Wednesday, October 2, 2013

Waterjetting 14a - an introduction to cavitation

Many chemical plants have heat exchangers built into the circuits through which their various chemical fluids flow. These are long tubes, within a surrounding shell, so that fluid at one temperature flows through the tubes within the unit (the tube side fluid), while fluid at another temperature flows in the shell around the tubes (the shell side fluid), passing heat between the two. It is an effective and economic way, for example, of taking the excess heat remaining in a fluid after processing and pre-heating the feed stock to that process before it reaches the reactor.

Figure 1. Illustration of a heat exchanger (Shell and Tube )

It is a similar construction to the tubes that carry water through a ship’s boiler, generating steam to drive the turbines of the vessel. The tubes and shell will, over time, build up deposits, since the fall in temperature will often cause material to precipitate out of the fluid, and cake the walls. With time the tubes clog, and without cleaning, the unit stops working. So how to remove the deposits? For many years workers would take hammers and long rods and drive them down through the tubes to break out the scale. The problem with this approach, apart from being very slow and messy, was that if the rod bent, then it could be driven through the wall of the tube, which meant that the tube could no longer be used and had to be sealed off, at both ends.

As I noted in a post written a year ago, Naval personnel were some of the first to discover that high pressure waterjets could be used to replace the hammer and rod. Not only was the water not capable of breaking through the tube, but it was much more effective in breaking up, and removing the scale. As a consequence jobs that might have taken 150 man-hours and $2,000 (in 1972) took 10 man-hours and cost $700.

The use of high-pressure water to clean heat exchangers is now ubiquitous, it is a much faster, and cheaper system, and though operating pressures have risen, the volume flow required to immediately carry all the debris out of the tubes has continued to be a requirement. But not all systems work equally well. Back in those early days the Navy held a competition between systems, each capable of delivering water at 10,000 psi at a flow rate of 20 gpm. When they ran competitive trials against similar tubes, they found a significant difference between the performance of five different companies bidding for the work.

Figure 2. Comparative cleaning performance (as a percentage of area cleaned) by competing waterjet systems on Navy boilers (Tursi, T.P. Jr., & Deleece, R.J. Jr, (1975) Development of Very High Pressure Waterjet for Cleaning Naval Boiler Tubes, Naval Ship Engineering Center, Philadelphia Division, Philadelphia, PA., 1975, pp. 18.)

Now there are quite a number of different nozzle designs that have been used by the different companies, and, more recently, rotating nozzle assemblies have also been used to improve efficiency, but back then nozzles were quite simple. So how could one company do that much better than the others?

I was not there for the demonstration, but in talking to operators in that time frame I learned that some of the more skilled folk (and this is back in the day when most of this was done with hand-held equipment, rather than today’s automated systems) would put their (gloved) hand over the edge of feed line, partially blocking the flow of water out of the tube. This filled the tube with water, and the cleaning jets worked better! Why this is, well let me start to explain the mysteries, dangers and potential benefits from what we call Cavitation, since that is what causes the change.

Cavitation occurs when, through one process or another, water is stretched. Because water isn’t very strong in tension, too much of a pull will cause it to form little bubbles of sensibly vacuum within the flow (cavities), as the water is ripped apart. These cavities in the flow aren’t stable and rapidly collapse as soon as the surrounding fluid reaches any significant pressure. The bubbles themselves are individually tiny, but their destructive power can be quite dramatic, as I will explain in a later post.

It is a concern when dealing with high-pressure water systems, because water has to flow relatively rapidly into the high-pressure cylinders of a pump, as the piston pulls back at the beginning of a cycle. If the inlet valves are too small, or the supply pressure is too low, then the water cannot enter the cylinder fast enough to fill the void left as the piston is drawn back. This causes small cavities to form in the water inside the cylinder. One the piston reverses, and the fluid inside the piston is pressured on the pressure stroke, the cavities collapse.

Figure 3. High-pressure pump schematic

No problem, you might think, since one is merely refilling a hole. But that is not the case. Because the bubble does not close symmetrically as it collapses. Rather, at a certain point it will fold over (the term is “involute”) and a tiny jet, known as a Munroe jet, is formed by the convergence of the collapsing walls.

Figure 4. Schematic showing the development of a micro-jet within the collapsing cavity.

These bubbles collapse very rapidly, and are normally very small, but, under very rigorous conditions Al Ellis at USCD was able to capture photographs of bubble collapse showing stages within the above sequence:

Figure 5. Photographs of bubble collapse (Al Ellis UCSD)

The acceleration effect from adjacent wall convergence and collapse has been known and used for years in the development of shaped charges for demolition.

Figure 6. Schematic showing the development of a cutting jet (Munroe jet)

Dr. Baird at MS&T has taken photographs showing the development of this jet from a linear shaped charge, showing that it is not quite as smooth a cutting blade at that scale as it is in the much smaller collapse of the cavitation bubble. The explosive is initiated from the rhs and as the initiation moves along the charge the copper walls of the inner liner collapse together to form the jet. The colors on the liner were used to help monitor the relative displacement of different points on the charge as it reacted.

Figure 7. Formation of a jet during the collapse of a linear shaped charge (Dr. Jason Baird MS&T)

The jet formed in a linear charge is sufficiently powerful that it can, virtually instantaneously, cut through steel bars and armor plate, and is thus useful, for example, in bridge demolition. The small jet from a cavitation bubble collapse is much smaller, though as I will show in a later post, is also extremely powerful.

If cavitation is allowed to continue for long within a high-pressure pump system it will destroy the pump. That is why, where there is concern that it might occur, the high-pressure pump is itself supplied by a lower pressure unit (we typically used one that delivered at 60 psi) that ensures that there is always positive pressure within the pump cylinders. (This was also discussed in an earlier post).

But I will return to explain more about this in the next few posts.


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