Bernoulli vs. the pool guy

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Sure, but there are a lot of variables. Turns per day, the RPM and hours at the RPM, etc. So there are a lot of answers to your question. 2600 RPM is a bit unrealistic for the Intelliflo and doesn't take advantage of the lower speeds. But assuming all of the run time was at 2600 rpm and 1 turn per day:

2": 30k / 4.2 = 7.1 kwh per turn * $0.11 / kwh = $0.79/turn/day or $23.57/month
1.5": 30k / 3.6 = 8.3 kwh per turn * $0.11 / kwh = $0.92/turn/day or $27.50/month

Not really much in terms of $. With the lower speeds, the cost difference will be minimal.
 
So.. for almost 4 pages of this thread and all the time spent by the posters, all that over a half inch difference in pipe size for a savings of less than a pack of smokes over the course of a month? And remember, most peoples electric rates are not what you pay in california either. Most are closer to the 10-15 cent range.
:hammer:

From my simple way of thinking, its not worth much to hassle a builder to use 2 inch plumbing when 1.5 inch most use works just fine :wink:
 
mas985: Wow, thank you. It's not every day that I ask a noob question and get a consulting engineer's report in response!

ChemGeek: You're right, I left out the two returns in the steps. I was thinking of them more as jets for the kids to play with, but they'll normally be on, and should have been included. They're fed by a single 1.5" line from the pad.

4JawChuck: Thank you for responding so patiently. I'm a little sorry to have provoked a disagreement, but only a little - it was worth it to get such exceptional feedback.

bk406: Point taken. However, if the experts hadn't generously contributed their time and knowledge, we'd have no insight into why the answers are what they are, or where the edge cases manifest. My pool happens not to be an edge case, but I had no way of knowing that.

Anyway, I'm going back to my design and getting ready to ask questions about concrete now. I'll feel cheated if I don't get a stochastic analysis of hydration reactions somewhere in the thread ;)

Sim
 
Simbilis said:
bk406: Point taken. However, if the experts hadn't generously contributed their time and knowledge, we'd have no insight into why the answers are what they are, or where the edge cases manifest. My pool happens not to be an edge case, but I had no way of knowing that.
Click to expand...

Oh yea, i agree, Sim. My contention every time i see this argument being made about how 2" plumbing is so superior to 1.5" is that for the most part it's superfulous. Now, granted the numbers Mark put ou there are for 2600 rpm, and the kW costs i put out may be different for others, but the point is the money saved is miniscule. The bigger money saver by far is the 2 speed or VS pump, not the plumbing.
I see poster after poster chiding builders and folks building a simple vinyl pool, or gunite pool for that matter, to insist on 2 inch plumbing and if a builder uses 1.5" he's the dumbest human on the planet and find another builder. It's just not so.

I think sometimes people get caught up in theoretical discussions (which is fine) but forget what the actual practical thing is they are trying to figure out.
 
It wasn't until this post when there were details about how close the pump was to the deep end of the pool and that there were separate lines to each return so it was then clear that piping wasn't a significant factor in Sim's system. Every system is different and ones with longer runs, solar systems and cartridge filters (like my pool) are going to have piping be a more significant factor.

I found this file that has head loss estimates for floor (main) drains and skimmers as well as wall inlet fittings. Note that the main drain has about half the head loss of a skimmer so this means there will normally be more flow through the floor drains than through the skimmer. A 3/4" wall inlet has around 3.7 times the head loss as a skimmer (3.2 PSI at 30 GPM) while a 1" wall inlet has just a little more head loss than a skimmer (it's not clear if these inlets are eyeballs -- they might not be).
 
Well I read Mas985's excellent response as 1.2Kw saving, not much to you gas guzzling energy consuming Yanks but to some of us it's a saving. If everyone cuts the pool electricity by 1.2Kw per turnover that is a little bit of the worlds resources left every single day.
I know you only think in monetary terms but there are other considerations too.
 
Although the $ amount is not all that much, I would still use 2" plumbing on the suction side and pad. The pad I think is worth it since most equipment these days take 2" fittings plus that is where most of the head loss is in this case. The suction side is worth it in case you will need to run only one of the three lines. For example, if you decide to lower the water or even drain the pool, a quick way to do that is use the pump on the main drain only to waste. With just 1 1/2" plumbing, the suction will get quite high and likely draw in lots of air and the pump may lose prime. Plus, 2" pipe is not all that much more expensive then 1 1/2".

Also, one thing that seems to get lost in the discussion of pipe size is the number of runs used. Simply saying that 1 1/2" pipe is bad, is not always true. Three parallel 2" pipes are equivalent to a single 3" pipe! Even three 1 1/2" pipes are equivalent to a single 2 1/4" pipe. I am a big fan of multiple runs for both the suction and return side plumbing. Not only does it let you use smaller pipe for the same performance but it also gives you the option to control flow to specfic parts of the pool. I think all pools should be designed this way but unfortunately, not all PBs will do this.
 

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Sure enough, at the last minute my builder was back to ordering 1.5" pipe for suction.

Perhaps I'm missing their actual meaning, but the conventional wisdom among many builders seems to be that flow is absolutely limited by the size of the smallest pipe. Even my preferred builder keeps insisting that total flow is limited by the 2" fittings at the pump and filter. I'm going to show him a Bernoulli's Principle animation and see what he says.
 
4JawChuck said:
Here is a link to a handy online D'Arcy-Weisbach equation formula, just plug in the values and click calculate.

http://www.engineeringtoolbox.com/darcy-weisbach-equation-d_646.html
Click to expand...
Assuming you're not being facetious, that's a bit of a pain to use since you have to put in friction coefficients and nominal inside diameter which is not the same as the nominal pipe size. You also have to convert GPM into velocity. It's much easier to use the tables I linked to earlier here or in graph form here or to use Figure D-4 in the reference you gave earlier though they both give higher head loss results, probably because they assume a higher roughness for the PVC.

If someone really wants to use the D'Arcy-Weisbach equation, they can use my Pipe Head Loss spreadsheet I created a few years ago that uses that equation with a Swamee-Jain initial estimate followed by multiple Colebrook iterations for an accurate solution. However, since the friction for turbulent flow is dependent on the relative roughness, this is just an approximate result since the roughness is estimated within a typical range which is why the use of an approximate formula such as Hazen-Williams is fine for most purposes.

Simbilis said:
Sure enough, at the last minute my builder was back to ordering 1.5" pipe for suction.
Click to expand...
With 3 suction lines and the short runs, 1.5" pipe is not a big deal. Even if you get a solar system down the road and maybe want or need to do higher flow rates, splitting such flows in 3 helps a lot though there will be a higher flow rate in the floor drain line since they have less resistance to flow than skimmers (and you've got two of them sharing a line as well).

Just make sure that if/when you do get a solar system you don't end up putting the entire flow through 2" lines. Also, make sure he's not using 1.5" at the pad and is using 2" there instead (it sounds like he is).
 
Note too that calculating just head loss for the plumbing at an arbitrary flow rate is not enough to know how a specific pump will perform on specific plumbing. You have two separate systems, the pump and the plumbing, which can be modeled separately but if you want to know the operating point, you have to model them together and find where the plumbing curve crosses the pump's head curve. This can be done graphically or using an iterative solution which is what my model does. It models the pump head curve as well as the plumbing losses and finds where the two curves cross for a unique operating point.
 
For me, the bottom line is that my PB followed the conventional wisdom of using 1.5" lines to each of 3 returns/inlets and from each of 2 outlets/skimmer/drains as well as 2" for a single line to the 1.5" returns/inlet lines and 2" to/from the solar system and piping between panels (that was the solar company that did that last part). Now I pay the price of higher electricity costs because they did what was "common wisdom". A single-speed pump was put in as well and though variable-speed pumps weren't reasonably available 7 years ago, there were 2-speed pumps available that would have worked well with the solar on/off situation.

So while Sim's pool isn't significantly affected by using smaller pipe, my pool is and it is the same conventional wisdom that caused that problem. If I knew then what I know now, I would have insisted on at least 2.5" pipe to/from the solar and between panel sections (roof hips) on the roof and 2" pipe for each of the two lines going to the floor drains and skimmer and 2.5" pipe for the combo line from the pump to getting split for the three return/inlet lines. The least important would have been having 2" pipe to each return/inlet and to use 1" eyeballs instead of 3/4".

I have since cut my pump electricity costs and energy usage in half by replacing the main and booster pumps with an IntelliFlo VF, but I'm toast on everything else as much of the piping is buried, some of it under expensive hardscape. So spending a little extra time pondering what becomes fairly permanent may not have changed Sim's situation because it wasn't as critical, but could most certainly help someone else. Unfortunately, it's too late for me.
 
So if you had the 2 inch pipe, how much would you save, in dollars, per month? I dont need equations or such, just money saved per month given your local electrical rates, and your current pump and set up.

I understand it might save you a bit of money. I guess my point is that I really dont like it when the first thing a few people do is jump on someones pool build that has, or is proposing 1.5 inch pipe. For most, its fine. I just have observed time after time that builders and new posters get barrage of "2 inch is better, get 2 inch or find another builder". IMO, such blanket statements are irresponsible, and makes new members nervous about their new pool when the reality is they have nothing to worry about.
 

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