I built a DIY algae scrubber and dont have it running yet but the pump just came in and I'm worried it might be too strong. I have around 11" horizontal screen length and the pump is roughly 950gph.
Its the actual flow measured at the scrubbers entry point that you need to know. 11" x 35gph is 385 gph. That, or less is what you need. If the pump is pushing too much, perhaps you could put a T piece in line & bleed of the excess to get the flow correct.
Yup, I agree - that's likely a bit too much flow. However if the pump is rated at 950 GPH, that's a zero head. What you will want to do is find the head-loss chart for that pump. If, for instance, it's a Mag 9.5 you would go here: Supreme Aqua-Mag Magnetic Drive Water Pumps | Danner Manufacturing Inc. And then the "manuals" tab, first PDF, second page there is a chart. Keep in mind the Danner Mag pumps have a very low head loss compared to just about every other pump of that type. Next you will want to calculate your head, which is the sum of the vertical + other losses. Vertical is easy, that's just the level difference between the water the pump is sitting in (not the depth, just from the surface - that's where "zero" starts) to the centerline of your slot pipe. Horizontal/other is a bit trickier, because it depends on your pipe diameter and number of elbows, choke points, etc. What I do just to estimate this is figure another 24" of loss for the slot pipe and everything else. So if your vertical is 12", then total is 12+24 = 36". This is pretty typical. for the Mag 9.5 that would put you at around 825 GPH. Other pumps are likely to be more like 700 GPH. But that's still rather high. I have pretty much ditched the 35 GPH/in rule in favor of a more simple approach: Start by dialing in the flow such that you have just enough flow to cover a majority of the screen. With a brand new screen, you usually want to give it a few days to get an initial coating of slime/etc so that there is some resistance. At first it's likely that you will get "channels" of water at the bottom, or it will "arc" across the screen, so it's hard to judge your proper flow when that's happening. Just give it a few days before making the initial adjustment. The flow rate you get with this above approach will typically be more like 25-30 GPH/in. Once you start getting growth that you can harvest on a regular basis (months later) then if - and only if - you need to push the scrubber a bit harder, that's when you start increasing the flow. HTH Let me know if I misinterpreted anything on your original message
