Friday, April 17, 2015

Waterjetting 32b - early work with polymers

If we lower the viscosity of the fluid in a jet hitting a target, then the jet can penetrate into the fissures and flaws of that surface more easily and in this way it will see an improved cutting performance. On the other hand the thinner fluids have less resistance to the air or other fluids through which the jet has to pass on its way from the nozzle to the target, and so the amount of energy that is delivered to the target can be lower than with a less viscose jet. Some of the changes in fluid chemistry can, however, be reached using small amounts of different chemical additives that can have quite a profound effect on performance without totally changing the fluid being used.

Consider that before the jet gets to the nozzle it has already completed most of the journey from the pump to the target. And along the way, as I pointed out in one of the earlier posts I wrote, pressure losses along the line, due largely to friction, can overwhelm the power supplied by the pump. There was an occasion where a colleague at another research group was running a system where, because of the small diameter of the tubing he was using, the pressure drop from the pump was more than 75% before the water even left the nozzle. A slight increase in tubing diameter would have made the system much more powerful, with no other increase in cost.

But sometimes that option isn’t really available. I remember my first introduction to the use of long-chain polymers was after reading an article in The Sunday Times which described an investigation by the New York Fire Department. One of the problems in fighting high-rise fires is that the firemen have to haul their hoses up the long flights of stairs and 2-inch diameter hoses become quite heavy as they are moved up from level to level. In the late 1960’s (in the administration of Mayor John Lindsay) tests were carried out to see if this could be improved. Normally, when a smaller 1.5 inch diameter hose was used, the pressure loss in pumping 100 gpm through 1.5-inch diameter hose is around 25 psi per hundred feet, with 98% of that loss being due to the turbulence of the water.

Figure 1. Demonstration of the benefit of polymers before John Lindsey, the Mayor of New York City (seated far right). May 13, 1969. The two jets are of equal size and fed from pumps at the same pressure. The jet barely reaching the holding tank is the standard water, the other contains polyox.

What the demonstration showed was that more water could be sent through the smaller hose, (roughly 70%) so that it could be used, instead of the larger hose, and with better effect on putting out nearby fires.

One interesting historical comment is that the demonstration occurred in 1969, however back at the University of Leeds, I was aware of the research in 1967, and had included, as chapter 5 of my dissertation, a study of the benefits of using the polymer. Similarly Dr. Norman Franz, then at the University of Michigan, (and thereafter at UBC) had also become aware of the work, and – being smarter – had patented his findings, that were then licensed by the fore-runner (McCartney Manufacturing Company) of KMT.

The benefits were not just that the polymer reduced friction in the line from the pump, transitioning the flow from turbulent to laminar, with the consequent reduction in friction losses, but these also extended beyond the nozzle. I have always suggested that the easiest way to think of this is to compare the benefits of using the polymers to that of picking up spaghetti from a plate. Very short strands of the pasta need the eater to use a spoon, or simple fork to try and grab a small amount for a bite. On the other hand, where the pasta strands are longer, and well mixed together, when one pulls one out of the pile it is attached to another few that also rise, and before long the fork is full of a seemingly endless set of attached strands.

Given that the molecular weight of Polymerized Ethylene Oxide (Polyox) (which was first introduced by Union Carbide and is now a Dow product) can range up into the 8 million range, it is easy to make the analogy. As a result the intertwining of the long molecules facilitates passage down the feed line, but also tends to maintain the jet coherence once it leaves the nozzle.

The jet is thus not only allowed to pass through the feed line at a lower (often significantly so) pressure loss than with conventional water, but also, once the jet leaves the nozzle, the stream remains coherent to a much greater distance. This can be seen in Figure 1 at low pressure, however that result has been similarly validated at pressure of up to 50 ksi in the years since that original work was completed.

Figure 2. Effect of adding a small amount of polymer to the cutting performance of a jet at 10,000 psi.

I will spend the next couple of posts discussing the general concept of polymer additions but will close this with another anecdote. After I had carried out the first test series – which yielded the graph shown in Figure 2, I wrote to Union Carbide, who had been kind enough to send me about half-a-pound of dry product for that study. Given the favorable results we wanted to expand the study – but I received a note back from the European sales office of the company, together with an additional box of roughly three-quarters of a pound of product. The note let me know that I had now received their entire allocation of product for the continent of Europe, and that, in consequence, my research needed to be conservative from thereon out, since there was unlikely to be any more supply for some considerable time. (Which was why the research could not be pursued at any greater length until I had moved to UMR and acquired my own system – which took an additional four years).

It was not until years later, as spreadsheets became ubiquitous, and data analysis facilitated from the hand-calculations that we were reduced to before the days of computers, that I was finally able to fully comprehend the data that we had, but I will leave that discussion until the next post.

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