Part 2 of 3 – Intro to Drilling with DT Reverse Air and Flooded Reverse Methods
Conventional DTH Hammers and DTH RC Hammers
Dual tube methods have become standard practice in mining, water well, and large borehole drilling in recent years. With the growing popularity of these methods it is hard to believe that basic information on how these methods work can be difficult to come by. This blog series is meant to simply explain the concepts behind some of the most common applications of Matrix’s dual tube product line as used by our customers. As you are reading, keep in mind that we are NOT drillers. We are a custom manufacturing facility who specializes in producing dual tube for the drilling industry. If you have questions or would like to know more please contact us.
In this part of the intro to drilling series we will continue looking at reverse circulation dual tube drilling with air using conventional DTH hammers and DTH RC hammers. If you haven’t already, check out the last installment in our “Introduction to Drilling with Matrix Dual Tube series.” Part 1: Reverse Circulation Drilling with Air using a Rock Bit (Tri-cone/PDC), and be on the lookout for part 3: Dual Tube Flooded Reverse Circulation Drilling. If you would like to read more about any of the Matrix components discussed in this article check out our products page.
Reverse Circulation Drilling with Air using a Conventional DTH Hammer
Reverse Circulation drilling with air is not restricted only to rock bits. With slight modifications to the down-hole assembly, drilling with DTH hammers is also possible. Dual tube reverse air rotary drilling with a conventional DTH hammer (as shown in Figure 2) incorporates an interchange (crossover sub) located directly above the pneumatic down-the-hole hammer. The interchange directs the compressed air down the center of the hammer and out the bit face. The air decompresses (violently) as it enters the borehole sweeping the cuttings around the outside of the hammer and into the interchange slot(s) which direct the cuttings up the inner tube.
A recent design improvement to these interchanges has been the addition of a small channel through the center of the sub that bleeds a controlled amount of compressed air into the return sample stream thus “jetting” the cuttings up the inner tube. This essentially creates a venturi effect within the system reducing the amount of pressure needed to run the hammer.
Figure 2: Sample Flow. Dual Tube Reverse Circulation Drilling with Air Using a Conventional DTH Hammer
Reverse Circulation Drilling with Air using a DTH RC Hammer
Another recent innovation in hammer drilling with dual tube has been the invention of the RC (reverse circulation) DTH hammer (sometimes referred to as a center sample return hammer.) Dual Tube reverse circulation drilling with air using an RC DTH hammer (as shown in figure 3) is similar to the conventional hammer method in the sense that when compressed air is introduced to the tool through the drill string it operates a piston before being expelled to flush the cuttings away. Unlike the conventional hammer, in an RC hammer air is expelled from the tool around the shank of the bit, and is then directed across the face of the bit and back up through the center of the hammer along with the cuttings. A back reamer (digout sub) equipped with abrasion resistant pads is placed directly above the RC hammer. This sub helps minimize the probability of washing out large cavities, prevents premature wear on the OD of the dual tube rods and stabilizes the borehole giving the driller straighter holes. RC hammers contain an inner tube through the center of the hammer which links up with a hole in the piston. This lighter weight piston usually requires greater air pressures than a conventional hammer in order to achieve adequate impact forces, therefore the use of booster compressors is commonplace when using an RC hammer.
Figure 3: Sample Flow. Dual Tube Reverse Circulation Drilling with Air using a DTH RC Hammer (Center Return)
Waiver: The views expressed in this article are the author’s opinions and are based on engineering education, skills, and experience gained in working with many different customers in the industry. No part of this article is intended to replace or supersede any information supplied by others. The contents of this article may not be used for any type of legal action.