Waterjetting 18b - Abrasive Effects

Abrasive particles were first fed into jet streams as a way of helping to clean surfaces. In many cases the intent was to remove a surface layer of paint/rust so that the underlying surface would be clean and able to accept a new coat of paint or other protective coating.

However, as with many applications of waterjets and abrasive waterjets, putting too much power into a single jet stream could be counter-productive. As an example consider the case where an air-powered abrasive jet (sand-blaster) is used to clean paint from a surface so that it can be recovered. There are several concerns with the use of the abrasive that are not always immediately obvious to the operator. In an earlier post I wrote about the problems of surface deformation. With abrasive particles in the jet stream the operator often works to achieve a smooth, clean surface on the work-piece. The problem that this causes is that the abrasive particles bend over small protrusions on that surface, so that contaminant is trapped under the bent metal and accelerates corrosion after treatment. As a result an abrasive cleaned surface can need repainting after 2 years, whereas with proper cleaning the intervals can be stretched to 5 years. This latter period can be achieved with the use of high-pressure water as a cleaning tool, since this is able to wash into the small crevices in the surface and wash the corrosion products out of the surface, and leaves it clean.

The problem that this creates, in turn, is that it removes all the protective coatings and if the surface is a metal this can lead to flash rusting of the surface and other problems if the surrounding air is corrosive and surface treatment is not carried out fast enough. (In some chemical plants the period available for this coating can be less than an hour). In other cases, such as for example when Linda Merk-Gould cleaned the Statue of Freedom from atop the Capitol Building in Washington, other coatings are required to return the surface to the desired color and texture after cleaning.

Figure 1. Looking at cleaned test panels on the Statue of Freedom with Linda Merk-Gould, during the cleaning.

Further, in that particular case, the antique nature of the metal surface meant that the pressure of the jet had to be precisely controlled in a narrow range where it would be sufficient to remove the corrosion, and yet not sufficient to eat into the weakened metal surface. (That pressure range was IIRC about 3,000 psi).

The second problem that can arise with the use of abrasive on a surface occurs if small particles of that abrasive become lodged in the surface during the cleaning. While in most cases the small individual sizes of the particles have little effect on subsequent performance there are cases where this can be a problem. One is where the surface will be enameled after cleaning, and if steel is used, rather than an iron grit, in the final cleaning any residual steel on the surface can mar the final coating used in the process. Similarly in the advanced welding of structures any garnet or similar abrasive that is left in the surface an affect the integrity of the weld that follows. In both cases, as mentioned earlier a final wash with high pressure water without abrasive can be effective in cleaning the surface to the level needed.

This brings up the topic of the nature of the abrasive itself. In the past sand bas been the most commonly used abrasive, although in more specialized applications small-particle slag, walnut shells and other specialized abrasive is used. This is often the case where (as with the walnut shells) there is a need to minimize the damage to the surface being cleaned – and historically they have been used in cleaning bronze monuments, as a way of minimizing the loss in detail that occurs with harder abrasive, although as the experience in testing for the Statue of Freedom showed, full detail could be retained with the high-pressure water clean, while abrasive would blur the finer detail. Walnut (and other nut shells and parts) were chosen because of their relatively soft nature in contrast with the underlying material beneath the coating being cleaned. The use of softer abrasive, and in some cases soluble abrasive, helps preserve the underlying substrate and can lower clean-up costs, although the abrasive itself might be more expensive.

There is, however, a difference in approach when the surface will be left as cleaned when it is put back into service, and that where it is to be painted or similarly protected before being used again. The need is for the surface to be toothed, or notched, so that the overlying coating can attach to the surface and become harder to remove. (In passing this is why high-pressure waterjets are effective in concrete repair, since they only partially expose the aggregate and the new pour can attach to this rough surface, giving shear as well as compressive and tensile strength. Where the surface has been ground with diamond or carbide wheels the interface is smooth and there is a much lower adhesion between the layers, and the repairs, as a result, don’t last as long).

Figure 2. Microphotograph of a steel surface on the edge of an abrasive jet cleaning path, showing the surface deformation due to individual particles.

In these cases the shape of the particle, the size of the particle and its relative hardness all play a part in control of the final result of the treatment. However, the velocity at which the particle hits the surface is also a factor, controlled by the pressure of the delivery system. Too much pressure will, as mentioned above, bend over the surface protrusions so that the surface becomes smoother and more polished. While this has an aesthetic appeal at the time of treatment the rougher, greyer surface with the protrusions left in place gives the better grip. As Plaster noted in a table presented in his book on Blast Cleaning and Allied Processes, increasing air pressure above a certain value is counter productive.

Figure 3. Change in bond strength as the air pressure during sand-blasting is increased. (Plaster ibid)

I will discuss different aspects of abrasive choice in the posts that follow.

Waterjetting 18a - air and abrasive

As high-pressure waterjet systems have continued to expand into broader fields of application, that increased range has been significantly expanded where abrasive has been added to the jet stream. Abrasive waterjets are more widely used in cutting those materials that are less easy or practically impossible to cut cleanly with water alone (although that is a relative statement, since metal, for example, can be cut at higher water pressure without abrasive).

But before there was high-pressure abrasive waterjet cutting there was sand blasting and other applications of abrasives existed in cutting and material removal – think, for example, of sandpaper. The first “powered” use of sand to remove material has been credited to B.C. Tilghman Jr. of Philadelphia whose British Patent was number 2,147, an indication of how long ago it was. (His American patent was number 104,408). In his review of the topic in 1972 Plaster noted that the patent was fairly comprehensive in regard to some later developments. It in included a system wherein the abrasive was carried by means of

a jet of steam, air water and other suitable gaseous or liquid medium . . . . .the sand may be propelled by a current of air produced by suction or a partial vacuum. . . . . . When a jet of water under heavy pressure is used, as in hydraulic mining, the addition of sand will cause it to cut away hard and close grained substances, upon which water alone would have little or no effect.

Figure 1. Illustration of the initial steam-injected sand blasting design (after Plaster)

At the time that the invention was made steam was the easiest fluid to provide the driving pressures and volumes needed to power the abrasive stream.

The original machine was operated by steam at a pressure of up to 400 psi, and sand was fed from a feed funnel, down through a length of hose into a narrow (0.17 inch diameter) tube centered within the half-inch diameter steam tube. This tube tapered down to a quarter-inch inner diameter as it reached the end of the sand feed line, leaving a narrow gap around the exit to the sand pipe. The high velocity of the steam, as it then flowed the chamber at the end of the sand pipe created the vacuum that pulled the sand into the stream. The resulting jet was collimated by a 6-inch piece of quarter-inch pipe.

It was found, experimentally, that putting a pair of aligned, 3-inch long flat plates on the end of the nozzle, aligned with its edges, gave a better jet, with less lateral spreading when grooves or straight cuts were required.

Steam, however, wet the sand, which would then attach itself to the pipes, causing blockages. Problems also arose because the steam caused poor visibility, and made for unpleasantly hot and wet working conditions. Thus there was an incentive to change, and by the turn of the century (1900) the increasing popularity of compressed air provided an impetus for this change and compressed air then became the main fluid transport for the abrasive throughout the 20th Century. By 1984 production rates for such systems of around 4 sq ft/minute could be achieved by a single operator working with a system driven by a 12 hp. compressor.

As the technology became more widespread so the design of the nozzle was improved through a series of modifications. These led to the inclusion of what is known as a de Laval nozzle into the design of the delivery system. The de Laval design was initially used to drive a small steam turbine in a creamery in 1897, by Gustaf de Laval.

Those who first sought to make steam turbines were also the first to have a large steady supply of an elastic medium that is very like a gas, steam. They soon found themselves using nozzles to produce high-speed flow and they started by using convergent nozzles and they mostly still do. This was the intuitive design with its forerunner in use in hydraulic machinery. They soon found that whilst they could increase the speed of the jet formed by a given convergent nozzle by increasing the supply pressure, no comparable increase could be produced by reducing the back-pressure. They described the nozzles as “choked”. It must have been totally counter-intuitive to find that the fitting of a divergent cone to a convergent nozzle got rid of the problem.

In its simplest form the nozzle takes the form of a convergent section, followed by a narrow constant diameter throat, and this is succeeded by a diverging section at the end of the nozzle.

Figure 2. Basic components of a venture nozzle for abrasive blasting with air.

The increasing diameter of the channel, after the throat, causes a drop in pressure in the nozzle. This, in turn, allows the air to accelerate with the abrasive and the velocity resulting is more than twice as high as it otherwise might reach, going from perhaps According to tests by Tetrabore in 1981, velocity changes from 275 ft/sec to 650 ft/sec have been measured. At the same time the improved velocity of the jet made it effective over a greater area of the target with effective cleaning reported as increasing by 30 - 40%.

A specific design was patented by Albert in 1955, where the transition lines are radiused rather than being linear.

Figure 3. Based on the nozzle design patented by Albert (Plaster ibid).

There are two other advantages to the design beyond the improved air velocity as it leaves the nozzle. The first of these is that the flow is more uniform coming out of the nozzle, so that the surface being cleaned is more evenly attacked, reducing the need for nozzle manipulation to ensure that the surface is completely covered during cleaning, and secondly the amount of abrasive that is required to clean a given surface might be reduced by as much as 20%.

It is in this control of the air component of abrasive blast streams that there thus remains some potential for further improvement. But we will discuss that and other aspects of abrasive use in the following parts of this section.

Waterjetting 7d - High-pressure Waterjet cleaning over sandblasting paint

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 because 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.


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. were hand tools 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 when the surface is restored to a nice shiny surface 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, where the metal is eaten away more in the middle or along the sides of the pocket.

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


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.


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.


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 smooths 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:


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.


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.


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).

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.