Note: Descriptions are shown in the official language in which they were submitted.
WO 95117642 ~ pCfIAU94100784
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EBPANDABLE PLUG AND CONTROL METHOD
BACKGROUND OF INVENTION
The present invention relates to an expandable plug
adapted to seal or to regulate pressure or flow within
a tube, and also to a method of control.
Expandable plugs of this general type find application
in cases where normal valuing of flow is inappropriate
for some reason, for example because the position of
the desired seal or regulation is not fixed with
respect to the tube, the desired seal is temporary, or
ld the exterior of the tube is inaccessible.
The general approach to the design and construction of
expandable plugs involves the use of an elastomeric
cylinder or bladder, which is caused to increase in
diameter. This may be done by a variety of mechanical
means, or by application of internal pressure.
In use the plug will need to withstand diametral
expansion and axial thrust, the latter being due to
friction (in cases where the plug moves relative to
the tube) and the differential tube fluid pressure
across the plug.
Mechanically expanded plugs are suitable only for
~ 25 relatively small ratios of expansion and/or low axial
thrusts. Larger expansion ratios may be required in
cases for example where undue hydraulic obstruction of
the tube cannot be tolerated when the plug is not
expanded, or where the tube may vary in diameter or be
3p caused to expand by fluid pressure behind the plug.
For such applications, expansion by internal pressure
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is preferred.
SUMMARY OF IN~7ENTION
The present invention relates, in its first form, to
an expandable plug of the inflatable type, and aims to
provide a plug construction which does not require
excessive pressure for inflation, yet is capable of
withstanding substantial axial thrusts.
This first aspect of the invention provides an
inflatable plug comprising an inflatable bladder
adapted for diametral expansion by application of
internal fluid pressure, fluid inlet means
communicating with the interior of the bladder for
connection to a.source of inflating fluid and
reinforcement means including flexible reinforcing
cords which provide axial reinforcement to the plug
and diametral support to the bladder, said cords each
spreading laterally on inflation of the bladder. The
cords are preferably formed of a plurality of strands
which are not twisted together, thus allowing the
strands of each cord to spread out laterally as the
bladder is inflated.
The cords preferably extend between the opposite axial
ends ofthe bladder to form an envelope surrounding
the outer surface of the bladder, offering little or
no resistance to diametral expansion of the bladder.
A second aspect of the invention relates to a method
for controlling the inflation of an expandable plug in
a tube so as to control the pressure differential in ,
the tube, or so as to allow leakage of fluid past the
plug, by controlling the internal pressure in the
plug according to the equation.
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. PP = D P + Pd
where PP = pressure applied to plug
D P = Pressure differential across plug
Pd = Pressure to expand plug to diameter
'~ 5 d without confinement (free expansion
pressure)
d = diameter of confining tube
BRIEF DESCRIPTION OF THE DRAWINGS
Further preferred embodiments of the invention shall
now be described with reference to the accompanying
drawings, in which:
Fig. 1 is a schematic side view, partly in cross
section, of a preferred plug construction;
Fig. 2 is a perspective view of a portion of the
bladder with the sheath removed to reveal the helical
reinforcing cord arrangement;
Figs. 3 and 4 schematically illustrate the spreading
of the cords as the plug is inflated; and
Figs. 5 to 7 are schematic view illustrating the
control method according to the second aspect of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The plug generally consists of a pair of end cap
members 10 between which a cylindrical bladder 12
extends. The upstream end cap member l0a has a fluid
inlet 14 for entry of liquid or gas to inflate the
bladder. -In practice, the end caps may be identical
and the downstream end cap blocked off or restricted
to provide back pressure. At least one of the end
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caps is physically connected to an external support to
provide reactive support against axial thrust on the
plug. This support may conveniently be provided by
the pipe (not shown) which provides the inflation
fluid.
The bladder 12 should be formed of a tube of
elastomeric material with a high strain capability and
low stiffness, in order to accept diametral expansion
without rupture. This is particularly important where
a high expansion ratio is required.
The elastomeric bladder is fixed to the end caps by
compression seals 16 or other suitable means. Each
seal is compressed between a compression flange 18 and
a spacer block 20 as nut 22 on the shaft 24 of the
compression flange is tightened.
The reinforcing envelope 26 is formed of flexible
cords 28 which surround the outer surface of the
bladder, the cords being better shown in Fig. 2. The
cord may be fixed by winding about a series of angled
anchor pins 30 (shown in Fig. 1) attached to each end
cap. The cords, of non-elastomeric material, encase
the bladder, offering little or no resistance to
diametral expansion. It will be appreciated that the
gap between the cords will tend to increase as the
plug circumference expands. Within this gap the
elastomeric bladder is unsupported, and therefore
numerous, closely positioned cords should be used.
The cords may be protected by a second elastomeric
sheath 34 over the outside of the plug if necessary.
In the embodiment of Fig. 2, the cords are laid at a
small helix angle, for example less than 15°, to the
axis of the plug, with two or more layers 32a,32b of
cords with opposing helices. This assists in the
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. positioning of the cords during expansion, although at
the cost of a slight increase in diametral resistance
due to the circumferential component of the cord
direction.
Most commercially available reinforcing cord is made
by twisting together a large number of individual
strands. The twist prevents spreading of the cord and
provides greater resilience. However, the Applicant
has found that in this particular application the
normal twisted cord is undesirable, as the cords cause
intense local stresses in the elastomer and tend to
cut the bladder. The Applicant has found that by
using untwisted cord the strands move laterally over
the expanding surface of the bladder, spreading the
load and thus preventing cutting of the bladder.
Figs. 3 and 4 illustrate this lateral movement of the
strands during inflation. Fig. 3 shows a parallel
. cord configuration of an uninflated plug. Fig. 4
shows the same plug when inflated. It can be seen
that the width of the untwisted cords increases as the
bladder expands. For the sake of clarity, Figs. 3 and
4 show the cords widely separated. In practice, the
cords are preferably more tightly packed with little
or no gap.
In principle the cords can be of any material with
appropriate characteristics, in general high tensile
strength, high elastic modulus, high flexibility, and
. suitable chemical compatibility with the fluid
involved. Cotton, metal, or synthetic fibre cords
could be used. Very successful plugs have been
constructed using aramid_cords, which consist of a
very large number of very fine strands, and can be
obtained in untwisted form.
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It is important in the construction of the plug that
the cords be all egual in tension and length. One
cord slightly looser than others will be displaced on
expansion and leave an unsupported gap on the
elastomeric bladder. A method by which equal tension
may be achieved is to fix cord anchor rings on an
appropriate rigid frame, and wind the cords using a
counter weight to ensure constant tension.
In an alternative embodiment, the reinforcing envelope
may be integrally formed with the elastomeric material
of the bladder or sheath. The use of the untwisted
cords allows lateral movement of the strands, reducing
the tendency of the cords to tear the elastomer matrix
of the composite material during expansion.
The construction according to the invention
provides a plug with highly anisotropic properties,
namely:
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1. the ability to accept diametral expansion without
rupture and without requiring excessive pressure,
that is, a high strain capability in the
circumferential direction and a low stiffness; and
2. the ability to carry axial loading resulting from
the pressure differential across the plug, that is,
very high strength and high stiffness in the axial
direction.
The diametral expansion properties of the plug result
in a relatively low free expansion pressure, that is,
the pressure needed to expand the plug to the tube
diameter without confinement. This allows fine
control of the plug inflation pressure, thus allowing
control of the upstream pressure or flow in the tube
as described below.
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Surprisingly, the Applicant has found that a precise
. and predictable relationship exists between the
pressure applied internally to an expandable plug and
the pressure which may be retained behind the plug, as
described in the following equation:
Pp = OP + pd
where pP = Pressure applied to plug
DP = Pressure differential across plug
Pd = Pressure to expand plug to diameter
d Without confinement (free expansion
pressure)
d = diameter of confining tube
Figs. 5 to 7 illustrate the parameters of the control
method. Fig. 5 shows the unexpanded plug, with
atmospheric pressure both inside and outside the plug.
In Fig. 6 the plug is expanded to diameter d without a
constraining tube, by application of the free
expansion pressure Pd to the inside of the plug.. In
Fig. 7 the plug is expanded inside a tube 36 of
internal diameter d and the volume inside the tube
upstream of the plug is pressurised to p P. The
minimum plug pressure, PP, needed to maintain the
pressure differential p P may be calculated is the sum
of the free expansion pressure and the pressure
differential.
The Applicant has found that, by controlling the
internal pressure in the plug substantially according
to the above equation, the tube pressure upstream of
the plug can be controlled precisely as fluid will
begin to leak past the plug at pressure differentials
exceeding Q P. Controlling the plug pressure
substantially according to the above equation is also
particularly useful in applications where there is
WO 95117642 ~ . . PC17AU94100784
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relative movement between the plug and the tube. In
such applications, the retained fluid can be caused to
flow past the surface of the plug forming a thin film,
thus lubricating the relative movement and reducing
friction forces on the outer surface of -the plug.
The sensitivity of the control depends on the free
expansion pressure of the plug. If this is very
large, then a large plug pressure is required to
effect a seal, which inhibits fine control of the
system. It is desired therefore that the free
expansion pressure be kept as low as possible, which
object is achieved by the plug construction according
to the first aspect of the invention.
For pressure control applications, a further important
feature of such a plug relates to its length to
diameter (L/D) ratio. It will be appreciated that the
maximum expanded diameter of the plug occurs when the
cords form a sphere. After this point is reached,
further increases in pressure cannot result in a
further increase in diameter. (With a pressure
differential across the plug in fact, this point can
never be reached). This therefore sets a lower limit-
on the length of the plug. As this lower limit is
approached, the sensitivity of the plug decreases,
that is, the free plug expansion pressure increases,
and in the limit, to infinity. It would therefore
seem desirable to have a very long plug. However, it
has been discovered that a stability problem arises if
the plug is too long, whereby the plug collapses
progressively from the upstream to the downstream end,
and the retained fluid is released in spurts. The
critical upper L/D ratio depends on a number of
factors, including the compliance of the
pressurisation system (controlled by the volume and
elasticity of pipe work-and pumps, as well as the
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nature of the fluid being retained and the inflation
fluid. For water, successful L/D ratios for the plug
have been found below about 8:1.
- 5 While particular embodiments of this invention have
been described, it will be evident to those skilled in
the art that the present invention may be embodied in
other specific forms without departing from the
essential characteristics thereof. The present
embodiments and examples are therefore to be
considered in all respects as illustrative and not
restrictive, the scope of the invention being
indicated by the appended claims rather than the
foregoing description, and all changes which come
within the meaning and range of equivalency of the
claims are therefore intended to be embraced therein.
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