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 – 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 – Cutting Plywood and Pork, and water jet safety

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 two posts I have tried to show that there is a benefit to running an occasional calibration test on equipment to ensure that it is giving the best performance. This does not mean that the nozzle needs to be tested every day, although some of the cheaper pressure washer nozzles, for example, will wear out in less than an hour. An operator will learn over time about how long a nozzle will last and can, after a while, tell when it is starting to lose performance. But in working on a number of different jobs in succession, that sense of the performance may be missed, and it can be handy to have a standard target that a jet can be pointed at and that it should be able to cut in a known time.

One simple target is plywood, and, to continue the saga of nozzle comparisons through a slightly different approach, Mike Woodward used plywood sheets to compare different nozzles in one of the earliest comparisons of performance. We since duplicated his test equipment and ran tests with a more modern selection of nozzles but the basic results and conclusions remain the same.

In its simplest form, the idea is to build a holding frame that will hold small squares of plywood at fixed distances from the nozzle. In the frame shown below, the plywood pieces are set at one-foot distances apart with the nozzle held at a fixed point at the end of the test frame. Tests showed that it takes around 2,700 psi to cut through the plywood.

A simple frame to hold plywood samples

Figure 1. A simple frame to hold plywood samples

The initial tests that Dr. Woodward ran were run on nozzles that were run at 10,000 psi with a nominal flow rate of 10 gpm. The nozzles that were used cost in the range from $10.00 to $250 a piece. (And these costs were reported in 1985 at the 3rd American Waterjet Conference). Tests such as this are simple to run. Plywood pieces are set into the frame, the nozzle is placed at the end of the frame and the jet run for ten seconds. Over that time, the jet will cut through any of the pieces of plywood that it reaches with enough power to cut through, and generally, the jet will punch a hole through several pieces.

The different designs of nozzle that Mike Woodward tested in 1985

Figure 2. The different designs of nozzle that Mike Woodward tested in 1985

The profiles show that there was only one of the common nozzles at the time that fitted smoothly onto the end of the feed pipe. In the other cases, there is a small gap between the nozzle piece and the feed tube so that turbulence would be generated just as water entered the acceleration section of the nozzle.

The hole size in each plate was then measured and that width plotted as a function of the distance from the nozzle so that a profile of the jet cutting path could be drawn.

Profiles cut into the different pieces of wood showing the cutting power of the different jets

Figure 3. Profiles cut into the different pieces of wood showing the cutting power of the different jets as a function of distance and the actual amount of water flow as measured

As an additional part of the testing, a rough measure was kept of the effective nozzle life. Some other performance parameters for the different nozzles can be put into a table.

Performance of the different nozzles

Figure 4: Performance of the different nozzles

Clearly, just going out and buying the most expensive nozzle on the block is not necessarily the best idea. But it also depends on the use to which the nozzle is going to be applied. There are two different applications: that of cleaning a surface and that of cutting into it. The broader path achieved by nozzle 1, for example, which also removed the largest volume of wood per horsepower, makes it a good selection for cleaning and for reaching further from the nozzle as would be needed if one were cleaning the pipes of a heat exchanger bundle.

On the other hand, the more coherent flow through nozzle 2, which gave a narrower cut, might be a more effective tool in a cutting operation. In other cleaning operations, where the nozzle is being operated very close to the surface, then nozzle 3, which has a wider path, might be a better choice, though that is lost if the target surface is further away. And though there was not a great deal of difference in performance between nozzles 1 and 5, there is a considerable difference in price.

A smaller, lighter nozzle may be a beneficial trade-off if the nozzle body is fitting on the end of a lance that will be operated manually for several hours at a time.

There is an alternate way of using plywood as a target that I have also used in teaching class. The student is using a manually operated high-pressure cleaning gun at 10,000 psi and is to swing the gun horizontally so that the jet cuts into a piece of plywood that is set almost parallel with the jet path, but with the stream hitting the wood from the side initially further from the operator. But as the swing completes the jet cuts up where the nozzle almost touches it and then sweeps on past.

The result is that, over the distance, the jet can cut into the wood and a groove is carved into it.

Horizontal cuts into plywood

Figure 5. Horizontal cuts into plywood. There were about half-a-dozen students who had swiped the nozzle so that it just cleared the left edge of this 4-ft wide piece of plywood, and you may note that the cuts extend roughly ¾ of the way along the surface

Once the students had seen this cut, I would ask them how far away they thought, based on that measurement, the jet would cut into a person. Typically they said about three feet, and then, as a precaution, I suggested they add a foot or so more.

Then I took them over to a metal frame where we had hung a piece of pork. We carefully measured off the “safe” distance from the end of the nozzle to the pork.

“Now assume that is you”, I would say, “swing the jet as fast as you can, so that it barely has time to hit “your arm”, and we’ll just check that distance is correct.”

Piece of pork that has been traversed by a 10,000 psi jet several times

Figure 6. Piece of pork that has been traversed by a 10,000 psi jet several times, with a typical stand-off distance from the nozzle of more than four feet.

Invariably we got the result shown in Figure 6. The jet would cut into the meat to a typical depth of around two inches and groove the underlying bone. It was a salutary way of getting their attention about the safe use of waterjet technology, and I noticed that the staff also got a bit more cautious after we ran this class every year.

Waterjet Technology – Testing Waterjet Nozzle wear by cutting foam

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

Last week’s post discussed a simple test which helps to show not only how to compare the effect of different operating conditions (varying abrasive type, nozzle design, AFR etc) as a way of finding a possibly better and cheaper cut. It is also often handy to know when a nozzle is starting to wear out, so that different cutting operations might be scheduled to allow the nozzle to continue to work, without threatening the quality of a critical product.

Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel

Figure 1. Change in the cutting depth of a jet stream at 50,000 psi when traversed over ASTM A108 steel as a function of the time that the nozzle had been in use

While we have found that nozzles from a given manufacturer roughly agree in cutting performance and times before they wear out, the pattern of wear and performance change differs from one nozzle design to another. Also there is some variation in performance between nozzles even of the same design and under the same conditions.

There are also times when cuts are made without abrasive or when the cutting/cleaning jet is hand-held – what to do in those cases? Mainly we have used foam as the cutting target, set up so that the jet won’t cut all the way down through the foam all the way along the cut, so that, as with the steel, some idea of not only cutting depth but also cut quality can be seen.

Cuts through thick stiff packing foam

Figure 2. Cuts through thick stiff packing foam. Note the rough edge at the bottom of the extracted pieces, but the good initial quality of cut that was achievable for some 14-inches.

There is a caution in cutting foam in that some of the softer varieties are going to fold into the cut and give a slightly inaccurate measure of true performance; although for a quick comparison to see how a nozzle is lasting that is not a real issue. When cutting thicker material and also when going for higher quality cuts that is, however, something that should be borne in mind.

The white expanded foam that is used as a packing material is also very easy to cut, even with the pressures that can be found with a pressure washer type of system. Thus, if you are going to clean a deck or other surface, it helps to check by swiping the jet across such a piece of material to be sure that you have a good nozzle on the end of your lance before you start.

This may seem fairly logical; after all you just went to the hardware store and bought a new packet of nozzles. Well, as with the other nozzles we have looked at, quality is only assured after testing. In this particular case, we ran as many different varieties of fan nozzles as we could to see how they would perform when cutting across a piece of packing foam. It is not hard to cut packing foam with a high pressure jet. And since domestic cleaning is usually carried out at either 1,000 psi or 2,000 psi, we ran tests at both levels.

Results from a good, top, and a poor nozzle with cuts at 1,000 and 2,000 psi and with the foam moved through the jet at a distance of 3 inches

Figure 3. Results from a good, top, and a poor nozzle with cuts at 1,000 and 2,000 psi and with the foam moved through the jet at a distance of 3 inches. The number identifies the nozzle. Note that at 3 inches, number 18 could barely remove the top of the foam.

A fan jet is defined by the amount of water that it will allow to pass at a set pressure and by the angle of the cone with which the jet spreads out from the orifice. In passing, we found that the cone angle that the jet actually spread at was a little larger than that designated on the package.

The worst nozzle design that we found had difficulty in cutting into the foam even at a very close range:

On the other hand, the best nozzle was still able to cut the material with the nozzle held some nine inches from the foam.

Cutting result with the good nozzle held at nine inches above the foam target

Figure 4. Cutting result with the good nozzle held at nine inches above the foam target. At this distance the jet is removing as much material as the poor jet did at a 3-inch standoff.

A very typical result would have the jet fail to cut into the foam much beyond four inches from the nozzle. (I’ll use some photographs in a couple of weeks to explain in more detail why that is). And as a short editorial comment to those of you who clean around your house with a domestic unit, how many of you hold the nozzle that close to the surface? (Or at the car wash?) If you don’t you are losing most of the power that you are paying for and you are in the company of most of the students that I ran this demonstration with in my classes).

However there is one other feature to the photographs of the cuts that I would point out. Fan jets distribute the water over a diverging fan shape. But the results of the design fell into two different types, one where most of the water still concentrated in the middle of the jet, (as in Figure 4) and those where it was focused more on the side.

Cutting pattern with the jet streams more at the side of the flow

Figure 5. Cutting pattern with the jet streams more at the side of the flow (arrow points). Note that the two pressure cuts are on the other sides of the sample here.

The benefit of using foam is that it allows this picture of the jet structure to be easily seen, with very little time taken to swipe the nozzle over a test piece of material at the start of work, to make sure that the jet is still working correctly.

This is both an advantage and a disadvantage. Because the foam is relatively easy for a jet to cut even at a lower pressure, this means that the cut can become more ragged with depth where deep cutting is required.

One of the programs that we ran, some years ago, looked at how deeply you could cut into the stiff packing foam that is used in some industrial plants, where the item being packed needs to be held firmly yet will be released easily when needed. This requires that the foam be cut to a very tight tolerance, and at the time, pieces were still being cut by hand and then glued together. (Figure 2 above)

We found that we could cut up to about a foot of material before the small cut particles became sufficiently caught up in the cutting jet that the edge quality of the cut fell below specification. But in order to get to that depth we did have to add a small amount of a polymer to the cutting water. This helped to hold the jet more coherent over a greater distance and also reduced the amount of particulate that got caught up in the jet allowing the greater cutting depth.

Foam works as a simple sample to give some sense of the jet shape where the pressures are lower. When they are higher, a stiffer material is needed though it should still be cut by water without the need for abrasive. Plywood is a useful target in this case, and I will write about those tests next time.