plumbing dilemma ... need advice quick

[fyton2v]

This is from my SoCal pool build thread, but I'm hoping to get a little more info quick since I'm meeting with my plumber tomorrow morning to go over any final changes. Gunite is next week.

Please see the pic below. The pool as 4 returns and the spa has 2 for a total of 6 returns from the filter. At this moment the 4 pool returns are in a serial run starting with a 2" pipe and ending with a 1" pipe. Each of the 4 pipes that enter the pool are 1". I mentioned to my plumber that I'd like to cut off the last 1" run and connect a 2" pipe all the way back to the original 2" run to create a loop... keeping the 4th return in place through a Tee. He has strong feelings against this because he doesn't think there will be enough pressure to move water in the pool. The suction side is fed by 2 separate 2.5" runs from 2 skimmers. One skimmer is also supporting the main drains.

Will return pressure be a problem if we loop it back? Also, are 1" outlets big enough? It just "feels" undersized, but I don't have science or experience to lean on here.

Thanks all.
[attachment=0:1o9cp720]pool return.jpg[/attachment:1o9cp720]

 

[PoolGuyNJ]

Too many returns. Three in the pool and one in the spa (only if there is a separate jet pump for the spa) is usually sufficient for an efficient return flow for pools up to 60,000 gallons.

I would suggest using 1.5" stubs and not 1" stubs for the returns. Keep the return line at 2" and extending it 18" beyond the last return's T connection will even the flow out all the returns. No need to bring all the way in a complete loop. Velocity kills. Don't sweat it. Wind has more of an effect on skimming. Set the skimmers accordingly. Run a separate line for the drains. Run a valved shortcut at the pad between the suction side and return side to you can shut the drain from the suction side and divert return flow to the drain. Useful for sending heat down there.

If the primary circulation pump is to be used for the spa's jets, a valve to send water to the pool return and spa loop is fine and the extra return inlet is not needed.

Scott

 

[fyton2v]

Interesting. Is pressure the same as velocity? At what point does it become too little pressure? Won't the pump have a tougher time operating at too low a pressure and where is the trade-off between pressure and efficiency? This plumbing thing is driving me crazy.

It turns out his returns are 2" for the spa. So I'm at 4 1" returns and 2 2". He installs the 2" returns in the spa for more efficient heating. He originally wanted 3 returns for the pool, but I asked him to install an additional run because most the info I've seen out there on the Interweb had more than 3 for a pool of this size.

I pretty much have this only weekend to make any changes.

 

[mas985]

Is pressure the same as velocity?

No. Velocity is speed of the water and pressure is the force of the water.

He has strong feelings against this because he doesn't think there will be enough pressure to move water in the pool.

Lower pressure can be a good thing. The pump creates the pressure by creating flow rate through the plumbing. A loop would help some to equalize the pressure and flow rate to each of the returns. Water will travel the path of least resistance (i.e. lowest head loss) and since it will have two paths with a loop, the flow rate will drop in each path, and the head loss will drop as well. This will reduce the total head loss the pump experiences and the overall flow rate through the system and through each return will increase. The loop helps because the furthest return is no longer the furthest return and the returns that are now furtherest from the pump, have two paths feeding them. Half the flow rate through a pipe reduces the head loss by 75%.

 

[fyton2v]

Maybe one more post will help me (probably not).

In a practical pool application can you have useful pressure without velocity? For instance please consider 2 scenarios below:

1. A single run 2" run supporting 4 returns at 1" each.
2. A single run with decreasing pipe sizes (from 2 to 1.5 to 1) over a distance of 100ft.

What factors control back pressure towards the filter and actual pressure out of the 1" pool returns? Is it the 1" outlets or would the decreasing pipe size have a big impact as well?

Thanks for tolerating my personal crisis.

 

[mas985]

In a practical pool application can you have useful pressure without velocity?

In plumbing, pressure and velocity go hand in hand. The higher the pressure, the higher the velocity through the plumbing. But for the same pressure you can get much higher velocity through the plumbing with larger pipes.

However, a pump behaves opposite to the plumbing. Higher pressure results in lower flow rates. So from the pump's perspective, lower pressure is better. Which then leads you back to using larger pipes.

When designing a plumbing system, you would like an operating point that is on the right side of the pump's head curve. This is the most efficient area of the head curve. But in order to do that you need plumbing with very low head loss. That in turn requires very large pipes. So the bottom lines is:

Big pipes => Low head loss => high flow rates => High efficiency.

Now to get equal flow rates at the returns, the pressure at each return needs to be the same and thus the head loss to each return needs to be the same. Ideally, you want most of the head loss to be in the return eyeball. The exit velocity of the eyeball will be proportional to the square root of the pressure at the eyeball.

There are several ways to equalize the pressure at the return:

1 - Using very large pipe helps because the head loss in the feeder pipe itself is very low and doesn't matter much. So the path lengths to each return can be different and still have approximately the same pressure at each return.

2 - Using a loop helps to equalize the pressure around the loop and to each return. Plus having two paths to each return reduces overall head loss. Although the returns furthest from the pump will still have somewhat lower pressure and flow rates than the closest returns.

3 - Using a binary tree with equal pipe lengths and fittings ensures that the head loss through each path to a return has nearly identical head loss. This is the best way to ensure equal flow rates and combining this with large pipe size will also have low head loss.

It is not necessary to have equal flow rates through the returns but you don't want a large imbalance either.

 

[fyton2v]

wow. thank you.

So basically you're saying that, assuming the pipes are of sufficient size, pressure is mostly determined by the eyeball on the return. Correct?

In my case, if I redid the 1.5" pipes with a 2" loop and kept the rest the same, qty 4 1" in the pool and qty 2 2" spa returns, will this be workable? My pool equipment is close to the pool and it's about 130ft around the pool back to the equipment. There's 18" of elevation change unless you include a 7.5" deep end.

many thanks!

 

[mas985]

So basically you're saying that, assuming the pipes are of sufficient size, pressure is mostly determined by the eyeball on the return. Correct?

Yes but you would probably need pipe much larger than 2" for the head loss to be that low where it doesn't matter.

In my case, if I redid the 1.5" pipes with a 2" loop and kept the rest the same, qty 4 1" in the pool and qty 2 2" spa returns, will this be workable? My pool equipment is close to the pool and it's about 130ft around the pool back to the equipment. There's 18" of elevation change unless you include a 7.5" deep end.

A 2" loop would be much much better than a 1.5" straight run. If the loop run is too long around the back, you could also use option 3 (equal splits) which should be just as good if not better.

I would also use short 1.5" pipe for the returns into the pool since I think that is the standard size for most eyeball fittings. It shouldn't matter much to the PB.

 

[PoolGuyNJ]

A complete loop is not needed and wasted trenching time and materials. See the drawing.

Note the 2" RTN has three 1.5" lines to the pool. Water will fill the 2" to nearly the end and then the three 1.5s will equalize, just like an air plenum.

Scott

 

[mas985]

As long as the feeder pipe is straight, the distance between returns is short, all three returns use tees, the 1.5" pipe is the same length for each return and there are no extra fittings in between the returns, that configuration should be fairly balanced. That works great on a rectangular pool with all of the returns on one edge but the problem is that the OPs pool is not rectangular so either the 1.5" pipe will be different lengths or you will need to introduce 45's/bends to curve around the pool. That is where the imbalance usually happens. However, with strategic placement of the returns and bends, it is possible to have balanced returns on a curved pool and no-loop. But it takes a little more work to design the plumbing that way.