Estimating TDH From Pump Curve?

[zachary.cayou]

Would it be valid to estimate TDH based on the pump curve and the RPM in where water from all return jets seems to stop? Not trying to achieve exact numbers, just ballpark. I see tons of sources using head to estimate flow, but not the other way around.

Seems intuitive, but I’m a little uncertain as I do get flow out of my returns all the way down to 750 rpm, which should put head at less than 10 ft… and I’ve read several places that average head for a pool is around 40-60 ft.

 

[ajw22]

When @mas985 passes by I think he can help you.

 

[mas985]

Would it be valid to estimate TDH based on the pump curve and the RPM in where water from all return jets seems to stop? Not trying to achieve exact numbers, just ballpark. I see tons of sources using head to estimate flow, but not the other way around.

No you can't do it that way. TDH is not a constant and it is actually a separate head curve much line the pump's head curve (see below). However, there are other ways of determining TDH.

If you have RPM and GPM, then you can look up TDH from the head curve. But you need two of the three parameter to do it that way. Otherwise, you need the plumbing curve which is not trivial to determine.

Another way to determine TDH, is to use the filter pressure and some knowledge of the suction side plumbing. I have a spreadsheet in my signature that does this.

But in most cases, knowledge of TDH is not really necessary. Why do you need to know it?

 

[zachary.cayou]

Not exactly sure what you mean. If you have RPM and GPM, then yes you can look up TDH from the head curve. But you need two of the three parameter to do it that way. Otherwise, you need the plumbing curve which is not trivial to determine.

Another way to determine TDH, is to use the filter pressure and some knowledge of the suction side plumbing. I have a spreadsheet in my signature that does this.

But in most cases, knowledge of TDH is not really necessary. Why do you need to know it?

Thanks. Since I don’t have a way to measure flow directly, the idea was maybe I could find the RPM where flow goes to zero (water stops returning from all jets) to figure out the TDH, and then use that value against the curves to estimate the flow rate at higher RPMs. That make sense?

 

[mas985]

Thanks. Since I don’t have a way to measure flow directly, the idea was maybe I could find the RPM where flow goes to zero (water stops returning from all jets) to figure out the TDH, and then use that value against the curves to estimate the flow rate at higher RPMs. That make sense?

No it doesn't. I had modified my answer above while you were posting you might want to read it again.

 

[zachary.cayou]

No it doesn't. I had modified my answer above while you were posting you might want to read it again.

See it now. Ah okay, I was mistaken thinking TDH remained constant across different RPMs. That makes sense, appreciate it!

 

[mas985]

Why the interest in TDH?

Many members make the mistake thinking that turnovers matter when they really don't so I just want to make sure that is not the reason.

 

[zachary.cayou]

Why the interest in TDH?

Many members make the mistake thinking that turnovers matter when they really don't so I just want to make sure that is not the reason.

I was more interested in calculating flow. Not for making adjustments for turnover, mostly curiosity and to make sure it is in spec/optimal when running the heater for example.