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Drilling parameters of DTH Hammer
Date:2018/9/20 10:36:59

Drilling parameters of DTH Hammer

1. Air volume, wind speed and wind pressure

It is generally believed that if the pressure of all compressed air is high, the efficiency of DTH percussion drilling is also high, and the service life of the bit is also long. Air supply is not only the basic condition to ensure the work of the impactor, but also an important factor to ensure that the drilling is normal discharge of powder, because in the case of dry air drilling, the effect of powder discharge is mainly related to the upper return wind speed, and the wind speed is directly related to the air supply.

The air volume is determined according to the performance of the impactor and the upstream and downstream wind speed required for well washing. Because cuttings have different suspension velocities in the airflow medium due to their own viscosity, density and shape, it is necessary to adopt upward and backward wind velocities larger than the suspension velocities of cuttings to effectively discharge the cuttings out of the hole and achieve a clean bottom. There are no such problems in Reverse Circulation DTH impactor recently. For general positive circulation drilling, the velocity of upward and backward flow of compressed air in the gap between drill pipe and borehole wall annulus is generally recommended at 15-30m/s. Some complex working environment on the recommendation of the construction team rock discharge air volume often exceeds the DTH impactor work air volume. When the diameter of the drill pipe is quite different from that of the drill bit, the DTH impactor can not generate enough air velocity because of insufficient gas flow, and the cuttings can not be discharged out of the hole in time and accumulated at the bottom of the hole, so it is easy to bury the drill and cause accidents in the hole.

Therefore, in the process of large diameter downhole impact, When the ratio of drill pipe diameter to drill pipe diameter is large, the air supply of DTH impactor can not meet the air demand for slag discharge. Therefore, the circular gap between the diameter of drill pipe with cuttings at the bottom of hole and the borehole wall is particularly important. The selection and determination of air supply is mainly to ensure a certain return wind speed over the annulus, which is calculated according to the following formula (positive cycle is greater than 15m/s):

Type: V- return wind speed m/s; Q- air supply m3/min; D- drilling diameter mm; d- drill pipe mm.

The key to selecting a good air volume, wind speed and wind pressure is how to grasp the two relationships: the relationship between air energy and circulating resistance; the relationship between upward and backward velocity and clearance effect; the relationship between medium density and drilling conditions. In order to solve the above problems, we should take corresponding technical measures, such as increasing air supply volume and supplying air pressure, reducing circulation section, selecting counter circulation percussion drilling when conditions are available, selecting the type of impactor reasonably, adjusting the density of the medium, and using gas liquid two phase medium circulation, such as foaming agent, atomization and so on. Other inflatable media. The rule is that the higher the wind pressure is, the faster the penetration rate will be. With the increase of hole depth, the required pressure also increases, such as drilling_120 mm borehole, the depth of 150 m requires the consumption of 1.4 Mpa wind pressure, depth of 200 m is the consumption of 1.7 Mpa wind pressure. In addition, the consumption of air pressure by air foam drilling is about 0.18Mpa higher than that of pure air. Under the same conditions, when the hole depth is 200m, the air pressure of punching air is 2.21Mpa, while that of pure air drilling is only 1.7Mpa. Different punching methods have different requirements for the capacity of air compressors. In addition, when drilling in water, the backpressure does not increase 10m to 0.1Mpa for the DTH hammer.

In order to reduce the accumulation of debris, it is also possible to install a precipitation pipe on the upper part of the DTH impactor, so that the effect can be achieved and the air consumption can be reduced.

2.axial pressure

According to the principle of rock fragmentation, the rock is broken mainly by the impact of dynamic load, so the drilling efficiency of DTH punch is mainly determined by the impact energy and the impact frequency, and the axial pressure is the auxiliary force to ensure the full play of impact energy, too large or small will affect the normal drilling. It will cause vibration of drilling tools, premature wear and tear of drilling bits, hard alloy tooth dropping and difficult rotation, too small will affect the effective transmission of impact energy.

Table 1 lists the recommended axial pressure ranges for several specifications of DTH drilling, and relief drilling should be used if the gravity of the drill string itself exceeds the range.

able 1 recommended axial pressure for pneumatic DTH hasmmer drilling

 

Diameter of DTH hammer /mm

Minimum drilling pressure /kN

Maximum drilling pressure /kN

76

1.5

3.0

102

2.5

5.0

127

4.0

9.0

152

5.0

15.0

203

8.0

20.0

305

16.0

35.0



DTH impactor drilling mainly relies on the piston to impact the drill bit, rather than relying on the drill string to increase the drilling speed. This different drawing wheel bit is drilled in turn. Therefore, in order to prevent the drill pipe from vibrating, the lower limit value should be selected as much as possible. If the drill pressure is too high, the impactor and the drill bit will be damaged, and the drilling speed will be lowered.

3. the speed

Drilling of down-the-hole impactor is a drilling method that belongs to slow rotation. Reasonable speed selection is crucial for the life of the drill bit and even the drilling cost. It is mainly related to the magnitude of the impact energy generated by the impactor, the frequency of the impactor, the form of the drill bit, and the mechanical properties of the rock. DTH drilling is mainly based on impact rock, so there is no need for too fast line speed. The speed is too fast, which is detrimental to the life of the drill bit. Especially in the abrasive layer, the rotating silicon block will cause the cutting teeth on the periphery of the drill to wear and chip quickly.

   If the speed is too slow, it will cause the column teeth to collide with the existing impact breaking point (pit), resulting in a decrease in the drilling speed. Conventionally, the harder the rock, the larger the diameter of the drill bit, the lower the speed required.

   Drilling in some severely fractured rock formations, sometimes to increase the speed of rotation to prevent stuck drilling. But also note that sometimes the stuck is because the drill bit has been excessively worn, and increasing the speed complicates the problem.

   For DTH hammer drilling, the most bit rotation speed is required to achieve effective drilling speed, balanced operation and economical bit life.


Rock formation classification

Reasonable speedr/min

Cover layer

40-60

Soft rock formation

30-50

Medium hard rock formation

20-40

Hard rock formation

10-30

 


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