Replumbing pump, avoiding cavitation

[IceShadow]

Current setup is below. I need to replace the Jandy valve on the suction side. A couple of the screws are stripped out and the plastic loops on the top of the cap where the screws go are broken on several screws. I got a new Jandy valve and am ready to start. I also plan on cutting the skimmer / side drain pipes shorter and lengthening the pipe going up from the pump so it can rest on the ground without a board and bricks.

When I run the pump at full speed with one side of the Jandy valve closed I notice some cavitation. To avoid this I’ve had to leave the Jandy valve never fully closing off one side or the other. I’d like to avoid that cavitation if I can. Is there something I can plumb to do so? Would using 2” pipe from the Jandy valve to the pump help, or would it just push the cavitation back to starting earlier?

 

[ajw22]

Why do you want to stop all suction from the side drain pipe?

It sounds like your skimmer cannot provide all the flow the Intelliflo can take. What is wrong with giving it more water from the other pipe?

What RPM does the cavitation start?

 

[JamesW]

Why do you think that it is cavitating?

If you see air, that’s not cavitation.

If you see air, that’s a suction side air leak.

Cavitation is usually not visible and it sounds like pumping rocks/gravel through the impeller.

You can measure the suction with a vacuum gauge to see what the vacuum pressure is.

If it really is cavitating, you should just lower the RPM until the cavitation stops.

Maybe the line is clogged or too long.

There’s not much you can do above the ground now to make any difference if it is real cavitation.

 

[JamesW]

It looks like the area floods periodically and I would keep the pump on a block to keep it elevated at least by 2 inches.

You can glue the block down to reduce vibration from the pump to the ground.

 

[Flying Tivo]

+1 on the pump above ground.

 

[JamesW]

For suction, you want to keep the water velocity below 6 ft/sec. For returns, you want to keep the water velocity below 8 ft/sec.

Size.......6 ft/sec......8 ft/sec.
1.5"...........38...............51 gpm
2"..............63...............84 gpm
2.5............90.............119 gpm
3.0".........138.............184 gpm

What flow rate are you trying to get?

If you want more than 38 gpm, you should open both lines.

 

[IceShadow]

Why do you want to stop all suction from the side drain pipe?

It sounds like your skimmer cannot provide all the flow the Intelliflo can take. What is wrong with giving it more water from the other pipe?

What RPM does the cavitation start?

I don’t know that the side port offers much benefit to normal use. I’m assuming it’s there as a second suction line and possibly for a suction-side cleaner the old owners may have had. If it’s all coming from the skimmer then skimming action would be improved right? And it seems vacuuming would be improved as well when I do that with a vacuum plate over the skimmer basket. Edit: I notice the cavitation when one of the two suction side pipes is stopped and the pump is above 3000 RPM or so.

Why do you think that it is cavitating?

If you see air, that’s not cavitation.

If you see air, that’s a suction side air leak.

Cavitation is usually not visible and it sounds like pumping rocks/gravel through the impeller.

You can measure the suction with a vacuum gauge to see what the vacuum pressure is.

If it really is cavitating, you should just lower the RPM until the cavitation stops.

Maybe the line is clogged or too long.

There’s not much you can do above the ground now to make any difference if it is real cavitation.

It’s the sound. Definite sound of gravel passing through as you mentioned and matches some videos I watched on cavitation.

The issue for RPM is that my pump sometimes stops, throws an error, then immediately restarts, and when it restarts it goes into prime mode and does full 3450 RPM. Never figured out why and not sure how to correct that issue so I’ve been living with it.

It looks like the area floods periodically and I would keep the pump on a block to keep it elevated at least by 2 inches.

You can glue the block down to reduce vibration from the pump to the ground.

I don’t know that the area floods (it’s next to my chimney, see photo below) but I can get some 2” pavers and put it below the pump as insurance.

 

[IceShadow]

For suction, you want to keep the water velocity below 6 ft/sec. For returns, you want to keep the water velocity below 8 ft/sec.

Size.......6 ft/sec......8 ft/sec.
1.5"...........38...............51 gpm
2"..............63...............84 gpm
2.5............90.............119 gpm
3.0".........138.............184 gpm

What flow rate are you trying to get?

If you want more than 38 gpm, you should open both lines.

I don’t have a way to measure flow rate or really know what I’m looking to achieve. The thought was just trying to avoid possible cavitation under the circumstances that cause it, which is one suction port closed off, and pump at full RPM. Once it’s down to about 3000 or below it doesn’t cavitate. (Is that a word? Spell check doesn’t think so!)

If there really isn’t a way at this point to avoid it I’ll just keep the side port slightly open as I have been and use standard 1-1/2” pipe and replumb it as it currently is with the pipe slightly lower. Just trying to take advantage of the work to change it up a bit if there’s a way to add some insurance against cavitation and potentially damaging the impeller or other pump parts.

 

[Flying Tivo]

when it restarts it goes into prime mode and does full 3450 RPM

You can re-program that on the pump as to not go full throttle. It does sound more like a suction side airleak.

 

[JamesW]

The issue for RPM is that my pump sometimes stops, throws an error, then immediately restarts, and when it restarts it goes into prime mode and does full 3450 RPM. Never figured out why and not sure how to correct that issue so I’ve been living with it.

 

[Andy49]

I remember reading somewhere that there should be a straight run of pipe into the pump (3 to 5 diameter pipe length) If you move your union either to the pump inlet or closer to the valve, it may work. Not sure where I read that.

 

[ajw22]

I remember reading somewhere that there should be a straight run of pipe into the pump (3 to 5 diameter pipe length) If you move your union either to the pump inlet or closer to the valve, it may work. Not sure where I read that.

That is in the Pentair pump installation manuals.

 

[JamesW]

The straight pipe rule is mostly irrelevant and it really does not matter in my opinion as long as the velocity is less than 6 ft/sec.

For a pump with no strainer basket where the pipe goes straight into the eye of the impeller, then it makes a difference, but not when you have a strainer basket.

 

[IceShadow]

Thanks guys - sounds like the solution is to disable automatic priming upon start. Then I when I need to prime, I can manually tell it to go full bore and open both suction side valves when the priming starts up.

 

[Oly]

That looks like a sweet brick house Dustin.
I have the same SuperFlo VS pump and disabled the prime setting. My Pump #1 is scheduled to start at 2300rpm for an hour before the long #2 run at 1400 for the SWCG then a final hour for #3 to polish the surface and displace any air under the pump lid and ready for the next run. No leakage overnight. And yes if I open my system then I prime manually with the quick clean button and ramp up the rpm until charged fully.

 

[JamesW]

Below is what the manual recommends.

You can use sweep 90s into the pump.

If possible, it is advisable to follow the manual for the straight run, but I would not worry about it if it was not easy to do.

Also, the pump should prime just fine at lower speeds like 2,400 rpm unless the pump is really high above the pool.

I would set the priming speed at about 2,400 rpm to about 2,750 rpm and that should work fine.

 

[JamesW]

As you can see in these cutaway views, the inlet does not line up with the impeller and the water needs to do two 90s to get to the impeller.

 

[JamesW]

With no strainer, you would want the pipe to go straight in.

The strainer acts as a reservoir and the inlet just fills the strainer.

In my opinion, as long as the water velocity is less than 6 feet per second, the straight pipe rule really does not make any significant difference.

 

[IceShadow]

That looks like a sweet brick house Dustin.
I have the same SuperFlo VS pump and disabled the prime setting. My Pump #1 is scheduled to start at 2300rpm for an hour before the long #2 run at 1400 for the SWCG then a final hour for #3 to polish the surface and displace any air under the pump lid and ready for the next run. No leakage overnight. And yes if I open my system then I prime manually with the quick clean button and ramp up the rpm until charged fully.

Thank you. We really lucked out when buying it. They must have spent a million to build this place back in the mid-90s but the city it’s in doesn’t support the property values so we got the house for less than half of that.

 

[jedigrover]

That is in the Pentair pump installation manuals.

True. It’s also standard engineering practice for centrifugal pumps. The reason for this is to get a good straight flow into the impeller. If you have disturbances too close to the impeller intake, the flow in the pipe will be imbalanced putting radial stresses on the impeller and shaft. Over time, it will kill the motor bearings.

Edit: as @JamesW points out, the pump basket basically violates all of that, so it’s much less important.