Waterjet Technology – Fitting the water jet nozzle to the system

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

Waterjet Technology – Fitting the water jet nozzle to the system

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)

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.

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)

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.

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.

Depths of cut into blocks of Berea sandstone as a function of distance from the nozzle, at two flow conditions (Barker and Selberg)

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.

Waterjetting Technology – Pipe Straighteners

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

One of the advantages that became clear, even in the early days of waterjet use in mining, was that the jets cut into the rock away from the miner. It was thus a safer method of working, since it moved the person away from the zone of immediate risk. Rock has a tendency to fall when the rock under it is removed, and by using the jets to carry out the removal, so the miner is no longer as vulnerable.

But, in the early days of jet use the range of the jet was quite limited. Part of the reason for this is that the water is generally brought to the working place along the floor. It then has to be raised, through bent pipes, to the level of the nozzle, and then turned so that the water in the pipe is flowing in the direction in which the nozzle is pointing.

Waterjet Mining Monitor 1

Figure 1. Sketch of an early Russian waterjet mining monitor

Even though the pressure of the jet is relatively low, the volume flow rates were high, and the bends leading into the nozzle set up considerable turbulence in the jet, so that the range of the jet was quite limited beyond the nozzle. There are a number of different ways of improving the range of the jet, and I will discuss these in later posts, and many of these techniques apply whether the jet is being used at high volume and low pressure for mining, or at higher pressures and lower flow rates for cutting into materials. But today the technique that I will discuss is the use of flow straighteners.

The two most dramatic instances that I immediately recall for their use were at the Sparwood mine in British Columbia, where the collimated jet was able to mine coal up to more than 100 ft. from the nozzle, and in an underground borehole mining application where a Bureau of Mines commissioned system was able to cut a cavity to more than 30 ft. from the nozzle, which was centrally located.

Collimating jets to get better performance is not restricted to the mining industry. A visit to Disney, for example, will find jumping jets that appear to bounce from place to place (video here) (this one shows the start of the surface waves along the jet, known as Taylor instability, which grow and cause the jet to break up; and if you want to make one Zachary Carpenter has two instructional videos on how they are made. (here and here.)

Essentially, as those Youtube segments show, the flow straightness is achieved by dispersing the water – using a sponge – so that it flows through a large number of drinking straws. These straws act to collimate the water flow, and it emerges as a glassy rod, which even acts as a light path so that light shone down it emerges at the far end. This can be used for a variety of different purposes, other than just for entertainment.

This then is the basic idea behind a flow collimator, although for larger mining flows drinking straws are too weak, and the flow volumes need to be larger. There are various designs that have been used for mining applications. Some of the earlier trials were at the Trelewis Drift mine, where the then British National Coal Board set up an experimental operation.

Hydraulic mining monitor

Figure 2. Sketch of Monitor used in the NCB Trials (after Jenkins, R.W., “Hydraulic Mining” The National Coal Board Experimental Installation at Trelewis Drift Mine in the No 3 Area of the South Western Division, M.Sc. Thesis, University of Wales, 1961.)

 A number of different designs were used for the flow straighteners that were located at the nozzle end of the straight pipe section leading into the nozzle:

Designs of waterjet pipe flow straighteners 3

Figure 3. Designs for the initial flow straighteners used at Trelewis Drift (after Jenkins, R.W., “Hydraulic Mining” The National Coal Board Experimental Installation at Trelewis Drift Mine in the No 3 Area of the South Western Division, M.Sc. Thesis, University of Wales, 1961.)

More recent designs, which vary according to pressure, flow rate and pipe diameter, are a combination of those on the left above, and those on the right. It was such a combination that allowed the Canadian miners at Sparwood to achieve production rates of 3,000 tons of coal per shift as an average over the operation of a mining section.

While the use of flow straighteners does not give any gain over having a long straight section of pipe leading into the nozzle, it can bring the flow condition up to that level in places where the geometry (or the resulting unwieldiness of the pipe) would make the long entry impractical.

One of the more interesting applications of this is in the borehole mining of minerals. Simplistically a hole is drilled, from the surface down to the seam of valuable mineral. Then a specially designed pipe is lowered through the hole with the pipe having a nozzle set on the side. Then, as the pipe rotates, and is raised and lowered, the jet mines out the valuable mineral, which flows to the cavity under the pipe, where it is sucked into a jet pump and carried to the surface.

Schematic of borehole mining operation 4

Figure 4. Schematic of a borehole mining operation (George Savanick)

 As I mentioned at the top of the article, the jet cut a cavity some 30 ft in radius, with the jet issuing through a nozzle some 0.5 inches in diameter. In order to achieve this range it was important that the jet was properly collimated, yet the nozzle was set so that there could be no straight section.

Borehole miner nozzle 5

Figure 5. Section showing the feed into the borehole miner nozzle. Note the vanes in the section leading into the nozzle (George Savanick).

The turning vanes to achieve the flow collimation were designed by Lohn and Brent (4th Jet Cutting Symposium) to produce a jet equivalent to that achieved had the nozzle been attached to a straight feed.

Turning vanes for waterjet cutting 6

Figure 6. Turning vanes used to achieve a jet capable of cutting coal to 30-ft from the nozzle. (P.D. Lohn and D.A. Brent “Design and Test of an Inlet Nozzle Device” paper D1, 4th Int Symp on Jet Cutting Technology, Canterbury, BHRA 1978)

Tests of the performance of the nozzle showed that it produced a jet that was at least equal in performance to a nozzle with a straight feed, up to a standoff distance of 45 ft.

In simpler applications the designs do not need to be that complicated, for many simple spraying nozzles, for example, the straightener is made up of a simple piece of folded metal.

Waterjet pipe straightener in low pressure applications 7

Figure 7. Simple flow straightener for use in low pressure and flow applications.

The water has now reached the nozzle, but that is not the end of the story of the feed system, as I will start to explain, next time.

Waterjet Hose Connections and Nozzle Feed

This Waterjet Weekly is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

This Waterjet Weekly Blog is written by Dr. David Summers, Curator Professor from The University of Missouri Science and Technology.

Waterjet Hose Connections and Nozzle Feed

There is often not a lot of choice, when laying out a project, over the path or distance that water must travel from the time it leaves the initial pump to the point where it reaches the nozzle and is usefully applied. Yet, over this length, if the right choices are made, some considerable improvement in performance might be achieved.

As discussed earlier, the simplest improvement is often just to increase the size of the delivery line, although there have been occasions where it was cheaper for us to use a double line rather than a larger single, in order to lower the friction loss to the nozzle, and keep operating pressure in the range that we needed.

There are other concerns with the layout of the feed line to the nozzle, If it is a hose, then any connections between sections should be whip-checked, so that should a coupling fail (which has been known to happen) then the released sections of hose will not whip around and cause injuries or other damage.

Waterjet hose connections under pressure

Hose that separated from a coupling, while under pressure.

The major risk comes at the moment of separation, while the pressure in the line is higher, before it drops under the larger area through which the water can now flow. To stop the whipping of the hose ends, the two should be restrained by attaching a cable with two loops that fit over either hose end making the connection.

Waterjet hose and whip connnection

Hose connection covered with a pressure dissipating sleeve (the blue cover) and with a whip connected to the hose ends on either side of the connection.

Apart from this, and remembering the possible risk about avoiding chaffing points and that, over time, high pressure tubing does fatigue (after many years of operation most of the pipe segments connecting between our ultra-high pressure pump and cutting table failed in a relatively short time period – but they all came at the same time, and were installed together and saw the same loading cycles). (Which emphasizes that, over time, the pressure rating of the tubing should be reduced).

Connections, T-joints and other fittings that are used in the feed line should be sized appropriately. Any time that the diameter of the flow channel changes, then there is a cost in terms of the delivered pressure. This is best checked with the manufacturer to ensure that this is accurately assessed in the flow and pressure calculations.

Moving down the line, this brings us to the end of the feed line, and the entrance to the nozzle. In later posts I will cover different pieces of equipment that can be used, for a variety of different tasks in manipulating the nozzle, but, for now, I would like to consider just the flow from the feed line into the nozzle (without discussing nozzle shape at this time).

Of all the systems I have examined, this is the one point in the assembly of a feed system that was most commonly ignored or badly constructed.

All of us, from time to time, get caught up in traffic flows through road construction. When the lanes are controlled, and traffic feeds are properly directed, it is possible to get through these relatively quickly. But in most cases that is not what happens. There are always drivers who do not ease into the required lanes soon enough, but rather drive rapidly as far as they can and then force their way into the remaining lanes, thereby breaking the steady flow into a process of stop-start-stop-start. The process becomes much less efficient, and instead of the traffic moving at a steady, but slow rate it often can take over half-an-hour or more to get through the restriction.

So it is with water flow down a pipe. If the flow can remain in a laminar mode, with the flow channel slowly constricted to speed the water up to the required velocity, then the resulting flow into and through the nozzle can give jet streams that can throw over 2,000 jet diameters. Instead in most cases the throw is about 125 jet diameters, and I will discuss how to find that distance in a post or two before long.

In this particular post the water has not reached the nozzle yet, and it could still be in a poor condition. The best way to explain why is to use a comparative graph showing jet pressure measurements after the jet has passed through the nozzle, to show how the structure is affected. The work was carried out by the Bureau of Mines (Kovscek, P.D., Taylor, C.D. and Thimons, E.D., Techniques to Increase Water Pressure for Improved Water-Jet-Assisted Cutting, US Bureau of Mines RI 9201, Report of Investigations, 1988, pp 10) and the only difference between the two different plots is that in the upper one a 4-inch straight length of pipe was connected just upstream of the nozzle to allow the flow to stabilize before it entered the nozzle.

The effect of flow conditioning the water, prior to passage into the nozzle.(

The effect of flow conditioning the water, prior to passage into the nozzle.(Kovscek, P.D., Taylor, C.D. and Thimons, E.D., Techniques to Increase Water Pressure for Improved Water-Jet-Assisted Cutting, US Bureau of Mines RI 9201, Report of Investigations, 1988, pp 10)

Figure 3. The effect of flow conditioning the water, prior to passage into the nozzle.(Kovscek, P.D., Taylor, C.D. and Thimons, E.D., Techniques to Increase Water Pressure for Improved Water-Jet-Assisted Cutting, US Bureau of Mines RI 9201, Report of Investigations, 1988, pp 10)

There is a caveat to this plot, and this is the assumption that the internal diameter of the feed pipe and the entrance diameter to the nozzle are the same size. In virtually every system that I have examined in the field this has not been true, and as I will show, in a later post, the difference that this can make is very large. For the above example the length is 4-inches, but this was for a specific nozzle size, and the more general condition is that the length should be in the range of 100 – 125 pipe diameters.

By the same token, if the nozzle does not make up with the end of the feed line, so that there is a little eddy pocket created, the ensuing jet will be of poor quality.

Which brings us to the final part of this post, because there are many situations where it is not possible, because of space restrictions, to fit that particular length of pipe just before the nozzle. The most glaring example of this that we have had to deal with is where high-pressure waterjets are fed down a borehole, and then used to drill lateral excavations out from the bottom.

But if the borehole is say 10-inches in diameter, and the jet is an inch in diameter, because it is being used for mining out valuable pockets of uranium, then it is not possible to get the required straight section. Again the pioneering work on this was carried out, first in Russia, and then by the U.S. Bureau of Mines, under George Savanick. By placing a shorter length of a flow straightening device within the flow path, just before the nozzle, the flow can be straightened over a much shorter distance, and this will be the topic of the next post. And when George did this he was able to cut cavities that extended more than 30-ft from the borehole (with some unexpected consequences – but we’ll cover that later – grin).