# Help on Submersible Pump! Basic question

1) Why head decreases with increase in flow rate ?
2)why input power increases linearly with increase in flow rate?
3)Why output power at first increase upto a max value and then decreases with increase of flow rate?
4)why does efficiency curve also show same nature as no.3?

## Replies

• shuilishi
1.the flow rate depends on the impeller with the same power,and when they are same with the same kw and same impeller ,more flow rate will with low head
2.flow rate increases linearly with the increase the power because same impeller with different power has the max AMP load
• Rohan_sK
For the Pump basic curves :

1. The HEAD v/s FLOWRATE curves are drawn for the SAME Impeller Diameter and CONSTANT RPM.

Thus when Flow Rate increases for constant impeller diameter and constant rpm, then the velocity head developed remains the same, but the Frictional head loss increases. This brings down the Total Output Head (Hd). Thus the HEAD v/s FLOWRATE curve seems to DROP by small amount with an increase in flowrate.

2. Input Power increases with Flow Rate because greater volume of fluid has to be pumped per unit time.

3. There are factors like proper NPSH value and cavitation which come into action especially after the BEP and thus affect the Efficiency curve giving it the characteristic mentioned.
• gopu3107
but i think Npsh is for for finding out cavitation factor at minimum head. The avilable NPSH must be greater than the req. NPSH to avoid cavitation.
• kskristinesmith
A submersible pump is a pump which has a hermetically sealed motor close-coupled to the pump body. The whole assembly is submerged in the fluid to be pumped. The advantage of this type of pump is that it can provide a significant lifting force as it does not rely on external air pressure to lift the fluid.

Kristine
• Rohan_sK
Yep, the centrifugal pumps used in the normal purposes cannot have a suction head more than 6m to 7m maximum in real operating conditions and NEVER more than 10m ideally.

The reason is that the external air pressure ie the atmospheric pressure actually pusheds the water upwards while initial suction to the pump center line.

This is because the 10m value is the equivalent of the atmospheric pressure head and no pump placed on the ground can do suction beyond this height. The maximum suction head value further reduces to 6-7m because of the reduction due to frictional head loss in the pipes.
• nirave
the equation for power for a pump is P=wQH, where w= weight density, Q= flow rate and H= head (meter)... now for same power Q is inversely proportional to H ( ans to Q1). for same H and w Q is directly proportional to P (ans to Q2).

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