UPDATE:
I decided while I was waiting for the replacement impeller for my Dart I would do some flow calculation. Thanks to crvz's formula I think I got at least a good estimate of flow throughout my system. The steps I took are as follows.
1. Adjust system flow to desired rate and allow system to normalize
2. Shut down main return pump and allow all drains/returns to drain completely
3. Siphon out 10 gallons of water to a separate reservoir
4. Turn on return pump and start stopwatch as soon as flow reaches display
5. Stop stopwatch when flow starts to overflow weir
The calculation crvz gave me is as follows...
(3600/seconds to refill) x 10
*I.E. 3600 seconds per hour / seconds to refill display x 10 gallons of siphoned volume*
My first calculation was for the maximum flow the system can handle. I adjusted the output of the Dart as to have the water level completely covering the teeth in both overflows, but not allowing it to go over the top of the overflow. I also checked for stability of the Durso drains. Once normalized I started the test. It took 24 seconds to refill the 10 gallons previously siphoned and start overflowing the weir. Using the formula I calculated my system can handle a maximum of 1500 GPH total flow or 750 GPH flow per overflow. The system was stable at this flow rate but I obviously wouldn't feel comfortable running it for any length of time since there is no headroom at this flow rate. Marineland claims their overflows can handle 700 GPH, so from my calculation I proved this to be true.
My second calculation was what I would consider normal operating condition. I adjusted my Dart so the flow was approximately half the way up the teeth in my overflow, or operating at 50% capacity. Once normalized I started testing. It took 40 seconds to refill the 10 gallons and start overflowing. The calculation showed it was flowing 900 GPH total or 450 GPH per overflow.
At 900 GPH, the system is right at 5x turnover through the sump. Even though 450 GPH is over half of the recommended flow rate for the overflows I would consider this to be acceptable as it would take a complete blockage of one drain to cause a serious issue. I'm guessing there would be enough headroom, in case of a partial obstruction, in the opposing overflow.
Both of the above tests were tested using the main return pump only, no recirc flow. Now, where it get's tricky to estimate total flow through the sump is when I start compounding flow between the main return and recirc line. Small adjustments to the recirc line cause large fluctuations in return flow. I've been trying different combinations between the main return and recirc to come up with the same results as I tested earlier. I presume the only way to know total flow through the sump while using the recirc line would be to use a flow meter on the recirc's output. Then you could add the two numbers together for total sump flow. Either that or I could try and fill a gallon jug from the recirc line while timing the results.
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