, as
shown by the bold line 82 (Fig. 5) extending axially between the
piston valve 54 and the guide plate 72. The cup shaped piston
valve 54 is provided with a central boss 84 which is centrally
bored for receiving one end portion of the pin 82. The guide
plate 72 is centrally bored for threadedly receiving a pin support
86 in turn centrally bored to form a cooperating socket 88
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receiving ths other end portion of the pin 82. The pin 82 is
fabricated to yield (bend) in response to a predetermined axlally
applied pressure.
In the event of a leak in the pipeline 18 under the
roadbed, within the casing 16 and between its sealed ends, excess
pressure above a predetermined value unseats the piston valve 54
by bending the pin 82, movlng the valve 54 toward its dotted line
position of Fig. 5. This bending or collapsing of the pin 82
releases excess pressure in the annulus 20 through the threaded
end 38 of the head 32. The pin 82 bending at a precise
predetermined value thus limits the amount of pressure, to not
exceed a predetermined value, within the annulus 20.
The piston valve 54 may be resealed with its seat 40
after being unseated and the problem, if any, has been corrected,
by manually removing the sleeve 46, removing the pin nesting
socket 86 and manually removing the damaged pin 82. Extra pins
82', only one being shown, are contained by hollow bores formed in
the several bolts 74. These extra pins 82' being m&intained
within the bolts by snap-on plastic caps 90 covering the bolt nuts
80.
The access valve means 36 comprises a vent access valve
94 which is threadedly connected with the head 32 by a threaded
bore 96 formed in the wall thereof. The access valve 94 is fully
disclosed in my Patent No. 3,794,289 and principally comprises a
generally cylindrical plug which is centrally bored and provided
with O-ring seals, not shown, for receiving an elongated
relatively small centrally bored probe such as is disclosed in my
Patent No. 3,630,080. A quantity of gas, not shown, may be
in;ected into the vent 22 and annulus 20 through either access
valve 94 on the valves 30 to pressurize the annulus to a certain
value, for example, two and one half pounds above atmospheric
pressure. This access vslve 94 thus permits charging the casing
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annulus 20 of the road crossing system with a selected gas as wel~
as permitting a testing of the quantity of gas contained by the
annulus 22 by a probe and pressure gauge such as is disclosed by
my above mentioned Patent No. 3,630,080. As will be described
later, the access valve 94 provides a convenient means for
monitoring the oxygen content of fluid being purged from the
casing annulus through one of the vents 22.
To prevent tampering with or removal of the sleeve means
34, a pair of security ring eyelets 98 are rigidly connected with
the wall of the head 32 and sleeve means 34, respectively, for
receiving a flexible element threaded through these two eyelets 98
and a companion eyelet 98' rigidly secured to the vertical portion
of the vent 22 for receiving a padlock 100.
A typical installation and use of the system described
herein is described as follows: If a cased pipeline crosslng is
showing signs of electrical shorting which would indicate the
possibility of electrolytic action within the cased crossing, the
procedure of this invention 1s to expose the ends of the cased
crossing. This would normally be accomplished by digging into the
earth over each end of the crossing to expose the opening into the
annulus at each end of the casing 16. Since it is not likely that
the casing annulus is sealed properly to hold a positive pressure
within the casing annulus, the old seals (if present) would be
replaced by a positive pressure holding seal. Most of the old
seals were designed to keep moisture from invading the casing
annulus from the earth surrounding the crossing but experience
indicates that almost none of the crossings which are investigated
have been maintained free of moisture, and it is not uncommon for
the annulus to be full of water and mud. In any event, the casing
~ annulus is cleared of any debris such as mud and water as well as
possible and is retrofitted with a seal that will hold a gas under
a low pressure within the casing annulus. In most cases vent
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lines to the atmosphere at the surface are provided on the cased
crossing but in the event they are not, vent lines usually of 2
inch diameter are connected with the casing annulus to communicate
with the interior of the annulus. A precise non-resetting valve
such as shown in detail in Fig. 5., is installed in the vent line
and is made accessible at the surface similarly to the arrangement
of Fig. 1. The access valve 36 on each vent valve 30 is opened
and on one of the vent valves 30 the valve piston ~4 can be
removed to permit maximum purging of the casing annulus. A
protective fluid such as argon gas is then introduced into the
other vent access valve and the introduction of such protective
fluid is continued until the casing annulus is substantially
completely purged of any ambient fluids such as water and air to
fill the annulus as completely as practically possible with the
protective fluid. Argon is an inert gas which satisfies the needs
of inhibiting any corrosive or oxidation activity and also has a
relatively high molecular weight so that it acts as a good
displacement of ambient iluids (prlmarily water and air) and will
further prevent the intrusion of any subsurface fluids into the
casing annulus such as by way of seals 17. Other fluids having a
weight or density sufficient to displace intruding fluids and
which do not support corrosive activity can also be used. Other
practical considerations such as toxicity or flammability would
also be taken into account when choosing such a fluid. An oxygen
analyzer is attached to the discharge vent access valve and the
level of oxygen in the fluid being displaced is monitored. If
substantially no oxygen is present, the purging process can be
considered to have been completed. The discharge vent valve is
then closed and the introduction of protective fluid is continued
until, for example t two and one half psi is placed on the casing
annulus. The introduction of fluid is ceased and the lntroducing
vent line ls closed to shut in the casing annulus at the
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relatively low pressure needed to prevent fluids from the soil
fro~ invading the casing annulus. In the example system disclosed
herein, the rupture pin valve 30 ls then set to fail the pin 82
and thereby open the valve 30 at a pressure of 7 psi. Thus, if
product fluids from the carrier pipe 18 should leak into the
casing annulus to the extent that the pressure is increased to 7
psi, the rupture valve 30 will operate to open the vent line and
is non-resetting so as to remain open even if the pressure within
the casing annulus subsides. In this manner any fluids escaping
lo from the product carrier line 18 will be permitted to vent to the
atmosphere or perhaps into a vessel (not shown) at the surface,
but in no event will escaping fluids be contained within the
casing annulus at a pressure above the present precise vent or
rupture pressure of the vent valve.
While particular embodiments of the present invention
have been shown and described, it is apparent that changes and
modificatlons may be made without departing from this invention in
its broader aspects, and therefore, the aim in the appended claims
is to cover all such changes and modifications as fall within the
true spirit and scope of the invention.
