Understanding head and NPSH in pumps

[theBEAST]

So I know that head is related to energy. And it is just another way to measure the energy at any given point in the flow (although in units of meters/feet/etc). So does that mean the head is equal at all points of a pipe? Since by conservation of energy and mass, and assuming the flow is steady, then the energy everywhere in the particles must be the same?

Also for NPSH (net positive suction head). It is defined as the head required at pump inlet to prevent cavitation:

NPSH = P_i/(ρg) + V_i²/(2g) - P_v/(ρg)

There are two types of NPSH, one is NPSH actual and one is NPSH required. Mathematically how are these different? Do they both follow the same equation above? If so, why would they be different?

See in this plot:

NPSH actual is always larger than NPSH required, why is this?

This was a topic that the professor quickly covered and I don't think we will be tested on it. But out of curiosity I really want to understand the concepts behind it. If anyone could help me it would be greatly appreciated.

 

[haruspex]

NPSH ... is defined as the head required at pump inlet to prevent cavitation

As I understand it, that's the definition of the required NPSH. The actual NPSH is an attribute of the actual circuit. Cavitation occurs when actual < required. See http://en.wikipedia.org/wiki/NPSH.

 

[theBEAST]

As I understand it, that's the definition of the required NPSH. The actual NPSH is an attribute of the actual circuit. Cavitation occurs when actual < required. See http://en.wikipedia.org/wiki/NPSH.

Yes I know that but what is the different between actual and required? Do they both use the same equations? I am very confused...

 

[haruspex]

NPSH is a function of where you are along the flow, following the formula you quoted. It's how much spare pressure you have at that point for cavitation not to be occurring there.
Given a pump with an inlet, NPSH interior to the pump is likely to be less at some points than at the inlet. The NPSHR specified for the pump is the NPSH required at inlet. It allows for the maximum difference. It represents how much the NPSH will drop from inlet to the most critical point within the pump. NPSHA is the available NPSH, i.e. the actual value at inlet.