PUMPING OF LIQUIDS. NPSH

Every person working in industries had to deal with the pumps. This is the reason for this technical article, it is a reminder for some important information about it, just like a revision.

I'm writing an article brought by an old engineer book: "FLUID FLOW FOR CHEMICAL ENGINEER"

Pumps are devices for supplying energy or head to a flowing liquid in order to overcome head losses due to friction and also, if necessary, to raise the liquid to a higher level.

The head imparted to a flowing liquid by a pump is know at the total head Δh.

If a pump is positioned between two different points, the head for steady flow are related by the following equation:

In this equation, we have the static, pressure and velocity head and hf is the head loss due to friction.

SYSTEM HEADS

For us is important to know suction, discharge, total and and available net positive suction heads.

Remember: If the liquid level on the suction side is below the centreline of the pump, Zs is NEGATIVE.

The total head which the pump is required to impart to the flowing liquid is the difference between the discharge and suction head.

There is another important coefficiente to keep in mind: NET POSITIVE SUCTION HEAD.

This equation gives the head available to get the liquid through the suction piping. Pvp is the vapour pressure of the liquid being pumped at the particular temperature in question. NPSH can also be written as :

The available NPSH in a system should always be positive, the suction head must always be capable of overcoming the vapour pressure.

Since the frictional head loss Hfs increases with increasing liquid flow rate, the available NPSH decrases with increasing liquid pumping rate.

At the boiling temperature of the liquid, Ps and Pvp are equal and the available NPSH becomes Zs-Hfs. In this case no suction lift is possible since Zs must be positive. 

If the term ( Ps - Pvp) is sufficiently large, liquid can be lifted from below the centreling of the pump. In this case Zs could be negative.