Sunday, November 18, 2012

Waterjettting 3c - Pressure Washers

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.


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.

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.

71 comments:

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