Waterjet Technology – Cleaning with Heat

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

Water is used almost everywhere as a way of cleaning surfaces. Several times a day, we typically rub our hands together with water and usually with some soap to clean them. Pediatricians and others suggest that children recite a short rhythm such as a chorus of “Happy Birthday” while doing so to allow the water, soap and mechanical actions to combine and effectively remove dirt. That teaches the child that it takes some 20 seconds for the cleaning action to be effective. The cleaning action is not to sterilize germs, viruses and other obnoxious things on the hands. Rather it is to ensure that they and other dirt particles are physically removed, leaving the hands clean. (This is a different action to the chemical washes that are becoming popular.)

This is not an instantaneous process since the soap and water must reach into all the dirt-collecting parts of the hand – hence the need for the nursery rhythm. The same basic sequence occurs in the cleaning action of a high-pressure waterjet on a surface, although the pressure of the spray means that the water can penetrate faster. But it is why, in using a car wash lance in cleaning a car, it is smart to spray the body of the car with a detergent first, and then allow this to work in creating micelle clusters around the dirt particles, so that the mechanical action of the subsequent jet spray will dislodge and remove them. Merely adding detergent to the cleaning water as it goes through the cleaning lance and strikes the car surface does not give the chemicals in the water time to act before they are gone. Bear in mind that the jet is moving at several hundred feet per second and that it hits and rebounds from the surface over a path length of perhaps an inch or two. As a result, the residence time of the jet on the surface is measured in fractions of a millisecond. This is not enough time for the chemicals to work. (On the other hand it does help keep the sewers under the car wash cleaner than might be otherwise expected.)

With an increase in jet pressure, the speed of the mechanical removal of dirt and other particles from a surface can be fast and effective. The ability of the jet to penetrate into and flush out surface cracks and joints means that it becomes a good tool for removing debris from the joints in concrete decks, and, at a little higher pressure, it can also be used to remove deteriorated concrete from surfaces. But I am going to leave that topic until next week.

The other “treatment” that we use when we wash our hands is to heat the water. When used with soap, it helps to remove the surface oils on the skin that act as a host to bacteria. Heat is becoming a less common tool than it used to be in high-pressure jet cleaning. At one time, steam cleaning which was followed by hot pressure-washing had a larger sector of the market. It is a bit more difficult to work with (the handles of the gun get hot, and the operator needs more protection) but for some work it is still the more effective way to go.

Steam, however, loses both heat and mechanical energy very quickly after it leaves the nozzle. It will, for example, lose some 30% of its temperature within a foot of the nozzle. Hot sprays of water can thus be more effective, but when cleaning grease and oils, a lower temperature spray will merely move the globs of grease around the surface. Heating the water to around 185 degrees Fahrenheit (or 85 degrees C) will stop that happening and works much more effectively in getting the surface clean.

The effect of water temperature on cleaning different surfaces of different types of dirt

Figure 1. The effect of water temperature on cleaning different surfaces (A, B and C) of different types of dirt.

But, as with many tools, heated water needs to be applied with a little bit of background knowledge. I mentioned that just pointing a large jet of water at, for the sake of discussion, a boulder covered with an oil spill would, at lower water temperatures, just move the oil around the surface. At higher temperatures, the oil would break into smaller fragments that are removed from the surface, but they need to be captured, otherwise the treatment is just spreading the problem over a larger area. This is why it becomes more effective to use smaller, higher pressure systems that have lower contained jet energy and which can be used with a vacuum collection system to pick up the displaced water, oil and debris.

Using hot, pressurized water streams in cleaning up after the Exxon Valdez oil spill

Figure 2. Using hot, pressurized water streams in cleaning up after the Exxon Valdez oil spill (NOAA )

With the streams used in the picture shown in Figure 2, the energy in the jet will move the oil, but without containment it was being washed down to the water, where it was collected using booms. This is not particularly effective since in the process, the jets also washed the silt out of the beach and drove some of the oil down into the underlying beach structure, so that it continued to emerge in later years contributing to an ongoing problem.

What is needed is to provide enough energy to drive the oil away from the surface and yet not enough to move it great distances or to disrupt the surrounding material. This can be achieved by using a higher-pressure but lower flow rate jet. Because some of the water will turn to steam as it leaves the nozzle, Short (PhD U Michigan, 1963) showed that the droplet size will fall from 250 microns to 50 microns when the water is heated above 100 degC.

Obviously, that also will reduce the distance that the jet is effective, and so a balance needs to be achieved between the heat put into the water and the size of the orifice(s) if the jets are to remove the contamination but in such a way that it can be captured. And here again there is a benefit from having a suction tool associated with the cleaning spray. Because of the problems that oil and grease can cause, it will require special care in designing the capture systems downstream. Incidentally, it is generally better if the water is heated downstream of the pump, since there are higher risks of cavitation in the inlet ports if the water is too hot.

And sometimes the two can be combined in ingenious ways. For example Bury (2nd BHRA ISJCT, Cambridge, 1974) added a steam shroud around a conventional waterjet at 5,000 psi as a way of cleaning hardened plastic from the insides of a chemical plant pipe.

Wrapping a conventional waterjet in a steam shroud

Figure 3. Wrapping a conventional waterjet in a steam shroud (Bury et al 2nd BHRA ISJCT, Cambridge, 1974)

Without the steam assist, the plastic was not removable even at higher jet pressures, but with the steam to soften the plastic the pipe was successfully cleaned.

High-pressure water fails to remove hardened plastic

Figure 4. High-pressure water fails to remove hardened plastic, (lhs) but with a steam shroud a lower-pressure jet effectively cleans the pipe (rhs). (Bury et al 2nd BHRA ISJCT, Cambridge, 1974).

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).

Waterjetting Technology – Water Quality

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

One of the problems with taking a research team into the field is that you have to be able to provide answers and a path forward when things go wrong. So it was on a project we once had in Indiana, and it took about a year for me to live down the tale. We had set up a 350-hp high-pressure triplex for a project that involved washing explosives out of shells. Everything had been set up and was ready to go, and so we switched on the water to the pump, started the diesel engine and almost immediately noticed that we weren’t getting enough water downstream of the pump. What was the problem? We checked all the valves, couplings and hoses, and they all seemed to be OK. It was, however, a bitterly cold day with a howling wind around where we had the pump unit. And so I came up with the idea that it was the wind, chilling the pistons, which operated with their length exposed during part of the stroke. If the wind chill was cooling the pistons, then perhaps they weren’t displacing enough volume because they had shrunk. It became known as “The Wind Chill Factor” explanation and, as those of you who have done this sort of thing realize, it was bunkum! After a while, one of the team wandered back to the filter unit, pulled out the partially plugged filters, changed them to new ones and we were in business.

There are a couple of reasons that I tell this bit of history and they relate both to the quality, and the quantity of water that is being supplied at a site. I remember talking to Wally Walstad, who ran McCartney Manufacturing before it became KMT Waterjet Systems, about their second commercial installation and how the different water chemistry just a few hundred miles away had caused maintenance issues on the pumps that they had not expected.

Parts for a multi-piston high pressure pump

Figure 1. Parts for a multi-piston high pressure pump

It may seem obvious that a pump should be supplied with enough water so that it can work effectively. But the requirement, as one moves to higher-pressure pumps, becomes a little more rigorous than that. Consider that the water supplied must enter the piston and fill it completely during the time that the piston is pulling back within the cylinder. Because the piston is pulling back if the water flow into the cylinder is not moving in enough, then the piston will pull on the water. Water does not have any tensile strength, and so small bubbles of vacuum will form. When the piston then starts back to push the water out of the cylinder these bubbles, which are known as Cavitation, will collapse. In a later post I will tell you how to use cavitation to improve material removal rates. But the last place that you want it is in the high-pressure cylinder, since the bubble collapse causes very tiny high (around 1 million psi) micro-jets to form that will very rapidly eat out the cylinder walls, or chew up the end of the piston. (Happened to us once).

There is a Youtube video which shows the cavitation clouds forming in a pump (the white blotches) as the flow to the pump falls below that needed.

To avoid that happening there is a term called Net Positive Suction Head, NPSH. I am not going to go into the details of the calculations, though they are given in the citation. In most cases it is not necessary to make them (unless you are designing the pump). Where the unit being operated is a pressure washer, then the pressure that drives the water out of the tap and into the hose is usually sufficient to overcome any problems with the inlet pressure.

When flow rates run above 5 gpm, however, or when there is a relatively narrow fluid passage into the pump cylinders, or where the water reservoir is below the pump, then the normal system pressure may not be enough. There are two values for the NPSH which are critical – the NPSH-Required (NPSHR) and the NPSH-Available (NPSHA). Let me give a simple example of where one could get into trouble.

For example consider the change which occurs when a pump, normally rated at 400 rpm is driven at 500 rpm, for a 25% increase in output. At 400 rpm the NPSHR for a triplex pump supplied through a 1.25-inch diameter pipe from an open tank will be 8 psi. At 500 rpm, as the flow increases from 26.4 gpm to 33 gpm, the NPSHR rises to 9 psi, which is only a 12.5% change.

However, under the same conditions the NPSHA, which begins at 11.5 psi with a 26.4 gpm demand, falls to 7.8 psi at 33 gpm. When the required suction head is set against that available there was an initial surplus of 45% over that needed. But this changes to a shortfall of 12% when the pump is run at the higher speed. The pump will cavitate, inadequate flow will reach the nozzle to provide full pump performance and the equipment lifetime will be markedly reduced.

This supply pressure required should thus be checked with the manufacturer of the pump. In most cases where we have run pumps at 10,000 psi and higher, we have fed the water into the pump at the designated flow rate, but using a supply pump that ensures that the pressure on the inlet side of the pump valves is at least 60 psi.

One of the problems, as mentioned at the top of the piece, is that when going to a new site the immediate quality of the water is not known. There are two things that need to be done. The first of these, of particular importance at higher pressures, is to check the water chemistry. It is important to do this before going to the site since it usually takes some time to get the results, and if there are some chemicals in the water that may react with pump or system parts, it is good to know this ahead of time so that the threatened parts can be changed to something that won’t be damaged.

There is a specific problem that comes with cutting systems in this regard, since at 50,000 psi or higher, water quality becomes more important, even just in the nozzle passages. And I will deal with this in a few weeks when I talk about different nozzle designs.

And equally important is the cleanliness of the water. Particularly when tapping into a water line that hasn’t been used for a while (as we did), there is a certain amount of debris that can be carried down the line when it is first used. The smart thing to do is to run water through the line for a while to make sure that any of the debris is flushed out, before the system is connected up. The second is to ensure that there is more than one filter in the line between that supply and the pump.

Many years ago, when prices were much lower than they are today, Paddy Swan looked at the costs of increasingly dirty water on part costs. The costs are in dollars per hour for standard parts in a 10,000 psi system and the graph is from the 2nd Waterjet Conference held in Rolla in 1983.

costs for parts when increasingly dirty water is run through a pump

Figure 2. 1982 costs for parts when increasingly dirty water is run through a pump (S.P.D. Swan “Economic considerations in Water Jet Cleaning,” 2nd US Water Jet Conference, Rolla, MO 1983, pp 433 – 439.)

Oh, and the moral of the opening story became one of our sayings in the Center, not that we were original, William of Ockham first came up with it about seven hundred years ago. It’s known as Ockham’s Razor, and simply put it means that the simplest answer is most likely the right one, or don’t make things more complicated than they need be!

Using Nature’s Crack System

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 this section of the series on Waterjetting, the focus is on the way in which high-pressure waterjets grow cracks in their target. As John Field showed, even the presence of microscopic cracks on a glass surface are enough to initiate the larger cracks that lead to failure. In many cases, however, the most useful growth can be achieved if the cracks only extend to the point that they remove a desired amount of material. This becomes important where there are weaknesses and flaws in the material – such as the layers between plies of wood, or even Kevlar – which should not be grown as the jet cuts down through the material. And in a later article, this topic will be a part of a discussion as exactly what happens as a jet drills a hole into a target. But, for today, I would like to talk about crack growths in rock and soil, both because it is one of the oldest ways in which water can penetrate into material, and also because it holds the potential to be one of the newest areas into which waterjetting is growing, and will likely further advance into a more significant business.

And to begin consider that, as water penetrates into the cracks in a rock and grows those cracks slowly under natural forces, rocks with minerals in them will see those mineral particles separately broken out. The classic example of this is with gold. One of the ways in which the Forty-Niners found the gold in California was by panning for the gold particles in the rivers and tracking the gold deposits back up-stream until they reached the original gold deposits of the Sierra Mountains. Not that this was the first time that water transport had helped in gold mining. One of my favorite stories to begin classes is to remind them of Jason and the Argonauts.

Movie poster for Jason and the Argonauts

Figure 1. Movie poster for the 1963 film version of Jason and the Argonauts (IMDb)

It is a theme that has been made into a movie several times, and tells the story of how the Greek Prince Jason and a band of companions go in search of the Golden Fleece, and the adventures that he has along the way. Despite the mythical creatures the story is thought to be likely based on some measure of truth, with the voyage taking place some time before 1300 B.C. But our focus is on the fleece rather than the voyagers.

Suggested path that Jason followed to get to the River Rhion in Georgia

Figure 2. Suggested path that Jason followed to get to the River Rhion in Georgia.(Google Earth)

Within the Caususus mountains of Georgia lies the modern town of Mestia, which was thought in Roman times to be the site of Colchis, where Jason found the Golden Fleece. The reality is not quite as dramatic as the legend since as the Roman historian Strabo noted

“It is said that in the country of Colchis, gold is carried down by mountain torrents, and that the barbarians obtain it by means of perforated troughs and fleecy skins, and that this is the origin of the myth of the Golden Fleece”

The torrents of water in the Svaneti valley outside Mestia

Figure 3. The torrents of water in the Svaneti valley outside Mestia, (Nika Shmeleva Google Earth at 43deg02’29.74”N, 42deg42’25.13E)

It is thought that the miners of the time directed the streams so that they flowed over the veins of gold and eroded out the particles so that the gold was carried down to the valley. Here it was fed through the troughs that Strabo described, and the heavy gold particles were captured as they tangled in the wool of the fleece. To recover the gold the miners would then hang the fleeces in trees, so that they would dry, and the gold could be shaken loose. Unfortunately as the fleeces hung in the trees they provided a tempting target for Greek thieves. (In a later version that I will write about in the next post the sheep fleece was replaced with brush that could be dried and burned to release the gold).

Water was thus, in one of the earliest “automated” mining processes, used to both dislodge and then carry the valuable mineral from the mining site. The overall power of water to move soil has been used to wash away material for over a hundred years. In the 1973 War between Egypt and Israel, the Egyptian Army gained a significant advantage in the early hours of the war by using waterjet monitors to wash away the defensive barrier along the edges of the Suez Canal, rather than using conventional mechanical excavators.

To deal with the massive earthen ramparts, the Egyptians used water cannons fashioned from hoses attached to dredging pumps in the canal. Other methods involving explosives, artillery, and bulldozers were too costly in time and required nearly ideal working conditions. For example, sixty men, 600 pounds of explosives, and one bulldozer required five to six hours, uninterrupted by Israeli fire, to clear 1,500 cubic meters of sand.

The quoted Sunday Times report of the time suggested that the Israeli Army had anticipated that it would take 24-hours to remove the barriers giving time for their Army to mobilize and arrive. However, using a set of five pumps per breech site the Egyptian Army was able to make an opening in as short as a 2-hour time, with the mobilized water cannon opening 81 breeches, and removing 106 million cubic feet of material in that first day of the war. They were thus able to initially advance into the Sinai with relatively little resistance.

The pressure of the water does not have to be high to disaggregate the soil, but large volumes were needed in that application both to break the soil loose and to move it out of the way. Moving the debris out of the way is an important part of the operation, and while, in the above case it could be just pushed to one side, in many more localized jobs, particularly in cities, that is not an answer. However if the soil can be collected with the water, then the fluid can help to move the soil down a pipe away from the working area. And, more importantly, if the soil can be captured as it is being broken loose, then both can be collected before the water has had a chance to penetrate into the soil around the hole, and so the walls of the hole will not get wet and will remain stable and not fall in.

One way that we have achieved this is to rotate a pair of waterjets relatively rapidly (depending on the material the jet pressure can range from 2,000 psi to 10,000 psi) so that the surface layer is removed, and to immediately take this away by combining the jet action with a vacuum for removal. (In the initial trials we used a Shop Vac to remove both water and debris). This combination has become known as hydro-excavation, and will be the topic of a couple of posts in the future.

Similarly the use of high pressure to break an ore down into its different parts, so that the valuable mineral can be separated from the host rock at the mining machine, is become a new way to reduce the costs of transporting and processing the ore and make mining more efficient. As yet this latter is still more of a laboratory development, though it will develop for greater use in the future, and there will be additional posts on this too in the future. But, in both cases, the use of waterjets to effectively rely on extending pre-existing cracks makes the systems work. In the next post I’ll write about a couple of other ways of getting enough cracks into the rock as ways of making it easier to separate and remove valuable materials from underground.