Ask the experts - how to determine head?

[dppitone]

To determine head for a powerhead in a sump, do you start counting feet from the outflow point of the powerhead or from the surface level of the water in the sump?

I'm curious, because it seems that when my sump level goes down, gph drops off at the endpoint of the flow, and when the sump level goes up, gph rises.

 

[Redfish]

"Head" as you are referring to, is composed of two components. The first is friction loss through the piping system All other things being equal, friction loss is dependent on velocity through the pipe.

The other component is static head. Static head is the difference between the discharge elevation and the suction side water surface level elevation.

In your case you are manipiulating the static head with the sump water surface elevation. A higher the sump elevation results in less static head and therefore higher pumping rates.

 

[parothead22]

to determine total dynamic head pressure, which is not only the vertical distance that a pump has to move the water but also the resistance to flow caused by piping, fittings and valves.
Alot of people think that to calculate tdh, you have to calculate the vertical distance from the intake of the pump to the surface of the water in your aquarium. IT SHOULD BE MEASURED FROM THE TOP OF THE WATER LEVEL IN THE SUMP TO THE TOP OF THE WATER LEVEL IN YOUR MAIN TANK!
The height of the water in the sump create pressure on the intake of the pump to help the water along upwards to your main tank.

 

[dppitone]

I see.

Generally speaking, for the static component - so then would the static head be the same if the pump was moved up to the level of the surface, versus being for example 2 feet below that same surface?

 

[dppitone]

OK Parothead. I see from your reply. I'm glad I asked. The pumps I'm installing just got more powerful (compared to my initial incorrect calculations).

 

[jdieck]

<a href=showthread.php?s=&postid=7455825#post7455825 target=_blank>Originally posted</a> by dppitone
I see.

Generally speaking, for the static component - so then would the static head be the same if the pump was moved up to the level of the surface, versus being for example 2 feet below that same surface?

Yes and no, yes if you assume the pump does not need a positive succion head pressure (some need more than others).
All centrifugal pumps and powerheads need some pressure in the suction (provided by the depth of water at the inlet) in order to prevent cavitation and counteract any dynamic pressure drop due to friction on inlet piping.
When the pump pulls the water in it might create vacuum in certain areas of the inpeller, this vaccum or a sudden drop in pressure creates gas bubbles that make the pump vibrate, makes it noisy and shorten it's life not counting on a drop in discharge head pressure.

Enjoy!