gravity flow as pumping 2

[yazguli]

If we suppose the flow at the outlet of a water tank to be the discharge of a pump,and if we connect a pump to this outlet it is as if we have two pumps in series.In such arrangement we don't expect the flow rate to be increased specially if the system curve is near to flat.But in practice the flow rate is increased, and to some extent the more the pump size we have more flow rate. what is the reason (or what is wrong with my antecedents)?

 

[yazguli]

PEDARRIN2,
You are right.Even in " static pressure loss dominant systems" the curve is only relatively flat.
By the way,I think the characteristics of the pump we put on the outlet of the tank is also important.You may select a pump with a max. flow equal or less than that of the drain.In that case we will not expect an increment in flow.

 

[PEDARRIN2]

yazguli,

You are correct. If the pump curve indicates a churn pressure less than the static pressure requirements of the system, no flow would occur. As the churn pressure increases, the pump curve approaches the system curve until the static and churn pressures are equal. There would still be no flow. Only as the pump curve continues to increase would flow occur.

Now add the pressure/flow from the tank. The tank acts like a pump in the system in that it adds energy in the form of static head. Adding a mechanical pump adds energy which will allow for increased system pressure and increased system flow.

But remember, according to Bernoulli, the energy of the sytem is both velocity and pressure (and elevation). 3000 gpm at 10 psi is not the same as 3000 gpm at 100 psi.

But like you said, if the maximum energy (velocity/pressure) the pump can produce is less than the energy the tank is producing, via elevation loss, it may not add anything if the velocity/pressure of the water coming into the pump is more than the velocity of the impeller. Then the pump adds pressure loss to the system. That is why we do not use small circulating pumps for fire suppression systems.

But we have to be careful with this assumption. A lot of municipal systems use water towers and very large pipes to distribute water to buildings in the vicinity. Hydrant tests can indicate flows well in excess of what we need for a building system, sometimes up to 3,000 gallons or more. The hydrant test merely indicates what pressure is available for suction on the pump at a given flow. If we specify a pump with greater flow than the maximum flow of the hydrant test, we will cavitate the pump because the system cannot provide the net positive suction head required. I think the maximum flow that can be designed is the flow at which the hydrant test indicates 20 psig.

I am not really strong on pump theory so I could have some facts incorrect, but I think what I am saying is sound.