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.

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.