How much GPM is my pump producing?

[mkelley]

I have a 2 1/2 HP Sta-rite Maxi-Pro pump and can't find information anywhere on it that says what the GPM is -- only the RPM (even on the web -- I can find stuff like this page:

http://www.topdog.web.aplus.net/Merchant2/merchant.mvc?Screen=PROD&Product_Code=STAP6E6G-208L&Category_Code=PUMPSAND-1-STA

which is close although my own pump is 1/2 HP larger)

I'm trying to figure out how long I need to run it each day but without that info I don't know if it's possible.

[mas985]

You need two things to determine GPM, head loss and the pump head curve. The pump head curve you can find in your manual or here:

http://www.poolplaza.com/Max-E-Pro-techspecs.shtml

Head loss is usually between 45-65 ft of head which means your flow is between 110-135 GPM. Your probably near the lower end given the size of the pump (higher head). If you want a better estimate then you need a more accurate head measurement.

Total Head = Return Head + Suction Head
Return Head = 2.31 * Filter PSI
Suction Head = 1.13 * Pump Suction (in. mg.)

If you can't measure suction head which requires a vacuum guage, use this formula:

Total Head = 3.2 * Filter PSI (assumes suction is 40% of return head)

2 1/2 HP seems excessive for a circulation pump. Your turnover will probably be less than 2-3 hr.

[mkelley]

Thanks, Mark. That helps a lot.

Yeah, I know the pump is oversized -- even the solar guy was amazed at how big mine was (so to speak :>). It was just what they put in.

I had figured with 40 GPM it would turnover in six hours, so your estimate of 2-3 hours is probably right on the money. What I *don't* understand is whether I should be running the pump that little -- something doesn't seem right.

So... I guess my next question is whether if turnover is 2 to 3 hours, is it actually possible to run a pump that little? Is there not some sort of minimum time a pump needs to run regardless of the GPM, or is this just simple math and therefore that's it (I should also mention our water is always sparkling clear -- but I *was* running at 8 hours per day and then cut it down to 6).

[mas985]

First, let's get the details so we can narrow it down some.

What is your filter PSI? Pool volume?

To fully clean the water, you should not need more than 2 turnovers, Based on turnover, you can run the pump that short of a time for filtering but other things could drive you to a longer run time such as skimming the surface, solar or chemical distribution.

[tenax]

let's step back for a sec..i have a starite pump and the plate that has rpm on it has the max gpm on it. (i.e. mine says 48gpm for 3/4 hp pump) ...also, the other huge variable is the size of your fittings. i can tell you right now if you have 1.5 inch fittings, maximum gpm you will be able to run through it regardless of pump horsepower is 44gpm...it's simply as much volume of water you can push through pipes that size.

i have only the pool to pump (no spa) and 1.5 fittings so really, the 3/4 hp is perfectly mated at maximum 48gpm rating.

[mas985]

Tenax,

I know you mean well but the flow rate can exceed 44 GPM in 1.5" PCV pipe. In fact there is no physical limit on GPM given enough HP there is only a PSI limit where the pipe will fail. The specs for maximum flow rates are to prevent long term damage. This is why you will see older pools that have high HP motors with leaky plumbing.

[duraleigh]

Tenax and Mark,

Not to beat a dead horse, but I have seen that same flow limitation published on an otherwise authoritative website. (I think it was about choosing the correct pump size)

It doesn't make sense (if you think about it carefully) but they give no explanation for that gpm limitation and, as nearly as I remember, refer to it as an absolute.

You simply have to be skeptical of virtually everything you read especially when written as an absolute, irreffutable fact.

[mkelley]

Okay, I seem to be getting more confused here.

First things first -- I have at least 2" piping, if that helps (actually, the OD seems to be around 2 1/2 but I'm guessing we're talking ID). The filter PSI is 30 and the pool is about 11,000 gallons. I have a salt chlorinator if that means anything.

For the last month or so I've been running three hours in the morning and three hours in the afternoon and this works fine for solar and skimming. Based on the information here I've tried lowering it now to 2 and 2 (for a total of 4) as that's plenty for solar. Side note: in the winter I've always run the pump for only 5 hours (I read somewhere you didn't need to run it as much in the winter) and this was never a problem but now that I have solar I'll probably run it more (paradoxically).

Leaving aside all the controversy about piping and such, is there any way I would know (visually or otherwise) if I run the pump too little? I mean, will the water be too cloudy, or the balance get off or some other symptom that will tell me I need to increase the pump time?

As we're on a fixed income (and running this huge pump is *very* expensive) it would be really nice if I could run this as little as possible.

[tenax]

Tenax and Mark,

Not to beat a dead horse, but I have seen that same flow limitation published on an otherwise authoritative website. (I think it was about choosing the correct pump size)

It doesn't make sense (if you think about it carefully) but they give no explanation for that gpm limitation and, as nearly as I remember, refer to it as an absolute.

You simply have to be skeptical of virtually everything you read especially when written as an absolute, irreffutable fact.

but they had such pretty graphs!:) point taken. i do have a question for you and rather than hijacking this thread, i'll pm or email you if that is possible..thanks

[mas985]

If the 30 PSI you gave is with solar, then you have excessive return head and you cannot use the approximations given earlier. So your head loss is probably closer to 80 feet than 90 feet. This would result in about 70 GPM which gives you a turnover of about 2.5 hours.

I think your run times are more than enough as long as the pool stays clean.



As for velocity limits in PVC pipe, here are a few good links:

http://www.engineeringtoolbox.com/pvc-cpvc-pipes-pressures-d_796.html

The above link shows the bursting PSI which is really the limit for PVC piping and not flow rates. The 44 GPM comes from a recommended velocity limit in PVC piping of 7 ft/sec. However, this is not a hard limit.


http://www.engineeringtoolbox.com/pvc-pipes-friction-loss-d_802.html

This one you can see that the head loss just increase with GPM. The table stops at 60 GPM for 1 1/2" pipe but the following links can be used to calculate head loss for any GPM.

http://www.engineeringtoolbox.com/hazen-williams-water-d_797.html
http://www.engineeringtoolbox.com/darcy-weisbach-equation-d_646.html

Engineeringtoolbox.com is a great site to learn all about hydraulics.

Here is one more that gives some background on the limits:

http://www.poolandspa.com/page825.htm#Hydraulics

Sizing

The running water not only encounters friction created by pipes and equipment, but the water itself is creating friction. This friction will strip copper from pipes and heater components causing all kinds of havoc,9see the section on chemistry, damages filter grids, and makes DE or sand inefficient (see the filter section).

Because of this, most building codes set maximum flow rates of 8 feet per second through copper pipe and 10 feet per second through PVC. Since heaters all use copper heat exchangers, use 8 feet per second even if the plumbing is PVC. Los Angeles County, for example, allows a maximum flow rate of 8 feet per second on suction pipes of any type. What is feet per second in terms of gallons per minute?

50 gpm in 1 1/2-inch pipe = 7.9 feet per second

50 gpm in 2-inch pipe = 4.8 feet per second

60 gpm in 1 1/2-inch pipe = 9.5 feet per second

60 gpm in 2-inch pipe = 5.7 feet per second

By the way, there are a few exceptions to the rules. Los Angeles County requires pumps to deliver the desired gallons per minute at 60 feet of head. When sizing pumps, you must assume at least 60 feet of head regardless of the actual calculations. In filters, on the other hand, you must use the actual head as measured. Altitude also affects these calculations. Over 3300 feet above sea level a motor runs hotter, so you will want to upgrade to the next horsepower.