Sunday, June 7, 2015

Waterjetting 34a - Drilling holes with water jets

Looking back over the Waterjet Index I realized that while I have addressed different aspects of drilling holes with waterjets in various ways, I haven’t really brought it all together as a focused topic. So, herewith, and in the next few posts, that coalescence. It begins with a bit of a recap.

It was clear, early in the work on waterjet applications, that one of the key problems to be addressed was that of the ingoing water having to fight its way past the spent water already exiting the hole that had been created. This is particularly true when making an initial pierce through a target material, where there is very little relative movement between the nozzle and the target. Given that the interaction between the two flows occurs within about a hundredth of a second, the effect on cutting efficiency is relatively immediate.

So how to overcome the problem? One way is to pulse the jet, and in some early work at Leeds we built such a pulsating unit and spun it in front of the nozzle, chopping the jet into segments and allowing one segment to leave the hole, before the next arrived.

Figure 1. Pulsating disc to rotate ahead of nozzle and “pulse” the jet.

This was inefficient, because the energy put into the segments diverted by the disc was lost, and it was also extremely noisy, to the point that tests had to be carried out after everyone else left the building.

The alternative was to rotate the sample (this was in the days before high-pressure swivels and couplings were available) and align the jet just off the axis of rotation, so that the jet cut a hole somewhat wider than itself, as it passed over the target and thus provided an open path for the rebounding spent water that did not interfere with the path of the fresh jet travelling to the impact point.

A comparison of relative penetration rates showed that while a fixed nozzle and steady jet had sensibly stopped penetrating after about a second, the pulsed jet continued to drill a hole, as did the rotating jet, but the latter was able to drill faster and remove more material.

Figure 2. Comparison of a steady jet, a pulsating jet and a rotating jet as they penetrate into rock over time. All jets are at the same diameter, pressure and standoff distance.

By moving the nozzle out to a greater radius the hole created could be enlarged. This allowed the nozzle to move forward into the cavity created and the process could be repeated. In this way, over several iterations, a waterjet at a pressure of about 9,500 psi drilled though a block of granite, uniaxial compressive strength around 30,000 psi, and a new drilling tool had been demonstrated.

Obviously it is impractical to keep enlarging the hole by reaming it wider from the surface to allow the nozzle body to enter the hole and advance to the bottom. The jet must, from the beginning, drill a hole large enough for the nozzle to advance. And the easy way to do this is to incline the nozzle (at an optimal angle of around 20 deg, depending on the pressure, the type of waterjet and the target material properties). And while we still did not have a rotating swivel, we could turn and raise the target, while directing the jet through a small inclined orifice.

Figure 3. Inclined jet drilling a hole through a rock

Skip forward a couple of years, and we were drilling rock, at a rate of about 4-inches a minute, with a single jet. Then Jim Blaine, the RMERC machinist at the time, misunderstood a drawing, and added a central axially aligned orifice to the nozzle geometry. And within days we had increased the speed of drilling by two orders of magnitude. (Though by this time we also had a working rotating coupling to help rotate the nozzle).

Figure 4. Modified drill nozzle geometry

Figure 5. Drilling rate of advance as a function of hole diameter and rotation speed.

When one is using a single jet to cut the required profile of the hole, which must exceed the diameter of the nozzle holder if the drill is to advance into the hole, the rotation speed of the drill must be fast enough, relative to the rotation that the ribs of material left between adjacent passes of the jet along the hole wall are either non-existent (where the adjacent passes overlap) or are small enough that the mechanical impact of the nozzle body can break them off with very little force. In the latter case, however, this can put a mechanical load onto the drill string. The drill is often made up of only a length of high-pressure tubing, with the nozzle threaded on the end, so that any significant mechanical force can distort it and cause the drill to misalign and no longer drill a straight hole, so that this contact is discouraged.

On the other hand the rock through which the drill passes will likely change in strength and composition quite frequently, and so the depth to which the jet will cut will also change. This means that the hole diameter may reduce, so that the hole is no longer large enough to allow the nozzle body to pass. To stop this becoming a problem a small ring is mounted ahead of the nozzle, in the plane that the jet reaches the hole diameter required in this particular case. Now when the jet fails to cut to that diameter then the ring will stop the drill advancing, while the jet cuts along the line of contact and enlarges the hole to the required size, then allowing the drill to move forward. This is helped where the drill is spring-loaded so that the compression of the spring stops the drill advance, and the relaxing of the spring, as the obstacle is removed, allows the drill to move forward again.

Figure 6. Gaging ring on the front of a drilling nozzle.

That drill development was relatively straightforward, and was demonstrated in the late 1970’s. Subsequently commercial drilling systems were developed that used high-pressure water to drill holes in mine rock. They had the advantage over conventional tools in that the hose feeding the nozzle could maneuver in a much smaller space than a conventional drill, and thus longer holes could be more easily drilled from narrow working areas.

Unfortunately it still proved difficult to drill all rock with a plain waterjet, despite the use of ultra-high pressure equipment, and two different approaches were then tried, which I will discuss in the next posts.


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