I have purchased a small selection of bulkhead fittings from US Plastics, an online supplier of those and evidently a large variety of other plastic-related items. I am currently gearing up to procure another round of supplies, at which point I will be very close to the point of being able to fill the system with water.
Since I had to purchase fittings, it seemed like a good time to do some calculations and make sure I was sizing the plumbing appropriately. I assume a pump rate of 800 gph (it's a 1000 gph pump, but I am attempting to account for head loss). I can't seem to find the calculations (they must be in another spreadsheet), but for the moment I am assuming that the media will occupy about 30% of the GB space. The GB's max operational capacity is approximately 100 gallons. Thus, we will suppose 70 gallons will filter into the GB before the siphons start.
Given the above, the anticipated static fill time (the time it takes to fill completely before draining starts) should be about 315 seconds. A 1 inch diameter drain pipe should pull the water out of the GB in about 200 seconds. Figure in the constant fill rate, and the drain time jumps to 547 seconds (we'll call this the dynamic drain time), meaning it would take far longer to drain than to fill. At 1.5 inches of drain diameter, we should expect 124 seconds of dynamic drain time. That's better. At 2 inches, it should drain all 70 gallons in about 59 seconds.
Now, I can only cut these holes once, and I am slightly concerned that the length of the bed will cause one side to become more stagnant than the other, should I place the drain in one corner. The alternative is therefore to add a second drain. Two 1.5" diameter drains equals one 2.12" diameter drain in drain area, so the dynamic drain time would be around 52 seconds. Two 2" drains (equivalent to one 2.83" diameter drain) will reduce the drain time to 27 seconds. These times do assume sufficient out-piping, which means I would probably need to enlarge at least a portion of the pipe running down to the DWC.
Whether or not I really need the second drain is up for debate, but by at least installing the bulkhead fitting now, I can always add the actual drain later (and otherwise just plug the hole if it need not be used. If I do go with the larger size pipe, or two of them for that matter, I will very likely need to contend with a siphon start issue. Thinking back to the pull-start siphon I experimented with several months ago, I decided it might be good to find a way to build that in. Putting two bulkheads next to each other, however, did not seem like a good idea, and the loss of GB real-estate was becoming bothersome.
One potential solution, which I will be experimenting with as soon as the fittings arrive, is to put the start-pipe assembly above the bulkhead, rather than below it. In concept, there would be only a single pipe and single bulkhead fitting. The pipe would tee immediately above the fitting, and proceed up at full diameter and out at the start-pipe diameter (1/2" most likely). The start pipe connection would do a bend and then run parallel to the main drain, terminating just below the top of the main. The start pipe would therefore set the max water height in the GB.
Two siphons should not pose any special difficulty, as long as they are drained together. If the main (pull-started) siphon is closest to the out-pipe, and the secondary siphon connects into it just before the out-pipe, the secondary siphon should get pull-started by the main siphon. That is, once the main siphon trips, it should pull sufficient vacuum on the drain plumbing to forced the second siphon to start. The effect will be a three-stage drain: start, primary, secondary.
It may be better to put the primary siphon upstream of the secondary, but my only concern is that the long horizontal run will cause problems for primary siphon start.
Regardless of how the siphons are started or how fast they flow, this action has an effect on the DWC tank: given the dimensions of the DWC, a 70 gallon ebb-and-flow will equate to approximately 5.6" of rise and fall. Consequently, the pump will need to gather water from as close to bottom as possible. It should be noted that this height changes is slightly more than 50% of the maximum water height for the DWC. Should it be determined that less water is required in the GB to trip the siphon, then this percentage will be reduced accordingly. (For instance, a 50 gallon ebb-and-flow will equate to a 4" rise and fall.)
I plan to position the water pump outside the DWC, piped so that its inlet it always submerged. I may need to find a decent filter material to keep debris from entering the pump. One of the bulkhead fittings I acquired is intended to go through the wall of the DWC. This is mainly to ensure that the pump can be properly primed before starting, as it is not self-priming.
Testing will hopefully commence soon!
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