Waterjetting 24b - Cleaning and Cutting concrete - a cautionary tale

The control of cut depth is one of the more difficult aspects of using high pressure waterjets in places where the aim is not to cut all the way through a part. The ability of an abrasive jet to continue cutting beyond the expected target depth can first be evident to an operator when they leave the jet running, but stop the motion while they go and do something else. On their return they discover that the jet has cut, not only through the part, but also the bottom of the cutting tank, and in some circumstances also into the concrete floor beneath it. Honesty compels me to admit that the table in my old lab had at least one (repaired) hole in the bottom and a concrete mark to show where. I know of a least one very prestigious university with a waterjet that has the same sort of feature (they actually did it before we did).

Cut depth control with a plain waterjet is a little easier, since the water will run out of energy – or the jet structure can be tailored to control its effective range more easily than with the higher density abrasive particles.

Life becomes a little more complicated where the traverse speeds are slower, where the bottom of the slot will become very irregular as the cutting jet tracks backwards and forwards as the nozzle moves at a steadier pace. Henning has divided the cut section into three zones:

Figure 1. The division of the cutting edge into three zones (Henning et al 18th ISJCT)

The fluctuating patterns if the jets are cutting down to zone three make it more difficult to retain control of depth, which is most easily achieved if the cutting is restricted to zone one and the abrasive is restricted to primary impact , without the additional cutting that comes where the jet and particles bounce further down the cut, as shown in the pictures on the right of figure 1.

Restricting the cutting depth in this way (and reaching the required depth of cut with multiple passes) works quite well for abrasive jet cutting of different materials and is the technique often used in milling pockets into a variety of materials, as discussed earlier.

There are, however, some risks to this in the use of plain jets, particularly when working with target items that are made up of different materials – such as concrete. One of the problems was identified fairly early on, in the use of high pressure jets to clean surface runways at airports.

The aim for jet use on runways is to remove the surface coating of rubber that is laid down on the tarmac when planes land and in that first instant of contact as the wheels come up to speed, a small amount of rubber is moved from the tire to the pavement. However, if the jet parameters for cleaning this surface layer are not picked correctly then the jet will remove not just the rubber, put also some of the cement from around the aggregate particles in the surface.

The problem that this raises is that the cement is rough, while the pebbles of cement are usually smoother (since the often come from river deposits). Thus if the cement around the surface exposure of the pebbles is removed, a smoother surface is left on the runway. This is not good, since the point of the rougher surface is to provide friction that will slow the plane down, and the polished surface removes that traction.

The pressure of the jet can be adjusted so that, at the point where it is hitting the cement it no longer has the power to remove it, but this is a value that is going to change with the pump operating pressure, the nozzle diameter, and the standoff distance between the nozzle and the runway. It will also vary with the type of materials that are in the runway itself, so it is very smart to try some test runs at different control values before going onto the field to do the actual removal.

Concrete properties change quite a lot from place to place. In some of the earlier work that was carried out on showing how jets could cut through concrete, tests were carried out at an airfield in the southern United States. For the purpose of the tests cuts had to be made through the pavement, so that pieces of it could be easily removed.

Our approach was similar to that used when we cut the walls at the University using a rotating waterjet on a small carrier (though as memory serves this was a modified riding lawn mower) to traverse back and forward over the cut, moving the nozzle down each time.

The problem that we ran into was that we wanted to cut a slot that was about 2 inches in width, which we had presumed would be wide enough to liberate the pebbles and give access to the deeper parts of the slab. Unfortunately in this case the pebbles that had been used in making the concrete were more than two-inches in size, and so when there were parts of these sticking out of each side of the opening there was not enough of a gap between them to get the assembly into the slot and to deepen the hole, without a lot of adjustments.

It was possible to cut through by making the cut slot wider by making a second, adjacent cut, and with the jets cutting down about 2 inches into the material on each pass, it was possible to work down through to the bottom of the slab, although the large size of the aggregate meant that the nozzle path itself had to be at a greater distance from the wall than we had planned. The combination meant that it was not nearly as rapid an operation as we had anticipated. (The traverse rate was about 2 ft/minute, which was much slower than expected to allow the jets to undercut the larger pebbles). Much more material had to be cut out of each slot in order to achieve full cutting through the slab and this slowed the cutting process – plus there was the time needed to work out how best to change the cutting patterns on site so as to make the process work at all. (And the pebbles were a quartzite aggregate so that even increasing the jet pressure would not have effectively cut them, without adding abrasive to the mix, which was not – at the time – a viable alternative).

The point in mentioning this is that, while the job seemed initially to be a relatively simple one, because we did not know enough about the target material we were caught off-guard when it turned out to differ from our assumptions. We have been caught that way a number of times. We were asked at one time to demonstrate precision cutting of a piece of metal – assumed it would be no more than two-inches thick, and set up a cutting time based on that assumption, and then were faced with a block of eight-inch thick Hastelloy. Which we did cut, as requested, but it took some changes in the cutting plan, which had not been built into the day’s schedule. Asking those few extra questions, in both cases, would have saved us some embarrassment and time.

Waterjetting 17a - Runways and discriminatory cleaning

Welcome back as we begin a New Year, and I hope that it brings Prosperity and Happiness to you and yours and that it brings much success.

Somewhere within the celebrations of the last couple of weeks water has played a considerable role, probably largest in the many different ways in which it was used to clean items –either before or after use. Water, often with a bit of soap, has been one of the earliest cleansers helping to loosen and dislodge dirt as the flow passes over the surface. There have been studies in the past which suggest that there is little advantage to the use of antibacterial soaps over less expensive conventional soap. Although the initial study was in 2005 it has only been within the last week that the FDA has told manufacturers of antibacterial soaps and washes that they have to demonstrate that they are both safe and effective. They note:

In fact, there currently is no evidence that over-the-counter (OTC) antibacterial soap products are any more effective at preventing illness than washing with plain soap and water, says Colleen Rogers, Ph.D., a lead microbiologist at FDA. Moreover, antibacterial soap products contain chemical ingredients, such as triclosan and triclocarban, which may carry unnecessary risks given that their benefits are unproven.

"New data suggest that the risks associated with long-term, daily use of antibacterial soaps may outweigh the benefits," Rogers says. There are indications that certain ingredients in these soaps may contribute to bacterial resistance to antibiotics, and may have unanticipated hormonal effects that are of concern to FDA.

In light of these data, the agency issued a proposed rule on Dec. 16, 2013 that would require manufacturers to provide more substantial data to demonstrate the safety and effectiveness of antibacterial soaps. The proposed rule covers only those consumer antibacterial soaps and body washes that are used with water. It does not apply to hand sanitizers, hand wipes or antibacterial soaps that are used in health care settings such as hospitals.

The debate over the use of chemicals with water, rather than just relying on the mechanical force of the water to dislodge dirt (used generically to include bacteria and other undesired coatings) or the combination of water with a mechanical action (using a brush or rubbing hands together) has been evaluated in many circumstances, with different results. One path will form the thread for the next few posts, and it will take us to a perhaps unexpected product line. Given that this is the season when a lot of us travel, let's begin at an airport.

Figure 1. Plane landing at airport – note the puff of smoke from the tires (Seattle Times)

When a plane first makes contact with the end of a runway, as it comes in to land, the wheels are not rotating very fast on contact, and so there is a small smear of rubber left on the end of the runway as the tires are dragged up to the right speed by the movement of the aircraft over the tarmac. (A plane can lose a pound of rubber per tire on landing).

Over a period of time this thin layer of rubber starts to build up over the surface, covering and smoothing the rough asperities that allow subsequent tire impacts to grip the runway, and making the surface slick and less tractive as a way of slowing the aircraft. As a result planes can start to hydroplane in rainy conditions and several major accidents have been blamed on this layer being allowed to grow too thick and become a dangerous surface.

For many years the practice was therefore to bring out different chemical trucks to spray the surface and a recent operation in Port-au-Prince describes the operation.

The chemicals that the team used were environmentally safe and effective in cleaning rubber deposits from the surface. The chemicals were sprayed onto the airfield and then scrubbed, brushed and worked into the rubber deposits for approximately four hours. The chemicals loosened the sticky rubber buildup into a soft gel that was then removed from the runway with a brush and water onto the shoulders of the runway. . . . . .

The AOS Research Group has developed a C-130 deployable chemical rubber removal system that is light, compact and requires no in-theater support other than fuel and water. AFCESA provided $125,000 in Research and Development funds to evaluate the rubber removal system in actual expeditionary environment. Four team members (Figure 2) traveled to Haiti and used the rubber removal technology to remove rubber build-up from the runway. In three days (approximately three eight hour shifts) they cleared 125,000 square feet on the west end of the runway and 75,000 square feet on the east end.

Figure 2. Results from chemical removal of rubber at Port au Prince (ARA)

The process is relatively slow and somewhat manually intensive, which may be an advantage in a labor-rich environment such as Haiti, but which makes the process very expensive in places such as Europe and the United States. And this provided an opportunity for a new business. As I wrote first in “Waterjetting Technology”:

One of the fascinating stories of the waterjet cleaning industry is that of Bob White, and his wife Donna. In 1972 Mr. White was a small painting contractor who unsuccessfully tried to clean the rubber deposits left from the wheels of landing aircraft from the runway at McClellan Air Force Base in California (now closed). Although able to remove material at only 60 sq ft/hr, he was convinced, particularly after seeing that the state of the art system was a chemical treatment, that waterjetting was the answer. Through a combination of loans from a variety of sources to the tune of $180,000 he built a four-pump, 24 nozzle spray bar system, initially operating at 10,000 psi and went into business. By driving himself through the night and thus being able to underbid the competition he took the first 25 of 28 jobs which were bid, and by the end of that year he had paid off his loans. By 1977 he had 5 rigs on the road around the country and was anticipating his first million dollar year.

Figure 3. Bob White (From the Duck Book).

After the first year on the road it was found, for most applications, that the jets removed the rubber better at 5,000 to 6,000 psi rather than at 10.000 psi and there was the added risk that at the higher pressure the water would either damage or polish the underlying concrete, depending on its quality and that of the aggregate contained. By supplying the flow from each pump to a six nozzle section of the spray bar it was possible to isolate a section should it have a problem, while still operating the rest of the units.

Figure 4. Bob's early rig (ibid)

The rig used spray bars fitted with 36 nozzles of hardened steel, 0.062 inch orifice diameters with a 30 degree spread, required to give 1 inch of coverage at a 0.75 inch standoff used. Other available equipment used 2,750 hp pump units to supply 0.078 inch diameter jets held one-inch apart, and with up to 96 nozzles on the spray bar. Such a unit could clean paint build up over 0.18 inch thick at a rate of 13,000 sq ft/hr, at a cost of $0.064/sq ft. Lowering the nozzle diameter to 0.04 inches resulted in a bar 94 inches long which when held an inch and a half above the surface, at 7,000 psi would allow cleaning at the rate of 60 ft/min, for a combined rate of 40,000 – 50,000 sq. ft./hr. The cost was estimated at $0.037 to $0.05 per sq ft.

In 1974 White was winning contracts at $0.045 per sq ft, and going below $0.032 per sq ft to get others. He had at that time 34 competitors, and yet grossed $280,000 that year. In 1975 he was bidding runway cleaning at less than $0.022 per sq ft for larger areas. Nozzle life was on the order of 50 hours for the stainless steel tips, and 250 hours for the steel holders. In January of 1978 Bob White discovered he had cancer and although successfully treated by 1980 he had sold off his rigs to highway painting contractors while he himself had turned to publishing.

He was sadly reported as being murdered in Belize in 1988. He was a true Pioneer of the industry.

Runway cleaning has progressed considerably since those early days and modern systems use higher pressures, in part to reduce the amount of contaminated water that is produced, since the cost of collection and disposal becomes significant. There is also a marriage of chemical pre-treatment and waterblasting which is shown in a video here.

But the ability of the waterjet system to discriminately remove the overlying coating (rubber) without damage to the underlying surface led on to other applications, which I will cover in the next few posts.

Waterjetting 8d - Choosing angles

How times change! I was reading a column in the British Farmer’s Weekly, and came upon this, where the author is discussing the need for a generator.:

It will also be vital to keep the fuel flowing into the tractors, and power the pressure washer, and light the security lights, and all the other essentials of an average arable farm.

It is an indication of how far the use of pressurized water has come, that it is now seen, at the lower end of its application, as a vital farming tool. Which is a good introduction to talk a little further about the use of cleaning streams, and how to interact with differing target materials.

There was an initial first step, when someone would send the lab a mystery block of material and asked – how do I cut it? Generally the samples were small, but we would find a flat surface on the material, and carefully point a jet nozzle perpendicular to this surface. (In the early stages this was hand-held). When a jet strikes a surface, but can’t penetrate it, then it will flow out laterally around the impact point, under the driving force of the following water.

The test began with the jet at low pressure, and this was slowly raised, until the point was reached when the pressure was high enough to just start cutting into the material. At this point the jet had made a small hole in the target, and so the water flowing into that hole had to get out of the way of the water following. The sides of the hole stop it flowing laterally, and so it now shoots back along the original jet path. This spray can hit the lance operator if the nozzle is hand-held, but it is a fairly graphic way of determining the threshold pressure at which the material starts to cut. (and I’ll get into what happens as the pressure continues to go up in a future series of posts).

But for the purpose of cleaning, the jet has to move over the surface, once it has made that initial hole, at pressure. But, in many materials, if the jet comes vertically down onto the target, then only the material directly under the jet will be removed. And so the jet has to be played on every square inch of the surface in order to ensure that it is cleaned, or that the coating/layer is removed. In some sandstones, for example, two jet paths could be laid down, almost touching one another, and yet the rib of material between them would remain standing.


Figure 1. Adjacent jet passes in sandstone, the cuts are about an inch deep, but note that even though the narrowest rib is about 1/8th of an inch wide, it is only when the cuts touch that the intervening material is removed.

Yet that rib of material was, in that case, so weak that it was easy to break it off with a finger. (This turns out to be a weakness in making delicate sculptures out of rock). To use the full pressure of the water can be a waste of energy, if the material is very thick, since it all must be eroded with such a direct attack.

Yet the minimum amount of material that needs to be removed is that that attaches the layer to the underlying material (the substrate concrete, steel etc) and this can be quite thin. Thus, in attacking a softer material, particularly one that can be cut with a fan jet, a shallow angle directed at the edge of the substrate can be more effective.


Figure 2. Round v fan cleaning from Hughes (2nd US Waterjet Conference)

Because there is a balance between cutting down through the material to be removed, and cutting along the edge to grow the separation crack between the materials, some practice is needed to find, for a given condition, what that angle would be.


Figure 3. Choice of angle from Hughes (2nd Waterjet Conference)

The more brittle the material, then the greater the angle to the surface, since rather than just erode the material, the jet may also shatter the layer into fragments that extend beyond the cut path. But otherwise using an angled jet to the surface can be more effective. Hughes (from whose paper at the 2nd Waterjet Conference I took these illustrations) has a simple test for orifice choice.


Figure 4. How target response influences nozzle selection. (Hughes 2nd Waterjet Conference)

Some of the more advanced cutting heads use a series of nozzles that spin within an outer protective cover, as they remove anything from layers of damaged concrete to thin layers of paint from ship hulls. Increasingly these are connected to vacuum systems that will draw away the spent water and debris from within the contained space, without it entering the work space, and creating problems for the worker.

In order to reduce any collateral damage to the surroundings these jets are often made very small (thousandths of an inch in diameter) so that their range is short, and they are inclined outward to cut to the edges of the confining shield.

We have had some success in turning those angles the other way, so that they cut into the shield, rather than away from the center, and also so that each jet is directed towards the path of the next jet around the circumference. The intent in this case is to allow the use of a slightly larger jet, with a greater cutting range. In this case the individual cleaning/cutting path is a little larger, but because the jet at then end of the cut moves into the range of the adjacent jet, then any remaining energy that it and the dislodged debris still have, will not be enough to get through this second jet.


Figure 5. Inclined jet and shroud design.

The action of each jet then becomes not only to cut into and remove material, but also to contain the spent material from the other jets dispersed around the cutting arm, and to hold the debris in the center of the confinement for the very short time needed for it to be caught up in the vacuum line.

In all cases the choice of pressure, nozzle size, and operational factors such as angle of attack, come down to the target materials, those that have to be removed, and those that need to be left undamaged. And it is why it is useful, at the start of any new job, to take the time to do a little testing first, to make sure that the right choices of nozzle and angle have been made to get the job done quickly and efficiently.

Incidentally the idea behind the test of effective pressure, that the jet flows laterally when it hits something it can’t cut, can help, for example in easing the meat from the bone when a jet cuts a deer leg.


Figure 6. Cut across a deer leg, note how the jet has cleaned off the meat from the bone, undercutting the flesh.

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 7b - more insight into jet structure

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.


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 it 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.


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.


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, and as single droplets moving 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).


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 effected 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 be along 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 have 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. See, for 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.)

Deepwater Oil Spill - future precautions

The decision as to how best proceed with the final stages of closure of the Deepwater well is still apparently being debated. The issues relate to the problem of dealing with a column of fluid in the annulus of the well that sensibly has nowhere to go, and no easy way to be circulated out of the well, following an intersection by the relief well. Admiral Allen, in his remarks on Monday noted that the protocol would be

The science team will meet later on today and then they will brief Secretary Chu and Secretary Salazar. And the science team and Secretary Chu will make a recommendation on how to proceed.

In the meantime we are continuing an over abundance of caution to make sure we have mitigated risks at each point prior to directing the intercept of the well and we will continue to do that.

There does not appear to have been an update on Tuesday, from the Admiral; BP or the Secretary of Energy’s oil spill page which suggests that while the relief well will still intersect the original well, the precautions to be taken before that occurs are still being hashed out. So, while we are waiting, I thought I would mention the state-of-the-art in the plans that the major oil companies have put together to ensure that this large a disaster does not happen again, and then a short additional technical note about something that even I failed to note, during work on the BOP.

ExxonMobil has posted a blog (H/t Jane Van Ryan) which covers a presentation of the proposed system which will be built and held in case of need, to “ensure safe and effective drilling practices in the Deepwater Gulf of Mexico.” It was presented before the Bureau of Ocean Energy Management and recognizes the need, by the industry, to restore confidence in the ability to drill safely and effectively. SpecificallyChevron, ConocoPhillips, ExxonMobil and Shell have started to develop a new system that will;

• Fully contain oil flow in the event of a potential future underwater blowout.
• Designed to address a variety of scenarios.
• New Specially designed equipment (will be) constructed, tested and (will be) available for rapid response.
• Can operate in deepwater depths up to 10,000 ft.
• Adds containment capability of 100,000 bd (4.2 million gallons per day), exceeding (the) size and scope of the Gulf spill.

The intent is to have the initial system, which is expected to cost around $1 billion, in place in 6 months, with further improvements and expansions developed as they become available over the following year.

Based on the development of what was needed, and what appeared to work from the current disaster, the primary initial emphasis is on the need to contain and capture the leaking oil as soon as possible. The overall layout of the system looks somewhat familiar:

(ExxonMobil)

The system takes the flow that is collected from the well, and then sends this, through separate risers attached to suction blocks, up to the surface for collection. There is an underwater dispersant system, and an accumulator, or power pack, to help provide additional power that may be needed.
To capture the oil/natural flow from the well has to be a flexible system, given that each failure is likely due to a unique circumstance, and will have its own geometry.

In all cases there will be a complete seal created around the well, but whether this comes from some form of containment that fits to the existing infrastructure of the well, or whether it requires that a separate caisson be created around the well, sealing into the surrounding rock; the equipment and protocols for its use will be available.

If there is a failure where the upper casings and liners of the well have retained their integrity, then, depending on what is available the new assembly (which will contain 3 shut-off rams as with the current stack, as well as flow lines for choke and kill circuits) can be attached either to the mandrel, the BOP, or to the riser itself.


If the integrity of the end of the well is compromised (i.e. the rock fragmented, the liners broken or the wellhead destroyed in some way) then a different approach will be required.

Here a caisson assembly will be constructed around the remnant BOP, large enough to surround the well at a point where the rock has enough strength and quality to allow a seal under the wall. This will then allow a cap to be placed on top of the caisson, with the containment system from the last slide, mounted on top of the cap, as a way of capturing the hydrocarbons that are bled off from the well, without their coming into contact with the seawater, and forming hydrates.

(ExxonMobil )

In both cases the oil and gas recovered from the well will be sent up to the surface using the flexible risers that were planned for use at the Deepwater Horizon site, but were never used. The use of flexible risers allows these feeds to be moved away from the immediate vicinity of the well, to allow other operations.


As with the surface vessels used in the present situation, the support vessels will be able to separate and flare the hydrocarbons, though the main intent is to be to flare the gas, while collecting and trans-shipping the oil.

The system will be available and thus will require the formation of a separate organization, the Marine Well Containment Company, to construct, maintain and operate the technology, as well as to maintain expertise in the most effective use of it.

And in closing, a small additional technical note. After the oil spill was being partially contained there was some problem with hydrate buildup on different parts of the BOP, and on the cap. I remember noting the ease with which one of the ROVs was moving a lance that dislodged the hydrate crystals, and thinking that they must have been very loosely attached. Turns out I was merely unobservant, the lance was an ultra-high pressure cleaning lance that was fed water at a pressure of 36,000 psi from a pump lowered to be adjacent to the operation. It was the resulting jet of seawater and liquefied gas that was actually doing the cleaning. There is a Youtube video for those who were wondering what that particular operation and structure were doing.