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Causes of DTH Hammer Failure
Aug 14, 2018

DTH hammer bit 1.jpg

According to the gas distribution method, DTH Hammer can be divided into two types: valve type and valveless type. In addition to the different gas distribution methods, the structure is slightly different, but the working principle is basically similar, but the valve type is less resistant to pollution.

       There are many reasons for the failure of the DTH Hammer, but the manifestation is mainly the impactor without impact, impact force and intermittent impact.

1 Processing defect


       The impactor piston and cylinder liner are tightly matched, and the mating length is long, and the machining accuracy and surface smoothness are required. This requires a very high cylindricity of the piston and the cylinder liner. If the cylindricity is not guaranteed, the piston will be directional or intermittently stuck, and eventually the drill pipe may have to be lifted and unloaded frequently to repair the impactor.

       In addition, the rigidity of the outer casing of the impactor is also an important factor limiting the service life of the impactor. If the rigidity is poor, the impactor will be deformed due to frequent collision with the well wall during the drilling process; when the impactor is not working, it is often necessary to vibrate and disassemble and clean the impactor, which will intensify the outer casing of the impactor. Deformation; deformation of the outer casing will cause the internal parts of the impactor to become stuck and cause disassembly, which will eventually cause the impactor to be scrapped.

2 Impactor tail backstop seal is not reliable

At present, the tail of the impactor is provided with a check valve, and its structure is as shown. The sealing form is mainly based on the compression deformation of the spherical rubber cap or the O-ring attached to the metal cone to perform the backstop sealing. Its backstop function is achieved by an elastomer, and the elastomer generally has a guiding device.

This type of sealing has the following problems:

(1) There is friction between the spring and the guiding device, which will affect the cut-off speed of the check valve;

(2) The frequent compression and friction of the rubber sealing material for a long time will cause excessive wear;

(3) Spring fatigue failure, resulting in failure of the check seal;

(4) When the gas is stopped, the internal pressure of the impactor suddenly decreases, causing the rock powder or liquid-solid mixture to be backflowed into the inner cavity of the impactor, which will cause the piston to become stuck;

(5) More seriously, the water carrying cuttings enters the valve position (valve impactor), so that the valve plate can not normally close the gas distribution, causing the impactor to only discharge the chips without impacting the work failure.


3 Impactor head without seal

The boring head of the impactor head is provided with a venting hole communicating with the bottom of the well, and the boring head and the impactor are connected by splines, and the matching clearance is large. In the case of encountering a diving surface during drilling or requiring the addition of cementing fluid due to difficulties in well formation, there is a large amount of liquid and solid mixture in the gap between the bottom of the well and the borehole wall and the drill pipe. When connecting/replacement of the drill pipe or filling When the cementing fluid is solidified, the gas supply will be stopped, so that the check valve at the tail of the impactor will be quickly closed. Due to the inertia, some liquid will be instantaneously backflowed into the impactor through two ways: the venting hole of the bit, the bit and The gap of the spline sleeve. Subsequently, the impactor is like an empty water bottle that is buckled in the liquid. The gas enclosed in the inner cavity of the impactor is bound to be compressed by the external liquid. The higher the external liquid level, the smaller the gas length of the inner cavity of the impactor, that is, the impact The more liquid in the lumen of the device. If there is too much water in the inner cavity of the impactor, some cuttings will be brought into the piston pair of the inner piston, which greatly increases the stuck frequency of the piston. At the same time, if the cuttings deposited between the piston and the end face of the bit are delayed (see Figure 3), the impact energy of the piston will be mostly absorbed by the cuttings, which cannot be effectively transmitted, that is, the impact is weak.


4 Bit stuck

The bit and the impactor are splined (see Figure 4), the mating clearance is relatively large, and many types of impactor spline tails can be exposed with the spline sleeve. If the cuttings are wet, it is easy to form a mud bag to adhere to the bit. If this state is not improved in time, the mud bag will enter the spline fit clearance, which will affect the effective impact of the impactor piston impact; more seriously, the bit and the spline sleeve may be stuck together.


5 Large solid phase particles in cementing fluid

Under normal circumstances, in the case of easy collapsed formations where wells are difficult to drill, it is necessary to use foam drilling, that is, frequently adding foaming agent to the well for cementing. However, in practice, during the construction of the squad, washing powder solution tends to be used in view of convenience and economy. Due to the quality of the washing powder or the quality of the water, there will be some insoluble particles. The cementing fluid is filled from the drill pipe to the bottom of the well, so that undissolved particles in the washing powder will directly enter the inner cavity of the impactor, affecting the normal operation of the valve (valve impactor), or increasing the piston card. The probability of lag.

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