Flow rates in above ground pumps??

poolfilterman

Member
May 25, 2015
6
Nokomis, Fl
I decided to check a popular 2500 GPH pump for above ground pools, to see what the actual flow rate was and used a separate tank so I could actually measure flow correctly.

Installed a pressure gage into the lid of the canister and measured flow at startup pressure of 0. I got 20 GPM or 1200 GPH and at 2 psi, got 720 GPH or 12 GPM. Actual max pressure output was 5 PSI.

Removed pump end to check the design and found the impeller to be much smaller than the pump housing (volute). The smaller impeller allows it to slip through water (not pumping). The slippage becomes worse as dirt builds up on the cartridge.

' The inlet hose size is 1 1/2" diameter which is adequate, however, the transfer pipe from the pump to the canister (inside the pump enclosure) is only 1" diameter. The small size pipe increases friction loss and drops flow rate considerably.

The return hose is 1 1/2" which is adequate for its length.

It does not take much, to plug a cartridge, at 5 PSI. and cleaning the cartridge does not remove all the small particles lodged in the pleat fabric. Consequently, cartridge life is short and needs to be thrown away.
 
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Jimrahbe

Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
26,099
Bedford, TX
PFM,

I am not sure I agree with the idea that low pressure clogs a filter faster.. I have an inground pool, but not sure that matters.. My filter pressure is 1 or two lbs and I get plenty of flow and my current cartridges are over 7 years old.

Filter pressure is the amount of effort it takes to force water through a filter. The lower the pressure the better.

Maybe I just don't fully understand your theory.. :scratch:

Thanks,

Jim R.
 

poolfilterman

Member
May 25, 2015
6
Nokomis, Fl
I'm sorry, low pressure means there is little back pressure from the filter cartridge and flow is maximum.

The issue I found was the above ground filter pumps maxed out at 5 PSI. The low maximum pressure allows the cartridge to plug faster. Your inground pump probably maxes out at 35 psi, meaning your cartridge lasts longer.
 

poolfilterman

Member
May 25, 2015
6
Nokomis, Fl
The maximum output of the above ground filter pump was only 5 PSI. That compares to your inground pump maxing out a 35 PSI. In both cases flow is high when pressure thru the cartridge is low, but, to get the filter to last longer between cleanings, you need higher pressure. Since 5 PSI is max on our above ground pump, it takes less dirt to plug it and consequently plugs it faster.
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
15,162
Pleasanton, CA
Pool Size
20000
Surface
Plaster
Chlorine
Salt Water Generator
All of that is fairly consistent with the really cheap AG pumps. They are not very powerful.

What is the actual model# of the pump?

I'm sorry, low pressure means there is little back pressure from the filter cartridge and flow is maximum.

The issue I found was the above ground filter pumps maxed out at 5 PSI. The low maximum pressure allows the cartridge to plug faster. Your inground pump probably maxes out at 35 psi, meaning your cartridge lasts longer.
The maximum operating pressure of a pump filter system really doesn't have much to do with how fast a filter will clog. The only thing that really determines how quickly a filter will clog is the size of the filter and the flow rate.

The maximum output of the above ground filter pump was only 5 PSI. That compares to your inground pump maxing out a 35 PSI. In both cases flow is high when pressure thru the cartridge is low, but, to get the filter to last longer between cleanings, you need higher pressure. Since 5 PSI is max on our above ground pump, it takes less dirt to plug it and consequently plugs it faster.
Not true. Again, it is the size of the filter and flow rate that matters, not the pressure. A pump operating at low flow rate and thus it will take longer to clog a filter than the pump running at higher flow rates (i.e. more water is being filtered).
 
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poolfilterman

Member
May 25, 2015
6
Nokomis, Fl
Sorry, let me try to explain more clearly:
1. I agree, you get maximum flow through the cartridge when there is little dirt build up on the cartridge, which means low back pressure
2. As dirt builds up on the cartridge, it creates resistance to flow.
3. Inground filter system pumps can deliver pressures up to 35 PSI and sustain flow rates, although lower, for longer periods of time (not recommended).
4. Cartridge fouling on above ground pool cartridges, affects flow dramatically because of the pumps design. The impeller slips through the water when it comes up against resistance, or back pressure. Since the maximum PSI is only 5 PSI and the impeller slippage, the cartridge does not last nearly as long as the inground pool filter cartridge. Thus, it plugs faster and must be changed frequently.

Hope I may have answered your questions and thank you for the thoughtful challenge.
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
15,162
Pleasanton, CA
Pool Size
20000
Surface
Plaster
Chlorine
Salt Water Generator
Still disagree with #3 & #4.

Since the maximum PSI is only 5 PSI and the impeller slippage, the cartridge does not last nearly as long as the inground pool filter cartridge. Thus, it plugs faster and must be changed frequently.
While there may be scenarios where that is true, I don't believe that is true in every scenario. There are just too many variables to make a general statement like that.

Pumps with steeper head curves will be affected more with a dirty filter than pumps with a flatter head curves. Also, smaller pumps have lower flow rates so it takes more pump run time to collect the same amount of debris as a larger pump would on the same filter. If you run the two pumps for the same amount of time per day and you only clean the filter with a 25% rise in PSI, as TFP recommends, it will take longer for the smaller pump to reach that level than it will for the larger pump.
 
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