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Old   July 3, 2001, 16:13
Default flow in piping system
Peter Luke
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Hi, I am designing a piping system to flow purified water in all our plant. I will use a 2in. dia. main pipe, and I need to make water circulate through a parallel loop (about 20 m length, 1in. dia.). I was thinking about using the Venturi principle at one end of the loop to aspirate water and make it circulate, but I don't know if it will work. Since water is purified, it must flow continuously in the loop as in the main pipe, and I would like to be sure that it does. So I need your advices!
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Old   July 4, 2001, 06:21
Default Re: flow in piping system
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Hi peter

why can't you just use a circulating pump? i saw many such plant using it.

Best regards

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Old   July 4, 2001, 13:36
Default Re: flow in piping system
Peter Luke
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Thanks Alain, but it's probably possible to make water circulate just by creating a depression at the end of the loop. The problem is to be sure that water flows with the proper (and relatively constant) velocity. I tried to make some calculations, but it's very difficult to analyze. Has anybody ever seen a plant using this phenomenon to make water circulate?
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Old   July 6, 2001, 08:06
Default Re: flow in piping system
Alton Reich
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All you have to do is remember that water will flow from an area that is at high pressure to an area of low pressure. I'm assuming that the main loop (2" pipe) will be driven by a pump. That pump will determine the direction of flow. In order to determine the flow rate in the main loop (or conversely, to size the pump), you need to calculate the pressure drop in the main loop. This can be done using the Bernouli equation in a basic fluid mechanics text book, or Crane Technical Paper #410 (my personal favorite reference for these kinds of problems). For a simple series loop, you should find that you can do the problem by hand.

The parallel loop complicates things a little. You have two choices. You can assume that since the parallel loop is small in comparison to the main loop that the flow through it is negligible (this is a better assumption if the main loop were 6" and not 2", but it isn't terrible). If you make that assumption, then the flow rate through the parallel loop can be found from based on the pressure difference between the points at which the parallel loop connects to the main loop. In other words, you would treat it as a seperate simple series flow problem.

What you really should do is treat it as a parallel loop. In a portion of the piping system, the flow will be split between the main and parallel loop, and the pressure drop through the two pipes has to be the same since they start and end at the same physical point. For solving these types of problems, I've found that a spreadsheet is useful for keeping all the numbers straight. Most spreadsheets also have an equation solving capability that lets it do all the hard work of finding the right flow rate.

Hopefully this makes sense. If it doesn't, let me know and I'll try again.

Regards, Alton
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