Waterjetting Technology-Water Quality Part One

David Summers WW

Waterjetting – Water Quality part one.

One of the problems with taking a research team into the field is that you have to be able to provide answer’s, 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, and 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

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 move’s 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.

Cost of dirty water is run through a pump.

Cost of 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!

 

Waterjetting Technology – High-pressure pump flow and pressure

David Summers WW

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 gpm 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.

HP Pump delivery options with two ends 1

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 (see table from Waterjetting 1c) the rest of the water leaving the manifold needed an alternate path.

Positive displacement pump 2

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 3 piston pump-3

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.

Figure 4. Depth of cut into sandstone, as a function of nozzle diameter and jet pressure.

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.

Posted by KMT Waterjet Systems 2013 all rights reserved.

KMT Waterjet Waterjetting-Pressure Washers and Industrial Cleaning

Dr. Summers Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Series

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.

Water 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. The first automated car wash dates from 1947, while the average unit today 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, that 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 just because a tool is widely available, and simple to assemble, does not 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.

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