Double check head loss/pump sizing

shmeeps

Member
Feb 23, 2021
6
Dallas, Texas
Pool Size
22000
Surface
Plaster
Apologies for the long post; I haven't been able to find a straight answer here and want to make sure I get all the info in. Long story short, winter freeze in Dallas caused some leaks in my main pump. When putting a go-kit in my Pentair Challenger CHII-N1-1-1/2A/346206 to fix its leak, I've found that the impeller bolt lock housing is degrading so the bolt can't stay fully tightened, the volute is slightly deformed, so not even a new set of gaskets stops it from leaking completely (but does help significantly), and the motor/shaft are both having some rust issues. I've pretty much settled on replacing it since it's super old and obviously hasn't been kept up with as well as it should be. I've got some questions on flow rates and head loss though, as it seems to me like my current pump is massively oversized for my pool (it's loud, and creates a "pebbles bouncing around" noise when running. The performance graphs seem to indicate it's probably trying to move too much water through 1.5" pipes?). I've read through the hydraulics sticky, among other sources, but keep getting different values from different places so I wanted to get a second opinion, or see if I'm just over-thinking this.

I was told the pool is 21k gallons, but I think it's a bit higher based on measuring. The main body is 16x29, 5.5 ft average depth for 19,140 gallons. I have two half-circle sections on either end that are 12x4. I plugged a 12x8 at 3ft for the shallow end into an oblong calculator, then took half for it to be a half-circle, then half again to account for the steps into the pool, which added 500 gallons. Did the same with a 12x8 at 8 ft for the deep end, halfed only once for the semi circle to add 2,650 gallons, so a total of about 22,290 gallons. So ideally I think I'd aim for about a ~45 GPM flow rate to get a turnover in a ~8.5 hours. I'd also be fine with aiming for ~37-38 GPM and running for 10 hours, as the pumps I've been looking into would still net about $150 in energy savings due to the lower amperage, even with the increased run time.

The pool came with the house so I don't have any designs or anything for the pipes in ground, but I've measured piping on how I think it would be laid out. This is all 1.5" PVC.
  • Skimmer 1: 80 linear ft. Assuming 2ft drop, 3 ft raise, 85 total​
  • Skimmer 2: 40 linear ft. Assuming 2ft drop, 3 ft raise, 45 total. Not sure where these two join, so I'm assuming they join underground closer to the pump.​
  • Main drain: 45 linear feet, assuming a max of 1ft drop, then max of 10 ft raise. Probably ~55 ft total​
  • Equipment lines: 12 linear feet. 2ft raise up to sand filter included​
  • Return line: 70 linear feet, total of about 3-4ft drop to pool​
I also have a booster pump return line that's 3/4" inch, but I believe I'm going to cap it, remove the booster, and get a robotic cleaner instead. It's a total of 70 ft, if it makes sense to leave the line connected without the booster just to have two return sources.

I've counted 10 elbows above ground. 1 is going away for sure, I believe I can remove another 2 placed right outside the filter as well. I speculate there are about 12 underground, so a total of 22 currently. I have one 3-way valve right before the pump, and the tee into the booster line but that will be removed if the booster line is going away. I've counted 18 couplings total, though I should be able to remove at least 8 when replumbing the stuff around the equipment. I'm sure there are more under ground with the length of some of those runs though.

Outside of that, my pump is about 1 ft above the pool surface, and my filter is a Hayward S244T2, and based on the head loss rates in its manual, 45 GPM would add ~13 ft of head loss to the system, or about ~9 ft at 38 GPM. I'm not sure if that includes the Hayward SP0714T multi-port valve on top, which adds in ~7 ft at 45 GPM, or ~5 ft at 38 GPM.

I plugged all that into Pentair's calculator (~275 linear feet, 1.5" diameter, 1 foot lift, 21 elbows. Then either 45 GPM/20 feet misc or 38 GPM/14 feet misc) for about ~66 feet at 45 GPM, or ~48 feet at 38 GPM. Is that roughly accurate? I've ran these numbers through the stuff on InyoPool's guide subbing in Schedule 40 flow rates I've found through either Sta-Rite or Watts Water using my intended lower flow rates instead of the 70 GPM they use in various places, but it ended up higher than the Pentair site. However, I've seen a few places say InyoPools guide gives higher numbers than expected. I've also run them through InTheSwim's guide, again trying to account for the differences in flow rates but get ~79 feet at 45 GPM and ~59 feet at 38 GPM. I'm not sure if the latter are over-estimating in general/the unions are adding up or the Pentair one is under estimating.

As far as pump selection, I've mainly been eyeing the Pentair Whisperflow as a replacement. If the Pentair calculator numbers are accurate, then I figure I can either pick up a 3/4 HP model and run it for ~8.5 hours (66 ft/45 GPM), and it will likely be a bit oversized, though that might be good incase there's anything underground I can't account for. I'm not sure how the 1/2 HP would work, assuming I'm aiming for a lower flow (48 ft/38 GPM), the chart indicates it would run at 55 GPM, which should then... result in increased head loss? Regardless, if the latter, larger numbers I got are more accurate it seems like a 1 HP version would be slightly underpowered at 79 ft/45 GPM, and a 1/2 HP model would be right on the money for 59 ft/38 GPM, but not leave any room for unknowns in the system.

I've read about using a vacuum and pressure gauge into the pump basket and pump volute, respectively, to measure the current TDH of the system, which might give me a better idea of what's going on underground, but I can't see anyway to get both gauges into the pump without separating the pump itself from the basket since the volute drain plug is between the volute and pump basket. Can you run these on a short hose and still get accurate results? Or will it even help if the pump itself is being replaced?

Any advice on better sources or head loss rates or pump models?
 

Jimrahbe

Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
21,152
Bedford, TX
S,

To be honest I did not read most of your post as it matters not in most cases..

I suggest a 3 HP IntelliFlo that you can just adjust to meet your needs. The IntelliFlo is considered by most to be the "Gold Standard" of pool pumps.

I run mine 24/7, mostly at 1200 RPM, for less than $20 bucks a month. (No reason to run 24/7, just something I like to do) The less you run it the less it will cost.

Let's see what @JamesW has to say...

Thanks,

Jim R.
 

JamesW

TFP Expert
Mar 2, 2011
23,864
You can't add up the suction lengths.

The flow will divide between the 3 suctions such that the head loss is equal for each line.

So if you have (1) 85' line and (2) 45' lines, about 26% will go through the 85' line and the (2) 45' lines will get about 37% each.

The maximum flow for a return should be 8 ft/sec, which is 51 gpm for a 1.5" return line.

A 75' return line at 51 gpm is 12 feet of head.

The suction head is about 1.14 feet of head.

The 85' suction line will get about 26% of 51 gpm, which is 13.3 gpm, which is about 1.14 feet of head.

The (2) 45' lines will get about 18.9 gpm each, which is also about 1.14 feet of head.
 

shmeeps

Member
Feb 23, 2021
6
Dallas, Texas
Pool Size
22000
Surface
Plaster
Apologies, I'm not sure if if I'm following here. I see calculating the return at the maximum recommended speed to get the max GPM of 51. From what I'm understanding, the suction should also pull 51 GPM but spread proportionally across each suction line, with the longer one having less flow. I'm assuming elbows would need to be factored into each line as well? So something like
  • Return: 75 ft, ~8 elbows @ 51 GPM - 16.9 ft
  • (1) 85' Suction: ~4 elbows @ 13.3 GPM - 1.3 ft
  • (2) 45' Suction: ~4 elbows @ 18.9 GPM - 3 ft
So I'm assuming for the total, I'd add all of those, then the 14 ft for the filter and 8 ft for the multiport valve @ 51 GPM, respectively, and the extra foot of lift for a total of 44.2 ft? At that rate it seems like the 1/2 HP Whisperflo would be more than enough, though it sounds like having the flexibility of a variable speed might be worthwhile either to keep the running costs or noise down.
 

Jimrahbe

Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
21,152
Bedford, TX
S,

Noise wise, when I first got my IntelliFlo, I would often go over and touch it just to make sure it was still running.. :mrgreen:

Keep in mind that the idea that you have to "turnover" your pool water X times a day is a myth. It is just not true. Chemicals keep your pool sanitized and clear, not the number of magical times the water passes through the filter. The filter is there to capture all the stuff that falls into your pool, like bugs and leaves etc. It is not there to prevent your pool from turning green.

The main disadvantage of a VS pump is the initial cost. The upside is that you can adjust them to fit your needs. Basically they are a Variable Horsepower pump. This is real handy when you have a situation where you have different water flow demands at different times.. So, you might want a slow speed for normal operation, a different speed when running the cleaner, and another speed to run your waterfall, etc..

Another option is a 2-speed pump where it runs at half speed on low and full speed on high. These pumps cost less than a VS pump and can still run off of a 2-speed mechanical timer.

Good luck with your "Pump Hunt"..

Thanks,

Jim R.
 

JamesW

TFP Expert
Mar 2, 2011
23,864
When you are calculating head loss for multiple pipes on suction or return, you have to figure out how much water goes through each pipe.

For example, if you have a pool that is too full and you want to siphon some water off:

You have 3 rolls of flex PVC pipe at 100 feet each.

One roll is 1”, one is 1.5” and one roll is 2” flex PVC.

The pool is on a hill and the water level is 5 feet above the level where each pipe ends.

The head loss for each pipe will be 5 feet since that is the driving force.

If you use head loss charts, you will see that the flow for each is as follows:

Size......gpm.....% of total

1”..........9...........10

1.5”.....28...........31

2”........53...........59

Total = 90 gpm.

The total flow will be 90 gpm.

If you change the water height above the end of the pipe to 10 feet, the head loss for each pipe has to be 10 feet.

Size......gpm.....% of total

1”..........13...........10

1.5”......40...........31

2”.........77...........59

Total = 130 gpm

Now, the total flow is 130 gpm, but each pipe carries the same percentage of water.

When figuring out the amount of water going through 3 suction lines of different lengths and/or diameters, you know that the head loss for each pipe has to be the same because they all start and end at the same place. The pipes start at the pool and end at the suction manifold.

This means that the problem is the same as the examples above except that you only know the total flow and that the head loss is equal on all pipes, but you don’t know what it is.

The pressure at the pool is zero and the pressure at the suction manifold is the head loss.

For your situation, you know the total flow and you know that the head loss for each pipe is equal.

This gives you enough information to solve for the unknown variables.

There is an equation for calculating head loss as shown on the page with the head loss calculator (see below), which can be used to solve the problem, but it’s easier to just assume a head loss and figure out what percentage of flow will go to each pipe.

For example if we know that the total flow was 110 gpm for the 3 rolls of PVC, we can calculate the head loss by assuming that 11 gpm would go through the 1” pipe, 34 gpm would go through the 1.5” pipe and 65 gpm would go through the 2” pipe.

If you check the head loss for each pipe, you see that they are all 7.5 feet of head loss.

So, 110 gpm going to 3 separate pipes at 100 feet each where one is 1”, one is 1.5” and one is 2” will have a head loss of 7.5 feet.

When determining the “Total Equivalent Length” of a pipe, you measure the total length of the pipe and add a specific amount for each 90 degree fitting. For a 1.5” 90, you add 7.5 feet. For a 2” 90, you add 8.6 feet.

For example, if a 2” pipe is 70 feet long with (5) 90 degree elbows, you would add 70 + (5 x 8.6) = 113 feet total equivalent length.

Each piece of equipment has its own head loss chart that should be published in the installation and operation manual or available from the manufacturer.

You have to figure out each piece of equipment separately at the desired flow.

You typically want to run as slow as possible while meeting any flow requirements.

Things that need specific flow rates can include:

1) Skimmers to get good skimming action.

2) Heaters as specified in the manual.

3) Solar as specified per panel and total.

4) Fountains to get the desired effect.

5) SWGs to trigger the flow switch.

Skimmers can usually get good skimming effect at 7 to 15 gpm each.

If your only requirement is skimming, you can adjust the flow until the skimmer action just meets what you would consider to be good action.

I recommend a variable speed pump.

Do you have any specific flow requirements for anything?




 

JamesW

TFP Expert
Mar 2, 2011
23,864
Outside of that, my pump is about 1 ft above the pool surface,
Don’t worry about the elevation of the pump relative to the pool. The rise and fall cancel out.
my filter is a Hayward S244T2, and based on the head loss rates in its manual, 45 GPM would add ~13 ft of head loss to the system, or about ~9 ft at 38 GPM.
That looks correct.
I'm not sure if that includes the Hayward SP0714T multi-port valve on top, which adds in ~7 ft at 45 GPM, or ~5 ft at 38 GPM.
The multiport adds more head loss and has to be added.
I've read about using a vacuum and pressure gauge into the pump basket and pump volute, respectively, to measure the current TDH of the system, which might give me a better idea of what's going on underground, but I can't see anyway to get both gauges into the pump without separating the pump itself from the basket since the volute drain plug is between the volute and pump basket. Can you run these on a short hose and still get accurate results?
You can use a vacuum gauge to measure the head loss on the suction side, but it changes depending on the flow rate.

If you can measure the flow rate and the vacuum pressure, you can get an idea about the suction side system curve.

You can do a system curve for the suction side, the return side and the total system curve.

The system curve always starts at 0,0 and goes up exponentially based on the pump “Affinity” law, which states that the pressure (head loss) increases by a square factor.

For example, if the head loss is 30 feet at 40 gpm, it will be 22 = 4 times the head loss at 80 gpm.

From there, you can create a system curve.

GPM.....head loss in feet of water.

10...............1.9........((10 ÷ 40)2) x 30.

20..............7.5

30 ...........17

40............30

50............47

60............67

70............92

80.........120

If you plot the system curve on the same graph as the pump performance curve, you will be able to see the intersection, which will be the operating point.

https://aquamagazine.com/service/how-to-read-pump-system-curves.html

At any rate, I would recommend a variable speed pump, which will give you good flexibility.
 

shmeeps

Member
Feb 23, 2021
6
Dallas, Texas
Pool Size
22000
Surface
Plaster
I appreciate the in depth explanation.

Do you have any specific flow requirements for anything?

None that I'm aware of outside of skimmers. My setup is simple, 2 skimmers and the floor drain and a filter. No heater, solar, fountains, water features, etc.

Given that, is the Intelliflo worth the extra cost in my setup? I've seen the differences as mainly higher max power, more speed settings, no 120V, and the built-in diagnostics. I believe I'm set up on 240V but will have to double check. Not sure if the rest of that justifies the ~$600 price difference for me.
 

JamesW

TFP Expert
Mar 2, 2011
23,864
You can go with a 1/2 hp pump or a two speed pump.

You don't need a lot of flow.

Maybe consider the Pentair SuperFlo VS, which will take 120 or 240 and it's a good pump.
 

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shmeeps

Member
Feb 23, 2021
6
Dallas, Texas
Pool Size
22000
Surface
Plaster
Pentair SuperFlo VS

Somehow didn't get that in the other post but was the other VS I was comparing, and with the current rebate it's half the price of an Intelliflo. Figure having one that can pump a little more is a good idea if it's only a bit more than the Whisperflo I was originally thinking of getting. I'll go ahead and order a SuperFlo then.

Thanks a ton!
 
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