IES60541B2 - An exhaust restrictor arrangement for a reverse circulation downhole hammer drill - Google Patents

Description

AM EXHAUST RESTRICTOR ARRAH6EHEMT FOR A REVERSE CIRCULATION OTWWLE 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 in such a drill.

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, 8605α - 2 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 20 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 - 3 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 the chuck.

In the prior art, the fixed sleeve secured to end depending from the chuck is the main air restrictor, but functions most effectively in restricting air flow directly to the face of the bit only when 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 f slidably mounted in the chuck and defining therebetween air exhaust passageways communicating with an alternating supply of pressure air V·· 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 the bit adapted so as to restrict the flow of pressure air directed through the passageways - 4 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 5 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. Preferably, the bit sleeve fits inside, and is capable of sliding movement relative to the chuck. Most preferably, the bit sleeve is 10 shrink-fitted to the bit. Advantageously, a sleeve is fitted to the end of the chuck to match the end outside diameter of the chuck to the outside diameter of the bit and bit sleeve at the head portion of the 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.

A preferred embodiment 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 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 same arrangement as in Figure 1, in which the bit is fully retracted with respect to the chuck.

Referring to Figures 1 and 2 of the drawings, the forward end of a reverse circulation downhole hammer drill comprises an outer wear sleeve 101, an inner cylinder and bush assembly 102, which houses a piston (not shown) which reciprocates within the inner cylinder to repeatedly strike the inner end 103 of the bit 104. The bit 104 is mounted within a chuck 105 and is capable of sliding forwardly within the chuck to the - 5 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. The invention provides a bit sleeve 110 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 welded or attached by other means over the sides of the bit 104. A sleeve 106 is fitted to the outside of the chuck to match the outside diameter of the bit sleeve 110 where it covers the head portion of the bit.

The bit 104 and spline mountings of the chuck 105 define therebetween air passageways 109 which normally communicate with grooves 108 defined in the sides of the bit which would normally be exposed and open to the sides of the bit when the bit is in the protracted position. The bit sleeve 110 directs the flow of pressure air through the air passageways 109, 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 together with sleeve 106. Thus, it will be seen that the bit sleeve 110 moves with the bit, and in effect, forms a telescopic air directional arrangement in conjunction with the chuck with the effect that the positions of the air exhausts 111 are fixed relative to the floating bit 104.

Furthermore, pressure air flowing through the air passageways 109 is confined to emit from air outlets 111 directly to the cutting face 112 of the bit by the bit sleeve 110» Pressure air is therefore confined in a smaller space to change direction via grooves 113 in the face of the bit to enter exhaust inlets 114 and return to the surface via bore 115 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 110, air is only emitting directly io the face of the bit when the bit is fully retracted with respect to the chuck.

Claims (5)

1. An air restrictor arrangement for a reverse circulation downhole hammer drill, which drill comprises a chuck (105) fitted to the working 5 end of the drill, a bit (104) slidably mounted in the chuck and defining therebetween air exhaust passageways (109) communicating with an alternating supply of pressure air provided by the drill and directing said supply air via air outlets (111) to the face (112) of the bit and thence to exhaust inlets (114) defined by the face of the 10 bit, characterised by sleeve means carried by the bit (104) adapted so as to direct the flow of pressure air through the passageways (109) from flowing out to the sides of the bit and to confine the flow to emit directly to the face of the bit.. 15

2. An air restrictor arrangement according to claim 1, wherein said sleeve means comprises a bit sleeve construction (110) fitting over the sides of the bit so as to cover any opening between the air supply passageways (109) formed by grooves (108) in the sides of the bit, thereby directing supply air directly to the face of the bit, wherein 20 the bit sleeve (110) fits inside, and is capable of sliding movement relative to the chuck (105),..

3. An air restrictor arrangement according to claim· 2, wherein the bit sleeve (Π0) is fitted mechanically, whether shrink-fitted and/or 25 positioned positively with a holding-pin or screw or other device and/or threaded and/or welded or attached by other means over the sides of the bit (104).

4. An air restrictor arrangement according to claim 3, wherein a 30 further sleeve (106) is fitted to the end of the chuck to match the end outside diameter of the chuck to the outside diameter of the bit and bit sleeve (110) at the head portion of the bit.

5... An air restrictor arrangement according to any of the preceding 35 claims, substantially as described herein with reference to Figures 1 and 2 of the accompanying drawings.