Which variable-speed pump for lots of head?

[Pegasus_RPG]

Hello everyone. Been lurking here for awhile and now have my own questions.

I currently have a single-speed pump that uses a lot of electricity and is oversized for running just the spa. (I get a ~10 PSI increase when I close off all supply lines except the 5 spa jets that are fed from a single 2" pipe.) So I'm looking at replacing the pump with a variable speed one to solve both problems.

As part of the process, I measured all of the pipe runs and since I can't see underground, assumed 90-degree elbows everywhere. At 70 GPM, this gives me a figure of 127 feet of head, which seems to be off the charts for every pump I've looked at. So do I need to re-check my math, or buy pressure & vacuum gauges and read the numbers off of the existing pump? (If total dynamic head is even the same thing as pipe friction head loss.)

 

[Pegasus_RPG]

Aargh, just discovered I screwed up the spreadsheet (moved columns around and the formulas didn't keep up.) I actually have 54.5 ft of head at 70GPM. Whew! That opens up a few more options.

 

[CJadamec]

Forget all the calculations about pipe length and all that nonsense.

You have a working system and you have a pressure gauge. 1psi on the filter = 2.3 ft of head on the pump its that simple.

If you want to go with a VS pump the intelliflo or similar 3 HP VS pump from other major brands would be the way to go. In practical terms you can't over size a VS pump.

 

[Pegasus_RPG]

You have a working system and you have a pressure gauge. 1psi on the filter = 2.3 ft of head on the pump its that simple.

Really? What about the vacuum pressure on the suction side? (And the losses from the three elbows between the pump and the filter? Yes, it's plumbed a bit inefficiently, especially the heater. Looks like the installer didn't know you could turn the control panel around.)

 

[jblizzle]

You are way overthinking this. There is hardly ever a need to calculate head loss. 10 psi on your filter is not very high at all

 

[Pegasus_RPG]

lol, sorry I'm an engineer. Overthinking is what I do.
With my filter clean and valves set in typical circulation, the gauge on my filter reads 20PSI. (When valves are set to spa-only on the return and supply side, it reads about 30PSI.) So by that math I've got about 46 feet of head in typical conditions and 69 when running the spa only.

Once I get the new pump, how do I determine the ideal (i.e. most efficient) speed for each of the valve configs? Get a flow gauge too and set the speed to whatever maximizes WEF (kgal/kWh)?

 

[jblizzle]

For filtration you want to run the pump as low as possible while still maintaining enough flow for skinning and or cleaning and or chlorine generation. For the spa, you adjust the pump speed to give you the type of jet action you like.

No need to overthink it

 

[Jimrahbe]

RPG,

Filter pressure is not a good thing... When I run my 3 HP pump at 1200 RPM, my filter pressure is only about 1 lb. yet I have plenty of return flow.. Granted, my pool is simple, but the same basic operation applies. When running the pool, slow is better.

I can't think of anything that you would need a flow gauge for.. It might be fun to have, but not at all necessary..

Thanks,

Jim R.

 

[CJadamec]

Really? What about the vacuum pressure on the suction side? (And the losses from the three elbows between the pump and the filter? Yes, it's plumbed a bit inefficiently, especially the heater. Looks like the installer didn't know you could turn the control panel around.)

At the end of the day once the pump is running and primed there is effectively 0 suction losses. Suction loss only occurs when the water isn't returning to where it was pulled from. Gravity cancels out.

As for everything else head loss is dynamic and relies on both pipe restriction and the amount of water trying to go thru the pipe. The real world implication and measurement of that dynamic effect is the system pressure on the discharge side of the pump.

As the others have said you have a simple pool set up and you really don't have any reason to be concerned with the head loss of the system.

If you get a VS pump you simply increase the RPM to get the flow you want out of a return or spa jet. Or set the RPM just a little bit higher than needed to close the flow switch on the heater.

 

[Oly]

Before I changed by pump I tapped into my system to measured the vaccume and pressure and calculated my head and friction losses. Then I was directed to some great information by mas985, Mark. Check out this information and play around with his spreadsheets at the second link, I am amazed at the info available.

Hydraulics 101 - Have you lost your head?

Edit - Before reading this article, you may want to first familiarize yourself with this Pool School article on Pump Basics Hydraulics 101 - Hydraulics, Pipes and Filters When building my pool, I did a significant amount of research on swimming pool plumbing and water hydraulics. I also found...

www.troublefreepool.com

Hydraulics 101 - Have you lost your head?

Edit - Before reading this article, you may want to first familiarize yourself with this Pool School article on Pump Basics Hydraulics 101 - Hydraulics, Pipes and Filters When building my pool, I did a significant amount of research on swimming pool plumbing and water hydraulics. I also found...

www.troublefreepool.com