By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology
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.
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.
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.
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.
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.