Waterjet Technology – An intro to water jet structure

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

Once a waterjet starts to move out of the nozzle with any significant speed as the pump pressure begins to build, it becomes more and more difficult to look at the stream of water and get any realistic idea of its structure. Mainly what is seen is the very fine mist that surrounds the main body of the jet, and while some idea of the structure can be obtained by making cuts through material, it can be quite expensive to actually see within that structure. Part of the problem is that though the mist is very fine, it is also moving at speeds in the range of a couple of thousand feet per second. The human eyeball isn’t quite that fast. But we can use a very high-speed flash (in this case it was on for two millionths of a second) which has the effect of “freezing” the motion.

40,000 psi jet issuing from a 0.005 inch diameter orifice, front lit

Figure 1. 40,000 psi jet issuing from a 0.005 inch diameter orifice, front lit.

However, this mist still hides the solid internal structure of the jet and does not change much in relative structure, even when the internal jet conditions can be quite different. Fundamentally, the internal structure was described by Yanaida at the 1974 BHR Group Waterjet Conference and his description has been validated by many studies since.

The break-up pattern of a waterjet

Figure 2. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

This structure holds for jets across a wide range of pressure and flow volumes, but it is difficult to determine the exact transition points of that structure conventionally. And this can lead to very unfortunate results. I have twice seen people back a nozzle away and then move their hand in front of the jet to show that even high-pressure jets (these were being used to cut paper products and had no abrasive in them at the time) could be “safe.” If both cases, the individuals were very lucky to escape injury (water can penetrate the pores of the skin and lacerate the internal parts without any surficial signs of injury, and, as I showed last time, if the nozzle is too close, it will slice through flesh and bone). I thought to take today’s post to show, though the use of photographs, why that was such a stupid action.

The photos that follow were taken in Baxter Springs, KS, which has been recognized as the Birthplace of Waterjet Cutting.

Baxter Springs, Kansas. Birthplace of Waterjet Cutting

Figure 3. Baxter Springs, Kansas. Birthplace of Waterjet Cutting

In the early 1970’s, we used what was then a McCartney Manufacturing waterjet intensifier (today KMT Waterjet Systems) to shoot jets of varying pressure and nozzle diameter along a path, so that we could see how coherent the jets were. As I mentioned above, the problem with looking directly at the jet is that the internal structure is hidden by the surrounding mist. To overcome that part of the problem, we shone the light along a ground glass screen (to diffuse it) that was placed behind the jet, so that we could see the outline of the internal structure.

Arrangement for taking photographs of a high-speed jet

Figure 4. Arrangement for taking photographs of a high-speed jet.

This more of the downstream mist from the photograph, and a much better idea of the internal structure of the jet, and where the solid section ended could be measured.

Backlit, 30,000 psi jet issuing from a 0.01 inch diameter nozzle, the distance across the photograph is 6 inches

Figure 5. Backlit, 30,000 psi jet issuing from a 0.01 inch diameter nozzle, the distance across the photograph is 6 inches.

The benefit of the technique is perhaps more evident when nozzles at different pressures and diameters and different chemistry are compared. First consider the change with an increase in jet diameter. From the front-lit view there is little difference in the jets. From the backlit, it is clear that the smaller diameter jet only reaches 3-inches across the screen, while the larger jet barely reaches the end of the range.

The effect of doubling the orifice diameter at the same jet pressure on jet range, the photo length is 6 inches

Figure 6. The effect of doubling the orifice diameter at the same jet pressure on jet range, the photo length is 6 inches.

One of the parts of the study we were carrying out in 1974 was to examine the effect that adding different long-chain polymers had on jet structure. The ones that we were looking at include some that are now used in the oil and natural gas industry to make the “slick water” that is used in the fracking industry to improve production from shale reservoirs. But it also has an advantage in “binding” the jet together. And so, in the study, Dr. Jacques Zakin and I tested a wide range of different polymers to see which would give the best jet.

There were a number of different things we were looking for. In cutting paper, soft tissue and water sensitive material for example, the polymer can bind the water sufficiently well as to further lower wetting to the point where it doesn’t have an effect. It also can improve jet cutting under water – but I’ll cover those in a few post on polymer effects that will come to later in the series.

The effect of a polymer (in this case an AP273) is shown in two tests where the only change was to add the polymer to the water for the lower one.

Jets with an orifice diameter of 0.01 inches at a pressure of 20,000 psi

Figure 7. Jets with an orifice diameter of 0.01 inches at a pressure of 20,000 psi, the range is 6 inches, and the lower jet has had the polymer AP273 added to the water.

The narrower stream in the lower frame is the effect that we were looking for. Putting change in diameter and the better polymers together gave, as an example, the following:

The effect of changing jet pressure, nozzle diameter and polymer content on jet cohesion

Figure 8. The effect of changing jet pressure, nozzle diameter and polymer content on jet cohesion.

It might be noted that the jet in the bottom frame has as much relative concentration (and power) at the end of the range as the top jet had at the beginning of the range.

Now it all depends on what you want the jet to do, as to which condition you wish to achieve. Inside abrasive mixing chambers, the object is much different than it is when the object is to cut a foot or more of foam with high quality edges. And there have been some interesting developments with different polymers over the years, but I’ll save those stories for another day.

But bear in mind that those individuals who could slide their fingers under the jet in the top frame of figure 8 would have had them all cut off if the jet had been running instead under the conditions of the bottom two frames, and in all three cases, to the naked eye the jets looked the same.

Waterjetting Technology – High-pressure pump flow and pressure

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

When I first began experimenting with a waterjet system back in 1965, I used a pump that could barely produce 10,000 psi. This limited the range of materials that we could cut (this was before the days when abrasive particles were added to the jet stream) and so it was with some anticipation that we received a new pump, after my move to Missouri in 1968. The new, 60-hp pump came with a high-pressure end that delivered 3.3 gpm at 30,000 psi. which meant that a 0.027 inch diameter orifice in the nozzle was needed to achieve full operating pressure.

However I could also obtain (and this is now a feature of a number of pumps from different suppliers) a second high-pressure end for the pump. By unbolting the first, and attaching the second, I could alter the plunger and cylinder diameters so that, for the same drive and motor rpm, the pump would now deliver some 10,000 psi at a flow rate of 10 gpm. This flow, at the lower pressure, could be used to feed four nozzles, each with a 0.029 inch diameter.

Delivery options from the same drive train with two different high-pressure ends

Figure 1. Delivery options from the same drive train with two different high-pressure ends.

The pressure range that this provided covers much of the range that was then available for high-pressure pumping units using the conventional multi-piston connection through a crankshaft to a single drive motor. Above that pressure, it was necessary to use an intensifier system, which I will cover in later posts.

However there were a couple of snags in using this system to explore the cutting capabilities of waterjet streams in a variety of targets. The first of these was when the larger flow system was attached to the unit. In order to compare “apples with apples” at different pressures, some of the tests were carried out with the same nozzle orifice. But the pump drive motor was a fixed speed unit which produced the same 10 gpm volume flow out of the delivery manifold regardless of delivery pressure (within the design limits). Because the single small nozzle would only handle a quarter of this flow at that pressure, the rest of the water leaving the manifold needed an alternate path.

Positive displacement pump with a bypass circuit

Figure 2. Positive displacement pump with a bypass circuit.

This was provided through a bypass circuit (Figure 2) so that, as the water left the high-pressure manifold, it passed through a “T” connection with the perpendicular channel to the main flow carrying the water back to the original water tank. A flow control valve on this secondary circuit would control the orifice size the water had to pass through to get back to the water tank, thereby adjusting the flow down the main line to the nozzle, and concurrently controlling the pressure at which the water was driven.

Thus, when a small nozzle was attached to the cutting lance, most of the flow would pass through the bypass channel. While this “works” when the pump is being used as a research tool, it is a very inefficient way of operating the pump. Bear in mind that the pump is being run at full pressure and flow delivery, but only 25% of the flow is being sent to the cutting system. This means that you are wasting 75% of the power of the system. There are a couple of other disadvantages that I will discuss later in more detail, but the first is that the passage through the valve will heat the water a little. Keep recirculating the water over time and the overall temperature will rise to levels that can be of concern (it melted a couple of fittings on one occasion). The other is that if you are using a chemical treatment in the water, then the recirculation can quite rapidly affect the results, usually negatively.

It would be better if the power of the pump were fully used in delivering the water flow rate required for the cutting conditions under which the pump was being used. With a fixed size of pistons and cylinders, this can be achieved – to an extent – by changing the rotation speed of the drive shaft. This can, in turn, be controlled through use of a suitable gearbox between the drive motor and the main shaft of the pump. As the speed of the motor increases, so the flow rate also rises. For a fixed nozzle size this means that the pressure will also rise. And the circuit must therefore contain a safety valve (or two) that will open at a designated pressure to stop the forces on the pump components from rising too high.

Output flows from a triplex (3-piston) pump in gpm

Figure 3. Output flows from a triplex (3-piston) pump in gpm, for varying piston size and pump rotation speed. Note that the maximum operating pressure declines as flow increases, to maintain a safe operating force on the crankshaft.

The most efficient way of removing different target materials varies with the nature of that material. But it should not be a surprise that neither a flow rate of 10 gpm at 10,000 psi, nor a flow rate of 3.3 gpm at 30,000 psi gave the most efficient cutting for most of the rock that we cut in those early experiments.

To illustrate this with a simple example: consider the case where the pump was used configured to produce 3.3 gpm at pressures up to 30,000 psi. At a nozzle diameter of 0.025 inches the pump registered a pressure of 30,000 psi for full flow through the nozzle. At a nozzle diameter of 0.03 inches the pump registered a pressure of 20,000 psi at full flow, and at a nozzle diameter of 0.04 inches the pressure of the pump was 8,000 psi. (The numbers don’t quite match the table because of water compression above 15,000 psi). Each of these jets was then used to cut a slot across a block of rock, cutting at the same traverse speed (the relative speed of the nozzle over the surface) and at the same distance between the nozzle and the rock. The depth of the cut was then averaged over the cut length.

Depth of cut into sandstone

Figure 4. Depth of cut into sandstone, as a function of nozzle diameter and jet pressure.

If the success of the jet cut is measured by the depth of the cut achieved, then the plot shows that the optimal cutting condition would likely be achieved with a nozzle diameter of around 0.032 inches, with a jet pressure of around 15,000 psi.

This cut is not made at the highest jet pressure achievable, nor is it at the largest diameter of the flow tested. Rather it is at some point in between, and it is this understanding and the ability to manipulate the pressures and flow rates of the waterjets produced from the pump that makes it more practical to optimize pump performance through the proper selection of gearing than it was when I got that early pump.

This does not hold true just for using a plain waterjet to cut into rock, but it has ramifications in other ways of using both plain and abrasive-laden waterjets, and so we will return to the topic as this series continues.