April 6, 2015
Here’s an education session (This may not be necessary for you but others might find it helpful):
Once a pump turns on, the discharge rate is a function of the "head" that the pump works against. The head is the elevation difference plus the flow resistance of the piping. The sump size (and the float switch settings) only determines the duration (run time) of pump operation. Here's an example of a typical 4/10 hp sewage pump that is commonly used for a subdrain in a single family residential application:
Examine the pump curve. Consider a head (vertical axis) of about 10 FEET. Go horizontally across to intersect the pump curve and then drop down to read a flow of about 53 GPM on the horizontal axis.
A pump could be "rated" at a high flow but that might be against a very low head. Always go to the performance curve of the actual pump and determine what the flow rate will be for the actual head conditions.
(PS-The Zoeller pump curve is only an example. ICC is not endorsing that product or any other product.)
If the chosen pump does discharge at 120 GPM against the actual head conditions then yes, a large size gravity pipe flow pipe where the pump (converted to) dfu plus the gravity system dfu is greater that the pipe sizing tables allow for a particular pipe size. Remember that hydraulic design of gravity drainage systems needs to maintain an “open channel flow condition” (pipe not flowing full) to avoid flow surges and significant pressure changes in the vent system. A pump discharging to a gravity system is really no different than a drainage pipe carrying the maximum probable flow (the total dfu) from a number of fixtures discharging to the gravity system. Designers wouldn’t be allowed to under size portions of a drainage system and connecting pumps to drainage systems is no different.
The code requires designing gravity piping for total dfu (maximum probable flow) whether those conditions will actually occur in the building. But should they ever occur, the system is designed for that event. You stated “We know that this never happens….” . In many single family dwellings, the total dfu flowing condition might indeed, be a rare event. But none-the-less, the system has to be designed for such a condition because it could happen. A “family” could be quite large and everyone “getting ready” at the same time while the ACW and DW are also running will tax some drainage system designs. Operate the ejector pump at the same time (caused by family members using basement bathroom(s) draining to the sump) and problems could occur if the receiving drainage pipe is undersized. For a commercial building, such maximum probable flow events might not be as rare as one would think. This is why the code provides an “approximate equivalency” (gpm to dfu) to make it easy to size drainage systems without further engineering.
April 13, 2015
Section 709.3 (2018 IPC) states to calculate the dfu's from an ejector pump the discharge would be calculated 1gpm is equivalent to 2 dfu's. The IRC says 1.5 dfu's per 1 gpm. The typical sewage ejector that is installed in a home in my area is rated at 120 gpm which is equivalent to 240 dfu's. A 4 inch horizontal branch will only carry 160 dfu's. If taken literally a drainage pipe larger than 4 inch would be required. We know this never happens in a residential home or even a commercial setting. Does the tank size come into play in any of these equations? What am I missing here?
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