Tuesday, January 29, 2013

Waterjetting 5c - flow straighteners

One of the advantages that became clear, even in the early days of waterjet use in mining, was that the jets cut into the rock away from the miner. It was thus a safer method of working, since it moved the person away from the zone of immediate risk. Rock has a tendency to fall when the rock under it is removed, and by using the jets to carry out the removal, so the miner is no longer as vulnerable.

But, in the early days of jet use the range of the jet was quite limited. Part of the reason for this is that the water is generally brought to the working place along the floor. It then has to be raised, through bent pipes, to the level of the nozzle, and then turned so that the water in the pipe is flowing in the direction in which the nozzle is pointing.

Figure 1. Sketch of an early Russian waterjet mining monitor

Even though the pressure of the jet is relatively low, the volume flow rates were high, and the bends leading into the nozzle set up considerable turbulence in the jet, so that the range of the jet was quite limited beyond the nozzle. There are a number of different ways of improving the range of the jet, and I will discuss these in later posts, and many of these techniques apply whether the jet is being used at high volume and low pressure for mining, or at higher pressures and lower flow rates for cutting into materials. But today the technique that I will discuss is the use of flow straighteners.

The two most dramatic instances that I immediately recall for their use were at the Sparwood mine in British Columbia, where the collimated jet was able to mine coal up to more than 100 ft. from the nozzle, and in an underground borehole mining application where a Bureau of Mines commissioned system was able to cut a cavity to more than 30 ft. from the nozzle, which was centrally located.

Collimating jets to get better performance is not restricted to the mining industry. A visit to Disney, for example, will find jumping jets that appear to bounce from place to place (video here) (this one shows the start of the surface waves along the jet, known as Taylor instability, which grow and cause the jet to break up; and if you want to make one Zachary Carpenter has two instructional videos on how they are made. (here and here.)

Essentially, as those Youtube segments show, the flow straightness is achieved by dispersing the water – using a sponge – so that it flows through a large number of drinking straws. These straws act to collimate the water flow, and it emerges as a glassy rod, which even acts as a light path so that light shone down it emerges at the far end. This can be used for a variety of different purposes, other than just for entertainment.

This then is the basic idea behind a flow collimator, although for larger mining flows drinking straws are too weak, and the flow volumes need to be larger. There are various designs that have been used for mining applications. Some of the earlier trials were at the Trelewis Drift mine, where the then British National Coal Board set up an experimental operation.

Figure 2. Sketch of Monitor used in the NCB Trials (after Jenkins, R.W., "Hydraulic Mining" The National Coal Board Experimental Installation at Trelewis Drift Mine in the No 3 Area of the South Western Division, M.Sc. Thesis, University of Wales, 1961.)

A number of different designs were used for the flow straighteners that were located at the nozzle end of the straight pipe section leading into the nozzle:

Figure 3. Designs for the initial flow straighteners used at Trelewis Drift (after Jenkins, R.W., "Hydraulic Mining" The National Coal Board Experimental Installation at Trelewis Drift Mine in the No 3 Area of the South Western Division, M.Sc. Thesis, University of Wales, 1961.)

More recent designs, which vary according to pressure, flow rate and pipe diameter, are a combination of those on the left above, and those on the right. It was such a combination that allowed the Canadian miners at Sparwood to achieve production rates of 3,000 tons of coal per shift as an average over the operation of a mining section.

While the use of flow straighteners does not give any gain over having a long straight section of pipe leading into the nozzle, it can bring the flow condition up to that level in places where the geometry (or the resulting unwieldiness of the pipe) would make the long entry impractical.

One of the more interesting applications of this is in the borehole mining of minerals. Simplistically a hole is drilled, from the surface down to the seam of valuable mineral. Then a specially designed pipe is lowered through the hole with the pipe having a nozzle set on the side. Then, as the pipe rotates, and is raised and lowered, the jet mines out the valuable mineral, which flows to the cavity under the pipe, where it is sucked into a jet pump and carried to the surface.

Figure 4. Schematic of a borehole mining operation (George Savanick)

As I mentioned at the top of the article, the jet cut a cavity some 30 ft in radius, with the jet issuing through a nozzle some 0.5 inches in diameter. In order to achieve this range it was important that the jet was properly collimated, yet the nozzle was set so that there could be no straight section.

Figure 5. Section showing the feed into the borehole miner nozzle. Note the vanes in the section leading into the nozzle (George Savanick).

The turning vanes to achieve the flow collimation were designed by Lohn and Brent (4th Jet Cutting Symposium) to produce a jet equivalent to that achieved had the nozzle been attached to a straight feed.

Figure 6. Turning vanes used to achieve a jet capable of cutting coal to 30-ft from the nozzle. (P.D. Lohn and D.A. Brent “Design and Test of an Inlet Nozzle Device” paper D1, 4th Int Symp on Jet Cutting Technology, Canterbury, BHRA 1978)

Tests of the performance of the nozzle showed that it produced a jet that was at least equal in performance to a nozzle with a straight feed, up to a standoff distance of 45 ft.

In simpler applications the designs do not need to be that complicated, for many simple spraying nozzles, for example, the straightener is made up of a simple piece of folded metal.

Figure 7. Simple flow straightener for use in low pressure and flow applications.

The water has now reached the nozzle, but that is not the end of the story of the feed system, as I will start to explain, next time.

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