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.

Waterjetting – Pressure Washers and Industrial Cleaning

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

It is sometimes easy in these days, when one can go down to the local hardware store and buy a Pressure Washer that will deliver flow rates of a few gallons a minute (gpm) at pressures up to 5,000 psi, to forget how recently that change came about.

One learns early in the day that the largest volume market for pressurized water systems lies in their use as a domestic/commercial cleaning tool. But even that development has happened within my professional lifetime. It is true that one can go back to the mid-1920’s and find pictures of pressure washers being used for cleaning cars, and not only did Glark Gable pressure-paint his fences, but I have seen an old film of him pressure washing his house in the 1930’s.

Pressure washing a car in 1928

Figure 1. Pressure washing a car in 1928 (courtesy FMC and Industrial Cleaning Technology by Harrington).

Yet it was not a common tool as the first automated car wash dates from 1947. Today, the average unit will service around 71,000 cars a year, and there are about 22,000 units in the country.

When I first went to the Liquid Waste Haulers show in Nashville (now the Pumper and Cleaner Environmental Expo International) the dominant method for cleaning sewer lines was with a spinning chain or serrated saw blade of the Roto-Rooter type. Over the past two decades, this has been supplanted by the growth of an increasing number of pressurized washer systems than can be sent down domestic and commercial sewer lines to clean out blockages and restore flow. As in a number of other applications, the pressure of the jet system can be adjusted so that the water can cut through the obstruction without doing damage to the enclosing pipe. The technology has even acquired its own term: We speak of “Moleing” a line. And, for those interested, there are a variety of videos that can now be viewed on Youtube showing some of the techniques (see for example this video). Unfortunately, the fact that a tool is widely available and simply to assemble does neither mean that it is immediately obvious how best to use it, nor that it is safe to do so, and I will comment on some sensible precautions to take when I deal with the use of cleaning systems later in this series.

For now, however, I would like to just discuss the use of pressure washers from the aspect that they are the lower end of the range in which the pressure of the water is artificially raised to some level in order to do constructive work. At this level of pressure, it is quite common to hook the base pump up to the water system at the house or plant. Flow rates are relatively low and can be met from a tap. The pressure of the water in the line is enough to keep water flowing without problems into the low-pressure side of the pump, although this can be a problem at higher pressures and flows as will be discussed in the article on the use of 10,000 psi systems.

The typical pressure washer that is used for domestic cleaning will operate at flow rates of around 2-5 gpm and at pressures up to 5,000 psi. Below 2,000 psi, the units are often driven by electric motors, while above that, the pumps are driven by small gasoline engines. In both cases, the engine will normally rotate at a constant speed. With the typical unit having three pistons, the pump will deliver a relatively constant volume of water into the delivery hose.

Today, pressure washing has evolved into commercial applications used for surface preparation, road stripe removal and industrial water blasting for many industries including automotive, aviation, marine, cement plants and many more pressure washing applications.

There are two ways of controlling the pressure that the pump produces. Because the flow into the high-pressure size of the pump is constant, the pressure is generally controlled by the size of the orifice through which the water must then flow. These nozzle sizes are typically set by the manufacturer, with the customer buying a suite of nozzles that are designed to produce jets of different shape and occasionally pressure.

An alternative way of controlling pressure is to add a small by-pass circuit to the delivery hose, so that, by opening and closing a valve in that line, the amount of water that flows to the delivery nozzle will be controlled and with that flow so also will the delivery pressure.

Because the three pistons that typically drive water from the low-pressure side of the pump to the high pressure side are attached at 120 degree increments around the crankshaft and because the pistons must each compress the water at the beginning of the stroke and bring it up to delivery pressure before the valve opens, there is a little fluctuation in the pressure that is delivered by the pump.

In a later article, I will write about some of the advantages of having a pulsating waterjet delivery system (as well as some of the disadvantages if you do it wrong – I seem to remember a piston being driven through the end of a pump cylinder in less than five-minutes of operation in one of the early trials of one such system). In some applications that pulsation can be an advantage, particularly in cleaning, but in others it can reduce the quality of the final product. With less expensive systems however, it is normally not possible to eradicate this pulsation.

The Cleaning Equipment Manufacturer’s Association (CEMA – now the Cleaning Equipment Trade Association) funded the Underwriters Laboratory to write a standard for the industry (UL 1776) almost 20-years ago. That standard is now being re-written to conform to international standards that are being developed for this industry. There are also standards for the quality of surfaces after they have been cleaned, but these largely deal with cleaning operations at higher pressures and so will form a topic for future posts, when discussing cleaning at pressures above 10,000 psi.

Sadly, although pressure washers are now found almost everywhere, very few folk fully understand enough about how a waterjet works to make their use most effective. Because most operators use a fan jet to cover the surfaces that they are cleaning, the pressure loss moving away from the nozzle can be very rapid. A simple test I run with most of my student classes is to have them direct the jet at a piece of mildewed concrete. Despite the fact that I have shown them in class that a typical cleaning nozzle produces a jet that is only effective for about four inches, most students start by holding the nozzle about a foot from the surface. All it is doing is getting the surface wet and promising a slow, ineffective cleaning operation.

No matter how efficient the pump, if the water is not delivered effectively through the delivery system and nozzle, then the investment is not being properly utilized. It is a topic I will return to on more than one occasion.

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.