Pool pump "feet of head" quandry...

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OK, I did a bit more work on this and came up with two plumbing scenarios. Best case (1/2 the fittings and 1/2 equipment loss) and worst case (same fittings and equipment as my pool). I also added a 3/4 HP Northstar pump vs the 1.5 HP Superpump which both have the same BHP and thus about the same energy use. Here is a chart showing the results.

SuperPump.gif


NS is Northstar
SP is Superpump

You can see even the 3/4 HP Northstar pump would work for your plumbing although the best case would be close to the flow limits. With this hopefully, it will help you make a decision on what to do next.

You can also see that the Northstar provides more GPM and Head loss for the same input power (BHP).
 
There are two ways you can do it. There are tables such as this but I can tell you from experience that the error is quite large. I believe that there is a large variance in manufacturing and surface roughness which can create a substantial error. Also, it is difficult to find data for skimmers, main drains, eyeballs, etc that are pool specific and which contribute to most of the head loss.

So what I did was work backwards. The things I know about my pool are:

Suction measured at pump
Suction pipe lengths
Pressure measured at pump
Pressure pipe lengths
Pressure measured after heater (used to get equivalent lengths of pad plumbing)
Total Head from pressure and suction measurement
Total flow rate - lookup on head curve

Using Hazen-Williams Equation, I can work backwards to find equilvalent length of pipe. It is a bit easier than the Darcy-Weisbach equation and accurate enough.

So by backing out the fixed pipe lengths for each section, you are left with the equivalent lengths of the fittings only. The fixed lengths are then replaced with what Backglass gave and the fittings equivalent lengths are scaled by the by the diameters. This is done since fittings equivalent lengths tend to be proportional to the pipe diameter.

So although not an ideal calculation, it is about as good as you can get with the available data. You just have to keep in mind that there is some uncertainty in the calculation but it is better than nothing.
 
Thanks everyone for all the replies. Since the numbers aren't awful, I am going to chance it. Although I could send it back for a smaller one, the kids are screaming to swim and dont want to wait another two weeks for dad to get started (not to mention added shipping - 40 pounds!). Since I have to re-pipe everything anyway (previous owner sawzalled out the old pump) I will put in a bypass section between the intake and output lines before the filter so I can dial down the filter psi if it's too high.

I will report back with filter PSI when it's up and running. Sound like a plan?
 
Backglass,

The bypass is a good solution but you will need to take that into account when figuring your run time since you are bypassing the filter. Also, the pressure guage may not read the same increase when the filter gets dirty.

Another way to reduce flow rates is to add return head loss somewhere (i.e ball valve inline with the return anywhere on the pad). In some ways this is preferable to a bypass since more head loss means less energy consumption in the pump for the filtering flow rate and your turnover is a bit easier to figure out. But my guess is that you won't need it.

Also, my guess on filter pressure is about 15 PSI.

[EDIT] Updated chart in previous post. I realized that I had the pad plumbing as 2" instead of 1.5". This adds more head loss so I think you will be more than ok.
 
mas985 said:
Also, my guess on filter pressure is about 15 PSI.
Click to expand...

Close...12psi clean! I finally got around to hooking everything up today...had to re-do a bunch of split PVC, evidently the previous owner didn't drain everything completely before winter.

I think I'll be OK with the 1.5hp pump. Everything is looking good...except for the lovely green color of the water. The pool hasn't been open in two seasons. :shock:

Thanks so much for all your advice!
 
I am glad it came out close since I was a little concerned that the pressure was lower. It could mean you had more suction head loss which is not a good thing. You still have quite a bit but unless there is air in the pump lid, I would not worry about it. Here are the head numbers.

Suction Head = 25.12 ft
Return Head = 31.32 ft.
Total Head = 56.44 ft.
Flow Rate = 42 GPM

You could get closer with a suction measurement but what's the point.

Suction head is very close to return head which is usually not a good situation but as long as the pump does not complain, then you are probably ok.
 
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