Npsh

NPSHA for those who hate that stuffy word
This article follows the same approach as the pump tutorial and if you are just starting in
this field I recommend you read it first.
The dreaded termNPSHA means Net Positive Suction Head Available. It`s a term that
most people find difficult to relate to in part because they have no idea what value it
should have. Before we discuss values and theexact definition of NPSHA, let`s get an
intuitive understanding of NPSHA. NPSHA is a measure that corresponds to the level of
pressure at the pump suction. The higher the pressure, the higher theNPSHA and the
better the pump will operate. Normally we measure pressure with a gauge that is
calibrated in psig (pound per square inch gauge) or kPa in the metric system. This
pressure scale is setat zero when there is no pressure or the pressure is equal to
atmospheric pressure. The atmospheric pressure at sea level as measured on an
absolute scale such as psia (pounds per square inchabsolute) is 14.7 psia. In the
absolute scale, pressure starts at zero which is the lowest possible pressure and means
that there is no molecules of matter in the environment that can create pressure suchas
in outer space. It then can have any value corresponding to a high pressure
environment.
The term head in NPSHA has been well explained in the pump tutorial, the head
component that we are mostinterested in here is the static head or the level of fluid
above the pump suction. Head is measured in feet in North America and in meters just
about everywhere else. The value of NPSHA will varybetween the lowest value of 0
feet, up to the value of the local atmospheric pressure head 34 feet plus the suction
static head minus a small quantity which we will get to shortly. 34 feet is thevalue of
atmospheric pressure at sea level expressed in terms of pressure head. If your tank has
10 feet of suction head, the NPSHA may be 34 + 10 = 44 feet which is ample. One
should start to...