Sunday, December 2, 2012

Waterjetting 3e - 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 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, 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, 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.


Figure 1. Generic 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.


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!

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