water flow

[helmke]

I'm in the process of putting together a 55g tank that I bought used. I've decided to go with a sump/refugium system and am waiting for that unit to arrive. For that, I'll need an overflow box and return pump. As I understand it, optimum flow in my tank should be 20x the volume of the tank. I have two 802 powerheads mounted each generating 400 gph. So ideally, I'll need 300 gph flowing through the sump. Assuming 6 feet of head pressure, if I put a pump rated at 640 gph and an overflow rated at 300 gph, am I asking for trouble? What are to drawbacks of using a 600 gph overflow box anywhy?

Thanks in advance for your help.

John

[Hal]

To answer your last question first, there are no drawbacks to using a 600 gph overflow. It just means that the water will only trickle over the overflow (which is fine) instead of using all of the area between the teeth. Again, that's fine. As a former engineer, I highly recommend that you use an overflow that exceeds the max flowrate of your pump. Keep in mind that over time the space between the teeth of your overflow will get gummed up with algae and whatnot unless/until you clean it. Also keep in mind that you'll need to have an overflow rated for the 600 gph as well as the drain plumbing rated for 600 gph, taking into account the pipe diameter and any bends/turns or other restrictions (e.g. a valve). I've been using a Lifereef overflow for 5 years and like it very much, especially that the siphon will automatically restart after a power outage. Given my druthers though, I'd drill my tank if I had to do it again.

In general, if you're using a 640gph with an overflow rated at 300 gph, you're asking for 640-300 = 340 gallons of water on the floor as it overflows the lip of your tank. You'll need to discount the 640gps by the 6 ft of head pressure of course, but even then you should have well in excess of 300 gph. You should be able to look up your pump flowrate at various head pressures.

For your sump, figure 3-4x the volume of the sump as your max flowrate through the sump. If you go any larger you're asking for significant noise issues as well as the possibility of creating microbubbles.

If you find that your return pump is too big based on the above calculations there are three options:
1. get a new pump
2. restrict the output of the pump with a gate valve. Close the valve until you
get the flowrate that you want. I don't recommend this technique since it puts strain on the pump and will shorten the pump life. Note that I don't recommend a ball valve either since it will be more difficult to get the fine control that a gate valve gives you.
3. put a Y on the output of the return pump. Plumb one of the arms of the Y through a gate valve and then back into the display tank. Plumb the other arm of the Y directly back into the sump. Adjust the gate valve to give you the desired flowrate back to the display tank. The other arm is the bypass arm, and will adjust itself based on the restriction of the gate valve. The nice thing about this technique is that it allows the pump to operate at its "sweet spot", without seeing any restriction on the output. This avoids stressing the pump and shortening the pump life. As an added bonus you can use the bypass arm as a manifold to feed any reactors or skimmers that you decide to setup.

20x flowrate in the tank is actually the minimum recommended. Lots of people will go 40x or even 60x, especially if you plan to keep sps corals. When I first entered the hobby I thought that 20x was ridiculously high, but now I'm pushing 40x in my tank when my Vortech's are on max and am wanting more. My fish and corals hardly notice 40x.

Lastly, you don't mention whether you have a protein skimmer. If not, I highly recommend that you get one. Check the Lighting, Filtration, & other Equipment forum for lots of threads on which ones are good and which ones to avoid. Bang for the buck, this will be the single most important piece of equipment that you can buy, so do it right and do it right the first time so that you don't have to upgrade later.

Good luck.

[helmke]

Awesome Hal!!!

Thanks for the detailed response. As I understand it, I need to limit my flow through the sump to about 300 gph. The control should be on the pump side of the equation and I probably overestimated the power loss due to head pressure.

I think I'll look into a smaller pump to match the 300gph overflow and then look at an additional powerhead or two to boost the total flow in the system.

by the way, I am waiting for a protein skimmer that I ordered as well. I opted for a CPR brand which seems on the low end of the line, but not the bottom.

Thanks again for you advice.

[Hal]

As I understand it, I need to limit my flow through the sump to about 300 gph.
That depends on the size of your sump. It's 3-4x the size of your sump. The size of your tank doesn't matter here. In reality, the flowrate in your sump should only be what you need to feed your various reactors/protein skimmer/refugium, and possibly to keep the detritus in suspension. I say possibly because some people prefer to have the detritus accumulate in the sump and then to vacuum it out. Other than that, flowrate is pointless in the sump.

The control should be on the pump side of the equation
Yup.

by the way, I am waiting for a protein skimmer that I ordered as well. I opted for a CPR brand which seems on the low end of the line, but not the bottom.
CPR is a good brand. I don't have any experience with it, but that's what I hear.

One other thing on the off chance that you haven't done this yet: when you setup the waterline in your sump, make sure that you leave enough extra room for the water that will siphon/drain down into the sump when the power is turned off.