The presence of a standpipe really makes no difference in how much water you can process. The limiting factor is almost always the teeth spacing of the overflow chamber. This is what limits how much water can enter the chamber. If your plumbing can handle the water flow without a standpipe it will handle it with a standpipe. However, many people have reported being able to push the drains beyond the rated limit by increasing the teeth spacing and reducing the vent hole size which pushes the standpipe to act more like a siphon than a gravity drain. Doing this is risky and you are on your own. I can't recommend it. But people have claimed to reach rates as high as 800 to 1000 Gallons Per Hour (GPH) with 1 inch bulkheads rated for 600 GPH.
A Durso Standpipe does not alter (reduce) the flow rate that can be processed in your system. Another limiting factor to consider is the bulkhead itself.
Conservative estimates for bulkhead flow rates are as follows:
- 3/4 inch - 350 gallons per hour per bulkhead (unconfirmed) (1325 liters per hour).
- 1 inch - 600 gallons per hour per bulkhead (2275 liters per hour).
- 1.5 inch - 1,500 gallons per hour per bulkhead (5675 liters per hour),
If you feel you are not processing the expected the flow rate with a Durso Standpipe installed, then remove the standpipe to test and confirm your plumbing can process the flow without a standpipe. If this test fails, then you need to reduce your flow rate to match your plumbing.