PB installed elbow right before pump inlet

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DanWC

Active member
Aug 17, 2023
38
Kentucky
Pool Size
17890
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-20
Was posting over on Reddit about something unrelated to this and someone brought this to my attention. I was unaware that there should be 5x the size of the pipe or straight run for the pump inlet.

My questions are:
1. Is this a huge issue that needs to be corrected?
2. What issues can this cause? The main thing I am reading from google is cavitation. From what I can see and hear I do not think this is happening currently.
3. Is fixing this as easy as adding a couple more 90’s to create space for 10” of straight run?

The pump is a Pentair variable speed pump. So I plan to run pump 24 hours a day at a lower setting most of the day.

I don’t necessarily want to ask the builder to redo plumbing in this spot, but I will if this will be a major issue.
 

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This is actually a very good question because I, too, know about the rule that the pipe entering the pump needs to be five times as long as its’ inner diameter.

As to the question of how much of an issue that really is, I think that’s difficult to completely and confidently answer. I believe it is more of a preventative measure because it is known that turbulent water is very chaotic and one should try to reduce turbulence as much as possible for the water entering the pump so the pump can properly prime and continue sucking and so your aforementioned cavitation will be reduced as much as possible, because cavitation is a very good friend of turbulence. Having a straight run of pipe before the pump is one of the easiest ways to help with the turbulence problem because forcing moving water in a pipe to turn 90 degrees is a recipe for turbulence.

Very curious what our experts have to say.
 
It makes no difference.

The requirement only matters if the pump does not have a strainer or if the water velocity is high.

As long as the velocity is below 6 feet per second, it makes no significant difference.

The 90 is no different from any other 90.

You can use sweep 90s to reduce the head loss and noise, but at low velocity, it makes virtually no difference.

Pipe Size and Flow Rates.​

You want to keep the water velocity below 6 ft/sec on the suction and below 8 ft/sec on the pressure side of the pump.

Pipe Size6 ft/sec8 ft/sec
1.5"38 GPM51 GPM
2"63 GPM84 GPM
2.590 GPM119 GPM
3.0"138 GPM184 GPM
4.0"235 GPM312 GPM


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PVC Pipes - Equivalent Length and Pressure Loss in Fittings

Minor loss in PVC and CPVC fittings expressed as equivalent length of straight pipe.
www.engineeringtoolbox.com www.engineeringtoolbox.com

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Dan,

Based on your plumbing layout, it is just a stupid way to connect the pump, but it will not matter in the least.

Half the pool pumps in the world have almost no straight pipe before the pump, and no one can tell the difference..

In my mind, running 24/7 at a low RPM is the only way to go.. :goodjob:

I'd rate Reddit as a 2 and TFP a 10... But I am a little biased.. You should check us out. :mrgreen:

Thanks,

Jim R.
 
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You should never need more than about 40 gpm.

Even if you went to full speed and you could get to 100 gpm, the head loss is only about 0.7755 feet for the 90 at 9.8 feet per second (5.17 x 15/100).

At full speed, you would generate a lot of noise and turbulence at every 90, especially if you had any sort of air in the lines, but it should not cavitate.

You could possibly cavitate if the head loss exceeded 17 feet for the entire suction line, but that should not be an issue.

2" pipe is fine up to about 63 GPM.

If you need more than 63 GPM, for any reason, then the line is probably undersized.

I would estimate that the top speed you would ever need would be about 2,200 RPM.

Do you have a heater?


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PVC Pipes Schedule 40 - Friction Loss vs. Water Flow

Water flow in thermoplastic PVC and CPVC pipes Schedule 40 - friction loss <i>(ft/100 ft, psi/100 ft)</i> and flow velocities at dimensions ranging 1/2 to 16 inches.
www.engineeringtoolbox.com www.engineeringtoolbox.com

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Jimrahbe said:
Dan,

Based on your plumbing layout, it is just a stupid way to connect the pump, but it will not matter in the least.

Half the pool pumps in the world have almost no straight pipe before the pump, and no one can tell the difference..

In my mind, running 24/7 at a low RPM is the only way to go.. :goodjob:

I'd rate Reddit as a 2 and TFP a 10... But I am a little biased.. You should check us out. :mrgreen:

Thanks,

Jim R.
Click to expand...
I appreciate your response!! Will definitely be coming here for any futures questions. Someone on Reddit actually suggested I post here as you guys are experts. :mrgreen:

I have been stalking the forum for a bit as I have been trying to learn more and more about pool maintenance.

We are not moved into our house yet so pool upkeep is still being done by pool company as they are still not 100% done with hardscape. But I want to be 100% ready when it’s finally turned over to me.
 
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For water velocities below 6 feet per second, 90s are basically not a problem.

At water velocities between 6 and 8 feet per second, 90s are a minor issue and sweep 90s are recommended.

At water velocities above 8 feet per second, the 90s become a significant issue.

At 80 GPM, the water velocity is about 7.8 feet per second.

Below estimates your flow range at any speed.

At full speed, your flow will probably be between about 57 and 80 GPM.

So, even at full speed, the 90 is only a minor issue.

I would try about 1,725 RPM, which is half speed and it should put the flow between about 30 to 40 GPM.


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