Wednesday, February 13, 2013

Waterjetting 6a - An introduction to testing nozzle performance

In the next few posts I will be writing about some of the tests that you can run to see how a nozzle is performing. But before getting into the details of the different tests, you should recognize that this is where a little homework will be required if you are to get the most benefit from the topic.

The world that encompasses waterjet use has grown beyond the simple categories by which we used to define it. New techniques make it possible to cut materials that used to be more difficult and expensive to produce, and as practical operational pressures have increased so the scale, precision and economics of new opportunities have developed.

It is this range of application that makes it impractical for me to give specific advice for every situation, and instead, by explaining how to make comparisons, and what some benchmarks might be, to allow you to better understand your system, its capabilities and both the initial performance of nozzles, and then the evaluation to decide when they may best be replaced.

One lesson I learned early was that nozzles from different companies behaved in different ways, and that drawing conclusions on optimal performance, for example the selection for which pressure level and nozzle size was best, using one design would not necessarily hold with a competing design. Further there were nozzles that began their life on our system doing very well relative to others, but which quickly declined in performance. Thus, as part of an evaluation of different designs, we would test the nozzle cutting performance, against a standard requirement, at fixed time intervals so that we would know when it was wearing out and should be replaced.

Figure 1. Change in the cutting depth of a jet stream, at 50,000 psi, when traversed over ASTM A108 steel as a function of the time that the nozzle had been in use.

Both the shape of the curve and the effective lifetimes of different competing nozzle designs varied quite significantly. And obviously, since most folk don’t spend a lot of their time cutting through more than an inch of steel, the operational lifetimes of nozzles will vary with the requirements for the particular job. Nevertheless the relative ages at which nozzles can no longer reach that target can differ significantly,

Figure 2. Comparative effective nozzle life over which, operated at a pressure of 50,000 psi, a jet could cleanly cut a path through a 1.4 inch thick steel target at a traverse rate of 1.5 inches/minute.

As mentioned, the tests were carried out using nozzles from several manufacturers and, at the beginning of the test the longest lasting nozzle was not necessarily the one that produced the fastest cut, but consistently, over the interval, and for about twice as long as the competition, it was able to achieve the goal.

Figure 3. Depths of cut in steel after (top) 1,000 minutes of nozzle use, and (bottom) after 1,500 minutes of nozzle use.

In the particular case in which we made the comparison the major interest was in achieving a clean separation of the parts, and the edge quality was not as significant a factor. In many uses of this tool that edge quality will be important and would have given a different set of numbers (as Figure 3 would indicate) than the ones that were found for our application. As a result the judgment that the nozzle is worn out will change to a different time, and the relative ranking of the different nozzle designs may also change.

The only way in which anyone can make a rational decision on which is the best nozzle for an application, and how long it will be effective is by testing the nozzle against the stated requirement. When we began the test we anticipated that the difference between nozzles from different manufacturers, when fed with water at the same flow rate, and with the same quantity and quality of abrasive would not differ that much. As Figure 2 shows, we were wrong in that idea.

There are a number of different impacts that a change in nozzle design (i.e. in most cases buying a competing design over that initially used) can bring to a cutting operation. However these impacts are also governed by the pressure at which the work is being carried out, the amount of abrasive that is used, the relative nozzle diameters (if using a conventional abrasive waterjet system) and the speed at which the cut is made. But an initial assessment of relative merit should be carried out with equivalent parameters for the different designs.

In general, however, we ran tests at a number of pressures, and with varying abrasive feed rates, to ensure that the comparative evaluations were fair, and consistent. As a result we found that there were a number of different factors that came into play, which are not always recognized, and which could bias the results that we observed.

In the posts that follow this I will first cover some of the different tests that can be used, and then go on to explain some of the results and why they sometimes make it difficult to accept a simple comparison of results when, for example, the abrasive is not the same in both cases. To give a simple example of this, consider a conventional abrasive waterjet nozzle that is operated at increasing pressure.

Increasing the pressure will improve the cutting speed and/or the cut quality, as a general rule. It will reduce the amount of abrasive that is needed, but this is where the “yes, but’s . . . .” start to appear. As the pressure of the jet increases, so the amount of abrasive that is broken within the mixing chamber will also increase, so that the average size of the particle coming out of the nozzle will become smaller. The amount of this size reduction is a function of the quality of the abrasive that is being used, and a function of the initial size of that abrasive.

Within a certain size range, that reduction in the particle size does not significantly change the cutting performance, but if the mix contains too many small particles, particularly if the distance to the work piece is also significant, then the cutting performance can be reduced because of the particle break-up. Different nozzle designs produce different amounts of very fine material even from the same feed rate of the same abrasive into the nozzle. When the initial feed rate of the abrasive, or a different abrasive is used, then estimating which design and set of operating pressures is best becomes more difficult, as an abstract estimation.

This is why, in the posts that follow, the comparisons are made are based on actual measurements and why I recommend that everyone test their system using more than one design/set of operating parameters so that they can be confident that the combination that they are using will provide the best combination for the job to be done.

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