Where does head start?

One thing that can get confusing is positive suction head. A submersed pump can have positive suction head meaning you would calculate the net head from the sumps water surface not from the centerline of the pump suction. This helps since many of us run about a foot of water in oure sumps so that comes off the total head number. You also do not want to use any suction strainers or foam filters on the suction side of the pump as that adds head since it is starving the pump and can lead to cavitation.

 

What is is actually called is Net Positive Suction Head. What it means is the static head of water column and the point at which the liquid will turn to vapor. You need a factor large enough to keep a pump from cavitating. The result is NPSH.

Static head is simply the distance you have to lift a fluid. From surface to surface... period. When a pump curve is given for a pump... it does not matter how much water it is sitting in. 10" or 10 feet. A cutoff head of 15 feet is 15' above the water surface. It is NOT 15' from the pump centerline.

 

Isn't that what I just said?

 

huh?

 

Ya, pretty much same thing I said before, but there is more to suction head than static head. So I just left it out.

Any given pump will create "x" amount of low pressure at the inlet. If there is not enough static head pressure to keep it above the fluids vapor pressure, the pump will cavitate. So there is more to suction head that static, but nobody ever talks about suction head and it is not that necessary when dealing with aquarium pumps. Most of out pumps will suck air before they actually cavitate.