Waterjet Technology – Starting to make water jet cut hole

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

This small sequence of posts describes the initial milliseconds during which a high-pressure waterjet penetrates into a target material. Because this work was largely developed using rock targets, most of the illustrations will be with that material, but the concept applies, to a degree, also with abrasive laden jets penetrating into materials such as glass.

For this post, I am going to discuss just what happens with the jet being fired down onto the target surface without either the nozzle or the target moving. Much of this work was carried out in the 1960’s in the UK though I will begin with some tests that Dr. Bill Cooley carried out using his modification to a Russian hydraulic cannon that he redesigned so that it was capable of firing at 500,000 psi – and yes I have seen it fired at that pressure (I took the photo).

The Cooley Cannon ready to fire at 500,000 psi in an underground mine

Figure 1. The Cooley Cannon ready to fire at 500,000 psi in an underground mine.

In order to see how effective different processes were in cutting into different materials, the international scientific community that was developing waterjet technology at the time needed a method to compare the different approaches. The metric that was used was to define the Specific Energy as the amount of energy that it took to remove a unit volume of the target material. (And in time that will be subject of some more specific posts).

Bill’s cannon used stored gas that was suddenly released as a way of driving the water at the desired pressure and measured the pressure indirectly by timing the break of two pencil leads in front of the nozzle. This gave the jet velocity, and pressure could be back-calculated from that value.

Dr. Cooley took results from his work and from other scientists working with similar devices, to produce the following graph.

Specific energy as a function of the impacting jet length, measured in nozzle diameters

Figure 2. Specific energy as a function of the impacting jet length, measured in nozzle diameters. (Cooley, W.C., “Correlation of Data on Erosion and Breakage of Rock by High Pressure Water Jets,” Chapter 33, Dynamic Rock Mechanics, ed., G.B. Clark, 12th Symposium on Rock Mechanics, University of Missouri-Rolla, November, 1970, pp. 653 – 665.)

For those running a conventional cutting table, the water orifice is around 10 thousandths of an inch in diameter. So what this graph is saying is that once the first thousand diameters of length (1000 x 0.001= 10 inches) has hit the surface, then the process starts to become significantly less efficient. If the jet is moving at 2,000 ft/sec, that length arrives in around 0.0005 seconds. Why this rising inefficiency after that time, and how do we get around it?

Earlier in this series I mentioned that one of the tests to find the pressure at which a waterjet penetrates a target is to note the point at which, instead of the water hitting the surface and flowing along it, it changed direction to flow back towards the nozzle. This is because as the jet penetrates, it makes a hole, and the only way out of that hole is back along the way the jet came. Unfortunately, there is more water still coming down into the hole, and so the water leaving the hole (at the same volume flow rate) meets the water coming into the hole. The rapidly moving water going out is moving about as fast as that coming in, and so, as the hole gets deeper, the pressure at the bottom of the hole gets less. This has been measured by a number of folk, but Dr. Stan Leach was the first, and produced this plot:

Depth at the bottom of a hole, as a function of the incoming jet pressure

Figure 3. Depth at the bottom of a hole, as a function of the incoming jet pressure. (Leach, S.J., and Walker, G.L., “The Application of High Speed Liquid Jets to Cutting,” Philosophical Transactions, Royal Society (London), Vol. 260 A, 1966,pp. 295 – 308.)

Because the holes were preformed of metal (to hold the transducer) and were sized to the nozzle diameter, this is not as it turns out totally accurate although it illustrates the problem.

It isn’t totally accurate because, as the illustration from the last two posts showed, the erosion occurs initially around the edge of the jet rather than under it, and thus the hole created is about twice to three times the jet diameter rather than being of the same size.

Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle

Figure 4. Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle.

Nevertheless, as the hole deepens, the pressure at the bottom of the hole gets less, and after a while the jet penetration slows to almost a halt.

Penetration as a function of time

Figure 5. Penetration as a function of time (My Dissertation)

The sides of the hole, however, continue to erode, but from the bottom upwards so that after a short while, the narrower entry hole starts to constrict the flow out, and pressure begins to build-up in the hole.

Remember that a waterjet works by growing existing cracks in the material. So that if there is a natural crack in the rock, which may be as small as a grain boundary, or the scratch made by an abrasive particle as it moves back out of a hole in glass, then the water entering that small crevice will pressurize the walls and cause the crack to grow. Often there is more than one, and the result can be, in rock:

Rock breakage around the jet impact point on a 1-ft block of sandstone

Figure 6. Rock breakage around the jet impact point on a 1-ft block of sandstone (after Moodie and Artingstall Moodie, K., Artingstall, G., “Some Experiments in the Application of High Pressure Water Jets for Mineral Excavation,” paper E3, 1st International Symp on Jet Cutting Technology, Coventry U.K., April, 1972, pp. E3 25 – E3 44.)

In rock, that might not be such a bad thing since in many cases the intent is just to break the rock out of the way, so that a tunnel can be created that folk can walk or drive through. But in the case of glass and other such brittle materials, where the object is just to make a very fine cut with no side cracks, cracking the sheet is disastrous. This can be illustrated by the results when a jet was fired along the central axis of a 2-inch diameter core of granite. The escape of water into the cracks allowed the cycle to repeat several times, and the hole was, as a result, much deeper than it would have been if the cracking had not occurred.

2-inch diameter granite core that split when a short jet pulse was fired into the core, along the axis

Figure 7. 2-inch diameter granite core that split when a short jet pulse was fired into the core, along the axis.

And so, next time, I’ll write about some of the ways in which we can get around this problem.

Waterjetting – The Effect of Standoff Distance

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 relying on photographs is that they are sometimes not of the quality that one would wish. This has happened with today’s topic, where the pictures are old, smaller and in poorer condition than I had remembered. However, with your indulgence, I am going to step through them. I do apologize for their poor quality, however.

The topic is the way in which a waterjet first attacks a target. I have mentioned different parts of this process in the past. But in this post I want to show that it matters where the target is, relative to the nozzle, because the structure of the jet itself changes with that distance, which I call the standoff distance between the jet orifice and the initial target surface.

The break-up pattern of a waterjet

Figure 1. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

As I mentioned last time, when the target is close to the nozzle, then the erosion pattern can, in the first few seconds of contact, be seen to be like a butterfly in pattern. The central part of the target under the jet is not eroded but there is severe erosion around the edges of the jet diameter, where a grain will see high differential pressures across its width and will be subject to high lateral jet flows.

Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle

Figure 2. Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle

As the nozzle is moved away from the target surface, however, that pattern of erosion changes. As the jet structure picture shows, the central zone at the initial pressure reduces in radius, and there is an intermediate zone of rapidly diminishing pressure, with an outer shroud of fine droplets. The effect on the impacted target is that there continues to be a small zone with no erosion in the center, and that erosion is still concentrated around this zone, in that of high differential pressure, which now encroaches on that central sector.

Erosion of an aluminum target with the nozzle 2-inches above the surface

Figure 3. Erosion of an aluminum target with the nozzle 2-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

That central small plateau is reduced to a very small point by the 3-inch standoff, which is where the jet reaches the end of the distance where the pressure remains constant over the central section. Thus, by a 4-inch standoff, the central section, though still present, is being eroded.

Erosion of an aluminum target with the nozzle 4-inches above the surface

Figure 4. Erosion of an aluminum target with the nozzle 4-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

As the nozzle is moved further back from the surface, that central promontory disappears at around a six-inch standoff. It is interesting to note that at this point, the cavity is starting to get noticeably deeper.

Erosion of an aluminum target with the nozzle 6-inches above the surface

Figure 5. Erosion of an aluminum target with the nozzle 6-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice. (The lower of the two circular damage patterns was caused through experimental conditions and should be ignored). The presence of a central mound can barely be discerned.

By this time, the central section of the jet is beginning to break down into initially short strings that very rapidly break into droplets. The damage pattern that results shows a cavity that is slightly increasing both in diameter and depth.

Erosion of an aluminum target with the nozzle 8-inches above the surface

Figure 6. Erosion of an aluminum target with the nozzle 8-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

By this time, the jet is continuing as a series of relatively large droplets, still holding a central structure, though surrounded by a rapidly decelerating cloud of mist.

Erosion of an aluminum target with the nozzle 10-inches above the surface

Figure 7. Erosion of an aluminum target with the nozzle 10-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

It is one of the interesting oddities of the jet cutting business that the amount of material that is eroded from the target is a maximum at this distance.

However, and this was the subject of great debate back at the time that it was first presented, the ability to control the droplet size and condition as a function of distance and the reality that in most applications the target must be cut to depth meant that this has a very limited application. It can be used if the droplets are generated properly and used within the relatively narrow window that they exist to improve surface erosion of material.

However, as Mike Rochester found when he studied this, back at Cambridge in the early 1970’s, the presence of a layer of water on the surface, and as the hole deepens this is almost always there, rapidly diminishes the effect.

The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface

Figure 8. The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface. (After M.C. Rochester, J.H.Brunton “High Speed Impact of Liquid Jets on Solids” First BHRA Symp Jet Cutting Tech, April `972, Coventry UK, paper A1.)

There are ways of getting around this problem but the presence of water in the cavity that the jet has produced can also lead to problems, and these will be the topic of the next two posts.

Waterjetting Technology – Repairing Concrete

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

Some years ago, we were on a bridge in Michigan, working on a demonstration of the ability of high-pressure jets to remove damaged concrete from the surface of the bridge. Before the demonstration began, the state bridge inspector walked over the bridge armed with a length of chain. He would drop the lower links of the chain against the concrete at regular intervals and, depending on the sound made by the contact, would decide if the concrete was good or not. He then marked out the damaged zones on the concrete and suggested that we got to work and removed those patches.

Automated removal of damaged concrete with water pressure

Figure 1. Automated removal of damaged concrete with water pressure

The change in the sound that he heard and used to find the bad patches in the concrete was caused by the growth of cracks in that concrete. It was these longer cracks and delaminations in the concrete that made it sound “drummy” and which identified it as bad concrete.

Now here is the initial advantage that a high-pressure waterjet has in such a case. The water will penetrate into these cracks. As I mentioned in an earlier post, water removes material by growing existing cracks until they intersect and pieces of the surface are removed. The bigger the cracks in the surface, the lower the pressure that is needed to cause them to grow. This is because the water fills the crack and pressurizes the water – the longer the crack, the greater the resulting force, and thus the greater the ease in removing material.

At an operating waterjet pressure of between 11,000 and 12,500 psi for a normal bridge-deck concrete, the cracks that are long enough for an inspector to call the bridge “damaged” will grow and cause the damaged material to break off. The pressure is low enough, however, that it will not grow the smaller cracks in “good” concrete, which is therefore left in place.

Damaged area of bridge after jet passes

Figure 2. Damaged area of bridge after jet passes.

In order to cover the bridge effectively and at a reasonable speed, six jets were directed down from the ends of a set of rotating crossheads within a protective cover. The diameter of the path was around 2 feet, and the head was traversed over the bridge so that it took about a minute for the head to sweep the width of a traffic lane.

Scarifying jets with the head raised above the deck so that their location can be seen

Figure 3. Scarifying jets with the head raised above the deck so that their location can be seen. Normally, the nozzles are positioned just above the deck, so that the rebounding material is caught in the shroud.

Unfortunately, while this means that the rotating waterjet head could distinguish between good and bad, and remove the latter while leaving the former, it could not read marks on concrete. So where the bridge inspector was not totally accurate, the jet removal did not follow his recommendations. It was, however, quite good at removing damaged concrete from reinforcing bar in the concrete where the water migration along the rebar had also caused the metal to rust. And, since the pressure was low enough to remove the cement bonding without digging out or breaking the small pebbles in the concrete, they remained partially anchored in the residual concrete. As a result, when the new pour was made over the cleaned surface, the new cement could bond to the original pebbles, and this gave a rough non-laminar surface, which provided a much better bond than if the damaged material had been removed mechanically with a grinding tool.

Rebar cleaned by the action of the jet as it removes the surrounding damaged concrete

Figure 4. Rebar cleaned by the action of the jet as it removes the surrounding damaged concrete.

Waterjets had an additional advantage at this point: In contrast to the jackhammer that had previously been used to dig out the damaged region, but which vibrated the rebar when it was hit, so that damage spread along the bar outside the zone being repaired, the waterjet did not exert a similar force, so that the delamination was largely eliminated.

Now this ability to sense and remove all the damaged concrete is not an unmixed blessing. Consider that a bridge deck is typically several inches thick and it is usually sufficient to remove damaged concrete to a point just below the top layer of the reinforcing rods. Once the damaged material is removed, the new pour bonds to the underlying cement and the cleaned rebar. But the waterjets cannot read rulers either. So in early cases where the deck was more thoroughly damaged than the contractor knew at the time that the job began, the jet might remove all the damaged concrete, and this might mean the entire thickness of the bridge deck. And OOPS this could be very expensive in time and material to replace.

What was therefore needed was a tool that still retained some of the advantages of the existing waterjet system, namely that it cut through weakened concrete and cleaned the rebar without vibration, but that it did so with a more limited range so that the depth of material removal could be controlled.

There was an additional problem that also developed with the original concept. For though the jets removed damaged concrete well in this pressure range, the jets were characteristically quite large (about 0.04 inches or so). The damaged concrete is contaminated with grease and other deposits from the vehicles that passed over it. Thus any large volumes of cleaning water would also become contaminated and as a result will have to be collected and treated. That can be expensive, and so any way of reducing the water volume would be helpful.

The answer to both problems was to use smaller jets at higher pressures. Because of the smaller size, their range is limited and at the same time the amount of water involved can be dramatically reduced. It does mean that the jet is no longer as discriminatory between “good” concrete and “bad.” This is not, however, a totally bad thing, since when working to clean around the reinforcing rods, there has to be a large enough passage for the new fill to be able to easily spread into all the gaps and establish a good bond.

Thus the vast majority of concrete removal tools that are currently in use are operated at higher pressures and lower flow rates. This allows the floor to be relatively evenly removed down to a designated depth, and this makes the quantification of the amount of material to be used in repair to be better estimated and the costs of disposal of the spent fluid and material to be minimized.

Scarified garage floor showing the rough underlying surface

Figure 5. Scarified garage floor showing the rough underlying surface. This will give a good bond to the repair material, as will the cleaned rebar.

The higher pressure system has the incidental advantage of reducing the back thrust on the cutting heads so that the overall size of the equipment can be reduced allowing repair in more confined conditions.

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).