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Fault analysis of DTH Hammer
Date:2018/9/12 15:42:41

Fault analysis of DTH Hammer

 The DTH hammer can be divided into two types: valve impactor and valveless impactor. In addition to the different ways of valve distribution, its structure is slightly different, but its working principle is basically similar, but the valve type and its anti-pollution ability is worse.

  There are many reasons for the failure of DTH impactor, but the main manifestations are non-impact, impingement and intermittent impact. Analyzing the cause of the fault and improving the equipment A and B will be helpful to improve the structure of the impactor and the drilling technology. The failure factors can be classified and classified from five aspects:

   1.processing defects

The combination of piston and cylinder liner of impactor is close, and the fitting length is long, and the machining precision and surface smoothness are required to be higher, which requires very high cylindricity of piston and cylinder liner. If cylindricity is not guaranteed, the piston will appear directional or intermittent stagnation, and eventually it may not lift or unload the drill pipe at the airport to carry out impact device maintenance.

    In addition, the rigidity of the outer liner of the impactor is also an important factor that restricts the service life of the impactor. If the outer tube is of poor hardness. In the drilling process, the inflatable machine will frequently collide with the wellbore and cause secondary deformation; when the impactor is not working, it is often necessary to vibrate and dismantle and clean the impactor, which will aggravate the deformation of the outer casing of the impactor; and the deformation of the outer casing will lead to the jam of each part of the impactor and lead to the failure of dismantling, eventually directly resulting in the collapse of the impactor. The impactor is scrapped.

    Hunan New Diamond Construction Machinery Co., Ltd. has been working intensively for more than 20 years for the effective movement of the impactor cylinder and piston and the high precision production of the products.

 

    2. The rear end seal of the impactor is not reliable.

      At present, the rear end of the impactor is equipped with a check valve, and its structure is shown in Figure 1. The seal is mainly made by compression deformation of spherical rubber cap or O-ring mounted on metal conical cap. Its backstop function is realized by elastic body, and the elastomer generally has a guiding device.


The sealing method has the following problems:

(1) the friction between the spring and the guiding device will affect the cut-off speed of the check valve.

(2) long term compression and friction of rubber sealing materials will make them wear away.

(3) the fatigue failure of the spring results in the failure of the backstop seal.

(4) When the air stops, the pressure inside the impactor suddenly decreases, causing the rock powder or liquid-solid mixture back-pouring into the impactor cavity, which will cause the piston to stuck;

(5) What is more serious is that when water carries rock cuttings into the valve position (valve impactor), the valve disc can not normally seal the valve, resulting in the impactor only discharge cuttings without impacting work.

 

3. No seal on the head of the impactor.

The bits at the head of the impactor used in geophysical drilling are equipped with exhaust holes to communicate with the bottom hole, while the bits are connected with the impactor by spline, and the fit clearance is large. There are a lot of liquid and solid mixtures in the bottom hole and the gap between the borehole wall and the drill pipe when there is a diving surface in the drilling process or when the cementing fluid needs to be injected due to the difficulty of well completion. When the drill pipe is connected with/replaced or injected with the cementing fluid, the gas supply will be stopped, and the check valve at the tail of the impactor will close quickly. Because of the inertia, some liquid will be instantaneously backfilled into the impactor through two ways: the exhaust hole of the bit, the gap between the bit and the spline sleeve. Subsequently, the impactor, like an empty water cup, is inverted into the liquid, and the gas enclosed in the impactor's inner cavity is bound to be compressed by the external liquid, as shown in Figure 2.

 

The balance between the gas pressure inside the impactor and the external liquid pressure is equal. According to the internal and external balance, the height of the air in the impactor can be calculated as H:

 

Formula: Po is the pressure of the impactor chamber after gas shutdown; ho is the length of the impactor chamber; P is the density of bottom hole liquid; G is gravity acceleration; H is the height of bottom hole liquid level.

In formula (1), it can be seen that the higher the external liquid level is, the smaller the length of the sealed gas in the chamber of the impactor, that is, the more liquid enters the chamber of the impactor. If too much water enters the chamber of the impactor, some rock debris will be brought into the movement of the piston in the chamber, and hitting increases the frequency of the piston stagnation. At the same time, if the rock debris deposited between the piston and the bit contact face is too late to be eliminated (see Figure 3), the impact energy of the piston will be largely absorbed by the rock debris, and can not be effectively transmitted down, that is, the impact force.


4. Sticking of drill bit.

The bit fits with the impactor spline (see Fig. 4), and the fit clearance is relatively large, and the spline sleeve matching the spline tail of many types of impactor bits can be exposed. If the cuttings are moist, it is easy to form mud packets adhering to the drill bit. If this condition can not be improved in time, the mud package will enter the spline fit clearance, affecting the impact energy of the impactor piston effectively downstream; more seriously, the bit and the spline sleeve may be stuck together.

 

5. There are large solid particles in cementing fluid.

Under normal circumstances, when the formation is easy to collapse, it is necessary to use foam drilling, that is, frequently inject foam agent into the well for cementing. However, in fact, the team construction process, in view of convenience and economy, often tend to use detergent solution. Because of the washing powder quality or water quality, there will be some insoluble particles. When cementing fluid is injected from the drill pipe to the bottom of the well, the undissolved particles in the washing powder will directly enter the chamber of the impactor, affecting the normal operation of the valve (valve impactor), or increasing the probability of piston sticking.

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