Tuesday, January 7, 2014

Waterjetting 17b - Removing layers

In the last post I discussed the ability of high pressure water to penetrate through the rubber layers that coat a runway, after planes land, and then remove it, without damaging the underlying concrete.

This ability of waterjets to be able to remove individual layers of a coating, without damaging the layers beneath (the substrate) is one that has found a number of different applications within the cleaning industry. One of the more obvious is the ability to remove the paint layer from underlying surfaces, without damaging the material which the paint covers. In fact, if the treatment is careful enough we have, in some cases, been able to remove individual layers of paint while leaving the less weakened underlying paint primer in place. And to demonstrate a point we showed that it was possible to remove (as desired) the top paint coat, the primer coating and then if there is metal corrosion also to remove the top layer of metal from a surface. It just depends on what is desired and “how clean is clean.”

One significant difference between cleaning and paint removal relative to cutting is that the thickness of the target material to be penetrated is generally much less than that found in other applications. Given that there is also a concern with contamination of large volumes of water, that must be collected and disposed of, the use of higher-pressured systems has a number of advantages.

One significant one is that, with the use of very small nozzles the distance over which the jet has power becomes significantly less (consider that the effective range is likely to be less than 150 jet diameters and that orifices on the order of 0.005 inch in diameter are used) and this minimizes the amount of possible damage that the jets might cause through over-cutting if there is an unanticipated weakness in the substrate material. With small ultra-high pressure jets cutting through the small depth of the desired material at a faster rate, the optimal cutting traverse speed for the nozzle over the surface increases. This in turn means that there is often a considerable advantage to mounting the nozzle array on a bar that is then spun at high speed giving a greater areal coverage and also allowing the assembly to be contained within a vessel that can be attached to a vacuum system that extracts both the water and the debris.

Figure 1. Simplified illustration of the combination of a spinning waterjet system with vacuum extraction to leave a cleaned and relative dry surface after passage.

The marriage of high-pressure water systems with vacuum extraction of the water and debris is one of the changes that are now happening in the industry, but which are, as yet, only in their infancy relative to the potential that with be developed as their use becomes increasingly commercialized. The largest market at the moment is in hydro-excavation, where relatively low jet pressures (2,000 psi) are used to disaggregate soil. Note that while this video gives you some idea of the concept it is using way too much water and if the head and suction pipe are better integrated much more impressive volume removal rates with much lower water usage rates can be achieved. (And as a minor quibble vacuum systems work best with water targets where the head is no more than about quarter of an inch from the top of the water surface.)

The technology is being advanced as more self-rotating heads are integrated into the system, since these give a much faster coverage of the ground without the need for bulky drive unit down at the operating end of the system. One of the advantages of the system is that the water can be set at a pressure that will not damage underground cables, sewer pipes or other passageways that run through the soil, and which would otherwise be vulnerable to the blade of a conventional mechanical excavator.

But the potential for this combination goes much further than these early tools. On a larger scale they can be integrated into the systems that are now used to remove fouling and residual paint from ships hulls but the use of higher pressures also increases the range of materials that can be removed.

For example in hydro-demolition one can set the operating pressure of the system so that, at a pressure of around 11,000 psi the waterjets will remove the damaged material from a garage floor or bridge deck, but where the jet does not have enough pressure to cut into the healthy concrete in the main body of the slab. However, even with vacuum material removal, there are two initial disadvantages to this system. The first of these is because of the large volume of water that must be removed as the jets cut over the surface, the second comes with the range of the larger jets at these lower pressures which mean that, should the damage extend beyond the depth anticipated, the jets can cut down through the concrete slab and remove much more material than desired or budgeted for.

Raising the jet pressure, while removing the discriminatory nature of the jet cutting to some extent does overcome the two disadvantages in that the material is removed with much less water, and at a faster rate. Also, as mentioned above, the smaller jet size means that the range of the jet can be carefully controlled , so that if the specification is that concrete only be removed to half-an-inch below the rebar, for example, then the jets can be controlled to achieve this depth of removal on a consistent basis.

With those thoughts in mind we will continue to progress down this thread in the next post in this series.