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How to Choose Right Air Pressure for the DTH Drilling Tools?
Aug 07, 2018

DTH Hammer.JPG

Because the piston quickly hits theDTH Bit at high frequency, the DTH Hammer is required to maintain a certain pressure at the bottom of the well to press the downhole impactor to ensure its efficient operation.

The reason why GMD's DTH Hammer has different performances from other competing products in different operating environments is mainly due to the following reasons:

1. Piston weight - The mass of the piston is as close as possible to the mass of the drill bit to obtain better energy transfer. In general, the mass ratio of the piston to the drill bit is preferably between 0.8 and 1.1. Excessive mass ratio will reduce the overall output power of the impactor. Too low mass ratio will cause the piston to rebound greatly when hitting the drill bit, reducing energy transfer and working efficiency.

2. Simple structure - under the same drilling conditions, the simpler the structure, the lower the failure rate. The piston is made of carburized and quenched material, which has high requirements on the cold-worked shape of the surface. The overly complex shape will greatly reduce the fatigue strength of the piston and thus reduce the service life.

3. Larger impact energy - the determination of impact energy has a great relationship with the structure of theDTH Hammer. Generally speaking, the larger the piston mass, the larger the stroke, the higher the single impact energy, and the higher the hardness. On the contrary, if the mass of the piston is lowered, a higher impact frequency can be obtained, but at the same time, a single impact energy is also reduced. The output power of the DTH Hammer is determined by the product of the impact energy and the impact frequency. In this way, it is necessary to understand the construction conditions during the selection process of the impactor. Different types of DTH Hammers are selected according to different rock formations. The harder the rock is needed. The greater the crushing impact work, the higher the impact frequency and the higher the drilling efficiency under the critical conditions of breaking.

Due to these characteristics, to ensure efficient drilling, axial pressure needs to be controlled to prevent the impactor from generating unnecessary bounce in the well. However, the total weight of the deep hole drill bit may need to be calculated to find the correct amount of weight. One thing that needs special attention is that the gauge pressure on the rig does not represent the actual pressure of the downhole drill. In addition to the axial pressure exerted by the rig, there is also the weight of the drill pipe itself, which affects the pressure of the impactor and the bit under the well.

A good rule of thumb - start drilling 500 lbs. per inch drill diameter (9 kg per mm drill diameter)

For example: 360 6-1/2" drill bit drilling pressure is 500 × 6.5 = 3250 pounds

360 165mm drill bit with a drilling pressure of 9*165=1485 kg

Please remember that this is only a rule of thumb as the starting point. The drilling conditions will change at any time. When the depth increases, the axial pressure of the rig needs to be adjusted. The specific adjustment method is the opening pressure minus the weight of the drill pipe. Ensure that the drilling pressure of the impactor is always stable.

Adding more weight or footage pressure does not increase the penetration rate. Excessive weight drilling of the drill bit only reduces bit life and increases wear on the alloy column.

Although drilling with too much weight is unfavorable, under-drilling is also detrimental. Insufficient pressure on the drill bit can have the following effects:

The drill bit is loose or slips when drilling, resulting in a decrease in the verticality of the hole;

Drilling under these conditions can create several operational problems, etc. When the cemented carbide pops up, it will cause the piston to open or stop working;

The straight shank bit fails and the borehole is laterally offset, resulting in extreme temperatures between the spline spline, the spline and the bottom of the drill chuck, which is unavoidable once thermal damage occurs.

Residual stresses cause failure of individual parts of the impactor and may also result in uncontrolled vibration.

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