Wednesday, April 8, 2015

Waterjetting 32a - jet fluids

From time to time I am asked why we use water in almost every application of high-pressure waterjet use, rather than using any one of a number of other choices for the fluid. There are a couple of major reasons for this. The first is that most of the uses of the tool will require that we use a fairly large quantity of fluid, over time, and water is the cheapest and most readily available of the alternatives that can be used in such volumes. The other is that, in many circumstances, it works as well, if not better than those alternatives. As an additional point it can often be cleaned up relatively cheaply and simply before being disposed of after use.

But the conclusion isn’t an absolutely true one, and along the way there have been a number of different investigations to find alternatives that can provide a better answer than just water alone. Some of these choices suggest applications where a different fluid might work better, for example if the target is a food product that contains a lot of sugar then it might be better to cut with a form of vegetable oil that does not dissolve the sugar on contact. In other cases there is an advantage to adding a chemical to the water to change its properties.

The changes in performance that decide whether the change in cutting fluid is worth the effort are (as with so many other choices) going to be based on the particular job that has to be carried out. In some cases, as with cutting candy for example, the benefits of using an oil stream may make the process practical in a food that would otherwise not be feasible to be cut with a high-pressure jet, for aesthetic reasons if no other.

Figure 1. Effect of changing fluid properties on cut depth at an impact velocity of 330 ft/sec. (after Rochester and Brunton) on a nickel target.

Several different fluids were used to generate the curves shown in Figure 1, ranging up to mercury in density, and including four different oils as a way of investigating viscosity.

One of the down sides to the use of oil as a cutting fluid comes from the increased viscosity of the fluid, with some of the oils that were initially used as the tool went into commercial use for cutting confectionary items.

The increased viscosity of the oil raised the pressure required to drive the fluid from the pump through the delivery lines to the cutting nozzle arrays. This in turn lowered the cutting pressure at the nozzle, and made the differential between the operating pressure at the pump during the drive stroke and during the reversal significantly greater. This greater fluctuation in pressure produced a greater fatigue on the drive train, and in turn led to a more rapid failure of components within the system. While the initial answer was thought to be in using a different cutting oil, the final, easier, solution was to move the pump closer to the cutting zone, reducing the pressure losses and bringing the fatigue back to acceptable levels.

Changing fluid properties can therefore have a significant impact on system operation, even though they may have little impact on the actual cutting performance, although in many cases there are significant changes as the fluid properties are changed.

The chemical impact is one that is perhaps addressed more commonly in cleaning applications, where it is often suggested that chemical agents be added to the water as a way of weakening the bond between dirt and the underlying surface. While this is theoretically possible, it should be remembered that the jet, even at the relatively low pressure of most pressure washers and car wash units, will be travelling at speeds of hundreds of feet per second. The residence time of the chemical on the surface can thus be measured in milliseconds and is rarely long enough for much change to occur.

In these circumstances it is usually more fruitful (and less demanding of chemical) to spray the chemical cleaning solution onto the surface first, and then allow a short period of time for the interaction to fully occur before applying the pressure wash. In these conditions (as we saw when we watched as our house was pressure washed the other week) even though the nozzle was held too far from the surface for there to be much pressure on the wall by the time the jet reached it, the chemical cleaner had broken the bonds of the algae and dirt, and the house walls were rapidly cleaned with relatively little additional effort. (This is contrast with the time that I had cleaned it without chemical, where it was necessary to keep the nozzle within six inches of the wall for the jet, unassisted, to remove all the surface contamination).

In the case of a house cleaning this highlights an additional benefit from using the chemical, since the lower pressure jet impact on the building means that the jet will not be strong enough to erode any of the timber surfaces around the house that were starting to weather with the passage of time.

There are, however, some chemicals that are used in mining and civil construction that will interact relatively quickly, when introduced into the cracks within rocks, particularly if they can be pushed to the tips of the cracks where they can rapidly reduce the strength of the bonds across the tip of the crack, making it easier for the crack to grow at lower fluid pressures. The fluid characteristic that controls this effectiveness is often related to its Zeta Potential. However much discussion of that topic would rapidly take us a considerable distance from the current subject, and so I would recommend that any of those interested might want to follow along a line that might start with the work at Brookhaven.

This series will return with a look at some of the mechanical changes that can be made to a waterjet, to improve cutting of everything from shoes, to ships and similar subjects.

Rochester M. and Brunton J. The Influence of the Physical Properties of the Fluid on the Erosion of Solids, CVED/C-MAT/TR10, University of Cambridge, UK 1973. (This formed part of Mike’s doctoral dissertation).

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