IES58633B2 - An exhaust restrictor arrangement for a reverse circulatio downhole hammer drill - Google Patents

Description

AN EXHAUST RESTRICTOR ARRANGEMENT FOR A REVERSE CIRCULATION DOWNHOLE HAMMER DRILL The present invention relates to an exhaust restrictor arrangement for a reverse circulation downhole hammer drill, in particular to an exhaust restrictor sleeve construction fitted directly to the bit and/or to the chuck in such a drill, and to a novel bit construction which incorporates an integral side exhaust passageway structure which is identical in function to the combination of the restrictor sleeve and the bit.

In conventional reverse circulation downhole hammer drills pressure air is supplied to alternate sides of a hammer piston which acts as a self-reciprocating valve to control the flow of pressure air, but instead of exhausting air and cuttings around the annulus space between the drill and the borehole, exhaust inlet passageways opening to the face of the bit and communicating through a central bore in the piston carry exhaust air and cuttings through the drill and drill string to the surface. Pressure air must therefore be supplied to the face of the bit around the sides of the bit via air passageways, which in most conventional reverse circulation drills, open to the side of the bit and are formed as grooves in the side of the bit. To prevent excessive supply air from passing up the annulus between the drill and the borehole wall, an increased diameter portion or air restrictor is present, preferably as near to the face of the bit as possible, normally in the form of a sleeve secured to, and depending from the chuck in which the bit is free to slide. There must of course be a small clearance gap between the air restrictor and the borehole wall, i.e., the bit cutting face is of greater diameter than the air restrictor. However, when the bit falls forward in the chuck, the hammer is not drilling and all air passes through the hammer in the 'blow mode' in order to clean all cutting and debris from the hole. Pressure air now escapes to the sides of the bit allowing excessive cuttings and/or air to flow past the increased diameter air restrictor and up to surface via the outside annulus. This condition is more prevalent in unconsolidated ground conditions, as the walls of the drilled hole are unstable. If the air exhaust and restrictor are not close to the cutting face there is a high likelihood that the pressurised air will erode the sides of the hole thus increasing its diameter and negating the effect of the restriction. Conversely the closer the exhaust point and restriction the less chance there is of this occurring.

In reverse circulation drilling, it is therefore essential to ensure that cuttings and debris from the hole being drilled are returned to the surface via the internal bore arrangement through the drill and not via the annulus space. Furthermore, the closer the air restrictor is to the face of the bit, the less chance there is of air being diverted into cracks or cavities in the borehole wall which may lead to a loss of circulation and blockage of the exhaust inlets.

Also, it is important to supply air at all times during the drilling cycle as close as possible to the cutting face of the bit, as this facilitates the supply air to change direction at the cutting face to enter the exhaust inlets and return to the surface. This is more important when drilling in formations which are loose and unconsolidated, as the borehole wall may be unstable. If the supply air outlets and air restrictor are not close enough to the cutting face, there is a higher probability that the pressurised air will erode the sides of the borehole, thus increasing its diameter, and therefore negating the effect of the air restrictor.

As mentioned above it is important to supply air at all times [whether the hammer is cutting or cleaning the hole viz. 'operating mode' or blow mode] as close as possible to the face of the bit. This is the problem which is addressed by the present invention which provides a fixed position of both the restriction [of cuttings blowing up the outer annulus] and air exhaust relative to the floating bit and that fixed position is close to the cutting face of the bit. It is desirable to direct as much pressure air to the face of the bit and directly to the exhaust inlets as is possible. The present invention seeks to provide an improved restrictor arrangement to the sides of the drill bit in a reverse circulation hammer drill with the aim of directing a greater volume of supplied pressure air to the face of the bit. Furthermore, it is an object of the present invention to provide a restrictor sleeve which may be fitted to a conventional reverse circulation drill bit so as to modify and improve its performance and to cooperate with a fixed air restrictor sleeve fitted to the chuck.

In an alternative arrangement, a bit for a reverse circulation hammer drill is provided by the invention which performs the same function as the conventional bit in combination with the auxiliary restrictor sleeve referred to above, but wherein the extended enclosed air supply passages so formed in the combination are machined from the bit so as to form an integral structure.

In the prior art, the fixed sleeve secured to and depending from the chuck is the main air restrictor, but only functions to restrict air flow directly to the face of the bit once the bit is fully retracted, i.e. when the air passageways formed by grooves in the side of the bit are fully covered by the sleeve. As the bit moves away from its fully retracted position, a gap opens up around the sides of the bit allowing air to escape to the annulus space forward of the air restrictor. Thus, the inherent weakness of this system lies in the fact that the air restrictor sleeve is variable in position relative to the bit which reciprocates inside the chuck, during its various operating functions.

It has been proposed in the prior art to extend the air restrictor sleeve forwardly so as to cover this gap, but this extension skirt is prone to excessive wear and must be regarded as sacrificial, i.e. it will require frequent replacement and will only operate at maximum efficiency when new.

Accordingly, the present invention provides an air restrictor arrangement for a reverse circulation downhole hammer drill, which drill comprises a chuck fitted to the working end of the drill, a bit slidably mounted in the chuck and defining therebetween air exhaust passageways communicating with an alternating supply of pressure air provided by the drill and directing said supply air via air outlets to the face of the bit and thence to exhaust inlets defined by the face of the bit, characterised by sleeve means carried by or defined by the bit adapted so as to restrict the flow of pressure air directed through the passageways from flowing out to the sides of the bit and to confine the flow to emit directly to the face of the bit.

Preferably, said sleeve means comprises a bit sleeve construction fitting over the sides of the bit so as to cover any opening between the air supply passageways formed by grooves in the sides of the bit, thereby directing air supply directly to the face of the bit. Most preferably, the bit sleeve fits inside, and is capable of sliding movement relative to, a fixed air restrictor fitted to and depending from the chuck. Alternatively, the bit sleeve engaging with the bit may be adapted to extend rearwardly from the head of the bit to form a shroud portion which in use is adapted for sliding engagement in an annular groove defined in a collar engaging around the chuck.

The bit sleeve may comprise an outwardly curved restrictor engaging the distal end of the skirt when the bit is in the retracted position. Alternatively, the bit sleeve may have a tapered tip in which case the distal end of the skirt engages the retracted bit.

The advantage of the exhaust restrictor arrangement of the present invention is that it serves to direct air supply directly to the face of the bit at all stages of the cycle and blocks the escape of cuttings and excessive pressure air to the sides of the borehole wall even when the bit is fully protracted from the chuck.

In another aspect, the invention provides a bit for a reverse circulation downhole hammer drill which defines pressure air supply passageways opening only to the front face of the bit, and the restriction is effected by a fixed sleeve attached to the chuck.

Preferred embodiments of an air restrictor arrangement for a reverse circulation downhole hammer drill will now be described with reference to the accompanying drawings, in which: Figure 1 shows a cross-sectional view of a first embodiment of the invention, in which a bit sleeve is fitted to the sides of a bit, which is shown in a fully protracted position with respect to the chuck.

Figure 2 shows the first embodiment of the invention as in Figure 1, in which the bit is fully retracted with respect to the chuck.

Figure 3 shows a second embodiment of an air restrictor sleeve fitted to the sides of a bit in which the bit is fully protracted with respect to the chuck.

Figure 4 shows the second embodiment of the invention as in Figure 3, in which the bit is fully retracted with respect to the chuck.

Figure 5 shows a third embodiment of the invention which is a bit formed with integral pressure air supply passageways opening only to the face of the bit, in which the bit is in a fully protracted position relative to the chuck.

Figure 6 shows the third embodiment of the invention, as in Figure 5, in which the bit is fully retracted with respect to the chuck.

Figures 7 and 8 show a fourth embodiment of the invention which is similar to the third embodiment shown respectively in Figures 5 and 6.

Figures 9 and 10 show a fifth embodiment of the invention similar to the first embodiment shown in Figures 1 and 2, but including a rearwardly-extending shroud portion on the restrictor sleeve.

Figures 11 and 12 show a sixth embodiment of the invention similar to the fifth embodiment shown in Figures 9 and 10, in which a fixed air restrictor sleeve fitted to the chuck is replaced by a collar for cooperation with the shroud portion, and Figures 13 and 14 show a seventh embodiment of the invention similar to the sixth embodiment shown in Figures 11 and 12, with a shortened shroud portion.

Referring to Figures 1 and 2 of the drawings, the forward end of a reverse circulation downhole hammer drill comprises an outer wear sleeve 1, an inner cylinder 2, which houses a piston (not shown) which reciprocates within the inner cylinder to repeatedly strike the inner end 3 of the bit 4. The bit 4 is mounted within a chuck 5 and is capable of sliding forwardly within the chuck to the position as shown in Figure 1 where it is fully protracted, or to a position as shown in Figure 2 where it is fully retracted, with respect to the chuck. An increased diameter air restrictor sleeve 6 is fitted to the outside of the chuck and includes a skirt portion 7 which depends from the forward end of the chuck.

The bit 4 and spline mountings of the chuck 5 define therebetween air passageways 9 which normally communicate with grooves 8 defined in the sides of the bit which would be covered by the skirt portion 7 of the air restrictor 6 when the bit is retracted, but which would normally be exposed and open to the sides of the bit when the bit is in the protracted position. However, the invention provides a bit sleeve 10 which is fitted mechanically, whether shrink-fitted and/or positioned positively with a holding-pin or screw or other device and/or threaded and/or attached by other means over the sides of the bit 4. The bit sleeve 10 directs the flow of pressure air through the air passageways 9, even when the bit is in the protracted position as shown in Figure 1 and concurrently provides a primary restriction on the flow of air up the outside annulus. Thus, it will be seen that the bit sleeve 10 moves with the bit, and in effect, forms a telescopic air directional arrangement in conjunction with the restrictor 27 and the fixed air restrictor sleeve 6, as it slides and fits within the depending skirt portion 7 of the air restrictor sleeve 6 with the fi effect that the positions of both the restrictor 27 and the air exhaust 11 are fixed relative to the floating bit 4. (Alternatively the bit may be manufactured as one piece having the bit sleeve 10 machined out of solid bar or forging).

Furthermore, pressure air flowing through the air passageways 9 is confined to emit from air outlets 11 directly to the cutting face 12 of the bit by the primary restrictor 27. Pressure air is therefore confined in a smaller space to change direction via grooves 13 in the 10 face of the bit to enter exhaust inlets 14 and return to the surface via bore 15 in the bit. Thus, throughout the cycle of operations pressure air is always confined to emit directly to the face of the bit. Without the sleeve 10, air is only emitting directly to the face of the bit when the bit is fully retracted with respect to the chuck.

Alternatively, as shown in Figures 3 and 4, a bit sleeve 18 is either mechanically fitted or manufactured from solid bar or forging. It will be noted that the tapered tip 22 avoids a constriction at the exhaust air outlets 11. In this case the restrictor 27 is not on the 20 bit sleeve 18 but on the fixed air restrictor 6, and skirt portion 7 but the air exhaust point 11 is fixed in relation to the floating bit.

Third and fourth embodiments of the invention are shown with reference to Figures 5 and 6, in which the head of the bit 20 is machined so as to form integral air exhaust outlet passageways 21 which perform the same function as the conventional bit in combination with the auxiliary air restrictor sleeve 10 as described above with reference to Figures 1 and 2. Figures 7 and 8 show another arrangement in which a greater amount of air is directed to the sides of the bottom 30 hole.

Figures 9 and 10 show a fifth embodiment of the invention similar to the first embodiment described above with reference to Figures 1 and 2, but including a rearwardly-extending shroud portion 25 on the restrictor sleeve 10, adapted to slide within an annular groove 23 defined between the fixed air restrictor sleeve 6 and the chuck 5. The depending skirt portion 7 of the fixed air restrictor sleeve 6 covers and protects shroud portion 25 at all stages of the cycle between full protraction (Fig. 9) and full retraction (Fig 10) of the bit. In this embodiment the skirt portion 7 acts only as a protection to shroud portion 25.

Figures Π and 12 show a sixth embodiment of the invention similar to the fifth embodiment described above with reference to Figures 9 and 10, in which the fixed air restrictor sleeve 6 fitted to the chuck does not have the skirt portion 7 attached and is replaced by a collar 24 for cooperation with the shroud portion 25, again defining an annular groove 23a between the collar and the chuck 5. Thus the fixed air skirt portion 7 is not an essential part of the arrangement.

Figures 13 and 14 show a seventh embodiment of the invention similar to the sixth embodiment described above with reference to Figures 11 and 12, but with a shortened shroud portion 25 which clears the groove 23a in the fully protracted position shown in Fig. 13, allowing some pressure air to emit through gaps 26 when the bit has dropped fully forward to clean the sides of the hole, in use.

Claims (5)

1. An air restrictor arrangement for a reverse circulation downhole hammer drill, which drill comprises a chuck (5) fitted to the working end of the drill, a bit (4) slidably mounted in the chuck and defining therebetween air exhaust passageways (9) communicating with an alternating supply of pressure air provided by the drill and directing said supply air via air outlets (11) to the face (12) of the bit and thence to exhaust inlets (14) defined by the face of the bit, characterised by sleeve means carried by or defined by the bit (4) adapted so as to direct the flow of pressure air through the passageways (9) from flowing out to the sides of the bit and to confine the flow to emit directly to the face of the bit.

2. An air restrictor arrangement according to claim 1, wherein said sleeve means comprises a bit sleeve construction (10) fitting over the sides of the bit so as to cover any opening between the air supply passageways (9) formed by grooves (8) in the sides of the bit, thereby directing supply air directly to the face of the bit, wherein the bit sleeve (10) fits inside, and is capable of sliding movement relative to,, a fixed air restrictor (6) fitted to the chuck (5).

3. An air restrictor arrangement according to claim 2, wherein the sleeve (10) engaging with the bit may be adapted to extend rearwardly from the head of the bit to form a shroud portion (25) which in use is adapted for sliding engagement in an annular groove (23,23a) defined in the fixed air restrictor (6) or in a collar (24) engaging around the chuck(5).

4. An air restrictor arrangement according to claim 1, wherein the bit defines pressure air supply passageways (21) opening only to the front face of the bit (20).

5. An air restrictor arrangement according to any of the preceding claims, substantially as described herein with reference to any of Figures 1 and 2, 3 and 4, 5 and 6, 7 and 8, 9 and 10, 11 and 12, or 13 and 14 of the accompanying drawings.