dth training manual - el dorado sistemas · 2016-06-03 · the second chapter looks at hammer care....

Copyright ©Mincon 2010, All Rights Reserved.

DT

H T

raining Manual

Mincon – “The Driller’s Choice”

CONTENTS

Introduction ............................................................................................................................................................................................... 4

Chapter 1: About compressed Air .............................................................................................................................................................. 5

What is a Compressor ............................................................................................................................................................................ 5

The Affect of Temperature and Altitude on Compressed Air ................................................................................................................ 5

Up-hole Air Bailing Velocity ................................................................................................................................................................... 6

Compressor Modulation and Air Pressure ............................................................................................................................................ 6

Backpressure ......................................................................................................................................................................................... 7

Multiple Compressors ........................................................................................................................................................................... 7

Chapter 2 Hammer Care ............................................................................................................................................................................ 8

Storage .................................................................................................................................................................................................. 8

Commissioning ...................................................................................................................................................................................... 8

Lubrication ............................................................................................................................................................................................. 9

Effects of Corrosion ............................................................................................................................................................................... 9

What is Galling? ................................................................................................................................................................................... 11

Chapter 3: Safety Considerations............................................................................................................................................................. 13

Chapter 4: DTH Hammer Set Up .............................................................................................................................................................. 14

Choke Selection ................................................................................................................................................................................... 14

Drill Bit Care and Installation ............................................................................................................................................................... 14

Drill Bit Handling ............................................................................................................................................................................. 14

Drill Bits: Prior to Use ...................................................................................................................................................................... 15

Drill Bit Installation .......................................................................................................................................................................... 15

Torquing Up a DTH Hammer ............................................................................................................................................................... 16

Chapter 5: DTH Hammer Operation ........................................................................................................................................................ 18

Lubrication Check ................................................................................................................................................................................ 18

Commissioning a New DTH Hammer................................................................................................................................................... 18

Collaring the Hole ................................................................................................................................................................................ 18


Feed Force/Pressure ............................................................................................................................................................................ 19

Rotation ............................................................................................................................................................................................... 20

Hole Cleaning, Flushing and Dust Suppression .................................................................................................................................... 21

Chapter 6: Drill Bit Failures and Causes ................................................................................................................................................... 22

Chapter 7: Drill Bit Sharpening................................................................................................................................................................. 29

Restore Carbide Shape ........................................................................................................................................................................ 30

Restore Gauge Flat .............................................................................................................................................................................. 32

Restore Taper Relief ............................................................................................................................................................................ 32

Recondition Flushing Grooves ............................................................................................................................................................. 33

Appendix 1: Conversion Chart ................................................................................................................................................................. 34

Appendix 2: Glossary of Abbreviations .................................................................................................................................................... 35

Appendix 3: Thread Identification API Standards .................................................................................................................................... 36

Appendix 4: Recommended Torque for Tool Joint Connections ............................................................................................................. 37


INTRODUCTION

This training manual looks at the complete drilling process. In the first chapter it discusses compressed air as a medium to operate “Down the Hole” (DTH) hammers. It looks at the effect of temperature and altitude on compressors ability to compress air. Up-hole bailing velocity is dealt with next looking at the minimum requirements for DTH drilling and providing a method to calculate the up-hole velocity for a given any set of circumstances. Air modulation and air pressure is looked into and the effect it has on performance. Finally, back pressure and it’s affect on drilling is tackled finishing off with multiple compressors tagged together in series.

The second chapter looks at hammer care. It deals with the procedures that should be followed when storing a DTH hammer, the steps to follow when commissioning a new hammer and lubrication of a DTH hammer. Following that the effects of corrosion and the topic of galling is discussed to understand their detrimental effect on DTH hammers.

The next chapter briefly looks at some safety considerations. Chapter 4 deals with the setup of a DTH hammer. Choke selection is discussed first. Next drill bit care including handling, inspection and installation is dealt with. Finally, the importance of proper closure of the hammer is covered focusing on correct torque and the effects of not completing this task properly.

Chapter 5 discusses the operation of a DTH hammer. Checking to ensure proper lubrication starts the chapter followed by commissioning a new DTH hammer. Collaring the hole, feed pressure and rotation are discussed finishing off with the importance of hole flushing.

Chapter 6 looks at drill bit failures and the causes. Understanding why certain failure occurs helps a driller to recognize them when they happen and what drilling or maintenance procedures to change to avoid them in the future. The final chapter looks at the correct procedures for sharpening drill bits to ensure correct operation and maximum performance. The appendices contain useful drilling information to aid the driller or supervisor.


CHAPTER 1: ABOUT COMPRESSED AIR

WHAT IS A COMPRESSOR

A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. Many compressor types are available for a number of different applications. Within the drilling industry, rotary screw compressors are the most popular. Rotary screw compressors use two meshed rotating positive-displacement helical screws to force the gas into a smaller space. The high pressure compressors used in modern drill machines employ a two stage method of compression. They also make use of cooling lubricant to remove the heat of compression. The rating of compressors is expressed as delivering a certain volume of air operating at a given pressure, for example, 900 cfm at 350 psi.

THE AFFECT OF TEMPERATURE AND ALTITUDE ON COMPRESSED AIR

Both temperature and altitude have an effect on air and consequently on compressed air. Higher temperatures and higher altitudes result in air becoming thinner, less dense, and the effect of this is a reduction in a compressor’s delivery pressure.

The table below shows just how much operating pressure can be affected by these two factors. For example, if we take a compressor which will deliver 1000 cfm at sea level, this same compressor will only deliver 745 cfm at 9,000ft, given a temperature of 40 degrees Fahrenheit (4.4 degrees Celsius).

°F °C Sea Level 1000ft 3000ft 5000ft 7000ft 9000ft 11000ft 13000ft 15000ft

Sea Level 305m 915m 1524m 2134m 2744m 3354m 3963m 4573m

-40 -40 0.805 0.835 0.898 0.968 1.043 1.127 1.217 1.317 1.426 -30 -34.4 0.824 0.855 0.920 0.991 1.068 1.154 1.246 1.349 1.460 -20 -28.9 0.844 0.875 0.941 1.014 1.092 1.180 1.275 1.380 1.494 -10 -23.3 0.863 0.895 0.962 1.037 1.117 1.207 1.304 1.411 1.528 0 -17.8 0.882 0.915 0.984 1.060 1.142 1.234 1.333 1.443 1.562

10 -12.2 0.901 0.935 1.005 1.083 1.167 1.261 1.362 1.474 1.596 20 -6.7 0.920 0.954 1.026 1.106 1.192 1.288 1.391 1.506 1.630 30 -1.1 0.939 0.974 1.048 1.129 1.217 1.315 1.420 1.537 1.664 40 4.4 0.959 0.994 1.069 1.152 1.241 1.341 1.449 1.568 1.698 50 10 0.978 1.014 1.091 1.175 1.266 1.368 1.478 1.600 1.732 60 15.6 0.997 1.034 1.112 1.198 1.291 1.395 1.507 1.631 1.766 70 21.1 1.016 1.054 1.133 1.221 1.316 1.422 1.536 1.662 1.800 80 26.7 1.035 1.074 1.155 1.244 1.341 1.449 1.565 1.694 1.834 90 32.2 1.055 1.094 1.176 1.267 1.365 1.475 1.594 1.725 1.868

100 37.8 1.074 1.114 1.198 1.290 1.390 1.502 1.623 1.756 1.902 110 43.3 1.093 1.133 1.219 1.313 1.415 1.529 1.652 1.783 1.936 120 48.9 1.112 1.153 1.240 1.336 1.440 1.556 1.681 1.819 1.970


UP-HOLE AIR BAILING VELOCITY

The up-hole air bailing velocity is the velocity of the air as it exhausts from the hole and returns to the surface carrying cuttings. Sufficient volume of compressed air is required to maintain pressure at the hammer and to maintain sufficient bailing energy to clean the hole.

Bailing velocity requirements vary with bailing rates, material hardness, density and depth of hole. The higher the drilling rate and density of the material the higher the bailing velocity must be.

Bailing velocity for hammers operating pressure in the range of 200 to 350 psi should be 3000 feet (900m) per minute minimum. Recommended minimum bailing velocity is 5000-7000 feet (1500-2100m) per minute. Velocities in excess of 7000 feet (2100m) per minute are not uncommon but in abrasive material this can cause rapid erosion of the bit body and hammer.

Bailing velocities are calculated by using hole diameter, drill pipe diameter and the actual volume of air delivered down the hole. Make sure to allow for the affect of altitude and temperature when calculating the actual volume.

• VF = Velocity in feet per minute. • Y(CFM) = CFM of air passed by

hammer • DL2 = diameter of hole squared

in inches • di2 = diameter of drill tube

squared in inches

• VM = Velocity in meters per minute

• X(M3) = M3/min of air passed by hammer

• DM2 = diameter of hole squared in mm

• dm2 = diameter of drill tube squared in mm

COMPRESSOR MODULATION AND AIR PRESSURE

Screw compressors with pressure sensing and a ‘running blow down’ system maintain the rated pressure by restricting the air intake and bypassing compressed air directly to atmosphere. At maximum rpm the compressor output is governed continuously by the pressure sensing system to deliver compressed air at the rated pressure to the maximum rated volume or modulate and deliver an allowed volume of air as demanded at the same pressure.

On a compressor rated to 350 psi, the air sensing adjustment on the compressor should be adjusted to commence closing the air intake valve as near as possible to 350 psi pressure, that is adjust to partially restrict air intake to the compressor at a pressure less that 350 psi and close at 350 psi pressure. This restriction may occur at lower pressures than 330 psi on incorrectly adjusted compressors.

A compressor rated to deliver 900 cfm at 350 psi will modulate if used with a hammer consuming 800 cfm at 350 psi and may start to modulate at 330 psi slowly increasing air pressure to 350 psi but restricting the volume of air delivered to the hammer’s requirement of 800 cfm. Alternatively it may unload at 350 psi pressure continuously varying pressure between 330 psi and 350 psi


pressure with the hammer receiving less than 800 cfm. The compressor will run below rated capacity and the hammer will operate at less efficiency than it would if all the air was available with reduced bailing velocity and drilling performance in the hole being drilled. When compressors continually modulate, chokes assist in reducing modulation, hold higher pressure and pass more air to increase bailing velocities and drilling rates.

BACKPRESSURE

As drilling depth increases, so does back pressure due to increase volume of drill cuttings in the hole. This increases further when there are high volumes of water entering the hole. It requires 0.434 psi (0.03 bar) to overcome 1 foot (0.305m) of water. So, if there is 50ft (15.25m) of water in the hole there will be a backpressure of 22 psi (1.5 bar). Operating pressure will need to be increased to overcome the higher backpressures.

A simple way to approximate the backpressure created from water in the bore hole is to lift off bottom in the blowing mode and read the hammer pressure on the drill machine’s gauge. This will be approximately what the backpressure created by the water is.

If for example the hammer is operating at 360 psi in the drilling mode and the gauge reads 80 psi on lift off in the blowing mode, the actual pressure drop across the hammer is 280 psi and thus is the actual working pressure of the hammer. As the backpressure increases a point will occur when the pressure drop across the hammer is insufficient to operate the hammer and drilling will cease. In such cases using a choke and/or injecting foam will aid in increasing the up-hole velocity and lift the cuttings through the water and decrease the backpressure.

MULTIPLE COMPRESSORS

On large diameter holes, deep holes or holes with a large quantity of water are encountered two or more compressors may be connected together feeding one common airline. It is essential to ensure that each compressor is protected by a non-return valve. The resultant combined output pressure will be equal to the compressor with the lowest output pressure.


CHAPTER 2 HAMMER CARE

STORAGE

It is important to take the following steps in order to ensure smooth operation when hammer is to be re used. When the hole is completed and the hammer is to be inactive for some time the following steps should be taken.

Short Term Storage (1 -2 weeks)

• Using high pressure air, blow the hammer clear of all water.

• Turn automatic oiler up full and cycle hammer until oil is running out the shank of the drill bit.

• If there is not an automatic oiler, pour 1 liter (2 pints) of rock drill oil in to the back head/Top sub.

• Turn the air on and cycle for 10 seconds. This will lubricate the internal parts.

• Seal the hammer at the back head (top sub) and chuck (drive sub) end with a rag to exclude any dust or foreign particles.

• Store the hammer horizontally in a clean dry environment.

Long Term storage (1 month or longer)

• Using high pressure air, blow the hammer clear of all water.

• Break out the back head and chuck on the rig as it is easier to do it here than back in the workshop.

• Disassemble the hammer.

• Inspect and clean all hammer components.

• Lubricate all the internal components with rock drill oil.

• Reassemble the hammer, and seal the back head and chuck end.

• Store horizontally in a clean and dry environment.

Before restarting any hammer after prolonged periods of inactivity, disassemble and inspect all internal parts. Clean and remove any oxidation with an emery cloth. Re-lubricate all internal components with rock drill oil and reassemble the hammer.

COMMISSIONING

Always use reputable drill bits in good condition. Using overrun bits will effect penetration rates, and reduce chuck life significantly due to excess cuttings wash. Coat the hammer threads with grease for protection and easier dismantling. Prior to use, lubricate the hammer with ½ pint (¼ liter) of rock drill oil as described above.

Fit the hammer to the drilling rig ensuring no debris or dirt enters the hammer from the site, dirty tubes or from unclean air lines. Make sure that the coupling threads from the drill are of the same specification to that of the hammer and they are in good


condition. Run the hammer at half the air flow for a few minutes to allow the oil to flow through and for internal components to settle in.

LUBRICATION

It is vital for DTH hammers to receive a constant supply of proper rock drill oil to protect the internal components and to provide a good air seal between the piston and the inner cylinder, and the piston and the wear sleeve for efficient drilling. The correct consumption of oil is dependent upon the air volume and conditions. Please refer to the lubrication graph below for recommendations. When drilling in wet conditions the normal amount should be doubled. There should be visual evidence of oil around the drill bit shank and within the tube joints when changing tubes.

The recommended grade of oil is dependent on the ambient temperature in which drilling is taking place as well as the operating pressure. As a rule of thumb, ISO320 grade rock drill oil should be used whenever possible as the hammer is a high frequency tool, however, where the pump cannot pump the oil in colder conditions, a lower grade of oil can be used as per the graph below. Up to double the amount of rock drill oil should be used to protect the internal parts from premature wear and corrosion when injecting water.

Remember: Insufficient lubrication or incorrect lubrication grades may result in damage to the hammer and it’s components. Hydraulic oils, engine oils, gear oils and diesel are not recommended for lubricating DTH hammers.

EFFECTS OF CORROSION

Drillers must be aware that damage to a hammer’s parts occurs due to lack of correct lubrication, lack of maintenance and storage of the hammer without protection rather than from normal wear from drilling operations.


Hammers operate in an environment of dust and dirt, using foam or water injection, encountering rock formations that contain sulfides, chlorides, sulfuric and nitric acids. Salt water can also be encountered and the presence of carbon dioxide in water forms carbonic acid.

All these chemical compounds and the use of foam can quickly lead to corrosion pitting and hydrogen embrittlement of the metal parts of the hammer within hours of being encountered causing wear and premature failure of the hammer parts. To prevent such in use and after use damage occurring, operators should check the pH of water being used and service the hammer immediately before reusing or storing the hammer.

Water intended to be used for water injection should be clean and free of solids and contamination. Before injecting water increase the oil lubrication rate. The pH (power of Hydrogen) levels of water should be checked before use and at periodic intervals. The pH of a fluid is measured on a scale of 0, very acidic, to 14, very alkaline, with 7 being neutral. Each pH value below 7 is ten times stronger that the preceding value. A pH value of 6 indicates 10 times stronger acidity than 7, while a pH value of 10 indicates 1,000 times greater alkalinity than 7.

Pure water has a pH value of 7 and is considered neutral. Unfortunately, water with a pH value of 7 is still quite corrosive to steel. The pH of water must be at least 10 before it will not be corrosive to steel. Water used for injection purposes can be treated with a soda ash solution with a ratio of 12 oz. of soda ash per gallon of water in order to raise the pH to 9.5 and greatly reduce the corrosive effects of the liquid environment.

While a uniform attack, rusting, is the most common form of corrosion, the most devastating type of corrosion encountered is where and extremely localized chemical attack results in holes in the metal. Here the anode or anodic area remains in one spot and deep corrosion pits occur at this site. Because drilling foam is a negatively charged surface active agent mixture, it is especially prone to cause corrosion pitting.

Metals will crack even in the absence of corrosion if they are subjected to a high cyclic stress for a finite number of cycles. A graph which shows the decrease in resistance to Stress as the Number of fatigue cycles is increased is called and S-N curve. Below a certain value of stress represented by the horizontal part of the curve, the metal will last indefinitely.

When metals undergo cyclic stressing and are also subjected to a corrosive environment, the stress level for failure may be reduced many times. For example, the fatigue strength of a steel part may be 50,000 psi but the corrosion fatigue strength can be reduced

Failure Failure

No Failure

No Failure

Number of Cycles for Failure

Without Corrosion

Number of Cycles for Failure

With Corrosion


to 500 psi in the presence of just ordinary tap water. The main reason for this is that corrosion causes pits, trenches, or other type notches.

Like notching the surface of a wire to cause it to break after only a few bends, the notches caused by corrosion pitting also create stress concentrations. As the material is stressed, cracks will form at these pits and eventually join together to form a single crack which ultimately results in failure of the component by corrosion fatigue. The presence of many cracks joined together by steps is characteristic of corrosion fatigue. These steps often give the fracture surface of a component that has failed by corrosion fatigue a serrated appearance.

Corrosion failures in hammers and bits can effectively be controlled by maintaining a protective barrier between the bit and hammer components and the environment through proper and diligent use of readily available products such as rock drill oil and thread greases. In situations where the water is acidic in nature, it is not recommended to use copper based greases as this can trigger a galvanic reaction with corrosive effect to damage the root of the threads and cause failure.

WHAT IS GALLING?

Galling is a type of extreme or severe wear that takes place between two sliding surfaces. The damage that results manifests itself as scuffing, scoring and seizing of the two surfaces. The appearance of the two surfaces as well as their metallurgical properties can be changed by roughening, localized heating, gouging, cracking and displacement of material due to solid phase welding. Particles that are removed from one surface are deposited permanently or temporarily on the mating surface. There are a number of

Corrosion Pitting

Corrosion Pitting Initiation of Fatigue Cracking

Corrosion Pitting Step Cracking


surfaces in a down the hole hammer that galling can occur including the piston running surfaces, threads, wear sleeve running surfaces and chuck /bit splines. The sketch below is a representation of galling in action.


CHAPTER 3: SAFETY CONSIDERATIONS

Be sure to work safely at all times. Wear protective clothing and safety equipment and observe all safety regulations as prescribed by your employer, Government, or the site on which you work. Do not wear loose clothing that may get caught in rotating parts and cause serious personal injury.

Remember that a “Down-the-Hole” percussive hammer emits noise and you should therefore take every precaution to safeguard your hearing against damage by using proper ear protectors. Use eye protection at all times. Rock chips and dust which may be discharged from the face of the bit or bore hole at high velocities and can cause severe injury.

Hammers can be heavy – Always use proper and approved lifting equipment and take every precaution to safeguard yourself against injury. Keep hands clear at all times – Beware of getting fingers trapped between the chuck and bit and do not use hands or feet to clear the top of the borehole at any time.


CHAPTER 4: DTH HAMMER SET UP

It is important to remember that although the injection of water into the hammer will increase the pressure in the hammer, the water is a restriction in the hammer that will cause a drop in the drilling performance.

CHOKE SELECTION

Chokes increase the airflow through the hammer and vary the operating air pressure or air volume delivered from large compressors by reducing compressor modulation to increase hammer efficiency and bailing velocity in the hole. They do this by diverting a portion of the air flow directly through the hammer and bypassing the hammer cycle. Where a hammer is to be used on a drill machine that has an excess of cfm available, a choke size should be selected to run the hammer at an optimum pressure. To determine the amount of air that will be bypassed, the following formula can be used.

F = 9.71 x D2 x P

Where: F = Flow in scfm

D = The equivalent hole diameter

P = Pressure in psig

It is assumed that the flow coefficient is 0.7, temperature is 120°F and gas is air.

For example: If a 1/8” choke is installed in a hammer then the additional air required to operate the hammer at 350 psi would be:

F = 9.71 x 0.1252 x 350

F = 53.1 scfm

The use of chokes will assist to clean the face of the hole at the bit, minimizing ‘regrinds’ of cuttings in medium broken ground allowing the drill bit carbides to directly contact and break fresh rock and improve the drilling rate.

Operators should select a choke with the smallest orifice hole necessary as a hammer develops the highest energy with a blank choke. Larger orifice chokes should only be used in very soft ground formations or where large volumes of water are encountered as they increase back pressure in the hammer and reduce hammer performance. All Mincon hammers are supplied with a solid choke installed, and a selection of chokes that can be installed as needed.

DRILL BIT CARE AND INSTALLATION

Your Mincon DTH drill bit is at the business end of drilling and just like the hammer, it will perform extremely well if cared for. Good drill bit care and maintenance helps ensure the longest life and best performance from that bit. The following gives recommendations on how to best care for your Mincon Drill Bits and install them correctly to maximize productivity and longevity.

DRILL BIT HANDLING

A drill bit is capable of taking considerable punishment in drilling operations; however, care must be taken while handling them to ensure that the bit you are drilling with is in sound condition.


Drill bit foot valves or blow tubes are manufactured from hard plastics are can be easily broken if dropped or something heavy is dropped on them.

The tungsten carbide buttons on a bit are extremely hard to fracture the rock they are drilling in. Dropping the bits onto hard surfaces can cause the buttons to crack and break. All care must be made to ensure drill bit heads do not come in heavy contact as the buttons striking each other can also cause damage.

DRILL BITS: PRIOR TO USE

Prior to using a new or used drill bit it should be inspected for the following and lubricated with rock drill oil:

• Check the condition of all the carbides and ensure that no damage has occurred and that the carbides are sharp.

• Inspect the face and head of the bit for any cracking or damage that could be detrimental to the drill bit.

• Make sure the gauge row carbides have sufficient clearance from the head of the bit.

• Inspect the foot valve (blow tube) for damage or cracking.

• Check the length of the foot valve from the strike face of the bit to the top of the foot valve. This dimension is critical in the correct operation of the DTH hammer and drill bit.

• Inspect the splines of the drill bit and chuck. Remove any burrs that have formed. Excessive wear to the splines of the bit and/or chuck will cause the hammer to run loose and can cause broken foot valve, shanking of the bit, or damage to the strike face of the piston and/or drill bit.

DRILL BIT INSTALLATION

When installing a new drill bit into a new hammer lubricate the splines on the bit with rock drill oil. Place the chuck over the bit and install the bit retaining ring. Most models of Mincon DTH hammers have reversible bit retaining rings, however some do not, and it is important to make sure that they are inserted in the correct orientation. Failure to do this can result in poor performance and damage to the internals of the hammer.

When using the drill machines rotary head to screw the chuck into the hammer, take care not to cross thread the chuck. With the head of the bit in the bit basket, torque up the bit in the hammer. See later for torque recommendations.

When installing a new bit on a used hammer it is important to inspect the chuck for wear to the body and splines. In soft and broken drilling conditions there is often excessive bit travel that results in uneven wear to the splines. Where this wear is excessive the chuck should be replaced as failure to do this will result in premature wear to the new bit’s splines, and possible damage to other components in the hammer due to lateral movement of the drill bit in the hammer. The body of the chuck should also be inspected for any gouging or grooving that is caused by erosive wear. Where this has occurred index the chuck so that the grooves or gouges do not line up with the drill bits exhaust grooves.


In the case when a used or newly sharpened drill bit is installed on a used hammer (used bits should never be used on a new hammer), the same indexing principles occur. If the bit is to be removed, sharpened and replaced back on the hammer, mark the location of the current bit alignment on the chuck and drill bit prior to removal. After sharpening of the drill bit has been completed, index the chuck by one spline so that the drill bits exhaust grooves are moved to a new location on the chuck as shown below.

In drilling conditions that are very abrasive such granite, quartzite the chuck should be indexed more than one spline. The Mincon DTH hammer chucks are multiple lead threads so it may also be necessary to turn the chucks starting position so that any grooves that have progressed into the wear sleeve are also separated into new locations as demonstrated below.

TORQUING UP A DTH HAMMER

Mincon DTH hammers use a patented three piece seating ring system and a compression ring to ensure that the hammers top end components are held firmly in place. This is extremely important as any movement of these components will result in premature wear and loss of performance. When a Mincon hammer’s backhead is closed up by hand there is a gap between the wear sleeve and backhead shoulder known as stand-off. All Mincon hammers have 0.030” (0.76mm) stand-off except the Mincon XP120 12” hammer which has 0.080” (2.0mm) stand-off.


The stand-off must be completely closed as part of the locking system for the hammer. It is not recommended to use the hammer action of drilling to close up the gap as movement in the top end will occur that will be detrimental to the hammer and ultimately lead to premature wear and loss of performance.

Recommendations for closing the hammer are to have it torque to between 750 - 1000 ft.lb per inch of hammer diameter. For a 6DH hammer between 4,125 – 5,500 ft.lb of torque should be applied. Where the rotary head cannot apply the requisite torque, addition wrenches should be used.


CHAPTER 5: DTH HAMMER OPERATION

LUBRICATION CHECK

When operating a DTH hammer it is essential that an adequate supply of rock drill oil is required. The rock drill oil must coat all internal surfaces of the drill pipe prior to it reaching the hammer itself. When using new drill rod or drill rod that has not been used in some time and has dried out it is important to run the air for a time to coat the internal surfaces and ensure that a good supply of oil is being supplied to the hammer. A good practice is to pour a cup of rock drill oil down each of the new or dry pipes to aid in this process.

COMMISSIONING A NEW DTH HAMMER

Coat the drill bit shank and the hammer threads with rock drill oil for protection and easier dismantling. Prior to use, lubricate the hammer with ½ pint (¼ liter) of rock drill oil. The oil should be poured through the backhead and by using a long screwdriver inserted into the backhead; the check valve can be depressed to allow the oil to run down into the piston chamber. Alternatively, install the hammer onto the drill machine’s rotary head, place a piece of cardboard on the table, locate the hammer’s drill bit just over the cardboard and turn the air on low. When the cardboard becomes wet with rock drill oil all internal components should be sufficiently coated with oil.

Fit the hammer to the drilling rig ensuring no debris or dirt enters the hammer from the site, dirty tubes or from unclean air lines. Make sure that the coupling threads from the drill are of the same specification to that of the hammer and they are in good condition. Run the hammer at half the air flow for a few minutes to allow the oil to flow through and for internal components to settle in.

COLLARING THE HOLE

Collaring the hole is a very important aspect of the drilling process. In blast hole drilling the quality of top of the hole will often determine the how well a charge can be set, the straightness of water well, geothermal well and foundation holes is determined by the straightness and quality of hole collaring.

The following procedure should be used when collaring the hole in rock:

1. While rotating the hammer clockwise at very slow speed feed the hammer to a few inches from the surface.

2. Turn on the air at a reduced pressure and slowly feed the hammer into the ground and continue at low pressure and impact.

3. As the hammer drills into the hole increase the air pressure, feed pressure and rotation as the hole becomes stable. It is advisable to lift the hammer off bottom occasionally to clean the hole as necessary. The conditions will dictate this and will be required more often in more broken conditions.

4. Once the hole has been stabilized continue drilling as normal.

The following procedure should be used when collaring the hole in overburden:

1. While rotating the hammer clockwise at very slow speed feed the hammer to a few inches from the surface.


2. Turn on the air at a reduced pressure and slowly feed the hammer into the ground and continue at low pressure and impact.

3. As the hammer drills further into the ground increase rotation and feed pressure. Lift the hammer off bottom occasionally to clean the hole as necessary as drilling through the overburden continues. Use of water and/or foam may be necessary to maintain the integrity of the bore-hole walls.

4. Continue until the hammer enters consolidated rock and case the hole. Be sure to follow all regulations regarding depth of casing into rock.

5. In situations where casing in extremely difficult conditions where the stability of the bore-hole walls cannot be maintained, such as gravel, heaving or flowing sands etc. the use of specialized foams, eccentric or concentric casing systems should be employed.

FEED FORCE/PRESSURE

For shallow holes the hydraulic down pressure applied to the Rotary Head through the pull down cylinders provides the necessary additional force. This additional pull down force is usually shown as a PSI value on the Rig gauges. To convert this figure to a weight value you will need to multiply the PSI reading by the area of the pull down cylinders. This can then be added to the weight of the Rods, Rotary Head, and Hammer minus Piston, to give the total “Weight on Bit”.

As the hole deepens with the addition of extra rods, a point is reached where the pull down force needs to be reversed to create a “hold back force” thus preventing too much weight being applied to the Bit. Since there are so many different types and sizes of rigs, it is not possible to give specific guidelines on the amount of additional down pressure as shown by the gauge. This becomes part of the necessary tuning by the drill operator. Regarding the actual figures for required weight the easiest way to calculate this is to use the two simplified formulae below:

1. When working with air pressure below 250PSI, (17.2bar) multiply the Bit Head Diameter by 500 for inches and by 9 for millimeters.

Example :- A Button Bit with a 6” (152mm) diameter Head would require 3000Lbs “weight on bit” or 1365Kg.

2. When working with air pressure at 250PSI (17.2bar) and above, multiply the Bit Head Diameter by 800 for inches and 14 for millimeters.

Example:- A Button Bit with a 6” (152mm) diameter Head would require 4800Lbs “weight on bit” or 2182Kg. Since these formulae have been simplified to allow for quick calculation you may find that, dependent upon ground conditions, slight alterations may need to be made to these values. Again this a necessary part of the tuning process performed by the drill operator.

It is also important to bear in mind that underfeeding, or insufficient weight on the bit will result in the bit “bouncing” up and down in the chuck. This will lead to a build-up of very high temperatures between the splines of the chuck and the bit which will eventually lead to “spot welding” and burning marks which will soon lead to bit shank failures.

Underfeeding also results in other types of failures, particularly button pop out and striking face failures. This is a result of the energy developed by the piston impacts not being transferred efficiently into the rock. As a result, a percentage of this energy is contained in the drill bit where it will eventually lead to failure through the weakest part of the bit.The percussive mechanism begins to operate as the air supply is turned on and when the drill bit is pushed firmly into the hammer. Excessive thrust pressures are not needed to make it work. The thrust controls on the drill should be adjusted to the correct pressure and should be readjusted to take


account of the weight of any extra tubes added so that the thrust pressure remains constant and not excessive. Insufficient thrust pressure will make the hammer drill erratically and less efficiently and cause premature wear to the bit and chuck splines with likely damage to the hammer components and threads.

The best way to ensure the correct feed pressure is to start drilling the hole and continue to increase the hold-down pressure until the drill string begins to bind in the hole and then reduce the hold-down pressure to achieve smooth drilling. If there is a concern for voids, the drilling should be closely monitored so that the hold-down pressure can be turned off and the hole cleaned before resuming drilling. Another method to identify the point at which optimal hold-down pressure is applied is to increase the hold-down pressure until the rotation pressure gauge starts to pulse and the reduce the pull down pressure until the rotation pressure gauge stops pulsing. A combination of hold-down and holdback can be used to stop the drill string from advancing too rapidly when a significant change in the rock conditions is encountered. Ultimately whichever method is used, the optimum situation is to have enough weight on the bit to drill as fast as possible without having excessive rotational torque.

When the hammer is lifted from the rock face, the drill bit extends from the chuck and the percussive action ceases. Extra air will pass through the hammer, which can be used to flush the hole clean.

ROTATION

The action of a percussive 'Down the hole button bit' is to make indentations in the rock which, with the rotation of the bit, make chippings and these need to be carried away to the surface by the hammer exhaust air as quickly as possible. Button bits have no cutting or tearing action as such and the effect of over rotation and overfeeding can have a serious effect on the life of the bit, especially in abrasive rock. This can wear away the outer carbide insert buttons or in more solid or dense material can cause the inserts to overheat causing micro-cracks which can lead to fracturing. If the drill string is rotated too slowly this will cause the button bit to re-drill previously cut chippings in the hole causing a drop in penetration rate and unnecessary wear on the buttons. As a guideline, the harder the rock and larger the bit diameter, the slower the rotation speed required. However it may be necessary to increase the RPM if in broken ground so as to prevent the bit jamming in the hole. Nevertheless, it should be remembered that this could also be caused by a badly worn bit and increasing RPM in these circumstances would only accelerate the problem. The controls of the drill should be adjusted in order to provide the largest drill chip size with the smoothest rotation and feed characteristics. Recommended rotation speeds would normally vary between 25 – 35 R.P.M. for most applications.

As the feed pressure is increased or decreased as rock conditions change, the rotation speed needs to be monitored and adjusted to maintain correct indexing of the bit face to new rock and maintain the largest chip size. To determine the correct rotation in relation to the feed pressure, adjust rotation so that it is ½” to 5/8” of penetration per rotation. The use of a piece of chalk or soapstone applied to the drill pipe during operation will create a spiral that indicates the amount of penetration per rotation. The rotation should be adjusted to keep this between the recommended to 5/8” of penetration per rotation.

The effect of incorrect rotation speeds can be seen by observing the wear patterns on the drill bits carbides. Where the rotation is too slow, wear will be observed on the leading edge of the carbides (side facing direction of rotation), and rotation that is too fast shows wear on the trailing edge of the carbide.

½”-5/8” of Penetration per rotation


It is essential to always keep the hammer rotating clockwise at all times during drilling and tool recovery to avoid uncoupling of the drill bit, hammer and drill pipe or damage to the drill bit from point loading on anything that may fall back into the bore-hole.

HOLE CLEANING, FLUSHING AND DUST SUPPRESSION

Effective hole cleaning is essential to maintain a maximum penetration rate and extend the life of the drill bit, hammer and drill pipe. Ensure that sufficient air volume is available to maintain an effective up-hole bailing velocity as described previously.

Use of a small quantity of water, usually less than 1 gallon per minute (3.785 lpm) can be used for dust suppression. Larger volumes of water injection are usually required in water well and deep holes for oil and gas drilling to keep the holes clean. In situations where water is encountered in the hole dry cuttings from the bottom of the hole meet the water as they are bailed from the hole and create mud rings that stick to the hole causing a restriction. Water injection is required to moisten the cuttings to prevent this from occurring.

When drilling in conditions that drill fast or contain clay larger volumes of water injection or no water injection are required if possible. If insufficient water is used, a tacky mud will form and clog the bore-hole. The correct amount will thin the mud so that it is effectively exhausted from the hole.

It is important to remember that water injected into the hammer is a restriction in the operating cycle of the hammer and as such should be kept to a minimum as allowed by the conditions during drilling operations.


CHAPTER 6: DRILL BIT FAILURES AND CAUSES

During the operation of the hammer and drill bit it is essential that the correct lubrication, feed force and rotation speeds be maintained. Please refer to the appropriate sections for details on these. The next sections will outline some of the common failures that can occur when proper care is not taken.

Failure - Drill Bit Shanking Splined Area

Cause - Lack of Lubrication, Worn Chuck, Excessive Torque

Lubrication is required between two steel surfaces that are running together to aid in the movement. Failure to do so results in a build-up of heat in the area that changes the heat treatment and metallurgical composition of the steel. Micro cracks develop in the heated area that significantly weakens the steel leading to failure.



When the splines of the chuck or bit get worn, excess movement occurs in the Drill bit causing unwanted additional friction between the steel surfaces that the lubrication cannot overcome. The friction causes excess heat in the steel and micro cracking occurs as described with a lack of lubrication. Using a new bit in a worn chuck will quickly mirror the wear on the bit to match the spline wear on the chuck and greatly reduce the life of the bit.

Blue heat marks indicate a lack of lubrication. Micro cracking in the shank causes the shank

to ultimately break.

Spline wear leads to excessive movement and over-heating of drill bit




A combination of excess feed pressure and rotation leads to rotational torque values that the lubrication of the hammer is unable to overcome. Galling occurs between the two running surfaces leading ultimately to drill bit failure.

Failure - Drill Bit Shanking Retaining Area

Cause - Lack of Lubrication, Worn bit retaining Rings, Worn Bit Aligner, Insufficient Feed Force

The bit retaining rings and the bit run together during the operation of the hammer. Where insufficient lubrication of the hammer is used, the bit retaining rings and bit generate excess heat causing failure in that immediate area. Where the bit retaining rings are worn, movement of the bit causes the same phenomenon that the lubrication of the hammer cannot overcome.

Galling marks on the pressure side of the splines. Lubrication is insufficient to

overcome the excessive rotational torque

Contact witness marks on the retaining area of the bit indicates a

lack of lubrication or worn bit retaining rings.




In the case where the bit aligner (bit bearing) is worn, excess movement of the bit occurs in this location. The movement causes galling between the two running surfaces leading to a loss in pressure or in the extreme case shanking of the bit in this location.



Where insufficient feed force is used or in broken formations the drill bit runs in the drop open position excessively. In turn the piston impacts down on the aligner and bit retaining rings causing damage to these parts. This in turn causes the bottom end of the hammer to run loose and transmits excess forces in the drill bit from the retaining area upward. These forces are detrimental to the drill bit and can cause shanking or damage in this area as well as the damage to the aligner and bit retaining rings.

Failure of the drill bit in the bearing surface caused by movement between

the drill bit and aligner.

Hammer running off bottom causes damage to the top of the drill bit.




Other evidence of a drill bit running off bottom continuously is tapered wear to the top section of the drill bit. The same result occurs where the bottom end of the hammer becomes loose and damage and/or shanking occurs.

Failure - Carbide Failure – Cusp Type

Cause - Overloading of Carbide

Gauge row carbides can become overloaded due to too much feed pressure, insufficient rotation speed or a combination of both. In such instances carbide failure occurs in the following sequence.

Hammer running off bottom causes tapered wear to the top of the splines of the drill bit resulting in shank failure.

4. Surface of carbide remaining is cusp like

in nature

3. Leading edge breaks away

1. Carbide Overload

2. Trailing Edge of Carbide breaks away


Failure - Carbide Failure – Shear Type

Cause - Over-running of Carbide

Drill bit carbides are designed to be extremely tough when the forces acting upon them are compressive in nature. As the gauge row carbides wear during the drilling process the forces change from compressive to shear. Excessive wear will lead to the carbide shearing off.

The following are examples of carbide failure due to over-running of the drill bit. As can be seen, the flats on the carbides are significant leading to shearing forces that have broken carbide


Failure - Carbide Failure

Cause - Micro Cracking

When drilling in softer formations such as limestone where the carbide does not wear much, micro cracking can occur on the surface of the carbide. This cracking, also known as snake skinning or reptiling, can be very difficult to see and may need a magnifying glass. The cause of the cracking is due to heat build-up and fatigue where the cobalt of the carbide is burning away. If the carbide is not sharpened the cracking will penetrate deeper causing failure. This phenomenon is also seen in iron ore drilling where the conditions provide extreme heat and the cracking can be seen very early in the bits life.

This case of Micro cracking occurred during Iron Ore Drilling. A much defined snake skin cracking can be seen and carbide failure is imminent if sharpening of the carbide is not carried out.


Failure - Broken Foot Valve (Blow Tube)

Cause - Worn Chuck, Worn Aligner, Over Drilling

The Foot valve (blow tube) on a drill bit will most likely break when struck by the piston. Should the hammer come out of alignment due to a worn chuck or aligner the bit is not running true and the piston can strike the foot valve causing it to break. This can also happen when the hammer is over drilling in very soft or broken ground conditions where the piston is impacting down on the aligner or bit retaining rings. The bottom end becomes loose and the piston can break the foot valve in the same described above.

Broken Foot Valve caused by the bottom end of the hammer becoming loose. Damage has also occurred to the strike face of the drill bit as the piston impacts the bit off square.


CHAPTER 7: DRILL BIT SHARPENING

It is important to keep drill bits sharp and in good condition. Failure to sharpen drill bits in a timely manner can result in a myriad of problems such as:

• Reduction of penetration rate.

• Breaking gauge row carbides due to shear loading.

• Reduction in drill bit life due to increased material removal to recondition bit.

• Failure of carbide where micro-cracking occurs and is not remedied.

Before commencing drill bit sharpening ensure that all proper safety equipment is used and all safety precautions are taken. A drill bit should be sharpened when the flat that develops on the carbide is one third the diameter of the carbide as shown below and a maximum of half the diameter. The drill bits should also be sharpened if the carbide is not wearing but the steel of the body is washing so that the carbide is protruding too much.

Drill Bit extremely over run and in danger of carbide breakage as well as thermal cracking to the body due to loss of gauge clearance.

Flat One Third carbide Diameter.


There are a number of drill bit sharpening solutions available, ranging from a simple hand held die grinder up to sophisticated robotic bit sharpening machines. There are even bit sharpening units that can be attached to the drill rig. Whatever the method used to sharpen a drill bit the aim is to achieve the same result:

• Restore carbide back to shape.

• Ensure steel is relieved from the carbides.

• Restore gauge flat on bit.

• Restore taper relief to drill bit.

• Recondition flushing holes.

RESTORE CARBIDE SHAPE

Hand held carbide grinding tools are available that are specifically designed to return the carbide shape back to it’s original form. Grinding cups are available to restore different carbide shapes like dome, ballistic and parabolic. The system pumps water through the tool and delivers it at the point of contact with the carbide as a coolant.

To grind a button correctly, oscillate the grinder in an orbital motion around the axis of the button insert. When grinding buttons, the orbital motion should be at an angle sufficient enough to allow the rim of the grinding cup to remove necessary steel around the base of the button (i.e. normally 7-9°). Always use the correct size and profile of grinding cup. The button should never be sharpened too much. Leave a flat of approximately 1-2 mm (1/32” – 3/32”) depending on the size of the carbide. Once the carbide shape has been restored the steel of the body should be dressed to the same level as around the carbide. All carbides should be sharpened the same amount so that uneven wear or drilling does not occur.

Drill sharpening machine Pictures courtesy of CME Blasting & Mining Equipment.


Grind down any damaged buttons using a silicone carbide grinding wheel. If the grinding surface of the grinding cup becomes plugged up (i.e. Dull), run the grinding cup into a scrap grinding wheel with flushing on; thus dressing the grinding surface of the grinding cup (i.e. Exposing new diamonds).

Where a Die Grinder with a diamond stone is used to sharpen the bits, the following method should be employed. The goal of this method is to remove as little carbide as possible to maximize the life of the drill bit.

1. Mark the carbide down the center with a pencil line.

2. Carefully grind either side of the pencil line, leaving the line intact.

3. Carefully relieve the steel around the carbide to establish a cylindrical protrusion of the carbide from the bit body.

4. Once the carbide shape has been restored on either side, blend both sides together along the pencil line.

5. Dress the body to the same level as around the carbide.


RESTORE GAUGE FLAT

The original gauge flat gets worn away during the normal drill process. The buttons on the gauge of the bit must maintain a clearance/protrusion of minimum 0.020” (0.5mm) and ideally 0.050” (1.27mm). If necessary, remove steel from the entire gauge of the bit, below the row of gauge buttons, so that proper button gauge clearance/protrusion is maintained. Insufficient gauge button clearance/protrusion also referred to as negative taper, may result in the button bit jamming in the hole during drilling, and causing bit failure.

RESTORE TAPER RELIEF

If necessary grind the taper relief back on the bit to restore it to 5 degrees. Take care not to grind into the gauge flat on the drill bit.


RECONDITION FLUSHING GROOVES

During normal drilling operations the flushing grooves of the drill bit can deteriorate. Grind away any steel that has peened over and grind out the grooves to ensure a good air flow for cuttings removal. Also grind out the side flushing grooves on the drill bit.


APPENDIX 1: CONVERSION CHART

FACTOR EXAMPLE

Air Pressure 1 psi = 0.0689 bar 100 psi = 6.89 bar 1 psi =0.0703 kg/cm2 100 psi =7.03 kg/cm2

1 psi =0.0680 atm (Atmosphere) 100 psi =6.80 atm (Atmosphere) 1 bar =14.504 psi 10 bar =145.04 psi 1 bar =1.0197 kg/cm2 10 bar =10.197 kg/cm2 1 bar =0.9869 atm 10 bar =9.869 atm 1 atm = 14.696 psi 10 atm = 146.96 psi 1 atm =1.0132 bar 10 atm =10.132 bar 1atm =1.0332 kg/cm2 10 atm =10.332 kg/cm2 Air Flow

1 cfm =0.0283 m3/sec 750 cfm =21.23 m3/sec 900 cfm =25.47 m3/sec 1 cfm =0.4719 liter/sec 100 cfm =47.19 liter/sec 1 m3/sec =2118.88 cfm 10 m3/sec =21188.8 cfm 1 m3/min =35.315 cfm 10 m3/min =353.15 cfm 1 m3/min =16.667 liter/sec 10 m3/min =166.67 liter/sec 1 liter/sec =2.1189 cfm 100 liter/sec =211.89 cfm 1 liter/sec =0.0600 m3/min 100 liter/sec =6.00 m3/min Velocity or Penetration Rate

10 m/hour =32.808 ft/hour 100 m/hour =328.08 ft/hour 1 m/min =3.2808 ft/min 1000 m/min =3280.8 ft/min 1 m/hour =0.0167 m/sec 1000 m/hour =16.7 m/sec 1 ft/min =0.3048 m/min 1000 ft/min =304.8 m/min 1 ft/min =0.0051 m/sec 1000 ft/min =05.1 m/sec 1 m/sec =60.00 m/min 10 m/sec =600 m/min 1 m/sec =196.85 ft/min 10 m/sec =1968.5 ft/min Volume

1 cc =0.001 liter 1000 cc =1 liter 1 liter =0.001 m3 1000 liters =1 m3

1 imp. Gal. =4.54609 liters 100 imp. Gal. =454.609 liters 1 US gal. =3.7884 liters 100 US gal. =378.84 liters Torque

1 lbft =0.1383 kgm 1000 lbft =138.3 kgm 1 lbft =1.3558 Nm 1000 lbft =1355.8 Nm 1 lbft =12.00 lbin 1000 lbft =12000 lbin 1 kgm =7.233 lbft 100 kgm =723.3 lbft 1 kgm =9.8067 Nm 100 kgm =980.67 Nm 1 kgm =86.796 lbin 100 kgm =8679.6 lbin 1 lbin =0.0833 lbft 1000 lbin =83.3 lbft 1 lbin =0.0115 kgm 1000 lbin =11.5 kgm 1 lbin =0.1130 Nm 1000 lbin =113 Nm Power

1 Horsepower =33,000 ft-lbs/min 10 Horsepower =330,000 ft-lbs/min 1 Horsepower =0.746 kW 100 Horsepower =74.6 kW Weight

1 g =0.03527 oz 100 g =3.527 oz 1 oz =28.3495 g 100 oz =2834.95 g 1 lb =0.45359 kg 1000 lb =453.59 kg 1 kg =2.204 lb 1000 kg =2204 lb


APPENDIX 2: GLOSSARY OF ABBREVIATIONS

cfm = Cubic Feet per Minute

API = American Petroleum Institute

C = Centigrade

dia. = Diameter

deg. = Degree

F = Fahrenheit

ft. = Feet

ft.-lb = Foot Pounds

gpm = Gallons per Minute

in. = Inch

kg = Kilogram

l = Liter

lbs. = Pounds

lpm = Liters per Minute

m = Meter

mm = Millimeter

mm Hg = Millimeters of Mercury

m3/min = Cubic Meters per Minute

psi = Pounds per Square Inch

psig = Pounds per Square Inch Gauge Pressure

rpm = Revolutions per Minute

scfm = Standard Cubic Feet per Minute


APPENDIX 3: THREAD IDENTIFICATION API STANDARDS


APPENDIX 4: RECOMMENDED TORQUE FOR TOOL JOINT CONNECTIONS


Notes:

dth training manual - el dorado sistemas · 2016-06-03 · the second chapter looks at hammer care....

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