EP0397716B1 - Injection device for introducing fluids under pressure into cracked structures - Google Patents

Abstract

An injection device for introducing fluids, in particular cement suspensions, under pressure into a hole drilled in cracked structure, for example of concrete or masonry, comprises an injection packer (22) with a through channel (24) which connects an outer inlet region (26) with an outlet region (28), and a coupling device (20) connected to the injection packer (22) for connecting the inlet region (26) to a conduit containing the pressurized injection fluid. The sliding coupling device (20) has a radial pocket (36) adjacent to an outlet part (58) of an orifice for the passage of fluid. The outer inlet region (26) of the injection packer (22) has a disc (30) which projects radially beyond the adjacent region (72) of the injection packer (22) and whose shape matches that of the pocket (36).

Description

The invention relates to an injection device for the injection of liquid substances, in particular cement suspensions, by a in a cracked structure, e.g. B. from concrete or masonry, introduced borehole, with an injection packer, in which a continuous channel is formed, which connects an outer entry area with an exit area, and with a coupling device assigned to the injection packer for connecting the entry area to a delivery line, in which the liquid substance to be injected is under pressure.

Injection packers of this type are suitable for high-pressure injections with injection pressures of 200 bar and above, but also for injections at a pressure of a few bar. To fill a crack in a building, or also in grown rock, the crack is first drilled to the side, preferably at an angle, and then inserted into the hole in the injection packer. In a first step, this is first determined in the borehole in such a way that it cannot be pushed out of the borehole even by injection pressures of 200 bar and above (by the reaction pressure of the injected mass). In a second step, the liquid material, for example liquid plastic, is then pressed through the inner channel of the injection packer and the crack in the structure is thereby filled. The liquid substance hardens or sets after a certain period of time, which also eliminates the hydrostatic pressure on the injection packer. The injection packer is thus a borehole closure which must be anchored or clamped so firmly in the borehole that it withstands the reaction pressure of the injected mass during the actual injection process and afterwards until the plastic has hardened.

Injection devices of the type mentioned at the outset are known from German Offenlegungsschriften 26 33 434 or utility model 83 30 425, in which the injection packer has a truncated cone-shaped jacket and is thus designed as a round wedge. The borehole is only formed so large that the wedge-shaped shaft can be inserted somewhat into the borehole. The injection packer is then driven into the borehole by hammer blows and wedges there. Subsequently, an extension piece belonging to the injection packer, usually a nipple, must be screwed into the entry area so that a spray gun can be attached.

However, this known injection device has several disadvantages: screwing in the nipple takes time. In the case of injections of the type in question here, the injection packers (shafts with nipples) are placed at short intervals, for example 20 to 25 cm, for the compression of longer cracks, which typically occur, a large number of injection packers are required. The nipples must be screwed into each individual shaft; the shaft either has a pre-cut thread or the nipple has a self-tapping thread. There are always errors when screwing in the nipple, the respective shaft is then no longer usable.

Furthermore, injection devices of the type mentioned in the form of so-called assembly packers are known, see for example DE-PS 25 50 555, in which a tubular rubber piece, for example a section of a rubber hose, is clamped between two disks and by a screwing movement in which the two Move washers so that the rubber sleeve clamps in the borehole. The previously mentioned nipples are also used in this previously known injection device, which serve as an attachment piece for the attachment of a spray gun, which here is also the coupling device. These nipples must be screwed into an internal thread intended for them. When the part of the assembly packer protruding from the borehole is later removed, problems arise; simple knocking off is usually difficult and is only possible with the aid of a chisel. As with the foldable injection packer, it is Use the nipple only once, since after the injection it is filled with the liquid substance to be pressed in and in most cases it cannot be cleaned.

The commercially available nipples used in the known injection packers have relatively small passage cross sections for the liquid substances, so that not all liquid substances can be pressed with these injection devices. For example, cement suspensions cannot be pressed through a nipple and therefore cannot be processed with the known devices. Nowadays, however, there is an increasing tendency to use cement suspensions for pressing instead of plastics in order to carry out the renovation with the same hydraulic binder with which the actual building structure is constructed.

Cement suspensions consist of the finest (flour-fine) granular constituents and are just as liquid (have a corresponding viscosity) as synthetic resins, but it is not possible to press them through conventional grease nipples. It is possible to press in the sealing ball of the grease nipple, but the cement suspension does not flow around this ball sufficiently.

Finally, the passage cross sections of the known injection devices are greatly restricted in that the nipple offers only a small free opening cross section in the open state. Even in the case of injections of liquid synthetic resins, the known injection devices therefore have the disadvantage that only a limited amount of liquid substance can be pressed through the injection device per unit of time. The entire injection process takes up too much time and you have to use substances that remain sufficiently liquid throughout the injection process, so the substance to be injected must not begin the curing process or the setting process within the entire injection time. This in turn also delays the final setting or hardening of the injection material that can be used.

The nipple used according to the state of the art has a double function: on the one hand it serves with its outer area for the positive connection of a spray gun and on the other hand as a valve. In the invention, the valve function is eliminated, the interaction of the disc and the pocket brings about the mechanical hold and the seal between these two parts and thus between the delivery line for the substance to be injected and the shaft of the injection packer.

The object of the invention is to remedy this. It is based on the object, while maintaining the advantages of the previously known injection device of the type mentioned, in that it is also suitable for injecting cement suspensions, offers a larger passage cross section, as a result of which a larger amount of liquid injection material can flow through the injection device per unit of time and thus faster setting or faster curing injection materials can be used.

Starting from the injection device of the type mentioned at the outset, this object is achieved in that the coupling device designed as a sliding clutch has a radially extending pocket which borders on an outlet part of a mouth supplying the liquid substance and in that the outer entry region of the injection packer has a disk which is opposite protrudes laterally from the adjacent area of the injection packer and the shape of the bag is adapted.

In this injection device, the injection packer has a disc on its outer end region. It serves to connect and hold in the coupling device. The disc is designed so that it can be inserted radially into the pocket of the coupling device and is then axially fixed. By means of the outlet part, the seal with respect to the injection packer is achieved, at the same time the outlet part exerts a mechanical pressure on the disk, as a result of which the injection packer is held in the coupling device in a force-fitting or positive manner.

In practical use, the coupling device is placed from above onto the injected packer, the disc of which projects freely out of the borehole. The pocket of the coupling device is therefore open at the bottom. As a result, the coupling device also holds onto the injection packer due to its weight, to which the weight of the connected delivery line is added.

In the injection device according to the invention, the delivery channel for the liquid substance to be injected runs practically exclusively in a straight line. The flow of the liquid substance to be injected remains continuous, i.e. does not have to flow around any object and is not necessarily constricted by any means, such as, for example, the nipple, but the minimum cross section for the flow channel of the substance to be injected can be freely specified, since it is not limited by other components.

In contrast to the previously known injection devices cited above, the injection device according to the invention has no valve in the inlet area. The disc serves to hold and seal against the coupling device, but this does not achieve a valve function and is also not desirable at this point in order not to split the flow of the injecting material.

An advantage of the injection device according to the invention is that the coupling device can be reused. In contrast to the previously known injection devices, in which even if one unscrews the nipple time-consuming cleaning of the inner passage area of the nipple has to be practically ruled out, i.e. the nipple is lost after a single use, there is no permanent connection between the coupling device and the outlet part of the injection packer , in any case before the connection to be released, the coupling device can be easily cleaned - and therefore reused - because of its large passage cross-sections.

The injection device according to the invention can be designed so that the flow channel of the substance to be injected is straight and for example, not less than a minimum diameter of 3 or 5 mm. Because of this geometry, the injection device can be used for practically all currently known injection materials.

The pocket is preferably provided in the vicinity of an end region of the coupling device. As a result, the distance at which the pane of an injection packer that has been driven in or inserted must protrude relative to the structure is particularly small.

In one development, the coupling device is an essentially cylindrical part. This makes it easy to train. However, in order to be able to connect the injection packer to the coupling device even in places that are difficult to access, it has proven to be advantageous to produce kinked coupling devices for such special uses.

The pocket is preferably formed in a sleeve and radially delimited by a semicircular surface to which two mutually parallel surfaces tangentially adjoin, the spacing of which is somewhat larger than the outer diameter of the disk. An internal thread is preferably provided in the sleeve.

In a first, particularly preferred embodiment of the invention, the coupling device has such a sleeve, which is designed with an internal thread, and a feed pipe which interacts with this internal thread and which forms the outlet part in the form of an annular disk or seal at its front end region. The connection with the washer of the injection packer is achieved by screwing the supply pipe relative to the sleeve with the washer in the pocket until it comes into contact with the washer and its cutting edge or seal is pressed against the washer in such a way that a sealing seat is achieved between the outlet part and the disc.

In a particularly preferred embodiment, the outlet part, which is designed as an annular cutting edge, is frustoconical with a through hole for the liquid substance, the outer frustoconical The jacket penetrates the channel of the disk and wedges (in the sense of a round wedge) with the inner wall of this channel. In another embodiment, the outlet part has an annular sealing projection, which is formed either by an elastic rubber ring or by a metallic cutting edge and cooperates sealingly with the flat surface of the disk, in which the cutting edge is stamped into a soft material of the disk or the elastic sealing ring is sealing against the disc.

This (first) embodiment of the coupling device has the advantage of a particularly simple construction, as a result of which cleaning and thus reuse are also simple. A secure seal is achieved, by pressing the outlet part against the window of the injection packer, the degree of the seal can be set as desired and can also be improved during the injection.

In another (second) embodiment, the outlet part is an elastic ring which is connected to the sleeve and projects into the area of the pocket. By means of the ring, the seal with respect to the injection packer is achieved, at the same time the ring exerts a mechanical pressure on the disk, as a result of which the injection packer is held firmly in the coupling device. The connection between the injection packer and the coupling device is achieved only by radially inserting the packer disk into the pocket of the coupling device. As a result, the coupling device can be made very short. The ring is interchangeably housed in the coupling device, preferably it is part of a disc, which can also have a hat profile, and is made of a rubber-like material. In a preferred development, the ring is surrounded on its outside by a cap-shaped metal part and thus protected, which absorbs the shear forces that occur when the disk is inserted into the pocket and thereby counteracts wear and mechanically supports the ring, so that it does not tilt when inserted or can be moved.

Hammerable plastic injection packers are made by hammer blows placed on the free outer surface of the disc. In order to avoid that this surface, which is sometimes also required for the sealing with respect to the coupling device, is mechanically deformed in such a way that a seal between the outlet part and the disk is no longer secured, it is proposed that the injection packer be driven in by means of a striker. Practically the same coupling device can be used as the striking block - but without a connected delivery line - as is also used for feeding the liquid substance.

Fig. 1

Ein Montagebild einer Kupplungsvorrichtung, deren Einzelteile im Axialschnitt dargestellt sind,

Fig. 2

eine Draufsicht auf den dem Injektionspacker zugewandten und mit einer Tasche ausgebildeten Stirnbereich der Kupplungsvorrichtung gemäß Fig. 1,

Fig. 3

eine Seitenansicht des Gehäuses der Kupplungsvorrichtung, und

Fig. 4

ein axiales Schnittbild des zugehörigen Injektionspackers,

Fig. 5

eine Draufsicht auf den äußeren Eintrittsbereich des Injektionspackers gemäß Fig. 4,

Fig. 6

ein Detailbild entsprechend Fig. 4 für einweiteres Ausführungsbeispiel des Injektionspackers,

Fig. 7

ein Montagebild einer Injektionsvorrichtung nach der Erfindung mit einem Austrittsteil in Form einer kegelstumpfförmigen, hohlen Schneide und einem in einem Bohrloch angeordneten Injektionspacker, alle Einzelteile sind im Axialschnitt dargestellt,

Fig. 8

ein Ausschnitt aus der kompletten Injektionsvorrichtung gemäß Fig. 7 zu Verdeutlichung eines anderen Ausführungsbeispiels einer metallischen Schneide und

Fig. 9

einen Axialschnitt eines Montagepackers für eine Injektionsvorrichtung nach der Erfindung.

Further advantages and features of the invention emerge from the remaining claims and the following description of four non-limiting exemplary embodiments, which are explained in more detail with reference to the drawing. In this show:

Fig. 1

An assembly picture of a coupling device, the individual parts of which are shown in axial section,

Fig. 2

2 shows a plan view of the end region of the coupling device according to FIG. 1 facing the injection packer and formed with a pocket,

Fig. 3

a side view of the housing of the coupling device, and

Fig. 4

an axial sectional view of the associated injection packer,

Fig. 5

4 shows a plan view of the outer entry region of the injection packer according to FIG. 4,

Fig. 6

4 for a further embodiment of the injection packer,

Fig. 7

2 shows an assembly picture of an injection device according to the invention with an outlet part in the form of a frustoconical hollow cutting edge and an injection packer arranged in a borehole, all individual parts are shown in axial section,

Fig. 8

a section of the complete injection device according to FIG. 7 to illustrate another embodiment of a metallic cutting edge and

Fig. 9

an axial section of an assembly packer for an injection device according to the invention.

In all four exemplary embodiments, the injection device according to the invention consists of a coupling device 20 and an injection packer 22. The latter has a continuous channel 24, which connects an outer inlet area 26 to an outlet area 28. This channel 24 has a clear diameter of 4 to 6 mm (depending on the embodiment), which remains as unchanged as possible over the entire length of the channel. The entry area 26 is designed as a disk 30 which has a diameter of 22 mm (second exemplary embodiment, FIGS. 1 to 6) or 18 mm (the further exemplary embodiments). Its thickness is 2.5 mm (second exemplary embodiment) or 4 mm (other exemplary embodiments. A cylindrical region 72 of the outer jacket adjoins the disc 30, which is provided in order to allow the disc 30 to engage behind the disc device 20. This region 72 is cylindrical in order to be able to apply the coupling device 20 in all angular positions.

In the first exemplary embodiments, the injection packer 22 is in each case a plastic packer, the outer jacket of which tapers in steps from the entry area 26 to the exit area 28. In this area, three sawtooth-shaped ribs 32, which run in a ring, are formed in an evenly distributed manner. As seen from the entry area 26, they jump forward at a right angle of approximately 1.5 mm and then run back into the tapering contour after a total length of 5 mm. This packer is made of an impact-resistant plastic, for example manufactured by injection molding, its plastic is sufficiently strong that wedging in the borehole is made possible.

The coupling device 20 according to the second embodiment (Figures 1 to 6) is made up of several individual parts. It has a sleeve 34 on one end face (which faces in the drawing of FIG. 4) a pocket 36 for receiving the disk 30 is formed. The opening direction, from which this pocket 36 is accessible and in which the disk 30 is inserted, is indicated by the arrow 38. The pocket 36 is delimited in the radial direction by a semicircular surface 40 which is adjoined by two straight, rectangular surfaces 42 which extend tangentially to the ends of the semicircular surface 40.

The pocket 36 is axially delimited on the one hand by a surface 44 and on the other hand by a web 46 which is approximately 2 mm wide and has the same shape as the pocket 36, but forms a semicircular surface with a smaller diameter (difference 3 mm). The described geometry of the pocket etc. can also be seen from the illustrations in FIGS. 2 and 3. In the illustration according to FIG. 3, which only shows the sleeve 34, the pocket 36 is open at the top in relation to the paper plane, the disk 30 is therefore inserted from above in this illustration. In practice, these processes are rotated exactly 180 degrees.

The sleeve 34 has an internal bore 48 which is partially provided with an internal thread and which runs centrally to the semicircular surface 40 and is delimited by a ridge 50 opposite the pocket 36. When assembling the coupling device, a support ring 52 with the outermost regions of a rounded, in the illustration lower surface 54 bears against this ridge 50. It has an inner bore 56 for receiving an outlet part 58 in the form of a ring made of rubber, which extends in one piece into a hat-shaped profile. The freely projecting, lower part of the ring 58 has an axial length which is somewhat greater than the axial thickness of the support ring 52 and fits radially exactly into the inner bore 56 of the support ring 52. In the assembled state of both parts 52, 58, therefore, the lower surface 60 slightly in front of the corresponding surface of the support ring 52. The geometry is such that this lower surface 60 protrudes into the space of the pocket 36 when the support ring 52 and ring 58 are mounted, to the extent that when the disc 30 is inserted, it is held in a frictional manner by the ring 58.

The inner bore 48 is closed to the outside by a screw 64 provided with a longitudinal bore 62, a sealing ring seals between the upper end face of the sleeve 34 and the lower surface of the screw head of the screw 64, for him there is a corresponding recess in the upper end face of the sleeve 34 provided. The screw 64 has a thread in its lower region which cooperates with the internal thread of the internal bore 48. When screwed together, the screw 64 presses the ring 58 and the support ring 52 towards the pocket, so that the position described above is maintained.

The screw 64 has an internal thread 68 in the outside area of its inner bore, to which a delivery hose or a valve, for example a ball valve, to which the delivery line is in turn attached, can be attached. In order to secure the screw 64 within the inner bore 48, a radial threaded bore 70 for a grub screw (not shown) is provided in the sleeve 34.

The drawing is to scale, therefore all other dimensions can be determined from the dimensions already mentioned.

It is crucial for the interaction of coupling device 20 and injection packer 22 that the diameter of the semicircular surface 40 is essentially matched to the outside diameter of the disk 30 and the outside diameter of the cylinder region 72 adjoining the disk 30 essentially corresponds to the diameter of the semicircular area of the web 46. The two surfaces 42 are at a distance which is slightly larger than the outer diameter of the disk 30. The same applies to the analog surfaces of the web 46 with respect to the cylinder region 72.

Figures 4 and 5 show that the outside end wall of the injection packer 22 is a flat circular disc. It is advisable to round off its outer edge somewhat so as not to damage the surface 60 of the ring 58 when the disk 30 is inserted into the pocket 36.

The coupling device 20 is disassembled for cleaning or - if this should become necessary - for exchanging the ring 58, from the illustration it can be seen that no complex cleaning work is required since the parts are simple. The parts 34, 52 and 64 of the coupling device are preferably made of metal, but they can also be made of plastic. A sleeve 34 made in particular of metal can also be used as an anvil.

In the modified embodiment according to FIG. 6, the channel 24 of the injection packer is not open at the bottom, rather the lower end region is closed off, the approach region 28 is formed by a plurality of radially running bores at the same axial height. These will covered on the outside by an elastic piece of tubing 74, for example a piece of silicone tubing, which is inserted into a corresponding recess in the outer jacket and as a result does not protrude from the free contour of the outer jacket. In this way, a valve similar to the valve of a bicycle tube is obtained. During the injection, the pressurized liquid injection material lifts off the tube piece 74, so that the injection material can flow through the radial bores which form the access area 28. A backflow is not possible.

FIG. 1 shows that the channel through which the injection liquid flows through the coupling device 20 runs in a straight line and is radially essentially limited only by the inner diameter of the ring 58. In the assembled state of the injection device, coupling device 20 and injection packer 22 are coaxial. All individual parts of the coupling device 20, that is to say the parts 34, 52, 58 and 64, are also coaxial.

The coupling device 20 in the first exemplary embodiment according to FIG. 7 consists only of two individual parts, a sleeve 34 and a feed pipe 76. The sleeve is constructed similarly to the sleeve according to the first exemplary embodiment, in particular the pocket 36 is made comparable, but the axial length is Sleeve 34 about 2.5 times as large to have enough gripping surface on the outer jacket of the sleeve 34 for a hand screw connection, which will be explained later. If you want to do without this large gripping surface, the sleeve 34 can also be made as short as shown in FIG. 1. However, as shown in FIG. 7, a degree 50 is not present. The feed pipe 76 has essentially the shape of a hollow screw and has an external thread in its lower region, which cooperates with the internal thread of the sleeve 34. A head 80 of the feed pipe 76 also has a large outer surface area in order to be able to grip it well and to facilitate the screw connection between the sleeve 34 and the feed pipe 76 even at higher torques.

In the foremost area, the feed pipe 76 forms an outlet part 58 in the form of an integral, frustoconical, hollow cutting edge. The diameter of the frustoconical outer shell is smaller at the free end region than the inner diameter of the channel 24, but it widens to a diameter which is significantly larger than this inner diameter, so that a conical interference fit between the inlet region 26 of the channel 24 and the outlet part 58 is achieved when the feed pipe 76 is pressed against the disk 30 by screwing.

7 shows an example of a practical application: an injection packer, which is shown here made of plastic, but can also be a metal packer, is placed in a borehole 82 of a building body 84. It projects only the pane 30 and the area 72, as well as a short piece of the adjacent area, from the building body 84.

First, the two parts 34, 76 of the coupling device 20 are screwed together until the outlet part 58 does not yet protrude into the area of the pocket 36. In this state, the coupling device 20 is pushed radially over the injection packer 22 in the direction of the arrow 86, and the pocket 36 then receives the disk 30. Now the feed pipe 76 is screwed relative to the sleeve 34, the cutting-shaped outlet part 58 penetrates into the entry area of the channel 34 and displaces the plastic material, which is softer than the metallic cutting edge, until a tight press fit, which can be felt when screwing, is achieved. The injection process can now begin when a delivery hose, as described above, is attached to the internal thread 68 of the feed pipe 76.

Fig. 8 shows a modified embodiment, here the design of the outlet part 58 is different. It is an annular circumferential bead which forms the free end of the feed pipe 76 and is made in one piece from the metal of the feed pipe 76, as is known for vacuum seals. When screwing it digs into the plastic material of the washer and forms an annular sealing surface. 8 has the advantage that the outlet part 58 cannot be damaged as easily as in the exemplary embodiment according to FIG. 7.

At the same time, the passage cross section for the substance to be injected (longitudinal bore 62) is not reduced below the dimension of the diameter of the channel 24. A seal against metallic disks 30, which is still possible in the exemplary embodiment according to FIG. 7, in particular if the cone angle is chosen to be correspondingly small, is difficult. Here it is advisable to provide an O-ring instead of the bead and to connect it to the feed pipe 76. For example, an annular groove is provided for this purpose, into which a piece of hose that forms the O-ring is inserted.

9 finally shows an assembly packer which interacts with the coupling device 20. Its entry area 26 is a metallic turned part that forms the disk 30 and the cylindrical area 72. It has a threaded extension and is screwed to a sleeve piece with an external hexagon.

Claims (10)

1. Injection device for introducing fluids, in particular cement suspensions, under pressure through a hole drilled in cracked structures, e.g. made of concrete or masonry, with an injection packer (22) provided with a continuous duct (24) running through it and connecting an external inlet area (26) with an outlet area (28), and with a coupling device (20) matched to the injection packer (22) for the connection of the inlet area (26) to a feed line through which the fluid substance to be injected is supplied under pressure,
   characterised by the fact that the coupling device (20), which is designed as a sliding coupling, is provided with a radial pocket (36) bordering on an outlet component (58) of an opening through which the fluid substance is fed, and by the fact that the external inlet area (26) of the injection packer (22) is provided with a flange (30) that projects radially in relation to the bordering area (72) of the injection packer (22) and which is designed so as to engage in the pocket (36).
2. Injection device as claimed in Claim 1, characterised by the fact that the pocket (36) is located in the end area of the coupling device (20).
3. Injection device as claimed in Claim 1 or 2, characterised by the fact that the pocket (36) is formed in a bushing (34) and radially delimited by a semicircular surface (40) bordering tangentially on two surfaces (42) that are parallel to one another, the distance of said surfaces (42) from one another being slightly larger than the outside diameter of the flange (30).
4. Injection device as claimed in any of the Claims 1 through 3, characterised by the fact that the pocket (36) is delimited axially by a surface (44) on one side and by a lip (46) on the other side, the distance between which is slightly larger than the thickness of the flange (30), and by the fact that the lip (46) is formed to match a cylindrical area (72).
5. Injection device as claimed in any of the Claims 1 through 4, characterised by the fact that the outlet component (58) and the flange (30) are coaxial when the injection device is assembled.
6. Injection device as claimed in any of the Claims 1 through 5, characterised by the fact that the outlet component (58) is an elastic ring which is connected to the bushing (34) and which projects into the area of the pocket (36).
7. Injection device as claimed in Claim 6, characterised by the fact that the ring (58) is surrounded by a supporting ring (52) having a chamfered contour on the side towards the pocket (36).
8. Injection device as claimed in Claim 6 or Claim 7, characterised by the fact that the bushing (34) is provided on the inside with a boring (48) into which the ring (58) and supporting ring (52) fit perfectly, and that said boring (48) can be closed with a screw unit (62).
9. Injection device as claimed in Claim 8, characterised by the fact that the screw unit (64) is provided with an inside thread (68) for the connection of a feed line or a stop cock, e.g. a ball valve.
10. Injection device as claimed in any of Claims 1 through 5, characterised by the fact that the outlet component (58) is a ring-shaped bit or sealing ring provided at the end of a feed pipe (76) which is screw-connected to the bushing (34), or by the fact that the outlet component (58) is a hollow metal bit with the form of a truncated cone, the outer diameter of this bit at the free end being slightly smaller than the inner diameter of the duct (24) and widening progressively to an out diameter that is wider than that of the duct (24).

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