Thursday, April 24, 2014

Waterjetting 20c - Critical Distances intro

I remember going down the New Orleans just after Hurricane Katrina, and being a member of one of the first inspection teams that drove down the Delta inspecting the damage. One of the things that has remained in my memory was that of driving past one of the large coal piles being stored outside one of the local power stations, and being surprised that – despite the wind and rain – it was still there. Some years previously I had visited several places that had build ultra-ultra-high pressure jet generating devices. (Not sure what else you call a system that produces jet impact pressure that begin to approach 1 million psi). The one that has struck in my memory was visiting the underground test for Dr. Bill Cooley’s water cannon. Built to advance the concept of using high pressure gas to drive a pulse of water at a rock target (this was during the days examining new ways to drive underground tunnels) the device fired a small slug of water through a very carefully designed nozzle to generate a jet measured to produce 500,000 psi on impact.

We stood around and watched as the device was loaded, charged and fired and were, I suppose, a little disappointed at the damage induced around the impact point. Only a small amount of fragments were produced from the shot, and as you can see from the figure above, there was not a lot of evident surface damage from the shots. Actually, for the amount of energy involved, the amount of material removed was quite significant and some of the pieces were several inches in size:

Figure 2. Fragments recovered after a single shot of the cannon.

Later analysis explained why the damage had not been greater. The problem arises because of the way that the pressure is generated to drive the jet. Most of the devices of the time, whether our own, others in the US, the UK or in Russia, used a gas driver to generate the driving pressure, either using smokeless powder (as we did) or storing gas volumes under pressure and then suddenly releasing the accumulated volume to provide the driver.

The problem with this approach is that the driving pressure is not sustained as the water moves down the relatively long nozzle, and the driving pressure on the surface is thus very transient. One recording of such from the UK showed a typical pulse for their unit:

Figure 3. Pressure pulse for the water cannon device developed in the UK.

Dr. Cooley’s cannon had a much shorter and more rapidly decaying pulse, and this was largely the reason that the damage that it induced was not greater. The high impact energy was able to generate large cracks into the surface as the jet penetrated, some of which coalesced with the surface and allowed fragments to release, but the energy pulse was not long enough at the high pressure and with inadequate volume to fill the cracks produced and pressurize them to cause rapid extension and result in larger volume material removal.

The relatively low pressure impact of the rain on the coal in the Gulf, and the high pressure impact on the rock in the test mine both saw the ability of the water to penetrate into the surface layers of the material. But in both cases the impact pressure on the water that permeated into the cracks within that surface was not enough in the secondary phase of the failure process, to sustain internal pressures within the cracks that would lead to large volume material removal.

Now, as I mentioned last time, there are ways to enhance the effect of an impact, by creating secondary surfaces for the rock to break to. The best illustration of this, perhaps is to consider two small (six-inch side) blocks of plexiglas. We drilled a small hole through to the center of each and placed a detonating cap in that hole. We then fired the detonator.

Figure 4. Block of Plexiglas after a detonating cap has been fired in the middle of the block.

Note that the block was big enough to contain the crack damage. But note also that there is very little material removed, because the cracks that were formed by the explosion were contained within the block and did not interact very much. Now consider what happened when we first drilled a circular relief slot around a similar hole and charge. (We based the radius of the cut on the results of the first shot, shown above). After the detonation, this is what we achieved:

Figure 5. Similar bock to figure 4 except that a relief slot was first cut into the block

Note that there are (for those of us interested in driving tunnels) several interesting improvements. Firstly the cracks that radiated out to the walls of the first block now stopped within the central isolated core, and the wall of the excavation is now smooth and undamaged. Secondly the interaction of the radiating cracks within the core all reached the relief slot, and broke the core into fragments that were liberated. And that this also broke to the back of the slot, which was left relatively flat and at the end of the relief slot, so that it would be easy to start a new drilling and breaking operation to deepen the tunnel without having to redrill any damaged zone at the end of the previous excavated section.

But, for the purpose of today’s exercise, note that in the first place – even though there were free surfaces at the edge of the block, they were too far away for the cracks to reach them, and effectively break out the material. It was only when the relief slot was moved closer to the exploding detonator that the central volume was broken out and the effective result that was desired was achieved.

The critical parameter is the correct assessment of the distance over which the interaction between the event and the free surface (or in the case of last time’s discussion between two concurrent jet cuts) will take place.

As I will discuss in later posts, sometimes the distance that is critical to effective jet use is only on the order of fractions of an inch, and if done correctly the result (as above) is impressive, if the distance is too great, then nothing happens. But we’ll talk more about this, in many applications, in the future.

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