At some point in the adventure of reef keeping most of us want to take the step of running an external sump, the big leap. This leads to the question “How do I get water to the sump?” This is typically accomplished though an overflow. There are three general types of overflows. The first are the siphon type overflows (Figure 1). These work by a constant siphon kept over the rim of the tank with a small reservoir both on the inside and outside of the tank to prevent air from being introduced. As water in the tank rises above a certain level the water travels into the first reservoir, through the siphon and over the top of the tank to the reservoir on the backside of the tank. It then gravity drains via a bulkhead to the sump. The second type of overflow is the built in overflow (Figure 2), which generally gravity feeds a drain(s) out the bottom of the tank through a bulkhead to the sump. The third common overflow type is the through-the-back overflow, which consists of a box mounted inside of the tank with a bulkhead going through the back glass (Figure 3). These also gravity feed, directly to the sump. The focus here is the flow of water through the actual bulkhead, not the overflow box.
Many aquarists fall victim to the so-called reef
ready tanks, the through the bottom types, usually with a black plastic
column running the height of the tank. These tanks generally are at best
restrictive bulkheads hamper flow to the point of often causing pumps
to be throttled back, losing flow, or even cause water to overflow the
tank itself! The answer to
obtaining sufficient flow is either larger or more drains,
ideally both. But how much larger and how many more
drains? There is much
anecdotal discussion about the flow rates of various bulkheads, but
there is little empirical evidence cited. We set about to test the various
common sized bulkheads, ¾” through 2”, and determine their actual,
tested flow rates.
To conduct these tests, we used two 65-gallon aquariums and a tank drilled for the different sized bulkheads. One of the 65-gallon tanks was used to pump water into the drilled tank, which then overflowed into the other 65-gallon tank. The aquarium being filled had a mark indicating 50 gallons (Figure 4) which was calibrated by first filling with a measured 50 gallons of water. We chose a pump with the ability to outflow the largest bulkhead, and controlled its flow to maintain the water level just above the top of the bulkhead being tested. The time to flow 50 gallons of water was timed, and then the flow was calculated through the bulkhead using the following formula:
Time being the time elapsed in seconds to flow the volume, 50 gallons in this case. The rate of volume per hour is measured in gallons per hour (GPH). For example, if it took 130 seconds to flow 50 gallons of water the equation will look like this:
We repeated this a minimum of three times for each size bulkhead. A Megaflow tank was tested as well, as it is often a topic of conversation. It should be noted this setup included a water level 4” above the overflow inlet and used a Durso style standpipe. Unless otherwise noted, all tests were performed with a bulkhead through the back wall of the tank plumbed with a street elbow straight down 2 ½ feet to the capture tank. This test was conducted with fresh water. A picture of the testing apparatus can be seen in Figure 5.
A note in regards to siphoning in the aquarium: Siphoning is an effect that occurs when the water traveling down through a sealed tube creates a vacuum on the upper end of the tube. This vacuum in turn pulls in more water, ideally keeping the vacuum going. We used a vented elbow to prevent siphoning in most tests, with an unvented elbow used to show the effects of siphoning in other tests. By aspirating (allowing air into) the elbow the siphon in these tests are broken, leaving a 100% gravity fed overflow. Some tests were conducted with full or partial (not the full duration of the test) siphons to demonstrate the difference in flow with a siphon. The drawbacks to using siphoning in overflow drains are discussed below.
First, it should be noted the effect produced by adding horizontal runs and strainers to the line; flow can drastically be reduced. It was cut in half on the 1” drains, the most common overflow drain size out there! The bulk of these tests were conducted with typical overflow plumbing, basically a Durso style standpipe that runs straight down to the sump. Ideally siphoning should not be allowed to occur in an overflow (see below). We allowed siphoning in a few runs to demonstrate how it can increase the flow. By doing this, a person is “balancing flows” with a false sense of security.
These results demonstrate that with typical overflow plumbing (that is a straight shot to the sump, no strainers) a:
2 inch bulkhead will flow approximately 1350 GPH
1 ½ inch bulkhead will flow approximately 975 GPH
1 inch bulkhead will flow approximately 350 GPH
¾ inch bulkhead will flow approximately 165
These results indicate that the flow rate can more than double if a siphon effect occurs. This is rarely the case in ‘real world’ aquaria, as the siphon will cause the water level to fluctuate continuously. This can result in the compartment momentarily draining completely, then refilling, resulting in ‘flushing’ type noises, as often witnessed in siphon type overflows. Unless the incoming water being pumped into the tank is matched exactly to what is being drained out this will occur, which is virtually impossible to accomplish long term. It should also be noted that this cycle depends on the siphon starting every time the water level goes up, this does not always happen! The flow can also significantly be affected by water column height over the inlet, as demonstrated with the Megaflow test. Caution here should be given in that this again would entail running the bulkhead at full capacity, requiring the same flow balancing as with siphoning.
The difference in flow through a longer horizontal run was less than with short runs, and is worth noting. Similar results can be expected by adding many elbows or other fittings. The flow is reduced, decreasing slightly with each fitting or foot of pipe. However, the flow capacity by adding an intake strainer is greatly diminished. The tests show the smaller the bulkhead, the greater the difference. The one inch bulkhead capacity was cut in half! It is also worth observing the tests used new strainers, with no buildups or obstructions.
Having more throughputs for redundancy is highly recommended. If your bulkhead can handle your pumps flow, what happens if it gets plugged? Having a second drain will provide you the safety margin needed when this happens. Tanks can also be plumbed with drains with different input levels inside the tank. The lower can take the bulk of the flow, while the upper can run at a low capacity and provide a safety net should the lower drain fail. One note should be made regarding the growing popularity of “overclocking” overflows. This consists of one drain lower than the other, with the lower drain being a full siphon. The lower drain runs at full capacity and the upper drain at a minimum gravity fed flow. This allows the siphoning drain to do so without the constant flushing noise and the upper drain theoretically acts as a safety should the lower drain fail. This does work, but there is one caveat. If the lower, siphon drain, fails or clogs, the upper drain (unless much larger than the other) will not be able to account for the tank’s needed draining capacity, leaving the tank itself overflowing. Regardless if you use one or multiple drains it is recommended that flow rates be considerably less than those indicated above. This will allow for buildup inside the pipe, inhabitant inhabiting the plumbing, etc. It is just one of those instances where bigger and/or more is better.
This area of reef keeping requires planning ahead, before the tank is set up, or even bought. Tanks can and should be ordered with more and larger drains. All other equipment, with the exception of the stand, can be replaced without tearing down the tank. The point is to research and strategize your overflows and drains before start!!