Page 29 - Know and Understand Centrifugal Pumps

NPSH, Net

Positive Suction

Head

Introduction

When someone turns on an electric light, the natural tendency is to
look toward the light and consider the shine. We tend not to think
about the electric wires and the current running through the light bulb.
Equally, when someone starts an industrial pump, the tendency is to
look toward the discharge piping and consider the pressure and flow.
We tend not to think about the suction piping, or the liquid coming
into the eye of the impeller. We need to emphasize the necessity to
consider what’s happening in the suction of the pump. This area is the
source of problems, and probably is responsible for about 40% of all
pumps going into the shop today.
This chapter is dedicated to NPSH, Net Positive Suction Head. NPSH
is what the pump needs, the minimum requirement to perform its
duties. Therefore, NPSH is what happens in the suction side of the
pump, including what goes on in the eye of the impeller. NPSH takes
into consideration the suction piping and connections, the elevation
and absolute pressure of the fluid in the suction piping, the velocity of
the fluid and the temperature. For the moment we can say that some of
these factors add energy to the fluid as it moves into the pump, and
others subtract energy from the fluid. There must be sufficient energy
in the fluid for the impeller to convert this energy into pressure and
flow. If the energy is inadequate we say that the pump suffers
inadequate NPSH.
In simple terms we could say that NPSH is the reason that the suction
nozzle is generally larger than the discharge nozzle. If there is more
liquid leaving the pump faster than the liquid can enter into the pump,
then the pump is being starved of liquid.