EP0171214A1

EP0171214A1 - Apparatus for removing a deposit from within a tube

Info

Publication number: EP0171214A1
Application number: EP85305121A
Authority: EP
Inventors: Vincent Norman Harper
Original assignee: Goodway Tools Ltd
Current assignee: Goodway Tools Ltd
Priority date: 1984-07-19
Filing date: 1985-07-18
Publication date: 1986-02-12
Also published as: GB8418379D0; GB2161885A

Abstract

The invention proposes the cleaning of deposit from blocked tubes, for example of a heat exchanger, by drilling through the deposit using a gun-drill like drill formed in section which may selectively be coupled to one another. The drill 100 has a guide 102 spaced from its tip 10 to ensure that the drill follows the axis of the tube without damaging the walls of the tube, while permitting sufficient pressure to be applied to drill through even hard deposits.

Description

Field of the invention

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

Background of the invention

There are numerous applications in which a 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 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.
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 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 deposit is particularly hard.
It is known for the purpose of drilling 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 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. Such drills are of course known, but they are formed in one piece and not in sections.
In British Patent Specification No. 1 531 585, a rotary tool is described for removing deposits from tubular passages which comprises a flat blade on the forward end of a shaft, the blade having a point offset from the shaft axis. In this case, however, the blade is the only part of the drill to contact the walls of the passage being cleaned and the drill is therefore inadequately constrained to follow the passage. In the event of misalignment between the drill shaft and the passage, the latter can be severely damaged. Such misalignment in practice can be caused either by sagging of the drill string, tending to urge the blade through the walls of the passage, or by sagging of the walls of the passage whereupon the walls of the passage can still be damaged if the drill continues to follow a straight bore. The problem is particularly severe when the drill string is of significantly smaller diameter than the tube being cleaned.

Object of the invention

The present invention seeks to provide apparatus which can enable even a hard deposit to be removed from a tube without risking damage to the walls of the tube.

Summary of the invention

In accordance with the 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 shaft for transmission of a lubricant 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 within the tube, and guide means being provided on the drill string at distance from the tip to engage the cut bore and maintain the axis of the drill in alignment with the tube axis.
As mentioned above, there is a risk of sagging in the tube. It is therefore possible if a rigid drill string is used for the tip to pass through the side walls of the tube. In contrast with the single point contact of the prior art which relies entirely on the blade to effect guiding of the drill string, the drill string in the present invention makes contact with the tube walls both at its tip and at the guide means displaced along the length of the drill string from the tip. The drill therefore follows in the short term the straight line of least resistance but over a length of tube is capable of bending to follow a curve of a minimum radius determined by the distance between the tip and the guide means.
The guiding is assisted by the fact that though the deposit may be hard, it is 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 some ways superior to an original tube.
Thus in a preferred embodiment of the invention, the guide means for making the second contact with the walls of the tube comprises a second cutter or reamer for polishing the bore cut by the tip of the drill. Alternatively, however, the guide means may be in the form of a bearing surface for merely sliding along the cut surface of the tube bore.
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 drills 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 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 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 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 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.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which :

Figure 1 is a schematic view showing a drill,
Figure 2 is a more detailed perspective view of the tip of the drill in Figure 1,
Figure 3 shows a cross-section through a junction between two sections of drill string.

Description of the preferred embodiment

Figure 1 shows a drill 100 having a leading cutter at its tip 10 which is shown in more detail in Figure 2. At a distance from the tip 10 is a guide means 102 the main function of which is to ensure the drill makes two point contact along its length with the walls of the tube being cleaned thereby assuring accurate self-alignment of the drill in the tube. The guide means as illustrated is intended to function as a reamer or second cutter for polishing the tube bore but alternatively it may merely act as a guide for sliding along the bore without further working the surface of the bore. The trailing end of the drill is partly cut away in Figure 1 to reveal an internal thread 104 for coupling the extension tubes of the drill string.
The drill tip 10 shown in more detail in Figure 2 is generally 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 has an axial bore which opens at the leading faces of the drill via a small bore 16. Unlike a conventional gun drill, the axial bore also communicates with radial bores 22 which lubricate the cutting edge 18.
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 stationary 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 reasonably flexible and may bow during use.
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 purposes 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.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.
It will be noted that the return flow of the lubricant must pass by the guide means and it is important therefore that the latter, be they a cutter or a bearing should have a non-circular cross-section so as not to obstruct the bore. If the guide means are in the form of a second cutter it is preferable that lubricant other than the return flow from the tip of the drill be used to cool and lubricate the cutting edge.
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 corresponds 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.
Once 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. This operation is repeated until the entire length of the tube has been cleared.
The coupling between two sections of drill string is shown in Figure 3, where it is also illustrated that a non-circular washer 130 may be placed between the sections to act as further guide means to reduce any tendency for the weight of the drill string to mis-align the drill in the tube.
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 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 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 of the walls of the tubes being damaged. On the contrary, because the tubes are preferably polished they are more resistant to the formation of a fresh deposit and will therefore perform better than a replacement tube.

Claims

1. A tool for removing a deposit from the inner surface of a tube, comprising a drill string formed of a drill (100) and at least one releasable extension tube, the drill (100) being formed of a hollow shaft (12) for transmission of a lubricant fluid under pressure to the drill tip (10), and the tip (10) having an off-centre leading point (20) and at least one lateral cutting edge (18) extending generally parallel to the length of the tube, whereby the drill is self-centring within the tube, characterised in that guide means (102) are provided on the drill string at distance from the tip (10) to engage the cut bore and maintain the axis of the drill (100) in alignment with the tube axis.

2. A tool as claimed in claim 1, wherein the guide means (102) comprise a second cutter. 1

3. A tool as claimed in claim 1, wherein the guide means (102) comprise a bearing means for centring the drill on the bore cut by the cutting tip (10).

4. A tool as claimed in any preceding claim, wherein the cross-section of the guide means (102) is non-circular, whereby the drill may be guided by the bore without preventing return flow of lubricant fluid.

5. A tool as claimed in any preceding claim, wherein fluid outlets (22) are provided in the drill tip to enable radial flow of the lubricant fluid.