Trying to understand MAX FLOW for Sump Pit next to pool

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T

toofast

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May 9, 2013
993
North East Ohio
Ok, so I have a high water table and over the weekend, ground water was somehow gushing up my sump pit and flowing into the house.

PB said I should NEVER have that issue, but heck I did.

NOW I am trying to get the correctly sized sump pump and I know enough to be dangerous. PB wants to load in a giant 1HP 8,000 GPH monster...I think that is overkill and 1 1/2 inch PVC can't handle that much at lower pressures anyways.

In reading the 101 document I see the GPM at 38 or 51 (assume that is relative to pressure) - anyways if I use max of 51 x 60 min = 3060 GPH MAX ? Is my math sound ? I realize if so that does not even take into consideration head loss.

I read some other tables that say at average pressure the max flow in 1 1/2 is closer to 6,000 GPH (without head loss of course). If it matters I have about 8' lift, then 1 90 degree and another 8 ft run to the sewer drain.

Am I on the right track or totally screwed up. Please let me know what else you need to help.

Can someone help out ?

Thanks,
 
I wouldn't worry so much about the pump head as it will be very low and the flow rate will be close to the pump rating (GPH). The most important thing is get the water to the drain as fast as it appears. You said it was gushing. Is there any way to estimate the flow rate out of the pit? You would just need to match that to keep it from overflowing. Also, is there anyway to just connect the pit with a drain pipe so it gravity feeds to the sewer and a pump is not necessary? Or is the sewer the same level as the pit?
 
You can pump ANY GPH with ANY size pipe. There is no limit except what a pump can provide. So I would approach the problem in a differently. How much water comes out of the pit during one of these floods? Then size the pump appropriately. But you can't size the pump unless you know what you need in terms of flow rate.
 
As a follow up...my PB gave me his plumbers contact info to finish this off directly...I told him I actually wanted to install two 1/2 HP pumps (2500 GPH each at 10' lift) with one mounted higher than the other so in CASE of emergency waters I would get two pump running. He told me the ONLY way to do this is to plumb 2 separate 1 1/2 lines all the way out and that no way will two 1/2 HP pump work together and pump 5,000 GPH. He told me that two pumps together that were y'd into a single 1 1/2 inch PVC would still pump ONLY 2500 GPH.

I am so confused...

Is he right or am I right or are we both wrong, etc.

Thanks and I hope this makes sense !
 
Yes, he is right. You have to have identical independent parallel plumbing systems to get double the flow rate. Otherwise each pump will experience the head loss of the combined flow rate through the single pipe and the operating point will shift on the head curve so each pump will produce much less than the 2500 GPH and the total will be less than twice the individual. It is rarely a good idea to use parallel pumps unless the plumbing is duplicated as well.

But do you think you will actually get more than 2500 GPH? Are these flood waters coming from outside the property line? If not, there is a fairly easy way to figure out the requirement from the surface area of the property and the maximum rain rate.

Note too that because of the lift, you know that you will probably have at least 8-10' of head so depending on the pump, that may affect the flow rate but you have to check the pump's head curve.
 
mas985 said:
Yes, he is right. You have to have identical independent parallel plumbing systems to get double the flow rate. Otherwise each pump will experience the head loss of the combined flow rate through the single pipe and the operating point will shift on the head curve so each pump will produce much less than the 2500 GPH and the total will be less than twice the individual. It is rarely a good idea to use parallel pumps unless the plumbing is duplicated as well.

But do you think you will actually get more than 2500 GPH? Are these flood waters coming from outside the property line? If not, there is a fairly easy way to figure out the requirement from the surface area of the property and the maximum rain rate.

Note too that because of the lift, you know that you will probably have at least 8-10' of head so depending on the pump, that may affect the flow rate but you have to check the pump's head curve.
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Yes, I actually thought I would get more than 2500 GPH, I didn't think I would get 5,000 of course but more.

So as for the water...what is happening is that our house is built on an old farm, with lots of old fields. They old fields had clay tile drainage to prevent soggy fields and guess what, many of the clay tiles still work. So in heavy rains, all the surrounding 150 acres drain into my lot as I am situated sort of at the end of the run. So I have NO IDEA exactly how large an area I am draining, other than my sump pump (in my basement) runs nearly non stop and my pool pit is always full.
 
mas985 said:
What about redirecting the water around the property with a drainage ditch?
Click to expand...

Well the local homeowners association will not allow it...there is a drainage ditch in the "back of the lot", however it runs parallel with the drain tiles, so no good. I would have to dig deep trenches on both sides of my property (oh and I can't because then I will mess with the septic system)...VERY close to my properly line, but I can't because the engineer mounds for my septic are in the way.

So if I knew this when I bought the house, well I most likely would not have...now I am in damage control. It sounds like the ONLY option is to figure out how to add a second 1 1/2" PVC out.
 

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If the water is coming from outside of your property, then it should be HO responsibility to fix or at least allow you to.
 
mas985 said:
If the water is coming from outside of your property, then it should be HO responsibility to fix or at least allow you to.
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Yes I agree...but in order to do anything, I have to move a engineered mound for my septic system and the starting $$ on that is $20K. So I am trying to find my best alternative in the meantime.

So final question in the meantime...I am not looking for a perfect answer, but if I have 2 3000 GPH pumps and they are both running together out of a 1 1/2 pipe will I get 3500 GPH or anything more than 3000 GPH.

Similarly, what if I have a primary pump at 2000 GPH and Backup at 3500 GPH in case pump one can not keep up...will I at least get 3500 GPH or a bit more if they are both running ?
 
Sump pumps are usually not very efficient and produce very low head. I would not even consider another 1.5" line - go 2" or larger even if it is a short run. It would probably help some to upsize your existing 1.5" line. A 1" rain on 1 acre of land is nearly 27,000 gallons, so no telling what you could be dealing with. Maybe you could come up with a way to guestimate the excess when it backing up. Is there a way to stop the flow from some of the clay tiles?
 
Perry Whitlock said:
Sump pumps are usually not very efficient and produce very low head. I would not even consider another 1.5" line - go 2" or larger even if it is a short run. It would probably help some to upsize your existing 1.5" line. A 1" rain on 1 acre of land is nearly 27,000 gallons, so no telling what you could be dealing with. Maybe you could come up with a way to guestimate the excess when it backing up. Is there a way to stop the flow from some of the clay tiles?
Click to expand...

Problem is I can't run a 2" line without tearing out all my new deck/patio/fence that was just installed....so short term this is not an option. I am just trying to figure the max flow (GPH) pump that will flow into a 1 1/2" pipe
 
toofast said:
So final question in the meantime...I am not looking for a perfect answer, but if I have 2 3000 GPH pumps and they are both running together out of a 1 1/2 pipe will I get 3500 GPH or anything more than 3000 GPH.
Click to expand...
It depends on the pumps. Do you have any data on the pumps you are planning to use? Specifically a head curve.

Here is an example of what happens when you put two pumps in parallel without have dual plumbing. For this example, I will use the 1/2 HP SuperPump.

A single pump on plumbing Curve-C would produce about 49 GPM @ 19' of head. Putting two in parallel changes the operating point so each pump has an operating point of 32 GPM @ 34' of head so instead of 98 GPM total, you will get only about 64 GPM or a loss of about 35%. But it will be worse for a sump pump because the head curves are usually lower for these types of pumps.
 
Assuming you bring two pipes up out of the pit and then combine, the plumbing curve is:

Head Loss (ft) = 7 + 0.00197 * GPM^2

Which means you would have an operating point somewhere near 48 GPM @ 11.5' of head for a single pump

For two pumps in parallel, the operating point would be about 88 GPM (44 GPM each) @ 13' of head

So not too much flow rate loss.

Note that you will have to put a valve or check valve on one of the pumps so that you can prevent back flow through one of the pumps when it is off.
 
mas985 said:
Assuming you bring two pipes up out of the pit and then combine, the plumbing curve is:

Head Loss (ft) = 7 + 0.00197 * GPM^2

Which means you would have an operating point somewhere near 48 GPM @ 11.5' of head for a single pump

For two pumps in parallel, the operating point would be about 88 GPM (44 GPM each) @ 13' of head

So not too much flow rate loss.

Note that you will have to put a valve or check valve on one of the pumps so that you can prevent back flow through one of the pumps when it is off.
Click to expand...

Fantastic...exactly what I was trying to figure out!
 
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