Second discharge line?

[monipit]

I am planning on putting a second submersible sump pump into my sump hole as a backup. It will be on it's own 15 amp circuit. Is it alright to add to the pvc pipe that I'm using with the original pump, or should I make up a second line? In case of power loss I plan on running two 12 gauge extension cords from my 5000 watt portable generator. Make sense?

[HayZee518]

the discharge line for the first pump is sized for use with one pump. if for some reason they both kick in, the discharge line would be too small to handle both. a second line seems more plausible

[LazyPup]

Properly designing a sump pump discharge system is not nearly as simple as one might think.

The problem is that sump pumps are typically high volume/very low-pressure pumps.

For the purpose of illustration I checked the specifications on a Gould 1/2HP sump pump and found it is rated at 2,200gal/hr into a 12ft static head.

A one square inch column of water standing 12” vertical weighs .434lbs therefore the actual discharge line pressure measured at the pump discharge port would equal 12’ x .434lbs = 5.208psig.

From this we must first subtract the static head pressure of the vertical riser from the pump to the horizontal discharge line.

Let us assume the sump pit to be 2’ deep and the actual point where the discharge line connects to the pump is 18” below the basement floor. Allowing another 8’ of vertical rise from the basement floor to the floor joists in the ceiling we get a combined vertical rise of 1,5’ + 8’ = 9.5’. The physical weight of water standing in that vertical column and opposing the lift would then be 9.5’ x .434psig/ft = 4.123psig. This means that the resultant line pressure at the point where the vertical line changes to horizontal would now be 5.208psig(pump discharge pressure) minus 4.123psig vertical static head equals 1.085psig.

In addition to the vertical static head loss we also have friction head loss, which is the physical opposition to flow caused by the friction of the water with the pipe wall. Friction head loss increased proportionally to the velocity of flow so to determine friction head loss we must consult a friction head loss table.

The Gould pump was rated at 2,200GPH so dividing by 60 minutes per hour we find the flow is 2,200gal/60min=36.6gal/min. From a PVC schedule 40 Friction head loss table we find that in a 2" sched40 PVC pipe at 36gal/min the velocity of flow is 5.50feet/second and the friction head loss is 3.04psig per 100’ of pipe. This means that the friction head loss is 0.0304psig/ft of pipe and we had a dynamic head pressure of 1.085psig so the maximum length of the discharge line should not exceed 1.085/ 0.304= 35feet total developed length from the pump discharge port to the discharge line termination point if we expect to maintain the full 2,200gpm flow rate.

If you will look in your sump pump owners manual normally you will find that the manufactures have already done the math and printed a graph to show the resultant rate of flow for different vertical lift heights and discharge line lengths. Typically if the discharge line exceeds 40’ they recommend increasing the size of the discharge line by one nominal trade size of pipe, thus if your pump is rated for a 2” discharge line you should increase to 3” pipe to compensate for the additional friction loss.

Now in regards to your question about combining two pumps on the same discharge line.
With two pumps the combined cross sectional area of discharge and volume of flow is doubled but the actual discharge head pressure remains the same. On the other hand, the velocity of flow is now increased from 35gpm to 70gpm and consulting the PVC friction head loss table we find the velocity of flow would be 11.01ft/sec and the friction head loss is now 10.96psig.

There is a general rule that we should never size a line in such a manner as to cause a velocity of flow in excess of 10ft/sec or severe line wall erosion might occur.

We can also see that the resultant friction head loss is nearly double the discharge pressure of the pumps and the end result is that with both pumps combined it may not pump as much water as one single pump would have.


The solution:
1. You could run separate discharge lines for each pump.
or
2. You could increase the size of the discharge line by one nominal trade size, which will roughly double the volume of the pipe.

Here is another tip:
You should run the discharge line with a minimum of ¼” per foot downward slope from the top of the vertical riser to the point of discharge. This will not only help offset friction loss it will insure that no water will remain in the line when the pump stops. In cold climates the line should be pitched at ½”/ft for the last 4 to 5 feet before exiting the structure. This will aid in preventing frost closure during winter months.