Page 22 - Know and Understand Centrifugal Pumps

Basic Pump Principles

Gauge pressure (psig)

Gauge pressure is the pressure indicated on a simple pressure gauge.
Simple pressure gauges establish an artificial zero reference at
atmospheric pressure. The formula is: psig = psia - ATM.

Vacuum
___ - ~ ~~~
The term vacuum is used to express pressures less than atmospheric
pressure (sometimes represented as a negative psi on pressure gauges).
Another scale frequently used is ‘inches of mercury’. The conversion is:
14.7 psia = 29.92” Hg. Another scale gaining in popularity is the
..
kilopascal (Kp) scale. 14.7 psia = 100 Kp

Note that there are many ways to express vacuum. Simple gauges record vacuum as a
negative psig. Compound gauges record vacuum as a positive psia. The weatherman
uses inches of mercury in the daily forecast, and millibars (1000 millibars is
atmospheric pressure) to express the low-pressure zone in the eye of a hurricane.
Boiler operators use water column inches and millimeters of mercury to express
vacuum.

Pump manufacturers express vacuum in aspirated feet of water in a vertical column
(0 psia = -33.9 feet of water). The pharmaceutical and chemical industry uses
‘Pascals’ (100,000 Pascals = atmospheric pressure) and the term TORR. This
conglomeration of values and conversion rates causes confusion. In order to
understand pumps, it‘s best to think of vacuum as a positive number less than 14.7
psi. In our experience, we’ve found that considering vacuum in this form aids the
understanding of net positive suction head (NPSH), cavitation, suction specific speed
(Nss), and the ability of pumps to suck-up (actually pumps don’t suck, but this will do
for now) fluid from below. Remember that vacuum is the absence of atmospheric
pressure, but it is not a negative number.

Pump head

The term ‘pump head’ represents the net work performed on the liquid
by the pump. It is composed of four parts. They are: the static head
(Hs), or elevation; the pressure head (Hp) or the pressures to be
overcome; the friction head (Hf) and velocity head (Hf), which are
frictions and other resistances in the piping system. These heads are
discussed in Chapter 8. The head formula is the following:

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