Tuesday, March 26, 2013

Waterjetting 7c - higher pressure washing with power

In the last post, on surface cleaning, I showed how the jet from a fan nozzle spread very quickly once the water left the orifice. With this spread the stream got thinner, to the point that, very rapidly the jet broke into droplets. These droplets decelerate very rapidly in the air, and disintegrate into mist which rapidly slows down. That mist has little capacity but to get a surface wet, and thus, within a very short few inches, the jet loses power and the ability to clean.

How can we overcome this? Obviously the jet would work better if it could carry the energy to a greater distance. And the jet that does that (as we know from trips to Disney) is a cylindrical stream. In some parts of the cleaning trade this is known as a zero degree jet, to distinguish it from the fifteen degree or other angular designation of the fan jet nozzles that it is often sold with.

But the problem with a single cylindrical jet is that it has a very narrow point of application. Depending on the standoff from the nozzle to the target this will increase a little as the distance grows, but is still likely to be less than a tenth of an inch. That, by itself, would make cleaning a bridge deck a long and laborious job. But consider that if we spun the jet so that it is tilted out to cover a 15 degree cone, the same angle as the best of the fan jets, the water would travel further. With a good nozzle it is possible to extend the range to 3 ft, rather than the typical 4 inches of a fan jet.


Figure 1. The gain in performance when a fan spray is changed to a rotating cylindrical jet. (initially proposed by Veltrup, these are our numbers).

In both cases the water flows out of the orifice at the same volume and pressure. But with the rotating jet the water is able to carry the energy some 9 times as far. As a result the area covered is 9-times as wide, and the job is carried out faster.

You can also look at it another way. It takes only about 10% of the water and the power to clean the surface with the rotating jet, as opposed to the amount required to clean with the fan jet. This is even though the pump unit and the flow rates are the same in both cases. This is why, when you buy some of the smaller pressure washers, they include a nozzle that has a round orifice and which then oscillates within a holder. Not quite as efficient as a controlled movement, but at least it is a start.

Now, of course, life is never quite as simple as it at first appears. Because the jet is being rotated there is sometimes, if the jet is being spun fast enough, some breakup of the jet because of the speed of rotation. And so, in the above example, too high rotation speed would have a disadvantage. Doug Wright showed this in a paper he presented to the WJTA in 2007.
Figure 2. The effectiveness of a rotating jet, at two speeds and at different distances (Doug Wright 2007 WJTA Conference Houston).

On the other hand because the jet has to make a complete rotation before it comes back to the same point on the coverage width, if the lance is moving too fast relative to that turning speed, then the jet will miss part of the surface that it is supposed to be cleaning.

I can illustrate this with a sort of an example. To make it obvious the rotating jet has enough power to cut into the material that it is being spun, and moved over. If the rotation speed is too slow, relative to the speed that the head is moving over the surface, then the grooves cut into the surface won’t touch one another and small ribs of material are left in the surface. This is not a good thing, either from a cleaning or mining perspective. The material we were cutting in this case was a simulated radioactive waste, that an improved design later went on to extract as a “hot” material in a real world project. These materials tend to be unforgiving if they are not properly cleaned off.


Figure 3. Cutting path into simulant showing the grooves and ribs where the rotation speed is not properly matched to the speed of the head over the surface.

There is another answer, which is becoming more popular for a couple of different reasons. If the pressure of the water is increased, then the jet will remain coherent for a greater distance, at a higher rotation speed. Going to a higher rotation speed, also brings in an additional change in the design of the cleaning head.


Figure 4. Cleaning head concept sectioned to show vacuum capture of the debris through the suction line after the jet has removed the material and washed it into the blue cylinder.

As the pressure increases, so the energy of the water and the debris rebounding from the surface increase. To a point this is good, since once they are away from the surface it is relatively simple, if the cleaning operation is confined within a small space by a covering dome, to attach a vacuum line to the dome, and suck all the water and debris into a recovery line. The surface remains relatively dry, all the water and debris is captured, and the tool can be made small enough, and light enough, that it can be moved either by a man or on the end of a robotically controlled arm. (The arm we designed the head for was over 30-ft long, which means that the forces from the jets had to be quite small).

With the higher pressure also comes the advantage that the amount of water that is required, for example to remove a lead-bearing paint from a surface, is much lower. If the water becomes contaminated by the material being washed off, then not only has the total volume to be collected, which is an expense, but it also must be stored and then properly be disposed of. And that may cost several times the cost of the actual cleaning operation, if the contaminant is particularly nasty. So reducing the volume of the water is particularly useful.

A friend of mine called Andrew Conn came up with the idea, for removing asbestos coatings from buildings, of tailoring the pressure and the flow from the nozzles, so that the amount of water required was just enough that it was absorbed by the asbestos as it was removed. Simplified and reduced the costs of cleanup, where that was a significant part of the overall price.

And speaking of using higher-pressure water, this means that there is no need for the abrasive additive, when cleaning say a ship hull. And that means that there is no need to buy, collect, and dispose of the abrasive during the operation.


Figure 5. Spent cleaning abrasive at a shipyard.

There are other advantages to the use of high pressure water over abrasive when cleaning metal, and I’ll talk about that subject a little next time.

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