I haven't been able to figure out why the sump of my system empties faster than it fills.
I replaced the return hose from the sump pump and it is of a slightly greater diameter. I assumed that was the problem, so I tried restricting the flow through the tube by installing a PVC valve in the return.
While this allows me to balance the flow, it reduces the force of the return into the tank, drastically cutting the circulation it the tank.
I've also tried increasing the flow into the sump by installing a larger diameter hose with the same results.
Your problem is that your return pump chamber is far too small in relation to the amount of live water needed in the system. (Live water is the amount of water you need to keep the system running, and is also the amount of water that drains into the sump when the main return pump is shut off.
I'm going to guess, and say that I think that this is a DIY sump project.
If that is the case, you need to modify the sump making the return pump chamber a lot larger. On a typical manufactured sump, the return chamber is about 1/3 to 1/2 of the sump. Many DIY projects make this chamber far too small.
Just to expand on what Dave has already stated, you certainly want your drain to be larger than your supply. This is how it is with most hydraulics, not just your setup at home. If you think about it for a second, there really is no way that it can empty faster than it drains. The tank empties into the sump only as fast as you pump water into the tank. Of course, if there are restrictions (return hose not being larger than the supply hose) you can come into the problem you are having. Eventually, your tank would overflow on the floor, not into the sump. Also keep in mind that when your system is actually running, there is X amount of water volume inside the plumbing. When you shut your main pump off, this entire volume empties into the sump. If your sump is too small, it will not compensate for the water volume required to fill the plumbing.
This wet/dry system is rated for a tank up to 75gal. The actual tank is 55gal. It worked fine when I set it up the first time. When I broke the tank down, the hose that went from the return pump in the sump back to the tank was rather cruddy, so I threw it out and purchased a new one. The new hose is a bit larger in diameter and I am beginning to think this may be the problem. Wouldn't a smaller diameter hose restrict the flow out of the sump? Also this would increase the pressure at the nozzle which empties into the tank, incresasing circulation? I'll provide pictures.
A pump is going to move water through a 1 inch diameter (arbitrary diameter) pipe much faster than gravity would ever hope to do. This is why the drains are usually much larger than the supplies; to compensate for this disparity.
Here is a pic of my sump. The drains are 1.5 inches in diameter and the hoses coming out of the pump are 1 inch in diameter.
It is also important to not select a pump that pumps too much water volume to the point where your returns cannot keep up. Again, to use arbitrary numbers, if you had the same setup as I have and used a 10,000gph pump, problems like the ones you are having are certainly going to arise!! What is the rating of the pump you are using?
I intended to provide pics... now I can't post them. Apparently I had a trial membership that expired.
You can host them on Photobucket or you can pay for a RAG membership. Having access to DaveK is worth more than any reef keeping book you will ever buy.
Make sure you do the power failure test. Turn off all power, and wait until water stops draining from the main tank. Be sure that the return line can not flow in reverse and act as a siphon. Be sure your sump can not overfloe and cause a flood.
Can't Balance My Filtration System
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