Waterjetting 35c - Developing a waste removal shroud - video.

The short videotapes in this segment show the evolution of a combination of a waterjet and a suction line as a way of easily removing soil or sand relatively quickly. It is a subject covered in an earlier post. These video clips show some of the tests that helped us to develop that design.

As mentioned in that earlier post the central tube connects to a vacuum line which removes the loosened debris and water. An earlier series of tests had shown that the suction nozzle had to be within quarter of an inch of the surface for the suction to be most effective. The jets had, therefore, to clear a way for this nozzle by cutting down through the material and pushing it into the mouth of the tube, before the tube arrived.

For the first test a single nozzle (out of the three on the head) was used at relatively low pressure.

Figure 1. Clip showing a single jet cleaning through 2-inches of sand.

However if the jet pressure is raised to cut harder material, then the jet has enough power to wash the material under and past the suction tube so that only a small part of the solid is picked up and the path fills back up with the washed sand.

Figure 2. A higher-pressure cutting jet does not give the debris time to be sucked out of the tank.

If three jets are used, but with the jets directed so that the paths hit each other within the suction zone this stopping each jet going further for a long enough time that the suction can remove both water and debris. 

 Figure 3. A three-jet combination where the jets are held within the shroud, leaving a clean path.

For those unable to see the video the configuration of the jets meant that they met under the shroud as shown.

Figure 4 The jet configuration around the shroud.

When this is combined with a protective (flexible) outer shroud the final result was a tool that removes material without over-spraying into the surrounding sand and destabilizing it. Leaving a clean channel.

Figure 5. Larger head design removing a 2-inch thick layer of sand.

In a subsequent post I will include (when I can find it among the 200-odd hours of material) a video of a similar (though smaller) tool cutting a clean channel into a soft cement, and leaving a clean path behind it, as shown in the earlier post. For those interested the parts for these cleaning heads were assembled from plumbing supplies from our local hardware store at a cost, per head of around a hundred dollars or so. (back in 1995 when we ran the tests).

Waterjetting 22d - more on shrouds

The use of a shroud to capture the water and debris from waterjet use, feeding it to an exhaust hose, that will then carry it away from the site, has become more universally applied over the last ten years.

There are, however, different ways in which this new combination (which has been given different names depending on the usage) can be applied, and how the components can be best combined for most effective application.

At the low end of the pressure range, feeding a 2,000 psi waterjet at 2 gpm into the soil at the entrance to a suction hose has created a powerful new tool for deep soil excavation. The technique, known as hydro-excavation, has a variety of different applications – one of the simpler demonstrations was shown by Hydro Spy here on Youtube.

The demonstration lasts some five minutes, and helps show why there is still improvement needed in equipment design, since the vacuum intake is not extracting material at a constant rate, but is only being fed by a hand-held lance that it often not cutting very efficiently, while at the same time the head is either buried in debris or being held too high off the surface to effectively capture the loose material effectively.

Figure 1. Components of a hydro-excavation system, the jet is breaking the soil into pieces that are they removed from the hole with the water, through the suction line. (Hydro Spy )

Because the two actions (jet fragmentation and vacuum removal) are separate they are both working at much less efficiency than if the situation were modified. For example, in the video, the lance is used to pry pieces of soil from the wall and too much time is spent with the lance away from the suction line or with the intake to the line buried and not removing material.

Figure 2. Frame from video showing lance being used to pry soil block from the solid – the suction line is not getting any water or soil at this time.

It is often forgotten that, in soils, the jet penetrates to maximum depth in about one hundredth of a second. Thus, to be effective it has to be moved over the surface relatively fast ( as it is in parts of the video) in order to be most efficient. This is, however, most often best achieved by driving the head mechanically, rather than relying on an operator to move the nozzle as fast as it should be moved. In a simple situation such as this it may be, for example, much more effective to use a dual-jet self-rotating nozzle assembly (which can be obtained from one of several equipment manufacturers) since these designs spin the jets over the surface more rapidly and consistently, so that the material is more effectively broken into relatively small pieces.

However in this section we have been discussing the use of shrouds and intakes to the suction line, and this design becomes of equal importance in ensuring that the system works at its most efficient. If the entry to the suction line in blocked because it has been run up against the bottom of the hole, or into a tight cluster of large pieces of material, then there is no production, until the head is lifted away from that seal. (Or if a short rod is attached to the bottom of the inlet to ensure that there is always a gap between the lip of the line and the bottom of the hole).

On the other hand if the inlet is lifted too far away from the surface, say more than half-an-inch, then the suction force pulling the pieces into the line becomes significantly less effective and production will again suffer. This is made worse where the floor of the opening is very uneven, since this makes it more difficult to maintain the gap at which the suction is most effective.

It becomes more effective – whether removing soil in this way or removing paint from a ship hull at much higher pressures – to integrate the jet action with the design of the shroud/inlet to the suction line. The two cases are otherwise different in that in the softer material the jets are cutting quite deeply (though hopefully no more than about half-an-inch at a time) into the soil, which causes the jet to rebound back up into the shroud body and makes water and debris collection relatively easy.

This is not the case with the removal of paint and coatings, where the layers are often relatively thin, and the jet will rebound, often parallel with the underlying steel that it does not have the power to penetrate. (Nor is this desirable, other than for the jet to penetrate into any corrosion pits in the surface and clean them).

With thin coating removal, since the surface is otherwise relatively smooth, the shroud can be mounted on wheels that allow the operator to set the gap thickness between the shroud and the surface. (The closer the shroud lip to the surface, the higher the force that holds the shroud to that surface, but also the higher the force that the motors must apply to move the shroud against the friction forces that are created). The suction force in this case will hold the shroud against vertical walls and even against the underside of ships hulls, bridge decks, etc. provided that the geometry of the head is optimized to provide that balance of enough suction to hold the head, without it getting too high for the trouble the traversing motors).

There is one other, final thought, in those cases where the jets are cutting into and along paint and other coatings. In some cases the coating can be best removed where the jet is attacking along the surface, rather than almost perpendicular to it, as is quite often the case in head designs. This can give a better and more efficient surface cleaning, but if the jets are at too great an angle to the surface, the operator runs the risk of seeing the jets carry the debris out past the edge of the shroud, making it much more difficult to capture and remove.

One way of getting around this problem is to incline the jet path within the shroud, so that at the distant end of the jet path within the shroud it intersects the path of the next jet around the design, which has sufficient force to stop the jet moving further out. We have successfully demonstrated that this does work in an application, where the jet was cutting relatively shallow grooves in the surface, and with greater penetration the jet will rebound upwards out of the slot, and more easily captured by the overlying shroud.

Figure 3. Showing how, by aiming the jet path into that of the next stream around the shroud the energy of the jets can be contained within the shroud envelope and the splashing outside of that envelope is much reduced.

Waterjetting 22c - More on shroud design

Lowering the pressure in a hose connected to a cutting head, by connecting it to a vacuum pump, will pull a certain amount of the water and debris released from a cutting/cleaning event into that hose. However to ensure that all of this material is captured, rather than just a fraction, requires a little more care and effort in the system design.

At the end of the last post on this subject, I began discussion of the use of shrouds to help to contain the ejecta and to direct it towards the suction line.

Figure 1. Schematic section through a shroud of a device, designed to mine high-level radioactive waste.

There are a number of different lessons that we learned as we developed this tool, and this piece will discuss a number of them. During the development and demonstration of the device we had to use a simulant, and a relatively weak cement was chosen, which would allow us to design the tool to operate it where we could see it, and easily interact with it.

Figure 2. Cutting test under way (no shrouds were used in this early test series)

The easiest way to drive the nozzle system was to run the high pressure tubing through a fixture that contained a hollow shaft electrical motor. This saved a lot of space, and allowed the high pressure tubing to feed into a distribution manifold under the motor, which fed the high-pressure water to a set of rotating nozzles.

Figure 3. Test rig without jets to show the design of the test head.

In figure 2 the jets issue from two self-rotating nozzle sets, themselves fed through a rotating feed tube, itself rotating around a central axis, and driven through a belt drive and gearing.

Various different patterns were cut into the simulant, as the different heads were moved over the surface, with the pattern controlled by the different rotation speeds relative to the overall head speed over the surface.

Figure 4. Computer image of the jet paths over the surface, in one combination of parameters for a head similar to that shown in Figure 3.

The design of the head was aimed at producing a set of jet passes (given that each jet was slightly inclined to the surface) which would produce pieces of simulant that were never larger than half-an-inch in size. Yet at the same time the goal was to remove 4 cu. ft. of waste each minute. The larger we could break out the particles, the less cutting we would have to make into the waste itself, saving energy and time, while at the same time increasing the overall volume we could release in that time.

Figure 5. Deeper cut into the simulant.

At the same time if the cut depth was too great, then several new problems would arise, apart from the initially obvious one of producing particles that would be bigger than the suction hose could easily handle. (Though we overcame that hurdle by running the particles through a high-pressure jet pump that effectively cut any oversize particles down to an acceptable size as part of its design).

The suction line needed more than just the water from the cut, to be able to pick up all the debris from the cutting operation. Air had to be drawn in around the sides of the shroud, yet at the same time the walls of the shroud had to come down to restrict the amount of that air and keep the suction strong enough at the surface to remove all loose material. This is done by fitting a rim of bristles (such as form the head of a paint brush) around the edge of the shroud that come down to brush over the outer edge of the cut, stopping a lot of the material from escaping out from the edge, while limiting the amount of air that feeds into the shroud, and in this way holding the suction pressure inside the shroud.

Figure 6. Early test showing a square shroud with bristles around the edge as it cuts into the waste. (Part of a previous pass has been filled with clay as part of the test). The shroud was larger to ensure that all ejecta was captured – as shown.

During the tests we found that the metal rim should, optimally, be no more than half-an inch from the surface of the material, after it had been cut, to pull all the material from the bottom of the crevices. But the edge of the head has also to pass over the surface in successive passes. So that high points left by deep cutting (Figure 5) will catch on the head, and can interfere with the rotation of the head on the next pass.

The aim of the cutting head design was, therefore, to leave a relatively smooth surface (of the sort shown in figure 3) over the waste after each pass, so that the head could be fed automatically down a fixed amount without any risk of it catching on large peaks left by the previous cut. This risk could also be lowered a little by slightly tilting the head backwards as it moves over the surface, since this allows slightly larger points to enter the head, where they are attacked by the jets before the driving mechanism has to pass over them. This tilting also makes it easier for the head to clean right up to the walls of the tank, where otherwise the edge of the shroud would hit the wall and stop the jets from removing that last rind of material from the edge. (Though it could be cleaned by a subsequent pass with the head turned up parallel to the wall and moved over it in that way – though this wouldn’t capture all the material as easily, due to wall curvature.)

Tilting the jets at a high angle so as to cut material at the edge of the shroud was also a possible problem, since it made it easier for the water to escape from the edge of the shroud, and out into the main body of the tank, which was undesirable. But I’ll talk about that in a later piece. Let me just note that, when these factors were all combined no material escaped from the edge of the shroud.

Figure 7. Test late in development, where a head similar to that shown in figure 1 is cutting over waste, without any material being ejected from around the shroud edges.

Deepwater Oil Spill - Improving the Clean-up

Flow recovery update from the Gulf:

For the first 12 hours on June 20 (midnight to noon), approximately 6,790 barrels of oil were collected and approximately 4,280 barrels of oil and 23.2 million cubic feet of natural gas were flared.

• On June 19, total oil recovered was approximately 21,040 barrels:
• approx. 11,050 barrels of oil were collected,
• approx. 9,990 barrels of oil were flared,
• and approx. 43.4 million cubic feet of natural gas were flared.

One of the more innovative recent technical advances that you have likely never heard of is called Hydro-excavation. It combines the use of a water stream of the sort you find at a car wash (i.e. 2,000 psi water at about 2 gpm) with a vacuum hose attached to a vacuum truck. It turns out to combine into a new tool that is quite effective for disaggregating soil and removing it to leave a relatively dry cavity down to significant depth, quite fast. It has a lot of advantages if you are excavating in an area where there are fiber-optic cables and other underground conduits that might take unkindly to the use of a back-hoe, (Including gas lines).

I mention this, because there is a certain amount of the clean-up now going on in the Gulf that can be done with waterjet lances, and some parts of the spill can be cleaned up with suction hoses. (And I have some experience with both, courtesy of helping in the development mentioned in the first paragraph). One of the classes that I have taught involved the use of pressure washers in a demonstration laboratory that supported the class lectures. So I thought I would take a moment to mention some of the things that need to be considered when using pressure wands and suction hoses.

The first obvious, but neglected point, is that the operator rarely can judge exactly where the tip is relative to the surface being cleaned. This is actually quite critical because the average pressure wand comes with a fan tip on the end. The tips come with different angles of dispersion of the jet, and having photographed a fair number, most jets are about 5 degrees broader than the jet pattern stamped into the nozzle. The second point is the one that is usually missed, but which relates to the distance of the nozzle from the surface.

When the jet comes out of a fan jet nozzle, the shape of the nozzle forms the stream into a sheet that gets wider with distance. Because the volume of water coming out at one time doesn’t change, the jet therefore gets thinner the further that it gets away from the jet.

Flash Picture of a fan jet

With a typical nozzle (usually called a tip) the sheet gets thin enough somewhere between 2 and 4 inches that the sheet perforates, and just as with a balloon when it pops, the material pulls back from the hole. In the case of the water sheet, this creates circles of larger droplets that continue to move forward. It is at this point that the jet is at its most effective, in many applications.

However, those droplets that are moving originally at about 550 ft/sec are moving into air that is relatively stationary. It breaks these large droplets up and decelerates them over the next couple of inches. As a result, the jet becomes virtually powerless within about 6-inches of the nozzle. (The distance varies with nozzle manufacturer, design flow rate and operating pressure – but that distance is typical). If you hold the nozzle further away from the target than that you are merely getting the surface wet, and not moving anything but surface dirt.

The problem that you, as an operator, have is that it is very difficult to judge that 6-inches. (Making it easy to “catch the student in error” at the beginning of the lab and reinforce the lesson). So the simple way to resolve the problem, is to touch the target surface before you start, position your feet accordingly, and then back the lance off a little and you are likely to be much more effective.

Having cleaned a wall of my house with an 18-ft extension lance to the normal pressure washer last week, I can also add that trying to maintain a 2-4 inch standoff while holding the lance above your head is an art that has to be learned.

OK, now the next problem is that most of the material being removed is going to be some form of hydrocarbon (oil. Oil emulsion or something similar). Some of these are quite sticky and hard to remove with just water pressure. In this case if the water is heated to about 185 deg F it will cut through and remove those coatings a lot more easily than at a lower temperature. You don’t want to heat it all the way to steam, since that loses the pressure of the jet at the nozzle too quickly, but with hot water the range can be extended.

One other way to extend the range is to use a spinning round jet nozzle (sometimes called a 0 deg tip). There are a number of these on the market and the cone of the jet is created by rapidly spinning the cylindrical jet that comes out of the nozzle. Depending on the quality of the nozzle (and diameter and pressure) these can increase the jet range to a foot or more. If you work out the amount of energy and water required to clean a surface both ourselves and some folks in Germany have shown that using this rather than a fan-jet can drop the amount of energy and water that you use to clean that surface by up to 90%.

The final point I want to make deals with the use of suction hoses. These are now appearing more frequently in some of the locally made systems being fielded along the Gulf. While the same basic argument about the operator not knowing where the tip is, still applies, there is a different reason as to why this is important.

In most cases the object is to pull a relatively thin film of oil from the surface of the water. The ideal place to hold the end of the hose is just above the oil: water level (less than half-an-inch). The air drawn into the gap helps pull the surface layer into the hose, and you don’t pull in a lot of water.


Unless you have really good control of your position (bearing in mind you are looking along the hose at the water) this is very hard to sense. If you push the hose into the water hardly at all then you pull in too much water and not that much oil.

If you tilt the hose then it tends to pull in a lot of air, and not a whole lot of water or oil.


The best way to control the position is to have the end of the hose attached to a piece of foam that will float and in this way the mouth of the nozzle can be placed where you want it. It is easier to do this if you bend the hose so that most of it is lying on the foam, and not pushing it into the water, but in this way, depending on the amount of oil, you can slide the nozzle up and down in the foam to get the best distance to recover the oil.


That half-inch distance is fairly critical for best performance, especially if you can keep just below it. (Yeah we actually did experiments where we adjusted it).

There are now whole books on this technology, and some safety recommendations on how to use some of the equipment, as the industry continues to grow. Hopefully this has been of some help. What one learns in one application can be quite usefully applied, often in others.