Pipe sizing

[Yoav]

Hello all.

I am trying to deal with an air leak for the second time now. I managed to seal the first a while back but this one seems to be somewhere underground in the suction side pipe.

Before going through digging up the pipe, I am considering buying a new pipe, installing it between the skimmer and the pump and verifying that it is indeed the problem.

Now that I'm about to get a new pipe, I was wondering about the proper size. I believe what I have now is a 1.5" piping (or 50 mm - I can't remember at this point which measurement is used for what...).
Since currently, the filter pressure is somewhere near 0, the return flow seems quite slow, and the pump seems to be straining, I am thinking of trying a 2" pipe on the suction side.

Is there a problem with using different sized piping on the suction and return lines?

I have noticed installations with multiple "in" valves (lines in parallel) and only a single return pipe. I even use the same configuration when manually vacuuming the pool bottom (during which the system seems to work better - pressure goes up to 6 psi, return flow is adequate, and the pump sounds better).

 

[Bama Rambler]

1½" pipe is 40mm, 2" pipe is 50mm. If I were replacing it I'd go with 50mm (2") as far as I could.

There's no problem having larger pipe on the suction side than on the discharge side. It's actually common practice in the industrial world.

Multiple suction runs from the pool to the equipment pad is actually preferred to a single run tying everything together at the pool. It gives you greater control not to mention lower suction head loss.

 

[Yoav]

Thanks Dave.

I clearly remember that I measured the exterior with a caliper at 50mm so I guess my piping is 2" all round. Which means I should use a 2.5" (63 mm) pipe at the suction side to lower the head loss there.

If I understand correctly, doing so would mean:
1. Water velocity on the suction side should either stay the same or drop slightly
2. Flow rate would increase in the system
3. Water velocity will increase on the return side

If that is correct, then I guess what I have to look out for are:
1. Make sure I don't raise the flow rate above what the filter can take
2. Make sure that the return water velocity is lower than 2.4 m/s (8 ft/s)

Lord knows how I'm going to do those things but I guess I need to find a way.
Yoav.

 

[mas985]

You are correct in your assessment. However, the limit of 8 ft/sec is really a recommendation and not so much a hard limit. Ideally, you would like velocity as low as possible to minimize head loss but otherwise it is not so critical.

 

[JohnT]

Thanks Dave.

I clearly remember that I measured the exterior with a caliper at 50mm so I guess my piping is 2" all round. Which means I should use a 2.5" (63 mm) pipe at the suction side to lower the head loss there.

The exterior dimensions of 1.5" Schedule 40PVC is 48.26mm, while the exterior dimension of 2" Schedule 40 PVC is 60.33mm. Most pipe like that is specified by inside diameter, which is roughly 40mm for 1.5" and 50mm for 2". Sounds like you have 1.5" pipe.

 

[Yoav]

Mmmm, clearly my memory is not to be relied upon

Thanks John, I went out and measured my pipes again. As you've said, considering that internal diameters are the crucial parameter here - my pipes are 1.5", and I should get a 2" for the suction side.

Another thing comes to mind - any thoughts as to flexible vs. rigid PVC piping? (using rigid pipes will probably require more joints...)

 

[JohnT]

I don't have a lot of experience with Flex-PVC, but I like it other than the cost.

 

[Yoav]

I am slowly preparing for the unavoidable dig-up of my suction side pipe.

During my checks I've noticed that the pipe connected to my skimmer is of a visibly smaller diameter than my regular 1.5" piping (not to mention it has 3 90-degree fittings before it connects to the 35 ft of pipe going to the pump).

This has raised several questions:
1. Has this been done intentionally to increase the water velocity in the skimmer?
2. How does a 1-2 ft of narrow pipe segment affect the dynamic head loss of a 30 ft pipe?

(Since I don't see how I can change that narrow 3-knee'd pipe section connected to the skimmer, I am not sure whether using a 2" pipe for the remaining 30 ft of pipe would be redundant or useful in decreasing head loss)

 

[JasonLion]

Using larger pipe will still help. What matters for water flow is the average pipe diameter. A short section of narrow pipe will reduce the flow, but not all that much. It is the diameter of the longest pipe runs that makes the largest difference.

 

[Guest]

Another thing comes to mind - any thoughts as to flexible vs. rigid PVC piping? (using rigid pipes will probably require more joints...)

Do not use flex pipe! No structural integrity, subterranean termites (if you have them there!) do chew on it, and it is relatively easy to penetrate. It is faster to plumb with flex, but not better. We follow a 7 fps flow or less on our pools, and only use rigid. The point is that even with the "added" fittings that may be used with rigid, we can still achieve these flow rates.

Dad always said do it right the first time. If he would have been a pool plumber, he would not have used flex either!