grinder ejector waste line from accessory dwelling connects where?

[Ted dummy/genius]

Hi All,
I've built an accessory dwelling unit with a full bathroom. The 3" waste line is pitched under the slab into a Liberty pump grinder/ejector (Provore 380-SD). The ejector is buried in the ground just outside the building structure and as per the installation instructions, a 2" SCH40 PVC pipe runs buried in a trench 100' back to the existing plumbing in a crawl space under my house.
Right now I have it stubbed out into the crawl space. I have two possible places to connect it. This is where I need help.

I hope I can figure out how to attach my pictures and drawings to illustrate...

The easiest place to connect is a straight shot to the 2" ABS master bath waste line which feeds into the 3"ABS toilet waste and then exits to the sewer. I'd come into it with a long turn Tee Wye fitting. Just not sure if that's allowable.
Second option is to run it further with a few more connections to get it to connect via a long-turn Tee Wye fitting to the 3" toilet waste line.

Any thoughts or guidance would be super appreciated.
Many thanks.
Ted

 

[Diehard]

Chances are that the pumps discharge rate will exceed the maximum allowed for a 2" gravity drain at 1/4"/ft.

You can estimate the pumps discharge rate by knowing the elevation head(Height distance from pump level to point of discharge.), the total distance to travel through that 2" discharge pipe(100 ft plus fittings)(for friction loss in feet of head) and comparing it to the pumps capacity curve. That will allow you to see what your actual flow would be when it reaches the point of discharge into the gravity system.

I looked up what appears to be the pumps performance curve and attached it below.

Typically the applicable plumbing code requires that the pumps discharge line connect through a wye fitting into the top of the gravity line.

EDIT: If this is confusing to you, tell me the height difference between the pump and the gravity line and confirm that this curve would be the one to use for your pump and I will tell you approximately what flow you can anticipate at the connection to the gravity line.
I don't anticipate the friction loss through a 2" PVC pipe, including a few bends, and check valve, will have a big impact on the head loss. So for an example, if the total head loss(elev head and friction loss) was about 10 feet, this pump would be pumping about 44 gpm.(Too high for a 2" gravity line.)

 

[Ted dummy/genius]

Diehard, Thanks so much for your time and thoughts.
This is confusing but I'll try here to give more information.

I estimate the height at the discharge port to be approximately even to the height where the 2" pipe enters under the existing house.
That height is about the same as the existing 2" gravity sink waste line I'm considering "Wying" in too. The other 3" toilet waste line I'm considering tying into is about 10" lower.

So is what you're saying is the pressure in the 2" discharge line is too much for 2" ABS sink waste line?
Therefore the connection should be made at the 3" toilet waste line?

Here is a drawing illustrating conditions.

And here is the pump performance for the provore-380-SD. It appears to be the same as you found.

 

[Diehard]

The elevation head of your system is actually measured to the point that it becomes a free falling flow(no longer pressurized), which is the top of the 3" gravity pipe. The fact that the pipe goes a bit higher than where it terminates does really matter once it's flowing. So it appears that you actually don't have much, if any, elevation head. Which means, if that's the correct pump curve, that you would be flowing about 46 gpm. The maximum that pump is capable of pumping. Now at 46 gpm flow, through a 2" PVC, only amounts to about 1 to 2 psi pressure loss or 2.3 to say 5 ft of head. Looking back at that pump curve that only reduces the flow to say 45 gpm. If at all!

So in answer to your questions. It's not the pressure but the flow that exceeds the capability of a 2" pipe. In fact, a 3" is borderline acceptable at that flow assuming it is at 1/4" per foot minimum. In other words, that 3" pipe word be running full at that flow, when actually typical design is for the gravity lines to run half full. If that gravity line was running a little slower than it theoretically should be, there's a good chance of backing up into some of the upstream gravity system.
Although realistically, you likely wouldn't be pumping the volume of water to cause things to back up.

It may not meet your local plumbing code and the codes typically approximates on the conservative side and uses DFU's(Drainage Fixture Units) rather than actual gpm. I use gpm as I use to dabble in engineered systems.

As it appears, you would be flowing at too high of a rate to dump it into a 2"gravity line. The 3" may work. Someone will have to check if it meets your code.
I strongly suggest you verify your pumps discharge flow.
I suppose, worse comes to worse you could probably reduce the flow rate with something, although it may be frowned upon.

 

[Ted dummy/genius]

Okay. I think I understand. Thank you Diehard. I had to read that twice out loud for it to sink in.
The 2" abs I'd be dumping into can't handle the volume so there is a possibility of backing up into the upstream system, the master sink. That was my concern. That would hurt my popularity around the house.
What if at where the 2"PVC enters under the house I add a 45*bend up and 45* bend over so the 2"PVC pitches down to the where I enter the 2"ABS sink drain.
Does having 15' of downward pitched pipe reduce the flow rate?
or to be safer, should I 45* up and 45* over and pitch at a steeper rate 18' over to the 3"waste line.
Again Diehard Thanks!

 

[Diehard]

Is 3" the size that leaves your house?
Adding another full bathroom would likely require a 4". ????
Looking at the California Plumbing Code I found the following info that dictates the required gravity pipe sizes. These as you'll see are based on DFU's.
Apparently if it can be deemed an "Intermittent Flow", 6 DFU's would be required for 30 to 50 gpm pumped flows, which would equate to a 3" line.
However, it it was deemed a "Continuous Flow",it must be figured based on 2 DFU per each gpm or 2 DFU X 45 gpm = 90 DFU. In which case a 4" gravity line would be required.


The only other thing would be how the pump package manufacturer deems their system to apply to. If you can, you should talk to them regarding the little to no elevation head, which is pushing you up into the maximum flow capability of the pump and subsequently (possibly) exceeding the allowable flow through your 3" line.

If you're referring to the 2"pump discharge pipe routing regarding pitching and added bends, it wouldn't help much, at all. The pipe angle, whether it's up or down doesn't matter in a force main(pump discharge). It's where it terminates to a free fall state and the added length doesn't add much to the pressure loss based on our gpm through a 2"PVC pipe.

What would have helped, because you have little to no elevation head to overcome, was a smaller pipe size. I know the pump manufacture required a 2" but I believe that was primarily based on the full range head vs flow that their system was designed for. Not the typical 2" diameter for solids handling. ????

EDIT: Funny coincidence but there's another current thread where a person has a spout of water coming up from his sink drain. In his case, someone before him had tied a sump pumps discharge to a sinks drain line. So he faced with either throttling the flow rate down or reroute the discharge line to a larger drain line down stream somewhere.