Waterjetting 22c - More on shroud design

Lowering the pressure in a hose connected to a cutting head, by connecting it to a vacuum pump, will pull a certain amount of the water and debris released from a cutting/cleaning event into that hose. However to ensure that all of this material is captured, rather than just a fraction, requires a little more care and effort in the system design.

At the end of the last post on this subject, I began discussion of the use of shrouds to help to contain the ejecta and to direct it towards the suction line.

Figure 1. Schematic section through a shroud of a device, designed to mine high-level radioactive waste.

There are a number of different lessons that we learned as we developed this tool, and this piece will discuss a number of them. During the development and demonstration of the device we had to use a simulant, and a relatively weak cement was chosen, which would allow us to design the tool to operate it where we could see it, and easily interact with it.

Figure 2. Cutting test under way (no shrouds were used in this early test series)

The easiest way to drive the nozzle system was to run the high pressure tubing through a fixture that contained a hollow shaft electrical motor. This saved a lot of space, and allowed the high pressure tubing to feed into a distribution manifold under the motor, which fed the high-pressure water to a set of rotating nozzles.

Figure 3. Test rig without jets to show the design of the test head.

In figure 2 the jets issue from two self-rotating nozzle sets, themselves fed through a rotating feed tube, itself rotating around a central axis, and driven through a belt drive and gearing.

Various different patterns were cut into the simulant, as the different heads were moved over the surface, with the pattern controlled by the different rotation speeds relative to the overall head speed over the surface.

Figure 4. Computer image of the jet paths over the surface, in one combination of parameters for a head similar to that shown in Figure 3.

The design of the head was aimed at producing a set of jet passes (given that each jet was slightly inclined to the surface) which would produce pieces of simulant that were never larger than half-an-inch in size. Yet at the same time the goal was to remove 4 cu. ft. of waste each minute. The larger we could break out the particles, the less cutting we would have to make into the waste itself, saving energy and time, while at the same time increasing the overall volume we could release in that time.

Figure 5. Deeper cut into the simulant.

At the same time if the cut depth was too great, then several new problems would arise, apart from the initially obvious one of producing particles that would be bigger than the suction hose could easily handle. (Though we overcame that hurdle by running the particles through a high-pressure jet pump that effectively cut any oversize particles down to an acceptable size as part of its design).

The suction line needed more than just the water from the cut, to be able to pick up all the debris from the cutting operation. Air had to be drawn in around the sides of the shroud, yet at the same time the walls of the shroud had to come down to restrict the amount of that air and keep the suction strong enough at the surface to remove all loose material. This is done by fitting a rim of bristles (such as form the head of a paint brush) around the edge of the shroud that come down to brush over the outer edge of the cut, stopping a lot of the material from escaping out from the edge, while limiting the amount of air that feeds into the shroud, and in this way holding the suction pressure inside the shroud.

Figure 6. Early test showing a square shroud with bristles around the edge as it cuts into the waste. (Part of a previous pass has been filled with clay as part of the test). The shroud was larger to ensure that all ejecta was captured – as shown.

During the tests we found that the metal rim should, optimally, be no more than half-an inch from the surface of the material, after it had been cut, to pull all the material from the bottom of the crevices. But the edge of the head has also to pass over the surface in successive passes. So that high points left by deep cutting (Figure 5) will catch on the head, and can interfere with the rotation of the head on the next pass.

The aim of the cutting head design was, therefore, to leave a relatively smooth surface (of the sort shown in figure 3) over the waste after each pass, so that the head could be fed automatically down a fixed amount without any risk of it catching on large peaks left by the previous cut. This risk could also be lowered a little by slightly tilting the head backwards as it moves over the surface, since this allows slightly larger points to enter the head, where they are attacked by the jets before the driving mechanism has to pass over them. This tilting also makes it easier for the head to clean right up to the walls of the tank, where otherwise the edge of the shroud would hit the wall and stop the jets from removing that last rind of material from the edge. (Though it could be cleaned by a subsequent pass with the head turned up parallel to the wall and moved over it in that way – though this wouldn’t capture all the material as easily, due to wall curvature.)

Tilting the jets at a high angle so as to cut material at the edge of the shroud was also a possible problem, since it made it easier for the water to escape from the edge of the shroud, and out into the main body of the tank, which was undesirable. But I’ll talk about that in a later piece. Let me just note that, when these factors were all combined no material escaped from the edge of the shroud.

Figure 7. Test late in development, where a head similar to that shown in figure 1 is cutting over waste, without any material being ejected from around the shroud edges.

Waterjetting 22b - Steep seams and shrouds

It was not until we started to move the auger that I described last time, that I realized how heavy and cumbersome it remained. It is true that we could lighten it considerably (and we did in the UK version), but it was still largely a platform mounted device that was relatively easy to move on the surface, but which would be much more difficult to move around in the confined spaces underground.

In this regard it is worth comparing two photographs.

Figure 1. Coal being transported in the pipes at the Hansa hydraulic mine in Germany.

Notice in this first one how the coal is confined, there is no dust, and the tunnel is relatively clean and clear. Contrast this with the typical mining operation where the coal is carried from the working face to at least the main haulage drifts, and often all the way out of the mine using conveyor belts. (The method of transport that would also likely be used in a typical underground auger section).

Figure 2. Conventional belt conveyor carrying coal underground. (Famur )

Note that in the second photo the belt occupies most of the space in the roadway making passage more difficult, and that the coal is openly exposed. The problem that this occasionally generates is that the coal transfers from one belt to another as it moves through the mine by falling off the end of one belt onto the next. This puts dust into the air, and if not properly maintained coal dust can accumulate under the belt and around the support rollers and drives. If not cleaned this can create heat through friction, and can lead to disastrous fires.

Figure 3. Studying a belt fire underground (Office of Mine Safety and Health Research )

Confining the coal, and using the water that mined it as a transport fluid – or at least part of such – has many advantages.

The Russians were aware of this as they developed some of their different mining machines. One of these was a small monitor that could be pushed up a seam, from a lower drift (without the need of a higher one), by adding segments as the unit was jacked forward.

Figure 4. Russian GVD monitor

The monitor could be advanced up the seam, cutting a channel about 3-ft wide, and to the height of the coal. This relatively narrow channel confined the water and the coal produced, so that both ran back down to the drift, where they could be either enclosed in a pipe, or run into an open flume, that would carry the coal away. Once the drive had reached the end of the section, then the two hydraulic cylinders that sat under the monitor could be turned, so that, from a protected section down-slope, the monitor could successively ream out strips on either side of the entry, until the roof collapsed, or the operation holed through to the previous panel.

Figure 5. Schematic showing the sequence of extraction in a) a coal seam with a relatively strong roof b) a seam with a weak roof, where a pillar of coal is left between successive lifts, typically around 30 ft, so as to provide additional support to the roof as the coal is mined out.

Production from these machines averaged about 76 tons/hour from seams that were in the range from 2 to 4 ft thick, this was more than double the production output achieved by more conventional means, with significantly less manpower.

However while there are some conditions where gravity helps to bring the coal and water back together, there are many cases where this is not possible due to other constraints. This is where it becomes necessary to use a shroud to confine the jets, debris and water so that they can be extracted together, often using a vacuum to assist in the process.

Figure 6. Rendition of the combination of three cutting jets rotating around a vacuum tube to slice into material and remove it.

The particular device shown in Figure 6 was developed as a way of remotely slicing into high-level radioactive waste for the Pacific Northwest National Laboratory. The waste is held in underground storage tanks, where the levels are too high for human access. As a result the waste had to be broken into pieces and removed remotely.

The initial problem was that the tanks, though holding perhaps half-a-million gallons of waste, had only small (18-inches or less) ports through which they could be accessed. Thus a relatively small tool was required, yet the mining rate had to exceed 4-cu. ft/minute. The situation required an excavation system on the end of a robotically controlled arm. But the arm would have to be more than 60-ft long (in the end a cousin of the arm used on the Space Shuttle was used). Any mechanical force applied at the end of such an arm would have tremendous leverage on the holding fixture, and a relatively low overall force would have to be found, yet one capable of cutting material perhaps as strong as a weak cement.

The answer came in the form of the device shown in Figure 6. By placing three cutting jets to rotate around a central tube, connected to a vacuum line, one could cut into the material and break out relatively small pieces, which could be aspirated away with the cutting water. (The requirements were that there would be no water left in the tank for any significant amount of time). Because the bottom of the tank was some 40 or 50 ft below ground level a small, high-pressure (10,000 psi) jet pump was developed by Michael Mann, capable of drawing particles of up to an inch in size into the line, and then projecting them up with sufficient energy to carry them out of the tank.

Figure 7. Basic system conceived to mine high-level radioactive waste and remove it from a storage tank.

While this description of the system makes the tool seem to be relatively simple to build and operate there are a number of features to the design that are fairly critical in order for it to work well, and I will cover some of those next time.

Waterjetting 7c - higher pressure washing with power

In the last post, on surface cleaning, I showed how the jet from a fan nozzle spread very quickly once the water left the orifice. With this spread the stream got thinner, to the point that, very rapidly the jet broke into droplets. These droplets decelerate very rapidly in the air, and disintegrate into mist which rapidly slows down. That mist has little capacity but to get a surface wet, and thus, within a very short few inches, the jet loses power and the ability to clean.

How can we overcome this? Obviously the jet would work better if it could carry the energy to a greater distance. And the jet that does that (as we know from trips to Disney) is a cylindrical stream. In some parts of the cleaning trade this is known as a zero degree jet, to distinguish it from the fifteen degree or other angular designation of the fan jet nozzles that it is often sold with.

But the problem with a single cylindrical jet is that it has a very narrow point of application. Depending on the standoff from the nozzle to the target this will increase a little as the distance grows, but is still likely to be less than a tenth of an inch. That, by itself, would make cleaning a bridge deck a long and laborious job. But consider that if we spun the jet so that it is tilted out to cover a 15 degree cone, the same angle as the best of the fan jets, the water would travel further. With a good nozzle it is possible to extend the range to 3 ft, rather than the typical 4 inches of a fan jet.


Figure 1. The gain in performance when a fan spray is changed to a rotating cylindrical jet. (initially proposed by Veltrup, these are our numbers).

In both cases the water flows out of the orifice at the same volume and pressure. But with the rotating jet the water is able to carry the energy some 9 times as far. As a result the area covered is 9-times as wide, and the job is carried out faster.

You can also look at it another way. It takes only about 10% of the water and the power to clean the surface with the rotating jet, as opposed to the amount required to clean with the fan jet. This is even though the pump unit and the flow rates are the same in both cases. This is why, when you buy some of the smaller pressure washers, they include a nozzle that has a round orifice and which then oscillates within a holder. Not quite as efficient as a controlled movement, but at least it is a start.

Now, of course, life is never quite as simple as it at first appears. Because the jet is being rotated there is sometimes, if the jet is being spun fast enough, some breakup of the jet because of the speed of rotation. And so, in the above example, too high rotation speed would have a disadvantage. Doug Wright showed this in a paper he presented to the WJTA in 2007.
Figure 2. The effectiveness of a rotating jet, at two speeds and at different distances (Doug Wright 2007 WJTA Conference Houston).

On the other hand because the jet has to make a complete rotation before it comes back to the same point on the coverage width, if the lance is moving too fast relative to that turning speed, then the jet will miss part of the surface that it is supposed to be cleaning.

I can illustrate this with a sort of an example. To make it obvious the rotating jet has enough power to cut into the material that it is being spun, and moved over. If the rotation speed is too slow, relative to the speed that the head is moving over the surface, then the grooves cut into the surface won’t touch one another and small ribs of material are left in the surface. This is not a good thing, either from a cleaning or mining perspective. The material we were cutting in this case was a simulated radioactive waste, that an improved design later went on to extract as a “hot” material in a real world project. These materials tend to be unforgiving if they are not properly cleaned off.


Figure 3. Cutting path into simulant showing the grooves and ribs where the rotation speed is not properly matched to the speed of the head over the surface.

There is another answer, which is becoming more popular for a couple of different reasons. If the pressure of the water is increased, then the jet will remain coherent for a greater distance, at a higher rotation speed. Going to a higher rotation speed, also brings in an additional change in the design of the cleaning head.


Figure 4. Cleaning head concept sectioned to show vacuum capture of the debris through the suction line after the jet has removed the material and washed it into the blue cylinder.

As the pressure increases, so the energy of the water and the debris rebounding from the surface increase. To a point this is good, since once they are away from the surface it is relatively simple, if the cleaning operation is confined within a small space by a covering dome, to attach a vacuum line to the dome, and suck all the water and debris into a recovery line. The surface remains relatively dry, all the water and debris is captured, and the tool can be made small enough, and light enough, that it can be moved either by a man or on the end of a robotically controlled arm. (The arm we designed the head for was over 30-ft long, which means that the forces from the jets had to be quite small).

With the higher pressure also comes the advantage that the amount of water that is required, for example to remove a lead-bearing paint from a surface, is much lower. If the water becomes contaminated by the material being washed off, then not only has the total volume to be collected, which is an expense, but it also must be stored and then properly be disposed of. And that may cost several times the cost of the actual cleaning operation, if the contaminant is particularly nasty. So reducing the volume of the water is particularly useful.

A friend of mine called Andrew Conn came up with the idea, for removing asbestos coatings from buildings, of tailoring the pressure and the flow from the nozzles, so that the amount of water required was just enough that it was absorbed by the asbestos as it was removed. Simplified and reduced the costs of cleanup, where that was a significant part of the overall price.

And speaking of using higher-pressure water, this means that there is no need for the abrasive additive, when cleaning say a ship hull. And that means that there is no need to buy, collect, and dispose of the abrasive during the operation.


Figure 5. Spent cleaning abrasive at a shipyard.

There are other advantages to the use of high pressure water over abrasive when cleaning metal, and I’ll talk about that subject a little next time.