Buying a high-pressure system requires a significant amount of money, and, as a result, most folk will make a serious attempt at comparing the quality of the different systems that they are considering buying, before they make that choice. Most of the expense goes into that part of the system that sits behind the nozzle, and which supplies the water and (where needed) the abrasive that form the cutting/cleaning system.
Often, however, while the upstream system is the subject of such scrutiny, the nozzles themselves, and the selection of abrasive often escape this level of evaluation. Both of these “parts” of the system are part of the wear cost of operations, and, as a result, the selection of the “best’ nozzle often involves operational cost considerations, with less emphasis on comparative evaluations of performance. To explain this most brutally, a company may spend $250,000 on a system, but then degrade the performance of that system by over 50% by choosing a nozzle system that saves the company 15% on purchase costs over that of a competitor. (I will show figures in a later post on this topic).
In the next few posts I am going to explain some of the tests that we, and others, have run to compare nozzle performance, and some of the results that we found. I don’t intend to “name names” because the tests that I will talk about are specific to certain specific objectives, and the reason that you are running a system will likely differ from the conditions and the performance parameters that we needed to match for some specific jobs. The evaluations will range over a number of different applications and will cover some quite expensive tests, as well as some very simple ones that can be run at little cost in time or money.
But, to begin, the first question relates to how you attach the nozzle to the end of the supply pipe. Here you are, if you have followed the train of thought of the last two posts on conditioning the water as it leaves the supply pipe, through a long lead section, or through a set of flow conditioning tubes, the water is nicely collimated and (as I will show) could under certain circumstances have a throw distance of perhaps 2,000 jet diameters or so. Yet the average jet has an effective distance of around 125 jet diameters. Why the difference? An illustrative sketch from Bruce Selberg and Clark Barker*, simply makes the point.
Figure 1. Comparison between a typical nozzle attachment and one where the flow channel is smoothed. (Barker and Selberg)
Right up to the point where the small focusing nozzle is attached to the pipe on the left (a) the flow has been conditioned to give a good jet. But then, just as the flow starts to enter into the acceleration cone in the nozzle it hits the little step at the lip of the nozzle where it attaches to the pipe.
As I will mention in a later post, when a jet hits a flat surface and can’t penetrate then it will flow out laterally along that surface. (This also happens with wind, and is why places such as Chicago are referred to as "The Windy City.") So the outer layer of the jet hits the lip, and where does it go? It runs right into the path of the central flowing jet into the nozzle and mixes right across it. So much for stable flow, that lateral disturbance turns the flow turbulent, so that it is rapidly dissipated once it gets out of the nozzle. Professors Selberg and Barker calculated the theoretical pressure of the jet coming out of the orifices, and compared it with pressure values that they measured.
Figure 2. Measured pressure profiles plotted against the theoretical pressure (small crosses) at different distances from a typical conventional nozzle with two orifices.
In comparison, as a way of ensuring that the flow path into the two orifices was smooth, the two authors added a small section made of brass between the end of the pipe and the entrance to the nozzle body ((b) in Figure 1). They inserted two pins to fit into alignment holes drilled into the end of the pipe, in the insert, and in the nozzle body itself.
Figure 3. Construction of a feed section between the nozzle body and the feed pipe to stabilize the flow (Barker and Selberg)
When the pressure profiles were taken with one of the new set of nozzles, the difference, as a function of distance, was quite marked.
Figure 4. Profiles from the nozzle design shown in (b) with a two-part nozzle (Barker and Selberg. Note that the standoff distance has increased for the two sets of profiles over that in Figure 2.
Further, when the depth of cut was measured after the jets were fired into blocks of Berea Sandstone at various distances from the nozzle, the improved performance was clear out to even further distances.
Figure 5. Depths of cut into blocks of Berea sandstone as a function of distance from the nozzle, at two flow conditions (Barker and Selberg)
The addition of the flow channeling section does make the nozzle a little longer, and the cone angle of the inside of the nozzle was continued out to the diameter of the feed pipe to reduce any steps that might induce turbulence. In addition the inside of both the transition section and the nozzle were polished to a surface finish of better than 6-microinches.
The nozzles themselves were specially constructed for us using electro-formed nickel on flame-polished mandrels and were thus quite expensive. Our particular purpose, however, was in the development of a mining machine that, with the nozzles that we used, was able to peel off a slab of coal, to the height of the seam, and to a depth of 3 ft, at a rate of advance of at least 10 ft/minute. (A later design in Germany went over 6 times as fast, when operated underground).
The advance rate was achievable because the jets were cutting a slot consistently about 2 ft ahead of the machine, and with two jets the coal between them was washed out without having to be mined. But that is a subject for a different post a t some time in the future.
Before I leave the subject, however, some folk might comment that their nozzles sit in holders that are then threaded onto the end of the pipe – thus they should be in alignment, and they are tightened until the holder is tight on the pipe. There are two caveats with this, the first is that this does not necessarily mean that the entry into the nozzle smoothly butts up against the end of the pipe, and in alignment with it. (Hence our use of pins.) In field visits we have measured, for other operators, the relative distances involved, and found that there can be a gap between the end of the nozzle body, and the end of the pipe, both contained within the holder. Even though the two diameters are the same, the presence of the larger chamber before the entry into the nozzle will again create turbulence and a poor jet.
The fix in both cases is a small transition piece, which is simple to design and insert to fill that gap, and smooth the passage. Though it does bring with it the second caveat. You need to make sure that the number of threads of engagement of the holder on the pipe remain enough so that the holder won’t blow off if the nozzle blocks. (One time one of ours did, but it was in a remote location, so thankfully no-one was hurt, although there was some damage as a result).
In the next post I will start to discuss the different ways that we have used, after the nozzle is in place, to make sure that the jets were doing what they were designed to and producing a jet of the quality needed.
* The information that I used in this article can be found, in more detail, in the paper: 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.