Waterjet Technology – Determining Angles of water blasting

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

How times change! I was reading a column in the British Farmer’s Weekly, and came upon this, where the author is discussing the need for a generator:

It will also be vital to keep the fuel flowing into the tractors, and power the pressure washer, and light the security lights, and all the other essentials of an average arable farm.

It is an indication of how far the use of pressurized water has come that it is now seen, at the lower end of its application, as a vital farming tool. Which is a good introduction to talk a little further about the use of cleaning streams and how to interact with differing target materials.

There was an initial first step when someone would send the lab a mystery block of material and asked – how do I cut it? Generally, the samples were small but we would find a flat surface on the material and carefully point a jet nozzle perpendicular to this surface. (In the early stages this was hand-held). When a jet strikes a surface but can’t penetrate it, then it will flow out laterally around the impact point under the driving force of the following water.

The test began with the jet at low pressure, and this was slowly raised until the point was reached when the pressure was high enough to just start cutting into the material. At this point, the jet had made a small hole in the target, and so the water flowing into that hole had to get out of the way of the water following. The sides of the hole stop it flowing laterally, and so it now shoots back along the original jet path. This spray can hit the lance operator if the nozzle is hand-held, but it is a fairly graphic way of determining the threshold pressure at which the material starts to cut (and I’ll get into what happens as the pressure continues to go up in a future series of posts).

But for the purpose of cleaning, the jet has to move over the surface, once it has made that initial hole, at pressure. But, in many materials, if the jet comes vertically down onto the target, then only the material directly under the jet will be removed. And so the jet has to be played on every square inch of the surface in order to ensure that it is cleaned or that the coating/layer is removed. In some sandstones, for example, two jet paths could be laid down, almost touching one another, and yet the rib of material between them would remain standing.

Adjacent jet passes in sandstone

Figure 1. Adjacent jet passes in sandstone; the cuts are about an inch deep, but note that even though the narrowest rib is about 1/8th of an inch wide, it is only when the cuts touch that the intervening material is removed.

Yet that rib of material was, in that case, so weak that it was easy to break it off with a finger. (This turns out to be a weakness in making delicate sculptures out of rock). To use the full pressure of the water can be a waste of energy if the material is very thick since it all must be eroded with such a direct attack.

Yet the minimum amount of material that needs to be removed is that that attaches the layer to the underlying material (the substrate concrete, steel etc) and this can be quite thin. Thus, in attacking a softer material, particularly one that can be cut with a fan jet, a shallow angle directed at the edge of the substrate can be more effective.

Round vs. fan cleaning from Hughes

Figure 2. Round vs. fan cleaning from Hughes (2nd US Waterjet Conference)

Because there is a balance between cutting down through the material to be removed and cutting along the edge to grow the separation crack between the materials, some practice is needed to find, for a given condition, what that angle would be.

Choice of angle from Hughes

Figure 3. Choice of angle from Hughes (2nd Waterjet Conference)

The more brittle the material, then the greater the angle to the surface, since rather than just erode the material, the jet may also shatter the layer into fragments that extend beyond the cut path. But otherwise using an angled jet to the surface can be more effective. Hughes (from whose paper at the 2nd Waterjet Conference I took these illustrations) has a simple test for orifice choice.

How target response influences nozzle selection

Figure 4. How target response influences nozzle selection. (Hughes 2nd Waterjet Conference)

Some of the more advanced cutting heads use a series of nozzles that spin within an outer protective cover, as they remove anything from layers of damaged concrete to thin layers of paint from ship hulls. Increasingly, these are connected to vacuum systems that will draw away the spent water and debris from within the contained space, without it entering the work space, and creating problems for the worker.

In order to reduce any collateral damage to the surroundings, these jets are often made very small (thousandths of an inch in diameter) so that their range is short, and they are inclined outward to cut to the edges of the confining shield.

We have had some success in turning those angles the other way, so that they cut into the shield, rather than away from the center, and also so that each jet is directed towards the path of the next jet around the circumference. The intent in this case is to allow the use of a slightly larger jet, with a greater cutting range. In this case the individual cleaning/cutting path is a little larger, but because the jet at the end of the cut moves into the range of the adjacent jet, then any remaining energy that it and the dislodged debris still have will not be enough to get through this second jet.

Inclined jet and shroud design

Figure 5. Inclined jet and shroud design.

The action of each jet then becomes not only to cut into and remove material, but also to contain the spent material from the other jets dispersed around the cutting arm, and to hold the debris in the center of the confinement for the very short time needed for it to be caught up in the vacuum line.

In all cases the choice of pressure, nozzle size, and operational factors such as angle of attack, come down to the target materials, those that have to be removed, and those that need to be left undamaged. And it is why it is useful, at the start of any new job, to take the time to do a little testing first, to make sure that the right choices of nozzle and angle have been made to get the job done quickly and efficiently.

Incidentally, the idea behind the test of effective pressure, which the jet flows laterally when it hits something it can’t cut, can help, for example, in easing the meat from the bone when a jet cuts a deer leg.

Cut across a deer leg

Figure 6. Cut across a deer leg, note how the jet has cleaned off the meat from the bone, undercutting the flesh.

Waterjet Technology – Removing Graffiti with Waterjet Blasting

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

When I began this series, I mentioned that the target material plays an important part in deciding which pressure and flow rate is best for a particular task. Sometimes, time also has a role and not always in the way of “faster is better”. I mention this because we made a mistake once. (Well we only made this mistake once, didn’t mean we haven’t made other mistakes). Almost thirty years ago, we carved the granite blocks that make up the University of Missouri S&T Stonehenge, a half-scale Americanized version of the British megalith. The campus Americanized it when Dr. Joe Senne, the Civil Engineering professor who designed it, incorporated an analemma based on the calendar developed by the Anesazi in New Mexico. This replaced the 19-stone inner bluestone ring of the original.

Missouri University Science and Technology Stonehenge replicated using waterjet technology

Figure 1. Missouri University Science and Technology Stonehenge replicated using waterjet technology.

The MS&T Stonehenge was chosen as one of the ten Outstanding Engineering Achievements of 1984, by the National Society of Professional Engineers, in part because the 160-ton 53-stone structure was carved from Georgia granite by high-pressure waterjets without the use of abrasive. It has, over the years, generated a lot of interest (even getting me onto the Tonight Show with Jay Leno) but that has also included the odd local “artist” who has adorned it with graffiti.

My initial response when this first happened was to go over to the monument immediately with a high-pressure pump and start to wash the paint off. And that was the mistake for two reasons. Firstly the paint was not totally dry, and secondly we had not protected the stone with an invisible protective coating to seal it. Thus, when we tried to wash the paint away, while we removed all the surface paint – and to a casual observer it remains clean – we had driven a small fraction of the still liquid part of the paint into the pores and grain boundaries of the granite. Thus, if you know where to look, there is still a slight discoloration where that first writing was removed.

Shortly after that, on the advice of the Georgia Granite Association, the campus found a coating that was applied to the rock sealing the pores and grain boundaries, and future cleaning was made a lot easier and more effective. However, it did not completely solve the problem since future cleaning had to be done in such a way as to remove the spray paint while leaving the protective coating.

And that reminds me of a funny story. Graffiti is a significant urban problem, and it costs cities like Albuquerque in New Mexico about $1.3 million a year in clean-up costs. Much of incentive for almost immediate removal is because it is a way for street gangs to mark their territory, and this motivates police to urge an aggressive treatment policy.

But what happens if it is art? There are street artists who have chosen to decorate generally abandoned buildings for free in various ways and not related to any gang activity. Perhaps the most famous of these is Banksy, who was making it to the news when a piece he painted on a London wall appeared in a Miami auction house. It was anticipated to be worth around $600,000 before being withdrawn from the auction.

Bansky “wall art” estimated to be worth up to $600,000

Figure 2. Bansky “wall art” estimated to be worth up to $600,000. (Banksy)

At one time, Albuquerque had a similar idea of hiring those who were spraying the town walls to instead create works of art on some of the otherwise blank concrete surfaces such as bridge abutments around town.

Unfortunately, this led to an awkward situation. One of the local artists had painted some of his art on a fly-over. Shortly thereafter, the city sent a crew out to cover up the remaining graffiti with a coat of whitewash. Unfortunately, the crew were not artistically trained, and so covered up the new work of art.

For some years, I had a photograph of a waterjetting crew working on that site. At first glance, they were removing graffiti, but in reality they were taking the white coating from the painting to re-expose it to public view.

And this is one of the advantages that waterjets possess in that they can, with care and training of the personnel, be used to preferentially remove individual layers of material, whether of dirt or paint, without doing any damage to the material underneath, the substrate of the surface.

This is important, for example, in removing paint from buildings where the underlying substrate may be a relatively weak wood surface, where any high jet pressure would be enough to eat away the softer parts of the wood turning a smooth wooden sill into an etched and rough surface far from the desired result. Thus, in these circumstances, there is a need for very fine pressure control if the desired result is to be obtained.

And sometimes, the material that is to be removed is not that easy to remove with water power alone at an acceptable rate because the pressure has to be lowered to the point that it only removes the paint at a slow and uneconomic rate. At that point, it is possible to add a relatively soft abrasive (something like a baking soda) that will not only be effective in removing the material but is soft enough that it will do relatively little damage to the surface. At the same time, many of these softer abrasives are also soluble, which means that the costs of clean-up can also be reduced.

In some cases the water pressure need be little more than tap pressure.

Low pressure waterjet graffiti removal using soluble abrasive

Figure 3. Low pressure waterjet graffiti removal using soluble abrasive.

For more information on waterblasting and industrial cleaning, visit the Aqua-Dyne Waterblasting website.

Waterjet Technology – Higher pressure washing with power

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

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.

The gain in performance when a fan spray is changed to a rotating cylindrical 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.

The effectiveness of a rotating jet at two speeds and at different distances

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. Neither from a cleaning nor from a mining perspective is this a good thing. 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.

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

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.

Cleaning head concept sectioned to show vacuum capture of the debris through the suction line

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 does the energy of the water and the debris rebounding from the surface. To a point this is good, since once they are away from the surface, it is relatively simple – providing 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 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.

For removing asbestos coatings from buildings, a friend of mine called Andrew Conn came up with the idea 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. This idea simplified and reduced the costs of cleanup, which 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.

Spent cleaning abrasive at a shipyard

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.

Waterjet Technology – Water Jet Stream Structure

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

In last week’s post I showed some high-speed photographs of the plain water jets that come from the small diamond and sapphire orifices and that are useful in cutting a wide variety of target materials. Before moving away from the subject of high-speed photography, this post will use results from that technique to talk about why pressure washer nozzles may not work well and have limited range. From there it will raise the topic of adding abrasive to a waterjet stream.

Most of us, I suspect, by this point in time, have used a pressure washer to do some cleaning, typically around the house or perhaps at a car wash. The jet that comes out of the end of the nozzle is typically a fan-shaped stream that widens as the water moves away from the orifice. This flattening of the jet stream and the resulting spreading jet is achieved by cutting a groove across the end of the nozzle to intersect either a conic or ball-ended feed channel from the back end of the nozzle.

Schematic of how a fan–generating orifice is often made

Figure 1. Schematic of how a fan–generating orifice is often made

One of the problems with this simple manufacturing process is that the very sharp edge that is produced to give a clean jet leaving the nozzle is very thin at the end. This means that with water that is not that clean (and most folk don’t filter or treat pressure washer water) the edge can wear rapidly. I have noted several designs (and we tested many) where the jet lost its performance within an hour of being installed, particularly with softer metal orifices. And in an earlier post, I did show the big difference between the performance of a good fan jet and a bad.

So how do photographs help understand the difference and explain why you should generally keep a fan jet nozzle within about 4-inches of a surface if you are trying to clean it? That does, however, depend on the cone angle that the jet diverges at once it leaves the nozzle. We found that a 15-degree angle seemed to work best of the different combinations that we tried. If the jet remained of sufficient power, this would mean that it would clean a swath about half-an inch wide with the nozzle held 2-inches above the surface. At 4-inch standoff it will clean a swath about an inch wide and at 6 inches this goes up to over an inch-and-a-half. But that would require that the jet be of good quality and evenly distributed.

Back-lit flash photograph of a fan jet

Figure 2. Back-lit flash photograph of a fan jet, at a jet pressure of around 1,000 psi. It is less than 6 inches from the end of the orifice to the rhs of the picture.

In Figure 2, the lack of water on the outer edges of the stream shows that the water is not being evenly distributed over the fan. As the water volume leaves the orifice, the sheet of water begins to spread out into the wider but thinner sheet that forms the fan. But as it gets wider it also gets thinner, and, like a balloon, water can only be spread so thin before the sheet begins to break up. As soon as it starts to do so, the surface tension in the water causes it to pull back into roughly circular rings of droplets.

Fan jet breakup from a spreading sheet into rings (or strings) of large droplets that rapidly break down into mist

Figure 3. Fan jet breakup from a spreading sheet into rings (or strings) of large droplets that rapidly break down into mist.

These droplets start out as relatively large in size, but they are moving at several hundred feet per second. As single droplets move through stationary air, the air rapidly breaks them up into smaller droplet sizes and then into mist while at the same time slowing the droplets down. The smaller they get, the quicker that deceleration occurs. When droplets get below 50 microns in size, they become ineffective. (From a study that was done on determining the effect of rain on supersonic aircraft).

Showing the stages of the fan jet breakup from a solid sheet to mist that does little but wet the surface that it strikes

Figure 4. Showing the stages of the fan jet breakup from a solid sheet to mist that does little but wet the surface that it strikes.

However, if the nozzle is held just in that short range where the droplets have formed but have not broken down, then the jet will be more effective than it would have been at any other point along its length. This is because of something that was first discovered when scientists at the Royal Aircraft Establishment-Farnborough and at the Cavendish Lab at Cambridge University were studying what would happen if they flew a Concorde into rain while it was still going supersonic. (They actually tried this in a heavy rain storm in Asia and found it was a seriously bad idea).

The pressures that can develop under the spherical droplet can exceed twice the water hammer pressure so that the impact pressure on the surface can exceed 20-times the driving pressure supplied by the pump. But the region affected is very small, and the effect diminishes as the surface gets wetter. And the problem, as with all waterjet streams, is that it is very hard to know where that critical half-inch range is. It varies even within the same nozzle design models due to small changes on the edge of the orifice. And as a very rough rule of thumb, a perfect droplet moving at a speed of around 1,000 ft/sec will travel 138 diameters before it is all mist. Most drops aren’t perfect and thus will travel around 30 – 50 diameters and once they turn into mist they will decelerate to having no power in less than quarter-of-an-inch. The implication of this, which we checked with field experiments, is that if you hold a pressure washer nozzle with a fan tip more than 4-6 inches from the target you are largely just wetting the surface, and spending a fair amount of money in creating turbulent air.

This story of jet breakup is a somewhat necessary introduction to two posts that I will publish before long. The first will be to discuss how we can use a different idea for nozzle designs to do a much better job at greater standoff distances and I will tie that in with some of the advantages of going to much higher pressure to do the cleaning job.

The other avenue that this discussion opens relates to how we mix abrasive within the mixing chamber of an abrasive nozzle design, and that will come along a little later.

(For those interested in more reading, there has been a series of Conferences on Rain Erosion, and then “Erosion by Solid and Liquid Impact” which were held under the aegis of John Field at Cambridge for many years, e.g. Field, J.E., Lesser, M.B. and Davies, P.N.H., “Theoretical and Experimental Studies of Two-Dimensional Liquid Impact,” paper 2, 5th International Conference on Erosion by Liquid and Solid Impact, Cambridge, UK, September, 1979, pp. 2-1 to 2-8. The founding conference was held under the imprimatur of the Royal Society, which devoted a volume to the Proceedings. Phil. Trans. Royal Society, London, Vol. 260A.)