OPERATING DTH EQUIPMENT
HALCO Rock Tools
*- YOUR DRILLING EXPERT -* [email protected] - [email protected] @HalcoRockTools - #Halco
This week we are offering you some insights on how to use and make the most of your DTH equipment, with further operating suggestions and cost-saving tips following next week. Read on and enjoy, lots of graphs and images ahead!
COMMISSIONING DTH HAMMERS AND DRILL BITS
SAFETY
Always wear the correct safety equipment.
SUB ADAPTOR
A Sub adaptor will be required if the hammer top thread differs from the drill tube thread.
BIT RETAINING RINGS
Never mix pairs of retaining rings which generally are manufactured as matched pairs and always re-fit them in the same position as when dismantled from hammer.
IDENTIFICATION NUMBERS
NON-RETURN VALVE
You may remove the non-return valve in dry conditions to give a slight increase in performance.
NEW HAMMER OR CHUCK WITH USED DRILL BIT
Check the drill bit splines for wear otherwise damage to the new chuck could occur.
HAMMERS EQUIPPED WITH SPLINE RIVE PINS
Always ensure that a full set of serviceable drive pins are fitted to these hammers before operating otherwise damage to splines will occur. In these circumstances, warranty from the manufacturer will not apply.
GREASE COMPONENTS
Grease all threads and splines when assembling drill bit into hammer.
CHECK DRILL BIT DIAMETER
Never try to use a drill bit which is larger in diameter than a partially drilled hole.
COMMISSIONING
VALVELESS HAMMER OPERATING CYCLE
1. With the piston resting on the bit and the hammer in the drilling position, high pressure air is directed on the piston striking face and lifts the piston. This movement commences the cycle.
2. As the piston travels upwards it covers the top exhaust chamber. Further piston movement covers the live air port to the bottom of the piston and uncovers the foot valve allowing the expanding air to exhaust through the bit. At the same time high pressure air is being directed above the piston from the piston reservoir chamber via the now unsealed air distributor which commences the power stroke.
3. During the power stroke the piston covers the foot valve and the expanding air below the piston stops exhausting through the bit. Further piston movement closes the piston reservoir chamber from the power chamber.
领英推荐
With the piston resting on the bit and the hammer in the drilling position, high pressure air is directed on the piston striking face and lifts the piston. This movement commences the cycle.
4. Lifting the hammer off the bottom of the hole allows the drill bit to drop on to the bit retaining rings. The piston follows the bit to rest on the bit striking face. High pressure air is then directed through the drill bit via the main exhaust holes in the piston and through the bottom chamber which normally feeds the piston for the return stroke.
The hammer is now in the maximum flushing position.
To resume drilling the hammer is again lowered to the bottom of the hole. This results in the bit and piston being pushed up into the normal drilling position. The hammer cycle then resumes automatically.
OPERATING DTH HAMMERS
Penetration rates with down-the-hole hammers are directly proportional to air pressure and, therefore, increasing the pressure will increase the drilling speed as illustrated in the graphs opposite.
Modern valveless hammers are products of precision engineering and are of a strong robust design with few internal parts making them economically attractive and easy to service. Air pressure is applied alternatively to both ends of the hammer piston by means of a system of ports and channels in the piston and cylinder to change the direction of the air.
TOOL LIFE
GRANITE / BASALT
In hard abrasive drilling conditions, it is wear on the external components which will govern the life of the hammer.
DTH hammers are primarily percussion tools and penetrate more by shattering materials than by tearing it. The same air which passes through the hammer causing the piston to reciprocate and strike the bit also serves to expel cuttings from the bore hole and therefore maximum utilisation of the air is achieved.
LIMESTONE
In non-abrasive drilling conditions, where the expected life of the hammer is governed by wear on the internal components, hammer life in excess of 15000 m (50,000 ft) is achievable if the hammer is correctly maintained and lubricated.
HEAVY DUTY HAMMERS
Heavy duty hammers which have thicker wall outer components can provide extra resistance to the onset of external wear. A recent case history revealed that a DOMINATOR 650HD heavy duty hammer, drilling in Ironstone, achieved a life in excess of 9000 m (30,000 ft) compared to little more than 4000 m (13,000 ft) achieved with a standard hammer.
AVERAGE PENETRATION RATES
ROTATION SPEEDS
Where drill bit life and cost is the prime consideration on a drill site, rotation speeds should be carefully monitored.
DTH drill bits are rotary – percussive tools with the emphasis on PERCUSSIVE. Their function is to fracture the material being drilled, which should then be immediately carried away by the exhaust air. Button bits have no cutting or tearing action as such and the effect of rapid rotation can be prejudicial rather than beneficial to the life of the bit, especially in abrasive rock which wears away fast-moving peripheral inserts or in solid dense material which causes the peripheral inserts to overheat and spall due to friction.
If the string is rotated too slowly, this will cause the buttons to impact previously chipped areas of the hole with a resultant drop in penetration speed. As a general guide - the harder the rock or the larger the bit diameter - the slower the rotation speed required. It may be necessary, however, to increase the rotation speed where the rock is badly fissured in order to prevent stalling. It should be remembered, however, that stalling in the bore hole could be the result of a very badly worn bit and increasing the rotation speed in these circumstances will only accelerate the problem.
THRUST (PULLDOWN) / HOLDBACK / TORQUE
Thrust should be kept as low as possible at all times avoiding excessive vibration in the drill string. Holdback should be increased more and more as additional rods are added, as drilling progresses. DTH drilling is primarily percussive drilling using the energy imparted by the hammer piston to the rock through the bit and any attempts to apply too much weight could damage the bit, hammer and drill string and impair the drilling rate.
Although the base of the hammer should maintain contact with the drill bit, there should be neither excess thrust nor vibration due to reaction between the hammer and drill bit. Insufficient thrust will cause the hammer to bounce resulting in a low blow energy to the rock causing vibration and also possible damage.
DRILLING DEPTH CAPABILITY
The depth capability with down-the-hole hammers is governed by two main factors, sufficient air volume to keep the hole clean and the drill rig’s lifting power i.e. its ability to withdraw the drill string from the finished bore hole. The question of hole cleaning or uphole velocity is dealt with in a previous article (UPHOLE VELOCITY).
Crawler mounted drill rigs working on quarries or open pit mining applications are generally designed with sufficient lifting power or pullback capability to lift the weight of the drill string from the completed bore hole which on these types of applications rarely exceeds 35 m (115 ft.) On deep hole applications, such as water well drilling, it is essential that the selected drill rig has sufficient lifting power with a reserve of power (safety factor) for contingencies such as the drill rig’s hydraulic system inefficiency, the weight of the rotary head, friction in the bore hole, potential hole collapse etc.
The above factors combine to reduce the amount of lifting power available for drill tubes and consequently the achievable drilling depth with any particular drill rig.
In reality only around half the gross lifting power can be used. The average weight of 90 -114 mm (3.1/2” - 4.1/2”) dia. drill tubes around 20 kgs. per m (13 lbs per ft) therefore to account for the 50% reduction for safety factor, this figure of 20 kgs. per m should be doubled to 40 kgs. per m (26 lbs per ft). For example, therefore, a drill rig with a gross lifting power of 8000 kgs. (17,600 lbs) would have a safe theoretical depth capability of 200 m (660 ft). Equally to drill a 350 m (1150 ft) deep well with a DTH hammer would require a rig with a gross lifting power of 14000 kgs. (30,800 lbs). A safety factor of 50% should be considered as a maximum.
With good drilling conditions and/or an experienced drilling crew, theoretical depths will be regularly exceeded. On very deep hole applications a common practice in order to increase the drilling depth capability of the drill rig is to attach the casing winch cable to the rotary head and pull simultaneously with the feed system and the winch. Whilst the theory behind this practice appears sound, the reality is that no make of drill rig can “holdback” more weight during drilling, than it can actually lift out on completion of the bore hole. Damage to DTH hammers & bits due to excess thrust (pulldown) being applied during drilling is a common result of this practice.