Difference between DTH(Down the Hole) Drilling and Top ...

The reason customer want to drill the hole is that drill and blast is the most efficient and economic way to break rock instead of excavating it.

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Blast hole drilling equipment

• Handheld rock drills

• Top hammer drills

  •  pneumatic

  • hydraulic

• Down-The-Hole drills
• Rotary drills

  • Rotary cutting

  • Rotary crushing

  • Rotary wearing

 

 

 

 

 

 

 

 

 

 

 

 

 

FOUR ACTIONS IN A DRILLING PROCESS&#;

• ENERGY TRANSFER &#; PERCUSSION / CRUSHING

• FEEDING

• ROTATION

• FLUSHING

 

 

 

 

 

 

 

 

 

Drilling methods        

• Top Hammer
•  Down the Hole
•  Rotary

 Percussion

• Does the real job = breaking the rock.
• Produced by rock drill or hammer&#;s impact energy and frequency.
• Typically between to hits per minute.
• Compared to pneumatic drills, hydraulic drills are capable of higher percussion power and faster penetration rates.
• Percussive drill rig is built around the hammer or rock drill
• in percussive drilling rock is broken with impact energy
• Impact Force per Blow : 20 to 60 Tons
• Blow Frequency:   30 &#; 60 Blows/Sec (Top Hammer), 25 &#; 35 Blows/Sec (DTH)
• Rod/Tube diameters: 23 &#; 140 mm

 

 

 

 

 

 

 

 

 

 

Top hammer drilling principle

• When the piston hits the shank, kinetic energy in the piston creates a compressive wave in the string of drilling tools, that will cause the bit to penetrate the rock
• Piston velocity at impact &#; 10m/s but the compressive wave in the drill steel travels at a velocity of m/s (velocity of sound in steel)
• The bit is forced forward and penetrates the rock around 1.0 mm
• Part of the energy not used to penetrate the rock goes back as a tensile wave and part of it as a compressive wave

 

 

 

 

 

 

 

 

Top hammer drilling energy and efficiency

• Example: Rock drill for TH drilling delivers 18 kW
• Energy is lost in places where the drill string diameter changes, like in joints and threads
• MF rods lost energy about 4% and rods and couplings lost energy around 6%

Efficiency 70% 60% 50% Drilling 12.6 kW 10.8 kW 9 kW Wear / Damage 5.4 kW 7.2 kW 9 kW

 

 

 

 

 

 

 

 

 

 

 

 

 

How rock breaks

• The cemented carbide buttons on the drill bit are put under load each time the piston in the hammer or rock drill strikes
• Rock penetration by percussive drilling can be divided into four phases:

  • Elastic deformation

  • Rock pulverization

  • Crack formation

  • Chipping

• All this happens of course very fast as the hammers strike at a rather high frequency, about 40 to 60 Hz (blows per second)
• How much the button penetrates at each blow depends on the type of rock and shape of the button, but typically it is in the range of 1 mm.

 Rock drill(Top hammer)and  Air hammer(DTH) configuration

 

 

 

 

 

 

Related links:
Factors To Consider When Buying A Water Well Drilling Rig

If you are looking for more details, kindly visit DTH drill pipes.

 

 

 

 

Percussion pressure or power

• Too high: short steel life
• Too low: poor penetration
• Lower pressure needed when
  • Collaring
  • Hitting fractured and layered rock
  • In soft rock

Feeding

• The purpose of the feed is to maintain the drill bit in close contact against with virgin rock across the vertical plane
• To ensure maximum impact energy transfer.
• The feed force must alway be matched to the percussion pressure
• Feed pressure must be always on right level

 

 

 

 

 

 

 

Feeding pressure

• Too high:
  • Bending of steels, 

    hole deviation

  • Increased rotation resistance,

    tight couplings

  • Risk of jamming

  • Difficult collaring

• Too low:
  • Reduced penetration due to poor rock contact
  • Loose couplings, thread burning
  • Poor shank and steel life
  • Higher drifter maintenance cost
• Drilling with a very worn bit or no rotation will cause a strong compressive wave travelling back in the string of tools
• By way of distinction from a tensile wave, the compressive wave will pass through joints which in this case means, it will &#;jump&#; into rock drill and rig
• Tensile waves are more harmfull to the drill steel than compressive waves A stabilizer takes care of the returning compressive wave that otherwise will reflect back in the drill stell as a tensile wave

Rotation

• Purpose of rotation is to turn the drill bit a suitable new position for the next blow. It means the bit must rotate so the buttons on the bit are moved between each blow
• Using the button bits, the periphery is turned about 10mm between blows.
• The optimum distance generates the biggest chips or cuttings and gives the most efficient drilling.
• Rotation pressure must be kept in reasonable for avoiding overtighted threads.

 

 

 

 

 

 

 

 

 

Drill bit rotation speed

• A good indicator of optimal rotation speed is the size of drill cuttings &#; The bigger the better
• Too high:
  •  Too high: Short bit life
• Too low:
  • Low penetration rate
  • Risk of jamming steels
• Rotation pressure (=torque) is a function of feed pressure; feed force adjustments influence the rotation resistance, recommended constant rotation pressure 90-110 bar

 

 

 

 

 

 

 

 

 

 

 

Flushing

• Fuction is to:

  • Remove drill cutting from the hole bottom to the surface by air blowing or water flushing.

  • To cool a bit.

• Medium can be air, water, mist/foam.
• Bailing velocity It is derived by dividing the volume of air by the annular space available between the hole and drill-string.
• Has a direct effect on drilling efficiency.

Bailing velocity

• Bailing Velocity in fpm = Q *( 144 x 4 ) / Π( D^2 &#; d^2)
  • Where,

  • Q = Volume of air available in CFM &#; usually the FAD of compressor after adjustment for altitude

  • D = Diameter of hole

  • d = diameter of drill-rod

  • BV in fpm = feet per minute

• Recommended BV values

  • For cuttings with high or wet density : ~ fpm

  • For cuttings with average &#; low or dry density : ~ fpm

  • Generally, optimum speed: &#; fpm

Insufficient air: for Top Hammer <15 m/s, for DTH(Down The Hole)< 15m/s

• Low bit penetration rates
• Poor percussion dynamics
• Interupt drilling to clean holes
• Plugged bit flushing holes
• Stuck drill steel
• &#;Circulating&#; big chip wear

Too much air: for Top Hammer  >  30 m/s; for DTH  > 60 m/s 

• Excessive drill steel wear
• Erosion of hole collaring point
• Extra dust emissions
• Increased fuel consumption
• Correction factors

• High density rock

• Badly fractured rock (air lost in fractures &#; use water or foam to mud up hole walls)

• High altitude (low density air)

• Large chips

More articles:

Pneumatic tools have been used to drill through rock since the 19th century. Back then, drillers would suspend themselves from a rope over a quarry face to put a drill from the right position and use small diameter holes. The process was inefficient and dangerous. It could send rocks flying and the accuracy was subject as much to luck as it was skill. After a while, drilling crews started carrying jackhammers on masts, beautiful boreholes were misaligned, the explosives that followed could be even more dangerous.

Thankfully, we live in the modern era, where down the hole (DTH) drilling has become one of the most efficient and safest methods since its invention in the mid s. Let&#;s explore what DTH drilling is, why it&#;s such a successful technique, and how it might be the perfect process for your drilling operation.

What is DTH Drilling and how does it work?

DTH drilling is a drilling method that&#;s used to crush rock by using a drilling hammer to hit hard rock repeatedly. DTH drills are pneumatic tools that provide power by driving compressed air into a piston inside the hammer. This type of drilling has made it much easier to drill wells quickly and efficiently by allowing contractors to switch from dirt boring to rock boring simply by adding a compressor and hammer to a drill bit.

Before DTH became a widespread drilling technique, contractors would desert a dirt bore project after encountering hard rock, leaving it to a different set of drillers to continue. With DTH drilling, the same drilling team can work through multiple materials.

At its most basic level, DTH drilling involves attaching a drilling hammer to the bottom of a drill string. Small, percussive hammers are placed above the drill bit, allowing drill pipes to exert a feed force necessary to rotate the hammer and bit together with fluid (usually air, water, or mud). This actuates the hammer and flushes the cutting. Drill pipes are then added to the drill string as the hole gets deeper.

Because the air-powered piston strikes the bit directly, energy is transferred down the hole, minimizing any lost energy. This allows drilling to greater depths.

DTH Drilling is most often used in the mining industry, where blast hole drilling is necessary for projects like water well drilling, building construction, and oil and gas processes.

Types of DTH Drilling Hammers

DTH hammers are fully fluid actuated. There are two primary parts: a valve which controls the flow and a piston that strikes the impact surface. The hammer body allows for stable, straight guidance of the bit. There are three types of hammers that depend on the actuation fluid:

• Air hammers

  &#; These are typically limited to shallow holes of less than 200 meters.

• Water hammers

  &#; These require a lost water flow of between 55 and 744 L/min depending on hammer size to actuate the hammer. These allow for better borehole control and greater depth than air hammers.

• Drilling mud hammers

  &#;  These are the newest types of drilling hammers, but they&#;ve shown a lot of promise so far. They&#;re able to drill on every existing drilling rig by using classical mud composition. They are fantastic choices for drilling into hard rock in geothermal projects due to their ability to drill extremely deep.

What are the benefits of DTH drilling?

DTH drilling is extremely effective, which is why we&#;ve been using it for the past seven decades. Some of the major benefits include:

• The use of air is extremely efficient. You don&#;t have to buy, source, treat, or transport it for use.

• DTH systems are simple to adapt to most directional drills, and it doesn&#;t take long.

• Very little rotational torque is required to cut rock that will allow the hammer to drill straight.

• The same drill can be used to drill through both dirt and rock.

• There isn&#;t as much wear and tear on a drill bit since the hammer is doing most of the work.

• There is a smaller carbon footprint than other techniques due to the use of less drilling mud and additives that need to be disposed of.

• It can be used more often in cities because it produces less noise and vibrations than alternatives. 

• Fewer amounts of additives and drilling fluids are required than for alternatives.

• The rate of penetrations is up to five times greater than rotary drilling techniques.

OK Bit has the DTH Drill Bits you need

If you&#;ve decided DTH is the right drilling method for your project, OK Bit can supply you with whatever bits you need to get the job done. If you aren&#;t sure if DTH drilling is the best choice or if you need help choosing a specific hammer or bit, our expert staff will be happy to help. Contact us today.