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 – Water Quality

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 problems with taking a research team into the field is that you have to be able to provide answers and a path forward when things go wrong. So it was on a project we once had in Indiana, and it took about a year for me to live down the tale. We had set up a 350-hp high-pressure triplex for a project that involved washing explosives out of shells. Everything had been set up and was ready to go, and so we switched on the water to the pump, started the diesel engine and almost immediately noticed that we weren’t getting enough water downstream of the pump. What was the problem? We checked all the valves, couplings and hoses, and they all seemed to be OK. It was, however, a bitterly cold day with a howling wind around where we had the pump unit. And so I came up with the idea that it was the wind, chilling the pistons, which operated with their length exposed during part of the stroke. If the wind chill was cooling the pistons, then perhaps they weren’t displacing enough volume because they had shrunk. It became known as “The Wind Chill Factor” explanation and, as those of you who have done this sort of thing realize, it was bunkum! After a while, one of the team wandered back to the filter unit, pulled out the partially plugged filters, changed them to new ones and we were in business.

There are a couple of reasons that I tell this bit of history and they relate both to the quality, and the quantity of water that is being supplied at a site. I remember talking to Wally Walstad, who ran McCartney Manufacturing before it became KMT Waterjet Systems, about their second commercial installation and how the different water chemistry just a few hundred miles away had caused maintenance issues on the pumps that they had not expected.

Parts for a multi-piston high pressure pump

Figure 1. Parts for a multi-piston high pressure pump

It may seem obvious that a pump should be supplied with enough water so that it can work effectively. But the requirement, as one moves to higher-pressure pumps, becomes a little more rigorous than that. Consider that the water supplied must enter the piston and fill it completely during the time that the piston is pulling back within the cylinder. Because the piston is pulling back if the water flow into the cylinder is not moving in enough, then the piston will pull on the water. Water does not have any tensile strength, and so small bubbles of vacuum will form. When the piston then starts back to push the water out of the cylinder these bubbles, which are known as Cavitation, will collapse. In a later post I will tell you how to use cavitation to improve material removal rates. But the last place that you want it is in the high-pressure cylinder, since the bubble collapse causes very tiny high (around 1 million psi) micro-jets to form that will very rapidly eat out the cylinder walls, or chew up the end of the piston. (Happened to us once).

There is a Youtube video which shows the cavitation clouds forming in a pump (the white blotches) as the flow to the pump falls below that needed.

To avoid that happening there is a term called Net Positive Suction Head, NPSH. I am not going to go into the details of the calculations, though they are given in the citation. In most cases it is not necessary to make them (unless you are designing the pump). Where the unit being operated is a pressure washer, then the pressure that drives the water out of the tap and into the hose is usually sufficient to overcome any problems with the inlet pressure.

When flow rates run above 5 gpm, however, or when there is a relatively narrow fluid passage into the pump cylinders, or where the water reservoir is below the pump, then the normal system pressure may not be enough. There are two values for the NPSH which are critical – the NPSH-Required (NPSHR) and the NPSH-Available (NPSHA). Let me give a simple example of where one could get into trouble.

For example consider the change which occurs when a pump, normally rated at 400 rpm is driven at 500 rpm, for a 25% increase in output. At 400 rpm the NPSHR for a triplex pump supplied through a 1.25-inch diameter pipe from an open tank will be 8 psi. At 500 rpm, as the flow increases from 26.4 gpm to 33 gpm, the NPSHR rises to 9 psi, which is only a 12.5% change.

However, under the same conditions the NPSHA, which begins at 11.5 psi with a 26.4 gpm demand, falls to 7.8 psi at 33 gpm. When the required suction head is set against that available there was an initial surplus of 45% over that needed. But this changes to a shortfall of 12% when the pump is run at the higher speed. The pump will cavitate, inadequate flow will reach the nozzle to provide full pump performance and the equipment lifetime will be markedly reduced.

This supply pressure required should thus be checked with the manufacturer of the pump. In most cases where we have run pumps at 10,000 psi and higher, we have fed the water into the pump at the designated flow rate, but using a supply pump that ensures that the pressure on the inlet side of the pump valves is at least 60 psi.

One of the problems, as mentioned at the top of the piece, is that when going to a new site the immediate quality of the water is not known. There are two things that need to be done. The first of these, of particular importance at higher pressures, is to check the water chemistry. It is important to do this before going to the site since it usually takes some time to get the results, and if there are some chemicals in the water that may react with pump or system parts, it is good to know this ahead of time so that the threatened parts can be changed to something that won’t be damaged.

There is a specific problem that comes with cutting systems in this regard, since at 50,000 psi or higher, water quality becomes more important, even just in the nozzle passages. And I will deal with this in a few weeks when I talk about different nozzle designs.

And equally important is the cleanliness of the water. Particularly when tapping into a water line that hasn’t been used for a while (as we did), there is a certain amount of debris that can be carried down the line when it is first used. The smart thing to do is to run water through the line for a while to make sure that any of the debris is flushed out, before the system is connected up. The second is to ensure that there is more than one filter in the line between that supply and the pump.

Many years ago, when prices were much lower than they are today, Paddy Swan looked at the costs of increasingly dirty water on part costs. The costs are in dollars per hour for standard parts in a 10,000 psi system and the graph is from the 2nd Waterjet Conference held in Rolla in 1983.

costs for parts when increasingly dirty water is run through a pump

Figure 2. 1982 costs for parts when increasingly dirty water is run through a pump (S.P.D. Swan “Economic considerations in Water Jet Cleaning,” 2nd US Water Jet Conference, Rolla, MO 1983, pp 433 – 439.)

Oh, and the moral of the opening story became one of our sayings in the Center, not that we were original, William of Ockham first came up with it about seven hundred years ago. It’s known as Ockham’s Razor, and simply put it means that the simplest answer is most likely the right one, or don’t make things more complicated than they need be!

Waterjet Pumps – Pump Pressure is not Cutting 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 began this series, I pointed out that whenever a waterjet is going to be used both the target material and the waterjet delivery system have to be considered if the work is to be done well.

In the last four posts I have tried to emphasize the role of cracks and flaws in the way in which water penetrates into and removes material. It is easier to see this with large-scale operations, such as in the removal of large volumes of soil, but it equally holds true in the abrasive cutting of glass. Now in this next series of KMT Waterjet Blogs, the focus is going to swing back to the ways in which high-pressure waterjets are developed, particularly in the different choices of equipment that can be used.

Because this KMT Waterjet Blog Series is meant to help folk understand how systems work and through that how to improve production and quality, it will tend to shy away from putting a lot of formulae into the presentations. There is a reason that I, an academic, don’t like having students learn equations by rote. It is that it becomes quite possible to misremember them. If you are used to looking them up (particularly true in today’s computer world where formulae can easily be used to generate tables) then you are less likely to mis-remember the exact relationships and to make a possible critical mistake.

But, as I showed here, when the tables of jet flow, horsepower and thrust were generated, there are a few critical equations that need to be born in mind. And the one that underlies the economics of many operations is tied up in the size of the power that is available to do the work. The basic power equation itself is relatively straightforward:

Relationship between hydraulic horsepower, pressure and flow

Figure 1. Relationship between hydraulic horsepower, pressure and flow.

But the calculation gives different values, depending on where the calculation is made in a circuit. To demonstrate this, let us use a very simple drawing of a flow circuit.

Components of a simple flow circuit

Figure 2. The components of a simple flow circuit. Water is drawn from the water tank, through the pump and delivered down a hose to a high-pressure lance, where the water is fed, through a nozzle and aimed at the target, where it does the work.

In the course of this small set of posts the different components that make up this circuit are going to be discussed in turn. But at the end of the first set, I mentioned that in an early comparison of the relative cleaning performance of 10,000 psi waterjets of nominally equal power, and flow (10 gpm IIRC) there was a dramatic difference in the cleaning efficiency, as the Navy reported at the time.

Relative cleaning efficiency in areal percentage cleaned of five competing systems in cleaning heat exchanger tubes in Navy boilers

Figure 3. Relative cleaning efficiency in areal percentage cleaned of five competing systems in cleaning heat exchanger tubes in Navy boilers. (Tursi, T.P. Jr., & Deleece, R.J. Jr, (1975) Development of Very High Pressure Waterjet for Cleaning Naval Boiler Tubes, Naval Ship Engineering Center, Philadelphia Division, Philadelphia, PA., 1975, pp. 18.)

Why such a difference? Consider how the power changes from the time that it first enters the pump motor, and then is converted into power along the line to the target. The numbers that I am going to use may seem extreme, but they actually mirror an early experimental set-up in our laboratory, before we learned better.

A water flow of 10 gallons a minute (gpm) at a pressure of 10,000 pounds per square inch (psi) pressure will contain – using the above equation;

10,000 x 10/1714 = 58.34 horsepower (hp)

But that is the power in the water. Pumps are not 100% efficient, and so there has to be some additional power put into the pump to allow for the relative efficiency of the pump itself. For the sake of illustration let us say that the pump converts the energy at 90% efficiency. Thus the power that is supplied to the drive shaft of the pump will need to be:

58.34/0.9 = 64.8 hp

But that is still not the power that we have to supply, since that power – usually – comes from an electric power cord that feeds into a motor, which then, in turn, drives the pump shaft. That motor itself is also not 100% efficient. Let us, for the sake of discussion, say that it is 92.6% efficient. Then the electrical power supplied will be:

64.8/0.926 = 70 hp

Now, as the calculation progresses, remember that this is the power that is being paid for. And so, in the first part of the flow, the power is transformed from electric power to water power, but at the pump.

The change in power from that input to the motor to that coming out of the pump

Figure 4. The change in power from that input to the motor to that coming out of the pump.

The water coming out of the pump then flows through either a length of pipe, or high-pressure tubing until it comes to the tool that holds the nozzle. There are a number of different factors that change the flow conditions to the point that it leaves the nozzle. The most critical, and often overlooked, is the size of the hose/tubing that carries the water. Particularly as pumps get larger and more powerful and the flow rates increase, it is important to ensure that the passage for the water is large enough so that it does not require too much pressure to overcome the friction acting against that flow. I have, myself, put an additional 10-ft length of tubing on a drilling lance, and seen the cutting pressure coming out the end fall from that which drilled a rock at 12 ft/minute to where it could not drill at all. (The pressure drop was around 200 psi per foot). I mentioned in that earlier post that a competitor, running at a pump pressure of 45,000 psi was losing 35,000 psi of that pressure just to overcome friction in pushing the water down through a tube that was too narrow. As a result the water coming out of the nozzle had barely enough pressure (10,000 psi) to cut into the rock.

At the same time, very few people pay a lot of attention to how their nozzle fits on the end of the feed line, or how well it is made. Think of this – you have just spent $200,000 on a system, and yet, because the nozzle is a disposable part, you look around for the cheapest source you can get. You don’t size it for a good fluid fit, nor do you check how well it is machined. And yet the entire performance of your system is controlled by that small item. The difference between a very good nozzle and a standard nozzle can give as much as a factor of 10 improvement on performance – but who checks. The one you use saved you $15 relative to what you would have paid if you had bought the competing product, what a bargain – right?

There are different ways in which pumps operate and produce the high-pressure flow. With a fixed size of orifice in the nozzle and with a given pressure drop along the feed line, the pressure at the nozzle will be correspondingly reduced. So that if, for example, we use a 0.063 inch diameter nozzle then the chart you developed after generating the table will show that this will carry a flow of 9.84 gpm at 10,000 psi. But let us suppose that the hose loses 20 psi per foot of length, and that the hose is 200 ft long, then the pressure drop along the hose will be 20 x 200 = 4,000 psi.

Thus the pressure of the water coming out of the hose will be only 6,000 psi. And at an orifice of 0.063 inches, the flow through the orifice will now only be 7.62 gpm. (The way in which the pressure is controlled is assumed to be through bypassing extra flow back to the reservoir through a bleed-off circuit).

Now the pump is still putting out 10 gpm at 10,000 psi, but now the flow out of the nozzle is only 7.62 gpm at 6,000 psi. The power in this jet is (7.62 x 6,000/1714) only 26.7 hp. This is only 38% of the energy going into the pump.

The power losses to the nozzle

Figure 5. The power losses to the nozzle.

Unfortunately this is not the end of the losses. Particularly in cleaning operations there is a tendency for the operator to hold the nozzle at a comfortable distance from the target, so that the effect can be seen. But, as I will show in later posts, the jet pressure can fall rapidly as stand-off distance increases, particularly with a poor nozzle. A good range for a normal nozzle in a cleaning operation is about 125 nozzle diameters. So that at a diameter of 0.063 inches this range is less than 8 inches. Many people hold the nozzle at least a foot from the target.

If the nozzle is held about that far from the target the pressure will have fallen by perhaps 65%. The water thus reaches the target at around 2,000 psi. The flow rate is 7.62 gpm, and the actual horsepower of the water doing the work is 8.89 hp. This is 12.7% of the power that is being paid for through the meter. And the unfortunate problem is that no-one can tell, just by looking at the jet, what the pressure and flow rates are. So that often these losses go undetected, and folk merely complain about how the target material is more resistant today, not recognizing that they are throwing away 87% of the power that they are paying for.

Power losses from the power cord to the target

Figure 6. Power losses from the power cord to the target.

One of the objects of this series is to help reduce these losses, by avoiding those mistakes that those of us who started in the industry some 40-odd years ago made all the time.