Pipe sizing

[pools_kalavo]

Hello all,
Just trying to design the piping for a deck level indoor pool which has been partially built. A lot of the piping has already been laid but the only pipe which can't be changed is the 2 inch pipe from the main drain to the surge tank. Ultimately though, they will all be covered in concrete. I'm trying to achieve a turnover time of 4.5 hours which will mean a flow of 66 GPM.
At present, my quandary is what size pipe to use from surge tank to pump (see drawing).



I'm new to all this, so have a few basic questions:
1) Can this pipe be too big?
i.e. a 2.5 inch pipe can achieve 90 gpm at 6ft/sec, but, what the ****, why not put in a 3 inch pipe. Then I know it is definitely big enough for all required flow rates. Are there any disadvantages to a big pipe other than cost?

2) Do the current pipe sizes look ok?
I assume the pipes which feed the surge tank from the pool via gravity can never be too big.

3) In the Hydraulics 101 thread ( http://www.troublefreepool.com/threads/830-Hydraulics-101-Have-you-lost-your-head ) it says:
"On the return side of the pump, multiple lines can help reduce head loss as well but the number of return lines should never exceed the number of suction or pump cavitation could become an issue."
I don't understand why. Please can someone explain.

4) Also in the Hydraulics 101 thread there is an "Equivalent Pipe/Fitting Size" table. I can't get my head round why where N=1 the equivalent pipe size isn't equal to the actual pipe size. I must be making a fundamental misunderstanding here but don't know what it is.

Thanks
Nick

 

[Bama Rambler]

Here's a few answers.

1) Practically no. On the suction side of the pump, bigger is always better. There's a point where it isn't practical to go larger, but it'd have to be pretty big to get to that point.

2) I would have went larger with the main drain piping, but that's the one you said can't be changed. Other than that they look fine. If I were going to change anything I'd go one size up on everything. You are correct about them never being too big.

3) What that means is that if your suction head is greater than your discharge (return) head you could discharge water faster than it can flow into the pump and therefore cause cavitation at the impeller. In other words, you always want the suction head to be lower than discharge head.

 

[mas985]

Welcome to the forum!

I'm trying to achieve a turnover time of 4.5 hours which will mean a flow of 66 GPM.

Why? Turnover generally has very little bearing on water quality. Read the pump run time study in my sig.

1) Diminishing returns. Sure you can go as big as you want but at some point, it doesn't matter. Every size increase reduces head loss by an ever decreasing amount.

2) Yes, I think the pipe size is more than sufficient.

3) What this means is that sometimes you can have too little head loss and if that occurs on the return side of the pump, it can cause issues like cavitation. But if you use 3" or even 2.5" pipe from the surge tank, I don't think that will be an issue.

4) That is because there is a labeled pipe size (column names) and then there is the actual I.D. A 2.5" pipe is actually 2.47" I.D. A 1.5" pipe has a 1.61" I.D. The table above that table has the actual pipe sizes.

 

[pools_kalavo]

Thanks for the answers Dave and Mark.
The 4.5 hours turnover was based purely on what I'd read that turnover should be 4-6 hours for residential pools. I'll read the Pump Run Time Study.
Cheers. Nick

 

[pools_kalavo]

What about the pipe sizes between "units" in the plant room?
Pumps, filters etc typically have 2 inch connections. Is there any value in going bigger than 2 inches for the relaatively small length of pipework between them?
Thanks. Nick

 

[mas985]

Going up to 2.5" on the pad will have a minimal impact. At a constant pump RPM, it might mean an additional 1-2% in flow rate. At a constant flow rate, it might mean a 4-6% reduction in energy use. However, that is only if you do not have to add more plumbing to accommodate the larger pipe size. Sometimes there isn't enough room for the larger pipe.

Also, design of this type of system can be very tricky. Did you do the design yourself or hire an engineer?

 

[JamesW]

Since the gutters are gravity fed to the surge tank, the flow rate is determined by the height differential from the surface of the water in the gutters to the surface of the water in the surge tank. Assuming 33 gpm, and 100 ' of 3" pipe, you would need about a 4" difference in height.

Can you describe the pump, filter and surge tank dimensions?

 

[pools_kalavo]

The pool has already been partially built by a PB. I'm currently making an initial design for the rest before seeking consultancy from another PB.

The final surge tank dimensions will be 3.51m (len) x 0.93m (width) x 1.51m (height) = 4.93 cubic metres = 1302 US gallons
The top of the surge tank will be 70cm (approx) below deck level. I would guess the gutter pipes will start 30cm approx below deck level and will enter the surge tank somewhere near the top. The original PB was just going to have one 3" pipe to cater for the flow from both the gutters. After checking the following link:
http://www.calctool.org/CALC/eng/civil/hazen-williams_g
I decided it would be safer with no real downside to have a 3" pipe for each side of the pool.
I'm planning on getting a Pentair IntelliFlo SW5P6R VSD pump, a Pentair Clean and Clear Plus 320sq ft filter and a Pentair IC40 SWG.
As Mark pointed out, a turnover time of 4.5 hours is not really necessary but I've decided to oversize components to reduce head loss and the regularity of cleaning the filter.
Below is a very rough and out of scale design for the surge tank.



Thanks. Nick