Saturday, June 28, 2014

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.

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