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.

Waterjet Technology – High Pressure Line losses

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

High-pressure water jet pumps are, as a general rule, quite efficient at bringing water up to the pressure required for a given task. And yet, time after time, the jet that reaches the target is no longer capable of achieving the work that was promised when the system was designed. More often than not this drop in performance can be traced to the way that the water travels through the delivery system, and out of the nozzle that forms the jet.

The water flows that are used in a broad range of operations are quite low. Ten gallons a minute (gpm) and flows below that volume are mainly used in cutting operations and higher-pressure cleaning. Further, there are few occasions where hand-held operations will use flows much above 20 gpm, because of the thrust levels involved. And low flow rates mean that there is little pressure loss between the pump and the nozzle, right? UM! Well not exactly.

The pressure losses due to overcoming friction in the feed lines (whether hose or tubing) from the pump to the nozzle can make a significant difference in the operation of the system, as I mentioned in one of the early posts of this series. In that post I pointed out that a well-known research team (not us) spent two weeks running a system with 45,000 psi water pressure going into a feed line, but with only around 10,000 psi being usefully available when the flow reached the far end. (And I will freely confess later in this piece to having made a similar mistake myself). So the question naturally arises as to how these losses can be avoided.

In a word – diameter! The smaller the diameter of the feed line through which the water must flow, then the higher the pressure that is required to drive the water through that line, regardless of the nozzle size at the delivery end. The diameter of concern is, further, the inner diameter of the hose or tubing, not the outer diameter (though the combination is important in ensuring that the line can contain the pressure that the water is carrying through the line).

There are concerns over the condition of the line, the fittings that join the different parts together and other factors that I will cover in the posts following this one, but this will deal just with the simple pressure drop that occurs along a tube at different flow volumes. There are formulae that can be used, but a reasonable estimate of the loss can be obtained either with the design tables that most manufacturers supply with their product or through a simple nomogram that I will place at the end of this piece.

To begin with, consider the basic equations that govern the pressure drop:

The equation relating pressure drop to flow volume and pipe diameter

Figure 1. The equation relating pressure drop to flow volume and pipe diameter

Note that in the above equation, the pressure drop is related to the fifth power of the diameter of the tube – such is the power that even a small change in flow channel diameter will have on the pressure drop in the line.

When flow begins through a channel, it is initially going to occur with the flow being laminar, in other words the water moves in layers. (There is an interesting video of this here and a video of one of the designs used, for example, to give the “solid” jet slugs that you might see jumping around the hedges at one of the Amusement Parks.)

The difference between laminar and turbulent flow

Figure 2. The difference between laminar and turbulent flow

As water speed increases, however, the flow will transition from laminar flow into turbulent flow, where the roughness of the flow channel wall becomes more important. The roughness, resulting friction factor and the flow volume all then combine to allow the calculation of the pressure required to overcome the friction in the pipe. This holds true whether the flow is at the one or two gpm used in cutting at high pressure, or the relatively low pressure, high volume flows used in fighting fires.

But (outside of us academics) few actually calculate the numbers. There really is no need, since most of the manufacturers provide the information in their catalogs. There are two ways of presenting the information. The older convention was just to provide a graph, from which one could read off the pressure drop, as a function of the pipe internal diameter, and for a given pipe length.

Pressure drop along a tube, as a function of flow rate and tube internal diameter

Figure 3. Pressure drop along a tube, as a function of flow rate and tube internal diameter. Note that the scales are logarithmic.

Charts such as this are a little difficult to read, and being on a log plot small mistakes in reading the value can give significantly wrong estimates so that a more spread-out method is often more helpful. The one that I prefer to use is a nomogram, where it is possible to do comparisons between different options on a single figure with a slightly expanded scale.

Consider, for example, this nomogram from the Parker Catalog which shows the relationship between the volume flowing down through a line, the inner diameter through which it is flowing, and the resulting velocity of the flow.

A nomogram to determine the best pipe diameter, based on the allowable velocity of the flow

Figure 4. A nomogram to determine the best pipe diameter, based on the allowable velocity of the flow. (Parker)

While this is not generally a concern in feed lines to nozzles (because of the high levels of filtration of the water) in lines that carry away spent water and debris the velocity can be of concern, and also in abrasive slurry systems, where flow rates above 40 ft/sec can lead to erosion of the line.

The more useful nomogram, however, is one that I have adapted from the U.S. Bureau of Mines (a Government agencies that is now, sadly, defunct).

Nomogram to calculate pressure loss along a 10-ft length of tubing

Figure 5. Nomogram to calculate pressure loss along a 10-ft length of tubing

Knowing the flow rate through the line, and setting a straight-edge (usually a ruler) to mark the level, the ruler is then positioned so that it also crosses the inner diameter of the tubing. In the example above that would align the ruler along the line shown, that runs from 20 gpm to 0.1875 inch pipe diameter (3/16ths of an inch). The point at which the line crosses the pressure drop gives the friction loss in the line. In this case that reads at 3,600 psi per 10 ft of pipe.

The example was taken from a field trial where we were drilling holes into the side of a rock pillar. We had no problem drilling the first ten feet, but when we added a second length of 10-ft tubing to allow us to drill holes 20-ft deep the drill did not work. It was not until late in the afternoon that we realized that by adding that second length of pipe we had dropped the cutting pressure coming out of the nozzle so that while the gage pressure was 10,000 psi, the initial jet pressure had been only 6,400 psi and when the second pipe length was added, the pressure fell to 2,800 psi. This was below the pressure at which it was possible to effectively cut the rock. And so we learned!