High-pressure Waterjet cleaning over sandblasting paint

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

Over the years, I have been caught up in “discussions” with several folk about how good high-pressure and ultra-high pressure waterjet streams were as a surface cleaning tool in contrast with chemical and abrasive use in removing paint and other surface layers. One debate was about cleaning some particularly toxic chemicals from various surfaces. The point that often comes up in these discussions is that of “how clean is clean?” And in this particular case it was stated that the surface could never be completely cleaned. The rationale for that position was that the chemicals would enter into any cracks and flaws in the paint and could therefore be retained either in the top coat or the underlying primer. My answer to that was to take a small sample and clean the surface over the first quarter, raise the pressure and remove the top coat on the second quarter, raise the pressure further and remove the primer down to bare metal on the third quarter and then, after adding a small amount of abrasive to the water, remove a thin surface coat of metal from the sample. It seemed to be a convincing demonstration, though I will come back to one problem in a later post, and for this post I will discuss taking the paint off.

It is now reasonably well known that high-pressure water can be cost effective as a way of removing paint, particularly from large structures such as bridges and ship hulls, but it took a while for some of the benefits to become evident.

Quebec Bridge

Figure 1. It was originally estimated that it would save some $1.75 Canadian per square foot to clean the Quebec Bridge with ultra-high pressure waterjets rather than sandblasting. That increases to $4.50 per sq. ft. if hand tools were the alternative (WJTA Jet News, March 2000)

There are 8-million square feet of surface in the bridge. As I noted at the end of the last post, the historic method for cleaning surfaces and removing deteriorated paint has been to suspend abrasive particles in an air stream and to use those particles to abrade and erode the paint from the surface. When the paint, rust and other coatings have been removed, the job is often considered finished as the surface is restored to a nice and shiny finish. There is, however, a snag when one does this. The numbers that I was once given were on the order of: from the time that a railroad wagon was put into service, it would take 5 years before it would require stripping and repainting. After that first treatment, however, the paint would deteriorate more quickly, and often within another 18 months, the wagon would have to be taken back for repainting.

So why is this, and why does high/ultra-high pressure paint removal help extend the life of that second paint coating? I and the industry are deeply indebted to Dr. Lydia Frenzel who did a lot of the pioneering work in helping to define the benefits of the technology and then spread the word about them. The problem begins as the surface begins to corrode, and I will continue to use the wagon as the example, though the result holds true for many surfaces. As the rust and damage continues to eat through the paint and into the underlying metal, that surface is not attacked evenly, but instead, small pockets of corrosion develop and the metal is eaten away mainly in the middle or along the sides of the pocket.

By the time that the surface is ready to be painted, it is therefore no longer smooth but rather pitted and covered in corrosion.

Exaggerated illustration of the condition of the surface

Figure 2. Exaggerated illustration of the condition of the surface, with the overlying corrosion shown in green.

When the surface is cleaned with an abrasive, typically driven using an air stream to sandblast the surface, the particles will impact and distort the surface. Thus, while the majority of the corrosion will be removed by the impact and scouring action of the abrasive, some will not. Further, the impact of the abrasive particles will bend over the weaker structures on the surface as well as peeling over some of the metal on the surface.

Electron microscope picture of a piece of metal on the edge of a pass by an abrasive laden stream

Figure 3. Electron microscope picture of a piece of metal on the edge of a pass by an abrasive laden stream, so that the action of the individual particles in cutting into and plowing the surface can be seen. Note that this peels over metal edges, for example at the arrows.

The peeling over of the surface and the flattening of it give the shine that used to be the sign that the job had been effectively done. There are, however, two disadvantages to this. The first is that by distorting the surface, the bending over of the metal traps small pockets of corrosion within the surface layer of the metal.

Representation of the metal surface after it has been cleaned with abrasive

Figure 4. Representation of the metal surface after it has been cleaned with abrasive. Note the folding over of metal to trap corrosion products. The abrasive particles are also not small enough to penetrate into the smallest tendrils of corrosion migrating into the metal, and these pockets (green) also are trapped.

With corrosion already embedded in the surface before it is painted, that will develop immediately and thus the relatively short time before it undercuts the paint and causes it to fall off. There is also another reason for this. As air pressure is increased to speed up the cleaning and give that “shinier” surface it smoothes the surface and makes it more difficult to anchor the paint on the metal. This was shown by F.W. Neville (and is quoted in the book “Blast Cleaning and Allied Processes, by H.J. Plaster) with this table:

Relative paint pull strength as a function of the pressure of the air driving the sandblasting stream in pre-cleaning the surface of the old paint

Figure 5. Relative paint pull strength as a function of the pressure of the air driving the sandblasting stream in pre-cleaning the surface of the old paint, prior to repainting.

As the table shows, the higher the air pressure then the smoother the surface, and the poorer the bond made with the paint.

Now consider what happens when a high-pressure jet cleans the surface. The water does not have the power to distort the metal, but rather does have the ability to penetrate all the cracks and pits on the surface, and flush them clean. As a result the surface is left rough (to give a good paint bond) and corrosion free.

Illustration of the relative condition in which a high-pressure waterjet will leave the surface

Figure 6. Illustration of the relative condition in which a high-pressure waterjet will leave the surface

One of the difficulties that early proponents such as Lydia had in getting the technique accepted, however, lay in the cleanliness of the surface. Because the metal had not been distorted back into a smooth upper surface, it does not reflect light in the “shiny” manner that an abrasive cleaned surface does. Thus to those trained to the latter, it did not appear clean. There had to be a considerable amount of demonstration, explanation and training before it was accepted that this “grey” surface was actually cleaner. And there are now standards issued by the Steel Structure Painting Council that recognize this.

A primer coated plate (left) that has been cleaned to white metal (right) using a high pressure waterjet

Figure 7. A primer coated plate (left) that has been cleaned to white metal (right) using a high pressure waterjet.

Note that actual microphotos of abrasive and waterjet cleaned metal surfaces can be found in the paper by Howlett and Dupuy (Howlett & Dupuy, NACE Corrosion/92, paper No. 253; Mat. Perf, Jan. 1993, p. 38, the waterjet pressure was 30,000 psi).

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.)

Waterjet Technology – An intro to water jet 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

Once a waterjet starts to move out of the nozzle with any significant speed as the pump pressure begins to build, it becomes more and more difficult to look at the stream of water and get any realistic idea of its structure. Mainly what is seen is the very fine mist that surrounds the main body of the jet, and while some idea of the structure can be obtained by making cuts through material, it can be quite expensive to actually see within that structure. Part of the problem is that though the mist is very fine, it is also moving at speeds in the range of a couple of thousand feet per second. The human eyeball isn’t quite that fast. But we can use a very high-speed flash (in this case it was on for two millionths of a second) which has the effect of “freezing” the motion.

40,000 psi jet issuing from a 0.005 inch diameter orifice, front lit

Figure 1. 40,000 psi jet issuing from a 0.005 inch diameter orifice, front lit.

However, this mist still hides the solid internal structure of the jet and does not change much in relative structure, even when the internal jet conditions can be quite different. Fundamentally, the internal structure was described by Yanaida at the 1974 BHR Group Waterjet Conference and his description has been validated by many studies since.

The break-up pattern of a waterjet

Figure 2. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

This structure holds for jets across a wide range of pressure and flow volumes, but it is difficult to determine the exact transition points of that structure conventionally. And this can lead to very unfortunate results. I have twice seen people back a nozzle away and then move their hand in front of the jet to show that even high-pressure jets (these were being used to cut paper products and had no abrasive in them at the time) could be “safe.” If both cases, the individuals were very lucky to escape injury (water can penetrate the pores of the skin and lacerate the internal parts without any surficial signs of injury, and, as I showed last time, if the nozzle is too close, it will slice through flesh and bone). I thought to take today’s post to show, though the use of photographs, why that was such a stupid action.

The photos that follow were taken in Baxter Springs, KS, which has been recognized as the Birthplace of Waterjet Cutting.

Baxter Springs, Kansas. Birthplace of Waterjet Cutting

Figure 3. Baxter Springs, Kansas. Birthplace of Waterjet Cutting

In the early 1970’s, we used what was then a McCartney Manufacturing waterjet intensifier (today KMT Waterjet Systems) to shoot jets of varying pressure and nozzle diameter along a path, so that we could see how coherent the jets were. As I mentioned above, the problem with looking directly at the jet is that the internal structure is hidden by the surrounding mist. To overcome that part of the problem, we shone the light along a ground glass screen (to diffuse it) that was placed behind the jet, so that we could see the outline of the internal structure.

Arrangement for taking photographs of a high-speed jet

Figure 4. Arrangement for taking photographs of a high-speed jet.

This more of the downstream mist from the photograph, and a much better idea of the internal structure of the jet, and where the solid section ended could be measured.

Backlit, 30,000 psi jet issuing from a 0.01 inch diameter nozzle, the distance across the photograph is 6 inches

Figure 5. Backlit, 30,000 psi jet issuing from a 0.01 inch diameter nozzle, the distance across the photograph is 6 inches.

The benefit of the technique is perhaps more evident when nozzles at different pressures and diameters and different chemistry are compared. First consider the change with an increase in jet diameter. From the front-lit view there is little difference in the jets. From the backlit, it is clear that the smaller diameter jet only reaches 3-inches across the screen, while the larger jet barely reaches the end of the range.

The effect of doubling the orifice diameter at the same jet pressure on jet range, the photo length is 6 inches

Figure 6. The effect of doubling the orifice diameter at the same jet pressure on jet range, the photo length is 6 inches.

One of the parts of the study we were carrying out in 1974 was to examine the effect that adding different long-chain polymers had on jet structure. The ones that we were looking at include some that are now used in the oil and natural gas industry to make the “slick water” that is used in the fracking industry to improve production from shale reservoirs. But it also has an advantage in “binding” the jet together. And so, in the study, Dr. Jacques Zakin and I tested a wide range of different polymers to see which would give the best jet.

There were a number of different things we were looking for. In cutting paper, soft tissue and water sensitive material for example, the polymer can bind the water sufficiently well as to further lower wetting to the point where it doesn’t have an effect. It also can improve jet cutting under water – but I’ll cover those in a few post on polymer effects that will come to later in the series.

The effect of a polymer (in this case an AP273) is shown in two tests where the only change was to add the polymer to the water for the lower one.

Jets with an orifice diameter of 0.01 inches at a pressure of 20,000 psi

Figure 7. Jets with an orifice diameter of 0.01 inches at a pressure of 20,000 psi, the range is 6 inches, and the lower jet has had the polymer AP273 added to the water.

The narrower stream in the lower frame is the effect that we were looking for. Putting change in diameter and the better polymers together gave, as an example, the following:

The effect of changing jet pressure, nozzle diameter and polymer content on jet cohesion

Figure 8. The effect of changing jet pressure, nozzle diameter and polymer content on jet cohesion.

It might be noted that the jet in the bottom frame has as much relative concentration (and power) at the end of the range as the top jet had at the beginning of the range.

Now it all depends on what you want the jet to do, as to which condition you wish to achieve. Inside abrasive mixing chambers, the object is much different than it is when the object is to cut a foot or more of foam with high quality edges. And there have been some interesting developments with different polymers over the years, but I’ll save those stories for another day.

But bear in mind that those individuals who could slide their fingers under the jet in the top frame of figure 8 would have had them all cut off if the jet had been running instead under the conditions of the bottom two frames, and in all three cases, to the naked eye the jets looked the same.