SELECTING YOUR EQUIPMENT

This week we have quite a bit of information available, as we are offering you a comprehensive guide to the things you should take into account when choosing the right equipment for your drilling needs. We will start off with the basics, covering the most crucial items in the form of hammers, bits, compressors, and lubricants. After this, it is time to branch out, discussing a variety of accessories from bit grinding equipment to shock absorbers. So read on!

NORMAL BIT DIAMETER / AIR PRESSURE SELECTION

Penetration rates are generally dictated by air pressure. The higher the air pressure, the faster the hammer will drill.

For example, if a hole diameter of 165 mm (6.1/2”) was needed with a penetration rate of 30 m/h drilling in granite, an air pressure of around 17 bar (250 psi) would be necessary.

Having selected the hole diameter required for the application in hand, for example 165 mm (6.1/2”), choose a hammer whose outside diameter is smaller than, but the closest to, the hole size to be drilled, for example Halco DOMINATOR 600. This will allow the use of a nominal or standard bit diameter for the chosen hammer in order to achieve the best transmission of blow energy and optimum penetration rate.

HAMMER SIZE / DRILL TUBE DIAMETER SELECTION

Having selected the main drilling elements for example - Hole dia.- 165 mm (6.1/2”) Hammer type - DOMINATOR 600 Drill Tube Dia. 114 mm (4.1/2”) Air Pressure - 17 bar (250 psi), the chosen compressor should have an adequate air volume. A DOMINATOR 600 operating at 17 bar (250 psi) air pressure would require a basic drilling air volume of 20 m3/min. (700 cfm). Most applications, however, dictate the need for greater air volume for contingencies such as flushing or altitude.

When drilling under a high head of water, higher air pressure may be required to overcome the effect of water back pressure. These instances are dealt with overleaf. If, for example, an existing 17 bar (250 psi) capacity compressor is available, but its output volume is insufficient, for example 17 m3/min. (600 cfm), a more fuel-efficient hammer may be selected, such as the MACH 60 which consumes less air than the DOMINATOR 600 at any given air pressure.

Individual drilling conditions, however, will always dictate the final hammer model choice, irrespective of compressor volume available, because any hammer of any make will perform differently in different rock conditions.

Where drilling conditions are extremely abrasive, a heavy-duty hammer with thicker outer casing components, such as the Halco S-DOM 650 HD, may be selected to further resist the onset of external wear due to abrasion. The need to use, wherever possible, a nominal or standard diameter bit rather than an oversize bit, cannot be stressed too strongly, in order to achieve overall optimum.

Oversize drill bits are generally used for overburden drilling in relatively soft conditions but are sometimes used for production drilling in hard rock, although they are not designed for hard rock drilling. A disproportionately high head/ shank diameter ratio increases the risk of excessive spline wear and/or shank breakage under high torque conditions.

DRILL BIT HEAD DESIGNS / INSERT TYPES

Many drill bit manufacturers offer a choice of both head designs and insert types to suit varied drilling applications.

COMPRESSOR – CONTINGENCIES AFFECTING FINAL CHOICE

Drilling at high altitude

Working at high altitude requires a higher air volume to sustain a given pressure merely because the air is thinner as altitude progressively increases.

Flushing

It is normal practice to have a reserve of air volume of at least an extra 20% above that needed for the normal operation of the hammer. This can be useful where repeated intermittent flushing is carried out to blow the hole clean in difficult conditions. Most DTH hammers can pass up to 70% extra air, when lifted up slightly so that the drill bit falls into the flushing position.

Vents / Chokes

When using vents or chokes in the hammer, it is important to ensure that there is sufficient extra air volume to bleed direct to exhaust. If the vent or choke facility bleeds too much air, the resultant air volume from the compressor may be insufficient to sustain the required air pressure at the hammer, leading to a drop in performance.

Drilling rig compressed air needs

If the drill rig is powered by compressed air, extra air volume will be required to cater for the needs of the drill rig’s motors. Generally, 1 m3/min. (35 cfm) is required for each 0.75 kw (1 hp) of motor power. An air powered rotary head with 4 kw (5.30 hp) would need approx. 5.40 m3/min. (190 cfm) at full power/rotation speed. An air powered Dust Collector generally needs 4.50 - 5.50 m3/min. (150-200 cfm). The drill rig’s feed system generally manages on less than 1 m3/min. (35 cfm). All rigs’ functions generally operate at an air pressure of 7-8 bar (100-125 psi).

Compressor life

To provide an optimum service life for a compressor, it should generally work at only 80% of its full capacity. It may be advisable therefore to select a compressor with a larger pressure capability than that needed in order to increase its service life.

Groundwater in the bore hole

Water in the bore hole exerts a back pressure of 1 bar (14.5 psi) for every 10 m (33 ft) of water present. To sustain therefore an air pressure of 17 bar (250 psi) under 70 m (230 ft) of water a compressor output pressure of 24 bar (350 psi) would be required although the air volume needed would only be that necessary to sustain an air pressure of 17 bar (250 psi).

Multiple compressors

On large diameters or deep hole applications, two or more compressors may be connected in tandem feeding into one common airline. It is important to ensure that each compressor is protected by a non-return valve and to remember that the resultant combined output pressure will be equal to that of the compressor with the lowest output pressure.

For example - 17 m3 @ 12 bar + 17 m3/min. @ 17 bar = 34 m3/min. @ 12 bar (600 cfm @ 170 psi + 600 cfm @ 250 psi = 1200 cfm @ 170 psi)

LUBRICANTS

Just like any other piece of precision machinery, the DTH hammer must be lubricated, and small quantities of oil should be injected into the air stream at regular interval whilst the hammer is working. Rock drill oils are recommended because these contain the emulsifying and viscosity additives necessary to deal with high pressure and high air flow conditions in which water is usually present, if only from condensation in the air line.

Oil not only provides slip to prevent pick up and premature failure of components, but it also acts as a seal on the surface of running parts to use air efficiently without pressure loss. It is therefore of paramount importance that the correct grade of oil is used at the appropriate consumption rate to suit volume and pressure, in line with the hammer manufacturers recommendation. Most modern valveless hammers, particularly when operating at high pressures need a heavy oil providing of course that ambient temperatures allow the oil to run through the airline. Halco for example recommend the following –

ACCESSORIES

Foam

Where drilling conditions dictate the need, foam should be made available together with a suitable injection pump and water supply. Details on applications/conditions where water/foam injection would be beneficial can be discussed in a further article.

Hammer lubricators

An air-line lubricator should be available to continually inject oil into the air stream whilst the hammer is operating. Not only should the lubricator be in working order, but it should be of a design which will cope with the compressor air pressure, which can be up to 24 bar (350 psi) or even higher.

Service tools

To avoid damage to hammers during servicing, it is advisable to ensure that the correct service tools are available.

Hammer stripping bench

A purpose-built hammer stripping bench is advisable in order to avoid damage to hammers.

Bit grinding equipment

In abrasive conditions, wear to the tungsten carbide buttons can destroy a drill bit after only a very short life. It is advisable therefore, when drilling in abrasive conditions that suitable grinding equipment is available.

Shock absorbers

Hammer shock absorbers are recommended for use particularly on lightweight rigs to protect the drill tube threads and rotary head from impact vibration, thereby reducing maintenance and drill tube costs. They can also be used to reduce the noise level from the hammer where environmental conditions demand this.

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