Waterjetting 25a - choosing jet parameters for range.

The range over which a waterjet is able to cut material can widely quite significantly, depending on a wide range of factors, including abrasive content. An earlier post described the way in which students in a waterjet class were shown some of the difficulties in assessing risks arising from the use of a waterjet, and the range over which it was dangerous. Simplistically the students first cut along a plywood panel to see how far from the nozzle the jet would remove wood.

Figure 1. By slightly tilting the 4-ft wide panel and then having students move the jet past the board along the left-hand edge, a measure of the range of the jet could be obtained.

However, after the students had decided that, for the 10,000 psi 0.03-inch diameter jet, the cutting range was about ¾ of the way across the width (i.e. 3 ft) they were then tasked to pass the jet, as fast as they could, over a piece of pork that was at least a foot further away from where they estimated the distance that the jet stopped cutting.

Figure 2. A piece of pork after being “sliced” by a 10,000 psi waterjet.

The pork was typically cut to a depth of over an inch, grooving into the bone at a distance that the student had previously decided was “safe.” It was pointed out to the class that the pork was a good simulator for human flesh.

The point of the demonstration was fairly obvious, but it does highlight that the distance at which a jet stops cutting one material because of insufficient energy, may still be quite a distance closer than that critical distance for other softer materials.

In one of the earlier scientific papers on waterjet cutting Leach and Walker plotted the drop in jet pressure from two different nozzle shapes, against the distance from the nozzle.

Figure 3. Decline in jet pressure with distance from the nozzle (Leach and Walker)

With poorer nozzle designs and in cutting many harder materials the critical distance at which the jet pressure falls below half the original pressure, and thus in many materials stops cutting, is at around 125 nozzle diameters. For a 0.03-inch diameter jet, cutting a distance of 36 inches takes the range to 1,200 diameters. And while that range is partly because we have significantly improved the fluid flow into the nozzle it relates, as noted, also to the strength of the material being cut.

One reason to mention this is that I have seen, both in photos and real life, people foolish enough to hold their hands in front of a 40,000 psi waterjet, as an illustration of the safety of the tool at even a short range. (Typically they were using jets of around 0.006 inches diameter with the hand about a foot from the nozzle). A slight increase in nozzle diameter, undetected by the operator, or a change in fluid content (such as by adding a long-chain polymer (such as Superwater) could extend the range of the jet several-fold, so that the unsuspecting operator might lose several fingers before realizing the change in conditions.

An earlier post described the work of Clark Barker and Bruce Selberg, who demonstrated that an increase in polish of the inner surfaces and a smooth transition path into the orifice could extend the cutting range of a jet in harder materials from 125 diameters to over 2,000.

Achieving a smooth flow path to the orifice is critical to superior performance, though – as I have mentioned before – it was at one time surprising to me how many contractors did not even have the nozzle insert mating with the end of the supply pipe. Rather, with the nozzle insert held in a holder, they just turned the latter until it was tight, not always achieving contact between the back of the nozzle insert and the pipe. In addition there have been many cases I have seen where the nozzle insert inlet diameter differs from that of the internal diameter of the connecting pipe Again this will interfere with performance away from the nozzle.

Assuming, however, that one has stabilized the flow into the nozzle, and that it is of the right shape, how can one increase the jet throw distance further? The obvious, and wrong, answer is to up the pressure that is driving the jet.

Why is this the wrong answer? Well, if one considers what happens when a jet shoots out into the air, as one can see in a high-speed flash photograph:

Figure 4. Flash photograph (exposure at about one-millionth of a second) of a high speed waterjet showing the structure.

As the jet travels through the air, so the relatively stationary air around the jet strips off, and decelerates, the jet in layers starting from the outside. These show up as backward pointing stringers flowing out from the main jet stream. As the outer layers are peeled off (as with stripping the layers from an onion) so the remaining diameter gets less until, as in the picture above, there is no jet left.

Consider that with a higher driving pressure that there is a greater differential between the air speed and that of the jet, and obviously the stripping action will occur more rapidly, reducing the overall range of the jet.

Now consider if, instead of putting that additional power into pressure/jet velocity one were, instead to put it into additional flow. Then there are more layers of the jet to strip away, and the differential is not as great. As a result, when one compares the performance of two jets one gets:

Figure 5. Comparing the performance of two jets.

Notice in this case that relatively close to the nozzle the two jets, cut to roughly the same depth, and in this range the higher pressure, smaller jet has advantages in that the thrust it applies to the holding tool is less, and the total amount of water used is also less (roughly 4.3 gpm rather than 7.3 gpm). However if one is cutting at a greater standoff distance between the wall and the target, then at about 4 ft from the nozzle (1000 diameters of the larger, 1500 diameters of the smaller) the lower pressured, higher flow rate jet becomes more effective.

This relative change in nozzle effectiveness with pressure and diameter was also reported from results at lower pressure when developing nozles for cutting coal in Germany.

Figure 6. Comparing the pressure profiles of jets at two different diameters and pressures, as a function of distance from the nozzle.(Benedum et al)

Note that here, again, at about 8 m from the nozzles, both jets are producing about the same impact pressure, while closer to the nozzle the smaller (blue line) jet has a better profile (at 0.78 inch diameter, and 1,300 psi) than the larger (black line) jet (at 1-inch diameter, and 1,000 psi). But at greater distances the lower pressure, larger diameter jet becomes more effective.

There is, in short, significant benefit to determining, before one starts, what the objective is and over what range the jet is expected to cut, since both will help decide what set of jet operating conditions will give the better result.

References Leach, S.J., and Walker, G.L., "Some Aspects of Rock Cutting by High Speed Water Jets," Phil. Trans. Royal Society, London, Vol. 260A, pp. 295 - 308.
Barker, C.R. and Selberg, B.P., "Water Jet Nozzle Performance Tests", paper A1, 4th International Symposium on Jet Cutting Technology, Canterbury, UK, April, 1978.
Benedum, W., Harzer, H., and Maurer, H., "The Development and Performance of two Hydromechanical Large Scale workings in the West German Coal Mining Industry," paper J2, Proc. 2nd Int. Symp. Jet Cutting Tech., BHRA.