Wednesday, January 16, 2013

Waterjetting 5b - hose connections and nozzle feed

There is often not a lot of choice, when laying out a project, over the path or distance that water must travel from the time it leaves the initial pump to the point where it reaches the nozzle and is usefully applied. Yet, over this length, if the right choices are made, some considerable improvement in performance might be achieved.

As discussed earlier, the simplest improvement is often just to increase the size of the delivery line, although there have been occasions where it was cheaper for us to use a double line rather than a larger single, in order to lower the friction loss to the nozzle, and keep operating pressure in the range that we needed.

There are other concerns with the layout of the feed line to the nozzle, If it is a hose, then any connections between sections should be whip-checked, so that should a coupling fail (which has been known to happen) then the released sections of hose will not whip around and cause injuries or other damage.

Figure 1. Hose that separated from a coupling, while under pressure.

The major risk comes at the moment of separation, while the pressure in the line is higher, before it drops under the larger area through which the water can now flow. To stop the whipping of the hose ends, the two should be restrained by attaching a cable with two loops that fit over either hose end making the connection.

Figure 2. Hose connection covered with a pressure dissipating sleeve (the blue cover) and with a whip connected to the hose ends on either side of the connection.

Apart from this, and remembering the possible risk about avoiding chaffing points and that, over time, high pressure tubing does fatigue (after many years of operation most of the pipe segments connecting between our ultra-high pressure pump and cutting table failed in a relatively short time period – but they all came at the same time, and were installed together and saw the same loading cycles). (Which emphasizes that, over time, the pressure rating of the tubing should be reduced).

Connections, T-joints and other fittings that are used in the feed line should be sized appropriately. Any time that the diameter of the flow channel changes, then there is a cost in terms of the delivered pressure. This is best checked with the manufacturer to ensure that this is accurately assessed in the flow and pressure calculations.

Moving down the line, this brings us to the end of the feed line, and the entrance to the nozzle. In later posts I will cover different pieces of equipment that can be used, for a variety of different tasks in manipulating the nozzle, but, for now, I would like to consider just the flow from the feed line into the nozzle (without discussing nozzle shape at this time).

Of all the systems I have examined, this is the one point in the assembly of a feed system that was most commonly ignored or badly constructed.

All of us, from time to time, get caught up in traffic flows through road construction. When the lanes are controlled, and traffic feeds are properly directed, it is possible to get through these relatively quickly. But in most cases that is not what happens. There are always drivers who do not ease into the required lanes soon enough, but rather drive rapidly as far as they can and then force their way into the remaining lanes, thereby breaking the steady flow into a process of stop-start-stop-start. The process becomes much less efficient, and instead of the traffic moving at a steady, but slow rate it often can take over half-an-hour or more to get through the restriction.

So it is with water flow down a pipe. If the flow can remain in a laminar mode, with the flow channel slowly constricted to speed the water up to the required velocity, then the resulting flow into and through the nozzle can give jet streams that can throw over 2,000 jet diameters. Instead in most cases the throw is about 125 jet diameters, and I will discuss how to find that distance in a post or two before long.

In this particular post the water has not reached the nozzle yet, and it could still be in a poor condition. The best way to explain why is to use a comparative graph showing jet pressure measurements after the jet has passed through the nozzle, to show how the structure is affected. The work was carried out by the Bureau of Mines (Kovscek, P.D., Taylor, C.D. and Thimons, E.D., Techniques to Increase Water Pressure for Improved Water-Jet-Assisted Cutting, US Bureau of Mines RI 9201, Report of Investigations, 1988, pp 10) and the only difference between the two different plots is that in the upper one a 4-inch straight length of pipe was connected just upstream of the nozzle to allow the flow to stabilize before it entered the nozzle.

Figure 3. The effect of flow conditioning the water, prior to passage into the nozzle.(Kovscek, P.D., Taylor, C.D. and Thimons, E.D., Techniques to Increase Water Pressure for Improved Water-Jet-Assisted Cutting, US Bureau of Mines RI 9201, Report of Investigations, 1988, pp 10)

There is a caveat to this plot, and this is the assumption that the internal diameter of the feed pipe and the entrance diameter to the nozzle are the same size. In virtually every system that I have examined in the field this has not been true, and as I will show, in a later post, the difference that this can make is very large. For the above example the length is 4-inches, but this was for a specific nozzle size, and the more general condition is that the length should be in the range of 100 – 125 pipe diameters.

By the same token, if the nozzle does not make up with the end of the feed line, so that there is a little eddy pocket created, the ensuing jet will be of poor quality.

Which brings us to the final part of this post, because there are many situations where it is not possible, because of space restrictions, to fit that particular length of pipe just before the nozzle. The most glaring example of this that we have had to deal with is where high-pressure waterjets are fed down a borehole, and then used to drill lateral excavations out from the bottom.

But if the borehole is say 10-inches in diameter, and the jet is an inch in diameter, because it is being used for mining out valuable pockets of uranium, then it is not possible to get the required straight section. Again the pioneering work on this was carried out, first in Russia, and then by the U.S. Bureau of Mines, under George Savanick. By placing a shorter length of a flow straightening device within the flow path, just before the nozzle, the flow can be straightened over a much shorter distance, and this will be the topic of the next post. And when George did this he was able to cut cavities that extended more than 30-ft from the borehole (with some unexpected consequences - but we'll cover that later - grin).

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