Waterjetting Technology – Pipe Straighteners

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

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.

Sketch of an early Russian waterjet mining monitor

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; 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.

Sketch of Monitor used in the NCB Trials

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 for the initial flow straighteners used at Trelewis Drift

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 a borehole mining operation

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.

Section showing the feed into the borehole miner nozzle

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 used to achieve a jet capable of cutting coal to 30-ft from the nozzle

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.

Simple flow straightener for use in low pressure and flow applications

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

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

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.

Hose that separated from a coupling while under pressure

Figure 1. 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.

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

Figure 2. 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 you have to remember 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, 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

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.

This brings us to the final part of this post because there are many situations where it is not possible due to 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).

Waterjet Technology – Making gift items by water jet cutting

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

There is a time which can come in late Winter and very early Spring when demand declines and there is some free time for the occasional home project. Although many of us now know and understand how well waterjets and abrasive waterjet streams can cut material, this is still not that widely recognized by the General Public. This slack time can help to remedy that problem.

Uninformed ignorance of jet capabilities was certainly true for many years on our campus and seems to become more so again as the years pass since I retired. Further, at Conferences, I often heard the complaint that the industry needs to get its message out more clearly to a wider audience. The vast majority of potential industrial users are unaware of how well waterjetting in one of its forms could help solve their problems.

Now there are lots of ways of solving that problem, but today I want to talk about just one, the one we used to help us with the problem. It had to be something that would be used by those we gave it to. It had to be small, relatively cheap and quick to make and yet demonstrate some of the capabilities we wanted to show off. The answer ended up as a business card holder.

Business Card Holder – Missouri Miner female figure

Figure 1. Business Card Holder – Missouri Miner female figure

University labs are generally cash strapped, and so the material had to be relatively cheap, so we used sheets of a light foam. This allowed us to cut out the figure parts using water alone (at around 20,000 psi) which significantly reduced the cost. Early in the design of the female figure (this was the third in a series where we cut a different shape each year) it was pointed out that relative body size was more critical with female figures, and so two different thicknesses of foam were used. The first was half-an-inch thick and used for the body and pins, while a quarter-inch sheet was used for the legs and arms.

Foam miner front view

Figure 2. Foam miner front view – showing the two thicknesses of material with waterjet cutting

Putting a small hole in the position of the eye allowed the model to show how precise and small a cut could be made through thicker material. The five pieces that made up the total were held together with two rectangular pins that were cut from the thicker stock and fitted through slots cut to their shape in the different parts.

One of the advantages of cutting these (and we cut parts for around 300 figures, and used virtually all of them each year) is that it was also possible, with relatively little trouble, to cut the campus identifier on a leg of the figure. With not a lot of space this was originally UMR and then changed to “S & T” when the campus changed its name.

A later model of the card holder with the campus ID cut into the leg by waterjet cutting

Figure 3. A later model of the card holder with the campus ID cut into the leg by waterjet cutting

For speed in cutting, we only cut the letters in half the legs, though you may note that in this later version we also cut the connecting pins as round rod, rather than rectangular. In this way the figure could be repositioned as the owner decided what they wanted to do with them.

Basically, however they served as card holders, and having passed them around, (and provided them to senior campus officials as place card holders for dinner meetings) it has been amusing to see how avidly they were sought and kept by some of those to whom they were given.

Now we did not get to these figures in one step. The initial idea was to carve something out of rock, since the overall department was known as The Rock Mechanics and Explosives Research Center. However, if you are making something out of rock, particularly a person’s shape, they need to be larger, because of the weak strength of the rock.

Comic-book Miner cut out of Missouri Granite with waterjet cutting

Figure 4. Comic-book Miner cut out of Missouri Granite with waterjet cutting

The cost was also high, since the cuts had to be made with abrasive, and the rock had to be polished before it was cut. (Trying to polish the pick points after cutting led to several breakages, and this is something that is either perfect or worthless).

There are several good ideas that individual companies have which help sell their name and capabilities where the gifts are of metal and can be used for opening bottles or of some other benefit. But we could not afford the cost to cut a lot of pieces using abrasive, and nothing that we tried in metal had the cachet of the small miners.

In this case, the mascot of the campus is the Missouri Miner, and while the first model that we cut followed along the shape of that cartoonish figure, many of our graduates were going into coal mining, which is also my background, and so the second and third versions had coal mining helmets, and as a further demonstration of capabilities, a small circular cut in the helmet allowed a yellow rod to be put into the helmet to illustrate the miner’s cap lamp.

Where we were asked to prepare small souvenirs for another event we did use the Missouri Granite, but had learned this time to buy tiles that were already polished. Then all we had to do was to cut the shape of the state into the tiles, and then put a University logo sticker on the piece and we had our memento for the guests.

Small memento of the state shape carved out of granite tile by cutting with waterjet technology

Figure 5. Small memento of the state shape carved out of granite tile by cutting with waterjet technology

This was for a specific occasion where the sponsor was willing to pay for both the cutting costs and the materials, but in order to keep costs down (since these were given away) the pieces had to be small. This particular run was one of the more difficult to keep inventory on, since several disappeared during the short time of the cutting runs (which we have found is an occupational hazard with “artistic” pieces where there are lots of temporary folk involved in our work).

Which is, I suspect, an entry for the last piece of advice on making such gifts, and that is to plan on making more than you think you need and, if possible, be able to make more if needed. In a later post I will write about where you can get some artistic help for relatively little cost to help with ideas such as this.

Waterjet Technology – Hoses and High Pressure tubing

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

One of the first decisions one makes in connecting a waterjet pump to a nozzle is to select the size of the high-pressure pipeline that will take the water from the pump to the cutting nozzle. This choice has become a little more involved as ultra-high pressure hoses have come on the market since they can be used at pressures that once could only be served with high-pressure tubing. However, at higher pressures, the flexibility of hoses becomes reduced – both because of that pressure and also because of the layers of protection that are built into the hose structure.

Much of the original plumbing in the earlier days of the technology used 3/16th inch inner diameter, 9/16th inch outer diameter steel tubing. One reason for this was that, at this diameter, the tubing could be quite easily bent and curved into spiral shapes. And that, in turn, made it possible to provide some flexibility into an assembly that would otherwise have been quite rigid.

Early cutting nozzle with spiral coils in the high-pressure waterjet feed line to the nozzle

Figure 1. Early cutting nozzle with spiral coils in the high-pressure waterjet feed line to the nozzle

When cutting nozzles were first introduced into industry, they were fixed in place because of the rigid connection to the pump. Therefore, the target material had to be fed underneath the nozzle since it was easier to move that than to add flexibility to the water supply line.

Early waterjet slitting operation

Figure 2. Early waterjet slitting operation (courtesy of KMT Waterjet Systems)

However, because feed stock can vary in geometry, some flexibility in the positioning of the cutting nozzle above the cutting table would allow the jet to do more than cut straight lines. A way had to be found to allow the nozzle to move, and this led into the development of a series of spiral turns that high-pressure tubing can be turned through, as it brings the water to the nozzle (See Figure 1). That, in turn, allowed a slight nozzle movement. By adding this flexibility to the nozzle, a very significant marriage could then take place between robotics and waterjet cutting.

The force required to hold a nozzle in a fixed location becomes quite small as the flow rate reduces and the pressure increases. (at 40,0000 psi and a flow rate of 1 gpm the thrust is about 10 lb). The first assembly robots that came into use were quite weak, and as their arms extended, the amount of thrust they could hold without wobbling was small, but critically more than 10 lb. And this gave an initial impetus to adding jet cutting heads to industrial robots of both the pedestal and gantry type to allow rapid cutting of shapes on a target material, such as a car carpet, where the ports for the various pedals and sticks need to be removed.

But this marriage between the robot and the jet required that the jet support pipeline be flexible, so that it could allow the nozzle to be moved over the target and positioned to cut, for example, the holes for retaining bolts without damaging the intervening material.

The pipe had to be able to turn and to extend and retract, within a reasonable range, so that it could carry out the needed tasks. Bending the pipe into a series of loops produced that flexibility.

A single full circular bend in the pipe will acquire sufficient flexibility that the end of the pipe (and thus the nozzle) can be moved over an arc of about 9 degrees.

Coils on a pedestal-mounted robot

Figure 3. Coils on a pedestal-mounted robot, allowing 3-dimensional positioning of the cutting nozzle

A large number of coils were required since the tubing has only a very limited amount of flexibility in every turn. For example, if one wanted to stretch the connection by lowering the nozzle, then the several coils would act in the same way that the steel in a spring would as it extended. The movement can perhaps be illustrated with the following representation of a set of spirals, with metric dimensions.

Schematic of a series of coils

Figure 4. Schematic of a series of coils, arranged to allow the nozzle to feed laterally

Each spiral will also allow a slight angular adjustment, and these add up as more spirals are added to the passage.

Angular movement allowed per spiral

Figure 5. Angular movement allowed per spiral. This should not exceed 9 degrees per turn

While, in many modern assemblies, this may seem to be a quaint way of solving the problem, back when these systems were first put together, it was very had to find high-pressure swivels that would operate at pressure for any length of time. In those days, we had one source that provided a swivel that would run for many hours provided that all the external forces could be removed from the swivel itself. But the moment an out-of-alignment force hit the swivel it was ruined. In another application, we had tested every swivel we could find that would fit down a six-inch diameter hole and had found one that would run for ten minutes. To finish our field demonstration, where we had to drill out 50-ft horizontally from a vertical access well, we had to continuously pour water onto the joint to keep it cool, and the manufacturer stood by with a pocket full of bearing washers that we had to replace every time one started to gall.

But that was over thirty years ago. Now the connections from the pump to the nozzle can flow through ultra-high-pressure hose with a flexibility that we could barely imagine. And ultra-high pressure swivels will run for well over a hundred hours each without showing any loss in performance. It was, however, a gradual transition from one to the other.

Ultra-high-pressure feed to a nozzle, using coils and swivels

Figure 6. Ultra-high-pressure feed to a nozzle, using coils and swivels

There are a couple of additional cautions that should be born in mind when laying these lines out. While a hose is more flexible, it is liable to pulsing and moving slightly on a bearing surface under pump cycling. In most places, this is not a problem, but if the hose is confined and bent, then it may cause the hose to rub against a nearby surface. Over time, this can generate heat and can even wear through the various hose layers.

Worn hose and the scuff mark where it was rubbing on a plate

Figure 7. Worn hose and the scuff mark where it was rubbing on a plate.

There are other issues with hoses: smaller high-pressure lines can kink when used in cleaning operations and this is a seriously BAD thing to happen. I will discuss that in a future article. Similarly, one must consider the weight of the hose, particularly in hand-held operations, where it is important to address hose handling as part of the procedure, but again this will be discussed later.