The basics of
sizing a pump to a tank involves knowing the tank volume, turnover rate, pump capacity, and what is known as the “pump curve”.
To
find the tank volume, you need to find the cubic feet. There are 7.5 gallons in a cubic foot. The formula in inches works out to be
(L/12 x W/12 x D/12) x 7.5 = tank gallons
This simplifies to
(L x W x D) x (7.5/1780) = tank gallons
or
(L x W x D) x (.00421) = tank gallons
For example, if your tank were 60x24x18 your tank gallons would be 109.2 gallons.
The term
“turnover” means the number of tank volumes that flow through the filter every hour. Typical turnover rates range from three to ten turnovers per hour. For fish originating in high current waters, you should lean towards ten turnovers. If you have fish that are from lakes or slow bodies of water, then you should have around three (or fewer) turnovers. Five turnovers for the above tank is calculated simply by multiplying the tank gallonage by the desired turnover rate. This would be 546 gallons. This is what we need to know to size the pump.
Pumps are
rated at zero head, meaning that there is no restriction and no working load attached to the pump. This rating makes the Marketing Department of the manufacturer happy because an inferior pump can now compete with the competitions ballsy pump out on the showroom floor. Unfortunately for us, this is how we have to buy pumps and we are left to find out what it is going to pump on our respective systems.
Before going further, let me define the word “head” as it is used in measuring pump performance.
Head is the unit of measurement that represents the working pressure and volume of a pump. If you were to erect a long pipe vertically and hook the pumps up to the bottom of it, each would be able to only pump up to a certain height. This is the max working load measured in “feet of head”. If the two pumps in the previous paragraph were compared in their working head, the ballsy pump would out pump the inferior one hands down. I have seen pumps with the same gallonage rating where one pump could pump three times higher than the lesser pump.
Lastly,
pump curves are demonstrated by the chart below. This chart contains all of the models in a line of pumps. The column to the left of the chart is the head measured in feet. The row at the bottom is the gallonage measured in gallons per minute. Each curve in the chart represents the performance of one pump model. Where a curve starts at the left, that point is the maximum working head. A pump will not pump higher than this figure. At this point, the height is maximized but there is no volume pumped. Now follow the curve to the bottom of the chart. This is the zero head reading. There is no restriction on the pump and the water is just flowing out the top of the pump. The volume is at the maximum but the working head is zero. Now go back up the pump curve to any point. By following a line to the left and another down, you can find the volume pumped at a certain height.
In our example tank, we need a pump that will pump 546 gallons per hour or 9 gallons per minute. This needs to be the gallonage at the top of the tank, not at the top of the pump. So assuming the height of the top of the tank is five feet, we go to the left hand column in the chart and find five feet. Now go right until you are above nine gallons per minute. This gives us the choice of two pumps the PM21 and the PM22. This chart is for Magnetic Drive Pumps by Iwaki.
Added for reference:
http://www.monsterfishkeepers.com/forums/showthread.php?t=205645
View attachment 56604
