Two Small Diameter Pipes in Parallel Same as One Large One? 2 Ratings (4.0 Stars).2 Ratings (4.0 Stars).2 Ratings (4.0 Stars).2 Ratings (4.0 Stars).2 Ratings (4.0 Stars).2 Ratings (4.0 Stars). 

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Two Small Diameter Pipes in Parallel Same as One Large One?

Postby HettyJacobs » 13 Sep 2010 19:09

I need to put in 580 meters of 63mm PN10 PVC for the final section of a 12km distribution line. We don't have any 63mm in stock so can I just use two 50mm pipes in parallel? The internal area of the 63mm pipe is about 22cm2 and the internal area of the 50mm pipe is about 12.5cm2 so two 50mm pipes should be about the same. Is my logic correct? :D

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13 Sep 2010 19:09

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Re: Are Two Smaller Diameter Pipes Same as One Large One?

Postby cdawggy » 13 Sep 2010 19:53

you also need to take into consideration the headlosses that the two pipes would have in comparison to the single pipe configuration.

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Re: Are Two Smaller Diameter Pipes Same as One Large One?

Postby HappyPlumber » 13 Sep 2010 22:47

Hetty, cdawggy is absolutely right, you need to go back to basics and calculate the headlosses. Two pipes giving the same equivalent cross-sectional area as a single pipe may appear to solve the problem but as we all know frictional losses a generated in the non-laminar region around the pipe walls and two pipes certainly have move wall surface area than a single pipe.

However, in this case I quickly set up a spreadsheet and ran both scenarios through the basic Hazen-Williams formula across a range of flow rates and you should be fine for all flows. Here's an example assuming a flow of 3l/s and a Hazen Williams Coefficient of 130.

For the 63mm pipe:

Headloss (m) = [10.9 x Length x (Flow ^ 1.85)] / [(Hazen Williams Coefficient ^ 1.85) x (Diameter ^ 4.87)]

Headloss (m) = [10.9 x 580 x (0.003 ^ 1.85)] / [(130 ^ 1.85) x (0.051 ^ 4.87)]

Headloss (m) = 32.8m

Along each of the 50mm pipes we use half the flowrate:

Headloss (m) = [10.9 x Length x (Flow ^ 1.85)] / [(Hazen Williams Coefficient ^ 1.85) x (Diameter ^ 4.87)]

Headloss (m) = [10.9 x 580 x (0.0015 ^ 1.85)] / [(130 ^ 1.85) x (0.041 ^ 4.87)]

Headloss (m) = 26.4m

i.e. with a flow of 3l/s, along 580m of 63mm pipe you would lose 32.8m of pressure head. If the flow was divided into two smaller pipes of 50mm, you would lose 26.4m along each.

Of course you may want to factor in secondary losses at the junction but it looks like you should be alright.

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Re: Are Two Smaller Diameter Pipes Same as One Large One?

Postby seveoxbotty » 14 Sep 2010 08:15

Another important factor to consider is the additional resilience that having two pipes will provide, as long as a means of isolation is provided. If isolation valves are provided then damage to one pipe can be isolated and a repair done whilst operation continues on the other pipe.

The snag is that there is always an extra cost in suppplying the two pipes and the fittings as well as the labour in laying them, especially if they are laid in separate trenches for maximum resilience.

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Re: Are Two Smaller Diameter Pipes Same as One Large One?

Postby rconner » 15 Sep 2010 15:25

You may wish to do a search under equivalent number or pipes or equation of pipes (see e.g.
the tables headed "Equivalent Number of PIpes" etc. at http://www.acipco.com/adip/products/Sect17.pdf ). [While likely not the optimum solution in all cases, I believe some folks even actually install multiple pipes for an element of "redundancy" they feel such schemes offer.]

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Re: Are Two Smaller Diameter Pipes Same as One Large One?

Postby HettyJacobs » 15 Sep 2010 16:24

HappyPlumber, cdawggy, rconner, seveoxbotty - many thanks for the rapid comments, just what I needed to know. Now I need to just crack the whip and get on with putting these pipes in the ground!

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Re: Two Small Diameter Pipes in Parallel Same as One Large One?

Postby Sumo » 19 Sep 2010 20:55

Hetty, I was bored and quickly ran this through EPANET for you.

Yep, the results are pretty much what we expected, two 50mm pipes in parallel are slightly better than one 63mm on its own.

Have fun layin pipe!

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