GB2161885A

GB2161885A - Removing a deposit from within a tube

Info

Publication number: GB2161885A
Application number: GB08418379A
Authority: GB
Inventors: Vincent Norman Harper
Original assignee: GOODWAY TOOLS Ltd
Current assignee: GOODWAY TOOLS Ltd
Priority date: 1984-07-19
Filing date: 1984-07-19
Publication date: 1986-01-22
Also published as: EP0171214A1; GB8418379D0

Description

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GB2161 885A

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SPECIFICATION

Method and apparatus for removing a deposit from within a tube

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The present invention relates to a method and apparatus for removing a deposit from within a tube.

There are numerous applications in which a 10 long tube tends to develope a deposit after extended periods of use. In particular, in heat exchangers, boilers, condensers, absorption machines and evaporators, there are a plurality of tubes which constantly conduct a fluid 15 and any impurity within the fluid can, over a period of time, form a deposit on the inner surface of the tube. With extended use, the cross section of the tube can be severely limited and in an extreme case totally blocked. 20 It has already been proposed to remove deposits from, for example, heat exchangers, by the use of brushes and cutters on a flexible shaft. Because of the flexibility of the shaft, it is not possible to apply pressure at the cutting 25 point and therefore, when the deposit is hard, removal of the deposit can take an extremely long time making the operation unprofitable. For this reason, it has been known to replace a heat exchanger rather than repair it if the 30 deposit is particularly hard.

The present invention seeks to provide a method and apparatus which can enable even a hard deposit to be removed from a tube.

In accordance with a first aspect of the 35 present invention, there is provided a tool for removing a deposit from the inner surface of a tube, comprising a drill string formed of a drill and at least one releasable extension tube, the drill being formed of a hollow tube for 40 transmission of a fluid under pressure to the drill tip, and the tip having an off-centre leading point and at least one lateral cutting edge extending generally parallel to the length of the tube, whereby the drill is self-centring 45 within the tube.

In accordance with a preferred feature of the invention there is provided a method of removing a deposit from the inner surface of a tube, which comprises reboring the tube using 50 a drill string formed of seprable rigid sections and having a self-centring drill tip operative to follow a line of least resistance through the tube.

It is known for the purpose of drilling 55 extremely long cavities to use a so-called gun drill, the name of which is indicative of its intended use. A gun drill is intended to be passed into the material being drilled after an initial guide bore has been formed. Because of 60 the eccentric position of the point of the gun drill, it is not guided by its tip but instead the walls of the hole already cut serve to guide the tip. Therefore, the drill tends to follow a straight line making it ideal for gun barrels. 65 Such drills are of course known, but they are formed in one piece and not in sections.

It has not hitherto been suggested to drill out a long tube, because the tubes do not always follow a straight line, tending on occasions to sag or bend slightly. It is therefore possible if a rigid drill string is used for the tip to pass through the side walls of the tube. The present invention is predicated, however, upon realisation that though the self-aligning action of a gun drill causes it to follow a straight line, it still adopts the line of least resistance. Through the deposit may be hard, it is still softer than the wall of the tube and therefore if the tip is of the nominal inner diameter of the tube it will tend to remove the deposit and align itself automatically with the centre of the tube along the entire of the tube to be cleared, even when the tube is subject to a small amount of sagging.

It is preferable for the drill to polish the interior of the tube at the same time as clearing it, by using a drill which is slightly oversized since a polished inner surface is more resistant to build-up of a new deposit. Hence, a tube after being cleared by the use of a tool of the invention is in someway superior to an original tube.

On occasions, the drill string may be of a significantly smaller diameter than the drill tip and over an extended length the drill string may sag tending to push the tip of the string towards the wall of the tube. In order to mitigate this problem, it is preferably to provide at a point spaced from the tip of the drill a bearing or support surface, which may if desired be a second cutting or grinding surface, whereby the drill makes two point contact with the tube so as to remain aligned with the tube bore even if the drill string may sag.

Because a gun drill like tip does not have a helical cutting edge, tending automatically to remove swarf, the drill is usually tubular and fluid is pumped under pressure to serve the dual purpose of cooling the drill and in particular the tip and removing swarf forcing the swarf backwards. Gun drill are designed to drill into a blind bore and thus the fluid comes out of the drill at its tip via an axial bore. The outer cross section of the drill is such as to allow a return of the coolant back over the outer surface of the drill.

in the present invention, the bore may not be totally blocked, whereupon fluid ejected axially would not have the desired cooling effect. The drill thus preferably includes radial bores for discharging fluid onto the cutting edges directly. The fluid exiting under pressure will follow the line of least resistance pushing the swarf forwards or backwards away from the tip at the same time as cooling the drill.

The individual sections of the drill string may be coupled to each other by means of a conventional screw coupling or by means of a

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bayonet. If desired a washer may be arranged at the coupling to act as a spacer tending to centre the drill string within the cut bore.

Drill strings are of course known from larger 5 scale drilling applications such as oil drills. The same or similar scaled down techniques may be employed for driving the drill string into the tube but in the preferred embodiment of the invention, the drill string may be driven 10 by means of a hand-held power tool with means for rotating the drill and feeding a fluid under pressure down the interior bore of the string.

The tubes in a heat exchanger tend to be 15 arranged in formation and when an automatically guided drill string is to be employed, it is possible to form a jig which is centred on the mouths of adjacent tubes whilst drilling out any one of the tubes of the heat exchanger. 20 The invention will now be described further, by way of example, with reference to the accompanying drawings, in which the single figure shows a perspective view of the tip of a drill of the invention.

25 The drill partially shown in the drawing is genereally similar to a conventional gun drill. The tip 10 is of tungsten carbide and is formed on the end of a shaft 12 having a recess 14 along its entire length. The shaft 30 has an axial bore which opens at the leading faces of the drill via a small bore 16. Unlike in a conventional gun drill, the axial bore also communicates with radial bore (not shown) which lubricate the cutting edge 18. 35 The leading point 20 on the tip of the drill does not lie on the axis of the bore. This is necessary in a gun drill for two reasons. In the first place, the cutting action is improved considerably in that a central point is station-40 ary when the drill is rotating and secondly, the drill tends to remain in a straight line. This is because the tip tends to centre itself on the bore already cut and is not sensitive to slight misalignment of the shaft. Indeed, the shaft is 45 reasonably flexible and may bow during use.

The drill in the drawing is the leading drill of a string which comprises further extension rods which may be coupled in any conventional manner to the leading drill. The cou-50 pling may be a bayonet coupling or a screw coupling.

All the sections of the drill string are hollow tubes to permit fluid under pressure to be pumped to the tip. The fluid is for the pur-55 poses of lubrication and cooling. Furthermore, because the cutting edge is not helical, the tip does not itself drive the swarf away from the cutting edge and fluid serves to extract the swarf from the bore. If the bore is entirely 60 blocked, then the shape of the shaft permits a return flow of the fluid over the outer surface of the drill string but where the bore is still clear then the swarf can also pushed ahead of the drill by the fluid emitted from the hole 16. 65 The drill need only have the cutter at its tip but if the bore is much larger than the shaft the bowing of the shaft if excessive may act to push the drill through the wall of the tube being bores. This may be avoided by provid-70 ing means for maintaining the shaft centred away from the tip. Such means may comprises a bearing formed on the leading drill at a distance from the tip or on each extension or interposed between sections at selected 75 couplings. The bearing should also be provided with axial passages not to obstruct the return flow of the fluid.

The means for centring the shaft at a distance from the tip may alternatively comprise 80 a second cutter or reamer of slightly larger diameter so that the bore is first cut then polished or enlarged. In this case, the second cutter should also be lubricated by the cooling fluid.

85 If the tube to be cleared is entirely blocked, it is necessary to drill first a pilot hole to remove the deposit from the end of the tube. The tip of the drill is then inserted into the bore and if the diameter or the tip corre-90 sponds to the nominal diameter of the bore then the drill will tend to follow the bore and centre itself automatically. The drill may therefore be turned by a hand held power tool during this operation.

95 One the drill has been driven to nearing its full extent, the drill is disconnected from the power tool and an extension tube is coupled to it, the power tool and the fluid supply now being connected to the end of the extension. 100 This operation is repeated until the entire length of the tube has been cleared.

It is possible to form a jig for driving the drill into the bore at a controlled rate and the jig may be centred on the tube in question by 105 feet which engage in the bores of adjacent tubes of a heat exchanger or the like tube assembly.

Because the string is rigid, force may be applied at the cutting point to remove the 110 deposit rapidly. This therefore permits even hard deposits to be removed at a sufficiently rapid rate to warrant repair as opposed to the current practice of replacement of severely blocked tubes. There is little risk nevertheless 115 of the walls of the tubes being damaged. On the contrary, because the tubes are polished they are more resistant to the formation of a fresh deposit and will therefore perform better than a replacement tuube.

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Claims

1. A tool for removing a deposit from the inner surface of a tube, comprising a drill string formed of a drill and at least one 125 releasable extension tube, the drill being formed of a hollow tube for transmission of a fluid under pressure to the drill tip, and the tip having an off-centre leading point and at least one lateral cutting edge extending gener-130 ally parallel to the length of the tube, whereby

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the drill is self-centring within the tube.

2. A tool as claimed in claim 1, wherein a guide is provided on the drill at distance from the tip to maintain the drill in alignment with

5 the tube axis.

3. A tool as claimed in claim 2, wherein the guide comprises a second cutter.

4. A tool as claimed in claim 2, wherein the guide comprises a bearing means for

10 centring the drill on the bore cut by the cutting tip.

5. A tool as claimed in any preceding claim, wherein the cross-sections of the drill and the shaft are non-circular and have a

15 maximum diameter substantially equal to the size of the cut bore, whereby the shaft may be guided by the bore without preventing return flow of the fluid.

6. A method of removing a deposit from

20 the inner surface of a tube, which comprises reboring the tube using a drill string formed of separable rigid sections and having a self-centring drill tip operative to follow a line of least resistance through the tube.

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7. A tool substantially as herein described with reference to and as illustrated in the accompanying drawings.

8. A method of removing a deposit from the inner surface of a tube, substantially as

30 herein described with reference to and as illustrated in the accompanying drawings.

Printed in the United Kingdom for

Her Majesty's Stationery Office, Dd 8818935, 1986. 4235. Published at The Patent Office, 25 Southampton Buildings,

London, WC2A 1AY, from which copies may be obtained.