AU712888B2

AU712888B2 - Control method for expandable plug

Info

Publication number: AU712888B2
Application number: AU97163/98A
Authority: AU
Inventors: Peter Glanville Chapman; Allan Kenneth Wallace
Original assignee: UPONOR BV; Vinidex Tubemakers Pty Ltd
Current assignee: Vinidex Pty Ltd
Priority date: 1993-12-21
Filing date: 1998-12-17
Publication date: 1999-11-18
Anticipated expiration: 2014-12-21
Also published as: AU9716398A

Description

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1 Australia Patents Act 1990

ORIGINAL

Complete Specification Standard Patent CONTROL METHOD FOR EXPANDABLE

PLUG

The following statement is a full description of this invention, including the best method of performing it known to me: The present invention relates to a control method for an expandable plug, to create a desired pressure differential within a tube.

Expandable plugs typically find application in completely shutting off flow in a tube where normal valving of flow is inappropriate for some reason, for example because the position of the desired seal is not fixed with respect to the tube, the desired seal is temporary, or the exterior of the tube is inaccessible.

•O•9 o The general approach to the design and construction of expandable plugs involves the use S 10 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 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 caused to expand by fluid pressure behind the plug. For such applications, expansion by internal pressure of an elastomeric device is preferred.

The present invention aims to provide a control method for an inflatable plug which enables the plug to be used to maintain a desired pressure differential across the plug within a tube, as distinct from prior art applications of inflatable plugs which simply shut off all flow regardless of variations in upstream pressure and hence pressure differential.

An aim of preferred forms of the invention is to provide a control method by which the desired pressure differential may be maintained while allowing leakage of fluid past the tube, for example to lubricate relative movement of the plug and tube.

4 A first aspect of the invention relates to a method of maintaining a desired pressure differential AP across an inflatable plug positioned in a tube of internal diameter d with S. relative movement between the plug and the tube, while allowing leakage of a lubricating S"flow past the plug of fluid within the tube to occur, including inflating the plug by applying and maintaining an internal pressure Pp, in the plug substantially according to the equation U 0 Pp AP Pd ,000 0 wherein Pd is the internal pressure required to expand the plug to tube diameter d without confinement.

o A second aspect of the invention relates to a method of maintaining a desired pressure differential AP across an inflatable plug positioned in a tube of internal diameter d including inflating the plug by applying and maintaining an internal pressure Pp, in the plug substantially according to the equation Pp AP Pd wherein Pd is the internal pressure required to expand the plug to tube diameter d without confinement.

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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 for use in the invention; Fig. 2 is a perspective view of a portion of the bladder with the sheath removed to reveal o the helical reinforcing cord arrangement; SFigs. 3 and 4 schematically illustrate the spreading of the cords as the plug is inflated; and ••o S Figs. 5 to 7 are schematic view illustrating the control method according to the invention.

The plug generally consists of a pair of end cap members 10 between which a cylindrical bladder 12 extends. The upstream end cap member 10a 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 oo the end 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 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

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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 ifnecessary.

S 10 In the embodiment ofFig. 2, the cords are laid at a small helix angle, for example less than 150, to the axis of the plug, with two or more layers 32a,32b of cords with opposing helices. This assists in the 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

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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.

It is important in the construction of the plug that the cords be all equal in tension and length. One cord slightly looser than others will be displaced on expansion and leave an S unsupported gap on the elastomeric bladder. A method by which equal tension may be 0 achieved is to fix cord anchor rings on an appropriate rigid frame, and wind the cords ••using a counter weight to ensure constant tension.

••O.oi S 10 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.

0 °The preferred construction provides a plug with highly anisotropic properties, namely: 1. the ability to accept diametral expansion without rupture and without requiring 0 excessive pressure, that is, a high strain capability in the circumferential direction and a s 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.

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 AP Pd where Pp, Pressure applied to plug AP Pressure differential across plug Pd Pressure to expand plug to diameter d without confinement (free expansion pressure) 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 S. 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 S 15 AP. The minimum plug pressure, Pp, needed to maintain the pressure differential AP 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 0 0'-6 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 AP.

Controlling the plug pressure substantially according to the above equation is also particularly useful in applications where there is 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 lubricating flow can be maintained by introducing fluid into the upstream zone, for example from a high pressure, substantially constant flow, source.

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 preferred embodiments.

For pressure control applications, a further important feature of such a plug relates to its length to expanded diameter 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, S° ""the sensitivity of the plug decreases, that is, the free plug expansion pressure increases, 0 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 15 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 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, preferably less than 3:1.

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.

Claims

1. A method of maintaining a desired pressure differential AP across an inflatable plug positioned in a tube of internal diameter d with relative movement between the plug and the tube, while allowing leakage of a lubricating flow past the plug of fluid within the tube to occur, including inflating the plug by applying and maintaining an internal pressure Pp in the plug substantially according to the equation 9 Pp AP Pd wherein Pd is the internal pressure required to expand the plug to tube diameter d without confinement.

2. A method according to claim 1 wherein the lubricating flow is a film of the fluid 15 flowing past the exterior of the plug.

3. A method of maintaining a desired pressure differential AP across an inflatable plug Spositioned in a tube of internal diameter d including inflating the plug by applying and °maintaining an internal pressure Pp in the plug substantially according to the equation Pp AP Pd wherein Pd is the internal pressure required to expand the plug to tube diameter d without confinement.

4. A method according to any preceding claim wherein the plug has a length to diameter ratio of less than 8:1. A method of maintaining a desired pressure differential in a tube, substantially as herein described with reference to Fig. 7. DATED this 17th day of December 1998 Vinidex Tubemakers Pty. Limited Uponor B.V. Agent for the Applicant HALFORD CO *V*o: 900:00 09 *0 0*0S 0*4