Reading Pump Charts

[wwhitney]

I'm familiar with the idea of a pump curve, and that if you have a distribution system performance curve (how much water will flow in the pipes to the outlets for any given constant pressure source attached to it), the system behavior is determined by where the two curves cross.

So what does it mean when a published pump curve doesn't intersect both axes? Say one endpoint is 5 gpm at 50 psi; why isn't the curve extended to show (to make up some numbers) 1 gpm @ 52 psi and 0 gpm @ 53 psi? Is the manufacturer indirectly saying "do not operate below 5 gpm?"

A related question on charts, like the one below that came up in another thread, for this pump: https://www.pentair.com/en-us/produ...2-hp-thermoplastic-sprinkler-system-pump.html That pump is specified as "maximum discharge pressure 50psi". So what happens if you hook up the 2HP model to a system that will flow 10 gpm at 50 psi and 14 gpm at 60 psi?

Or what happens if you connect the 1.5 HP model to a system that will flow 15 gpm at 45 psi and 18 gpm at 50 psi? Should the - entries be read as 0, so that with 5' suction head the 1.5 HP model has a maximum outlet pressure of somewhere between 45 psi and 50 psi?

Lastly, for pumps in general, does inlet suction pressure trade-off one to one with outlet pressure (for suction pressures below ambient air pressure)? I.e. if you have the pump 7.3' above the water source, is the outlet pressure 1 psi less than the 5' row for each of the flow rates given, as 2.3' = 1 psi? If so, can we conclude the 1.5 HP model would actually do ~ 8 gpm at 5' suction head and 50 psi outlet pressure, even though that table entry is not shown? [Should be close to the table value for 15' suction head and 45 psi outlet pressure, as 10' ~ 5 psi.]

Thanks,
Wayne

P.S. I hope the phrase "suction head" is OK, I'm just using head as a synonym for pressure.

 

[Reach4]

So what does it mean when a published pump curve doesn't intersect both axes? Say one endpoint is 5 gpm at 50 psi; why isn't the curve extended to show (to make up some numbers) 1 gpm @ 52 psi and 0 gpm @ 53 psi? Is the manufacturer indirectly saying "do not operate below 5 gpm?"

Something like that. It could be to keep you into the efficient part of operation, or in the case of high flows help you to avoid an upthrust condition.

 

[Sponsor]

 

[Valveman]

Charts are made from the pump curve. You can extrapolate all the way down to zero flow at shut off head.

 

[wwhitney]

Charts are made from the pump curve. You can extrapolate all the way down to zero flow at shut off head.

Great, thank you. Can you comment on whether inlet suction pressure is a 1-to-1 tradeoff with outlet discharge pressure?

And if so, any thought on why the above chart doesn't include a value like 6-8 gpm at 50 psi for the 5' suction head row on the 1.5 HP pump? Since it will do 8 gpm at 45 psi outlet for 15' suction head.

Cheers, Wayne

 

[Valveman]

Yes the suction lift comes right off the max head the pump can build. So, the 2HP in the chart probably has a max pressure of like 54 PSI because it doesn't show any flow at 50 PSI and 15 feet of lift. The 1.5HP probably maxes out at 52 or 53 PSI.

 

[wwhitney]

Yes the suction lift comes right off the max head the pump can build.

Great, thanks again. One more question about what looks like an error in the chart above:

Since 5 psi = 11.5' of head, for any entry in the chart on the 5' or 10' suction lift row, if you go down two rows (+10' suction lift) and left one column (-5 psi discharge pressure), I expect the flow rate to be the same or slightly higher. And that's true basically everywhere (sometimes the flow rate goes down 1 gpm) except in one area of the chart: for 15-20 psi discharge, 5-10 ft suction lift, and the 1.5 HP pump. There the difference is alot, the wrong way: 66 gpm vs 56 gpm ; 61 gpm vs 55 gpm; 58 gpm vs 46 gpm ; and 56 gpm vs 45 gpm.

So is that necessarily an error in the chart, or is there some other pump phenomenon that would explain the discrepancy?

Cheers, Wayne

 

[Valveman]

Lol! You probably know more about it than the guy who made the chart. I would have to see a curve to know if the numbers are correct. At high flow the pump is working way to the right of the curve and there could be several things effecting it like NPSH, recirculation, and even cavitation.

 

[Fitter30]

That chart only represents that pentair pump subtracts 2.31' per lb when height of water is submerged deeper that the 5'. All types of pumps are different.