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 High Pressure Pumps – Pump pulsations

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 pumps generally draw water into a cylindrical cavity and then expel it with a reciprocating piston. There are a number of different ways in which the piston can be driven. It can be connected eccentrically to a rotating shaft, so that, as the shaft rotates, the piston is pushed in and out. The pistons can be moved by the rotation of an inclined plate, so that, as the plate rotates, so the pistons are displaced.

Basic Components of a Swash Plate Pump

Figure 1. Basic Components of a Swash Plate Pump (after Sugino et al, 9th International Waterjet Symposium, Sendai, Japan, 1988)

And, more commonly at higher pressures, the piston can be of a dual size, so that, as a lower pressure fluid on one side of the piston pushes forward, so a higher pressure fluid on the smaller end of the piston is driven into the outlet manifold, and out of the pump. This latter pump design has become commonly known as an Intensifier Pump. The simplified basis for its operation might be shown using the line drawing that was used earlier.

Simplified Sketch showing the operation of an intensifier

Figure 2. Simplified Sketch showing the operation of an intensifier

When the intensifier is built, the simplified beauty of its construction is more evident.

Partially sectioned 90,000 psi intensifier showing the components and the small end of the reciprocating piston

Figure 3. Partially sectioned 90,000 psi intensifier showing the components and the small end of the reciprocating piston (Courtesy of KMT Waterjet Systems)

However, what I would like to discuss today is what happens when the pistons in these cylinders reaches the ends of their stroke, and it is a little easier to use an Intensifier as a starting point for this discussion, although (as I will show) it also relates to the other designs of high-pressure pumps that also use pistons.

Consider if there was only one side to the piston, rather than it producing high pressure in both directions. This design is known as a single acting Intensifier, and it might, schematically, look like this:

Simplified schematic of a single-acting Intensifier

Figure 4. Simplified schematic of a single-acting Intensifier

As the piston starts to move from the right-hand side of the cylinder toward the left, driven by the pressure on the large side of the piston, it displaces water from the smaller diameter cylinder on the left. Assume that the area ratio is 20:1 and that the low-pressure fluid is entering at 5,000 psi, then, simplistically, the fluid in the high pressure pump chamber will be discharged at 100,000 psi. But not immediately!

The outlet valve has been set, so that it will not open until the fluid has reached the required discharge pressure, and this will require a small initial movement of the piston (perhaps around 12%) to compress the water and raise it to that pressure before the valve opens. And, with a single intensifier piston, when the piston has moved all the way to the left and the high pressure end is emptied of water, then there will be no more flow from that cylinder, until the piston has been pushed back to the far end of the cylinder, and the process is ready to start again.

Some of that problem of continuous flow is overcome when the single-acting intensifier is made dual-acting, because at the end of the stroke to the left, fluid has entered the chamber on the right, and when the piston starts its return journey the cylinder on the right will discharge high pressure fluid. But again not immediately!

One way of overcoming this is to use two single-acting pistons, but with a drive that is timed (phased) so that the second piston starts to move just before the first piston reaches the end of its stroke. This takes out the dead time during the directional change. The two can be compared:

Difference in the pulsation between a phased set of single acting intensifiers, and a double-acting unit

Figure 5. Difference in the pulsation between a phased set of single acting intensifiers, and a double-acting unit. (Singh et al 11th International Waterjet Conference, 1992)

In cutting operations, reducing the pulsation from the jet is often important in minimizing variations in cut quality and thus, to dampen the pulsations with a dual-acting system, a different approach is taken and a small accumulator is put into the delivery line, so that the fluid in that volume can help maintain the pressure during the time of transition.

Effect of Accumulator volume on pressure variations

Figure 6. Effect of Accumulator volume on pressure variations (Chalmers 7th American Waterjet Conference, Seattle 1993)

A simplified schematic can again be used to show where an accumulator might be placed.

Location of the Accumulator in the waterjet intensifier line

Figure 7. Location of the Accumulator in the waterjet intensifier line

On the other hand, in cleaning applications particularly with water and no abrasive, there are occasions (which I will get to later) where a pulsation might improve the operation of the system. A three piston pump, without an accumulator will see a variation in the pressure output that may see an instantaneous drop to 12% below average and then a rise to 6% above average during a cycle. One way of reducing this is to increase the number of pistons that are being driven in the pump.

When one changes, for example, from the three pistons (triplex) to five pistons (quintupled), then the variation in outlet pressure is significantly less.

The effect of changing number of pump pistons on the variation in delivery pressure

Figure 8. The effect of changing number of pump pistons on the variation in delivery pressure. (De Santis 3rd American Waterjet Conference, Pittsburgh, 1985)

Part of the reason that longer steadier pulses of water, which come from the slower stroke of the intensifier, can be of advantage is that the water is a jet and comes out of the nozzle at a speed that is controlled by the driving pressure. A strong change in pressure means that there is a change in the velocity of the water stream along the jet. This means that slower sections of the jet are, at greater standoff distances, caught up with by the following faster slugs of the jet. This makes the jet more unstable. That can, however, be an advantage in some cases, and this will be discussed at some later time, when a better foundation has been established to explain what the effects are.

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.

Waterjet Technology – Pumps, Intensifiers and Cannons

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 we say a rock is hard it means something different, in terms of strength, to the meaning when we say that we want an egg hard boiled. Terms have to be – and usually are – defined through the way in which they are used. At the same time, each trade, industry or profession has certain terms that it adopts for its own with more specialized meanings than those which we, in the general public, are familiar.

Ask someone on the street what level of pressure they consider to be “high” and they might answer with numbers that range from 100 psi to perhaps 2-3,000 psi. And yet, within the industry those pressures are really quite low, relative to those most commonly used in cleaning and cutting. High-pressure waterjet systems are now available for water jet cutting metals, that will generate streams that run continuously at 90,000 psi, and the highest pressure jet that we generated in the MS&T Laboratories was at around 10 million psi.

Within that very broad range some simple divisions make it easier to group the ranges and applications of the different tools that are now common within different parts of the industry. At the same time, over the period of my professional life, the technology has moved forward a long way. Consider that when I wanted to run at test at 50,000 psi back around 1970, I had to use this particular set of equipment.

MS&T Water Cannon firing 12 gallons of water at 50,000psi

Figure 1. MS&T Water Cannon firing 12 gallons of water at 50,000 psi

The water cannon was made by cutting the end from a 90-mm howitzer and threading a one-inch nozzle on the end. Smaller orifices could then be attached beyond that to give different flow combinations. The pressure to drive the cannon was generated by putting 2,000 gm. of smokeless powder in a cartridge, and then loading and firing the cannon. We had been given the mount, which rotates around two axes by the then McDonnell Douglas (now Boeing), who had used it to hold and move the Gemini spacecraft while they were being inspected.

The pressure divisions which were debated and agreed by the Waterjet Technology Association back in the mid 1980’s broke the pressure range into three separate segments, which described the industry at the time.

The first range is that of the Pressure Washers. Operating pressures lie at and below 5,000 psi.

A small pressure washer being used to clean a drain

Figure 2. A small pressure washer being used to clean a drain (Mustang Water Jetters)

These are the types of unit which are often found in hardware stores for use in homes, and while I won’t get into this until some later posts on safety and on medical applications, it should be born in mind that it is possible to do serious injury even at these relatively low pressures. (Folk have been known to use the jets to clean off their shoes after work … need I say more – a waterjet cuts through skin at around 2,000 psi, and skin is tougher than the flesh underneath). At pressures below 2,000 psi, these are often electrically powered. A gasoline motor is often used to drive the portable units that operate above that pressure range.

High-pressure units are defined as those that operate in the pressure range from 5,000 psi to 35,000 psi. The drive motors are usually either electrical or use a diesel drive, and units of over 250 horsepower are now available. Many of these units are known as positive displacement pumps. That is taken to mean that the pump, being driven by a motor at a constant speed, will put out the same volume of water, regardless of the pressure that it is delivered at (up to the strength of the drive shaft).

To ensure that the pressure does not rise above the normal operating pressure, several safety devices are usually built into the flow circuit so that, should a nozzle block, for example, a safety valve would open allowing the flow to escape. Different flow volumes can be produced in larger units by placing a gear box between the pump and the motor. As the motor speed changes, for the same piston size in the pump, so the volume output changes also. However, because the pump can only deliver at a certain power the size of the pistons can also be changed so that, at higher delivery pressures, the same motor will produce a lower volume of water. I’ll go into that in a little more detail in a later piece.

Section through a high-pressure pump

Figure 3. Section through a high-pressure pump showing how the crankshaft drives the piston back and forth in the cylinder block, alternately drawing low pressure (LP) water in, and then discharging high pressure (HP) water out

Normally, there are a number of pistons connected at different points around the crankshaft so that, as it rotates, the pistons are at different points in their stroke. This evens the load on the crankshaft, and produces a relatively steady flow of water from the outlet. (Which, in itself, is a topic for further discussion).

As the need for higher pressures arose, the first pumps in the ultra-high pressure range (that above 35,000 psi) were intensifier pumps. These pumps are designed on the basic principal that the force exerted on a piston is equal to the pressure of the fluid multiplied by the area over which it is applied. Thus, a piston that is designed with two different diameters can produce pressures much higher than those supplied.

The basic elements of a waterjet intensifier

Figure 4. The basic elements of a waterjet intensifier

Fluid at a pressure of perhaps 5,000 psi is pumped into chamber C. As it flows in, the piston is pushed over to the left, drawing water into chamber B. At the same time the water in chamber D is being pushed out of the outlet channel, but because of the area ratio, the delivery pressure is much higher. If, for example, the ratio of the two areas is 12:1, then the pressure of the water leaving the pump will be at 12 x 5,000 = 60,000 psi.

Over the years, the materials that pumps are made from and the designs of the pumps themselves have changed considerably, so that pressure ranges are no longer as meaningful as they were some 25-years ago when we first set these definitions, but they continue to provide some guidance to the different sorts of equipment, and the range of uses of the tools within those divisions, so I will use these different pressure range definitions in the posts that follow.