Pumps

----- Original Message -----

From: WS

Can some one explain to me, preferably in words of one syllable, how you calculate the size of pump you need to move a certain volume of water.

Later in the year, I am thinking about installing a sump to my tank, mainly so that I can re-house some of the clutter. How do you work out what size pump you need to return the water to the tank at the same speed it is running out of the tank?

Also my son-in-law is installing a water feature in his garden and wishes to have a small waterfall. Again is there a way of working out, what size pump you need? Any help would be appreciated but please bear in mind that I am not technically minded and I do have trouble with jargon.

From: KN

I am not to experienced with the pump ratio but I currently added a Gen x external pump to my wet dry, filter, which pumps approximately 13.9 gallons a minute at zero head pressure. When I added the pump my overflow box could not keep up with the pump. I was sucking the sump of my wet dry filter dry.

I solved this problem by adding a bigger overflow. Now head pressure, if you do not know, is how far up the pump has to send the water. So figure how far up you are going to pushing the water. The pump you purchase should have a flow chart for example 150 gallons an hour at 6 feet. Match the amount of gallons per hour your pump will produce. Then buy an overflow bow that will handle that flow rate. Keep in mind your overflow box can be rated higher than the amount of water the pump will produce per hour. They are often adjustable.

From: HL

Actually, apart from "head loss", you don't have to do any calculations regarding the return pump. With a properly set-up (and adequately sized) siphon unit, it works like this:

The return pump will displace water from the sump to the tank until the water in the tank is level with the siphon outflow. Up till this point, no water will flow to the sump.

As the water level in the tank rises above the siphon return level, water will start to flow through the siphon, and down to the sump.

The water level in the tank will continue to rise until the outflow (to the sump) is the same as the inflow into the tank. At this point, the water level in the tank will stop rising, and the system will be in equilibrium. If the return pump capacity is increased (for instance if you replaced the pump with a more powerful one), the water level in the tank will increase until the inflow and outflow again reaches equilibrium. The only problem is when the water being pumped into the tank exceeds the capacity of the siphon. In this case, your tank will overflow, the sump will be pumped dry and your pump will most likely burn out.

So, as you can see, it is very important to get a siphon with adequate capacity, or to install a bypass valve in the return pipe. This valve can then be used to adjust the volume of water "bypassed" and returned to the sump (instead of to the tank). Check the sketch attached, it should make this clearer than my description. By opening the bypass valve you will allow less water to be pumped to the tank, and it should be a simple matter to adjust it to allow for the maximum flow through the siphon.

Two further things to keep in mind:

The depth of the return pipe (in the main tank). Should the power go off, or the return pump fail for some reason, the return pipe will become a siphon as well, if it's outlet in the main tank is below the water level. It will then rapidly siphon water from the tank, until the water level falls below the inlet. This could drain the tank, if the inlet is near the bottom of the tank, and obviously overflow the sump, leading to wet carpets, dead animals, heart attacks.... To prevent this, you should either keep the outlet above the tank's water level, or just below the water level, with enough capacity in the sump to accommodate the siphoned water. Alternatively, you could drill a small hole into the return pipe just below the water level. As soon as the water level drops to below this level, air will be sucked into the pipe, breaking the siphon.

Blocking, or breaking, of the outflow siphon. The siphon can become blocked by algae growth, a large snail, a wandering anemone, a spiny urchin, ... If this happens, the outflow from the tank to the sump will become restricted, causing your tank to overflow. If there is a power failure, the water will keep on siphoning out of the tank until the siphon breaks, or until the water level in the tank drops to the same level as the siphon outlet. As long as the sump can accommodate this volume of water no harm is done, as this is how it should operate. The (potential) problem comes in when the power is re-established, and the sump pump starts moving water back up into the tank. If the siphon was broken previously, there won't be any outflow, and the tank will overflow. A good siphon box prevents the siphon from breaking, and automatically re-starts the siphon as soon as the water level in the tank starts to rise. It is therefore vitally important to buy (or make) a proper siphon box.

Now, for those "head loss" calculations: It's actually quite simple if you have a master's degree in Calculus.... :) Actually, you don't need to calculate anything. All pumps worth using will give you a "free flow" capacity (the rated flow), and a graph or table showing the expected flow at different "heads" of back-pressure. So, if your outlet into the main tank is 1.4 meters above the return pump, you just read off the flow rate at this height.

I'll quote the figures of my current return pump (a Laguna Powerjet 3000):

The pump is rated as having a "no head" flow-rate of 4200 litres/hour (~1100 gal/h). At a head of 600mm (24") the flow-rate drops to 3400 litres/hour. At 1.2m it has a flow rate of 2400 litres/hour, and with a head of 1.8m (~6 foot) it has a flow-rate of only 1200 litres/hour (~300gal/h).

Obviously different pumps have different flow rated at different heights. You must firstly determine how much water you want to circulate to/from your sump. Then find a pump which can supply that volume of water at the "head" you need, and that's all there is to it.

If you have any further questions, or if something is not clear, you must please let me know.

From: WS

Thanks very much for your detailed explanation. I think that I managed to follow what you were saying, even though there was at least one word with four syllables. A couple of questions, if I may, which I hope are not too stupid.

You said it was very important to have a siphon with adequate capacity. How do you work out what is an adequate capacity?

The maximum size of sump that I could fit in would be 24*24*12 inches. My tank as you know is 55 UK gal. Would this sump be big enough?

With regard to pumps: How do you know how much water you want to circulate? I was planning on using a standard size pipe, the sort that is used in external filters. Do you have to measure how fast the water is coming down or is there an easier way? Presumably if the return pump is adjustable, you only need a rough idea; as you can adjust it.

From: HL

I don't believe there is such a thing as a "stupid question", if something is not clear, one could either ask, or remain "stupid" by not asking...

Firstly, you should determine what flow rate you want through the tank. A general rule of thumb is a minimum of 5 times the tank's volume per hour, for a "marginal reef" type set-up with mostly soft corals, and a minimum of 10 times the tank's volume for a "fore reef" type set-up with many SPS corals. Of course, if you're willing to keep some powerheads in the tank, you could lower this volume accordingly.

Secondly, choose a return pump capable of displacing the required amount of water at the required "head". Bear in mind that the pump must be reliable, and long lasting. It is also important that the pump runs quietly.

Thirdly, decide if you are going to buy a pre-maid siphon box, or build your own. If buying, choose a unit with a rating at least 25% more than your required flow rate. If you want to build your own, your siphon tubing should be at least twice the size of the pump's output diameter.

Building one's own siphon box is actually very simple. I've built mine using two Tupperware cereal containers expropriated from my wife's kitchen, a piece of plastic sheeting, PVC piping and fittings from the local hardware (or plumbing) supplier, and some silicone glue. If you want, I can e-mail you a sketch, and a photo of the unit in operation.

I would also recommend that you use a larger diameter, rigid PVC pipes for all your "plumbing". Small diameter pipes cause a substantial reduction in the through-flow capacity, due to friction loss. I would use a minimum of 1" diameter pipe for the return to the tank, and about a 2" diameter pipe for the outflow to the sump. As long as you have a "bypass" line and valve, allowing you to re-circulate some of the "return" water back to the sump, you should not be too concerned about measuring or calculating the flow rates. You can just adjust the bypass until the system runs at a safe maximum, and leave it at that.

As for the size of the sump: Anything is better than nothing, but the larger the better. The 24"x24"x12" should give you around 25 gallons, which would be quite adequate. If you could increase the height, though, it would make the sump a lot more versatile (I'm assuming the 12" dimension is the height - if it were 24" that would be more than deep enough).