Sump Pump GPM ratings vs Max Achievable flow for 1.5in pipe

[Kevinweg]

Hi,
I have a dumb fluid flow question.

I have a sump pump that has a max flow at 10' head of 3000 GPH. The pipe size is 1.5" ID PVC. When I look at a chart of max achievable flow for 1.5" ID PVC, I find 2600 GPH. Now, I realize there is a max achievable flow, I'm not necessarily trusting the chart I found on the internet.

I guess my basic question is... if the max flow is limited to 2600 GPH for this pipe that is supposed to be connected to this pump, why do they sell pumps that have flow specs ~3000 GPH?

Maybe I'm missing something here.

The problem I'm trying to solve is I need to help a friend get more flow out of their sump pit. People say "get a bigger pump", but I'm not convinced that's the answer. I'm going to check the actual flow tonight. I expect it to be far less than the max GPM on the pump tag. However, if it's at approximately the max value for the pipe size, nothing I do will help until I upsize the piping.

Perspective is appreciated.

 

[CarlB]

At 3000 gph (50 gpm), friction loss for 1.5" pvc piope is about 14'/100 feet of pipe. So if combined friction loss & lift (discharge point height above groundwater level) there is more than 10' of head, 3000 gph would not be posssible.

The 2600 gph value for 1.5" pipe is probably just a recommend maximum (practical) flowrate, but it is possible to get greater flows through it.

 

[SteveWag]

3000 GPH is 50 GPM. 50 GPM creates a velocity of about 8 FPS in a 1½ inch pipe. This is a high but not unusual velocity. Increasing the pipe size to 2-inch is advisable and reduces piping losses by about 3 times. Many times a pump manufacturer will make a discharge size that appears too small in order to reduce weight and costs and it is very common to upsize the discharge piping.
Steve

 

[Kevinweg]

I appreciate the feedback.

So, it appears that >2600 GPH is possible with that size pipe but if I'm reaching the calculated max, I should just increase the discharge pipe diameter. So, just to make sure, the discharge flange on the pump that is sized for 1.5 ID piping isn't the limiting factor. It's a threaded connection I imagine and so I'll have to use a coupling that expands 1.5 ID to 2 ID and that will increase my flow closer to the max value of the pump right?

I think he's got two 90 degree elbows in there too. Was going to swap those out for 45 degree curves.

 

[77JQX]

To properly advise you we need more information. Specifically, any elevation change between the sump and the discharge point, required bends, and distance between sump and discharge.

 

[Kevinweg]

"To properly advise you we need more information. Specifically, any elevation change between the sump and the discharge point, required bends, and distance between sump and discharge."

The pump base is 11 ft down from the discharge point. The standing water level is 1 foot up from the pump base so I'm assuming the head is 10ft so far plus the friction loss of 11 feet of straight pipe.

There is a 90 degree elbow at the top of the discharge tube, a run of 2 ft horizontal through the house wall and another 90 degree elbow pointing down into a drain tile that is used to carry water out into the yard.

I read that for 1.5 ID pipe, that each elbow is an equivalent of 4ft straight pipe friction loss. So, if I'm doing this right, I have 10+4+4 head so far not including friction loss for the 2ft straight horozontal piece and the 11 ft vertical piece. I think the 4ft of head loss estimated for elbows takes into account friction loss for that piece.

Don't know if I'm doing this right.

Thanks

 

[Kevinweg]

One other bit of explanation.

During heavy rains, the water flowing into that pit exceeds the system's capacity to pump it out. They have a driveway that slopes down to the house of 900 sq ft (equivalent). That driveway has a drain grate at the base so the majority of the water in the driveway ends up in the sump pit. I figured when it rains 5 in/hour, there's about 2805 GPH into that pit. Certainly, it will rain harder at times and that will significantly increase the water going into the system.

May be fighting a losing battle here with the way they have it set up. I have to go take a closer look at it tonight.

 

[TD2K]

You won't get 3000 gph with your current setup. The 10' for the pump is the total any elevation changes and line losses, you already have 10' just in elevation. With your piping, I'd estimate you have another 30' or so of line losses (the exit to atmosphere is a bit part of that).

So, you have an elevation lift of 10 plus your line losses through the pipe and fittings which if you keep 1.5" pipe should be about 3' of head loss giving you a total of 13' of head. I don't know how sensitive sump pumps are to additional discharge pressure to tell you how much less flow you should expect. The 2' of head loss through the pipe and fittings can be just about eliminated if you go up to 2" leaving you mostly the elevation gain.

Personally 2805 gph versus 3000 gph is too close to me, I'd be looking for a new sump pump and likely one with say 15' head capacity.

 

[cvg]

5 inches per hour seems a bit extreme. But if you really feel this is a realistic design parameter, you should increase the size of the sump and add a second pump or preferably divert the water somewhere else

 

[TD2K]

< With your piping, I'd estimate you have another 30' or so of line losses (the exit to atmosphere is a bit part of that).>

Sorry, that should have been 30' of equivalent piping. The estimated line losses through that of about 3' of head is still about right I think.

 

[Kevinweg]

Thank you for all the good advice. I went over to her house last night and did several quick tests on the sump with the existing parameters. The system pumped 5 gal of water in approx 6.8 seconds (best) out of 3 trials. About 2600 GPH. To do this, I filled the pit to the bottom of the drain tile inlet (it took about 5 gallons) and pumped it out. Wish I could have used more water, but had no way to measure the amount of water I was putting in other than the volume of the pit up to that point. I then pulled the pump to see what I was up against. It's a ridged ssp1000. They had no manuals there, but looked it up this morning and it's substantially stronger than what I stated above. At 10' of head, it's capable of 4200GPH.

Okay, while I was lead to believe worst case (new pump or 2nd pit), it turns out the inlet screen to this pump is heavily clogged with debris. I cleaned it up, put it back in and re-ran the tests. 4.8sec to 5.2sec for the same amount of water. Seems like a pretty reasonable gain. I'm still not ready to call it a success yet. We need another big rain to test it since my estimate of inflow into the pit was a total swag based upon the driveway drain.

In the mean time, going to route her downspouts away from her house and put a screen to catch debris going into the driveway drain (that's where the debris enter the pit).

Hopefully, this will shortly be a closed case. Thanks for all the advice. If the system does not perform adequately now, I'm armed with sufficient information that I can continue to improve the flow until we get what we need.

Thanks again!