Pool plumbing question (1.5' to 2')

VikingKnutMay 10, 2011

Thanks in advance for any feedback. I am looking for some confirmation of my analysis. I am an engineer, so I believe I understand the physics. However, I am not a plumber, nor am I a pool builder. I still feel my analysis is sound, but it conflicts with some pool builders. Hopefully, I won't screw up describing it. :)

While investigating whether my pool plumbing setup could support the new heat pump I just purchased, I noticed something concerning. I have a 32,000 gallon pool with a 60 sq inch DE filter and 2 HP motor. Unfortunately, the entire pool is plumbed with 1.5" piping. After researching, the theoretical "limit" on 1.5" PVC is 42 GPM. This means my pump is probably strained against the plumbing (thereby wasting electricity) and more importantly my turnover rate is suboptimal (probably 11 to 12 hours instead of 8 to 10).

After speaking to some pool builders, they suggest that it is impossible to fix without ripping out all the underground piping. This is where I totally disagree. Since all the intake pipes and return pipes are joined above ground, I believe a well designed and installed 2" solution would give the exact same benefit of 2" throughout. Here's why:

Since the theoretical limit of 1.5" is 42 GPM, each 1.5" will be limited by something close to that. However, I have 4 intake 1.5" (drains and 3 skimmers) and 3 return 1.5" (pool jets, stair jets, and swim out jets). Since each 1.5" intake can supply around 42 GPM, theoretically you can add each one for a 168 GPM flowrate. However, since these would be joined into a 2" junction, the limit would go back to the theoretical 73 GPM of a 2" pipe (as the slowest piping determines the limit). Therefore, whether I replaced all the piping to 2" or just the above ground piping to 2", by the laws of physics and simple engineering principles, my max flow rate would be exactly the same (again at or close to 73 GPM).

More importantly, for any pool that uses 2" for intake and return lines underground, they are probably wasting their money as they can't push more water than the 2" pipe that they all lead into (meaning into and out of the pump and filter). I have a couple of engineers that have checked my analysis and agree with me, but I haven't had any pool builders agree yet. After having a lengthy conversation with 1 pool builder, he did say that I am starting to make him rethink he thought for the last 25 years, but he wasn't ready to commit.

This website seems to back up my analysis. However, I would love to hear from some plumbers and pool builders that support my analysis (http://www.poolplaza.com/pool-pump-sizing-2.shtml). Or point out why it is wrong. If you need more details or I did a poor job of explaining, let me know.


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First of there is no theoretical limit to flow rates in pipes. Published limits that you often see are simply recommendations based upon friction loss and hydraulic shock. If you have a large enough pump there is no theoretical limit to flow rates in pipe. Head loss will simply increase by the square of the flow rate. As head loss increases, the pump flow rate decreases so it is in a way a self limiting system.

Head loss in general follows these rules:

Head loss is proportional to the square of flow rate
Head loss is proportional to the 5th power of pipe diameter
Head loss is proportional to the 4th power of fitting diameter.

So the biggest bang for your buck is to upsize pipe. If you are only upsizing pad pipe, that will help some but the pad head loss is typically on 1/3 of the total and the pipe is about half of that. So just changing the pipe on the pad will reduce the head loss around 20% or so.

You are correct in assessing the parallel pipes. Since the flow rate is split, head loss is much less than if you had a single pipe. In terms of equivalent head loss 2 - 1.5" pipes have about the same head loss as a single 2" pipe. 3 - 1.5" pipes have about the same head loss as a single 3" pipe.

Also, you will generally always improve pump efficiency with lower head loss so increasing pipe diameter will nearly always reduce energy costs.

Here is a link that might be useful: Hydraulics 101

    Bookmark   May 10, 2011 at 4:20PM
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