Pressure Defined - Emerson Daniel 700 Series Technical Manual

Technical Guide
DAN-LIQ-TG-44-rev0813
November 2013

Pressure Defined:

In order to determine the total force exerted on a surface, it is necessary to know the pressure or force on a unit of area. We
usually express this pressure in "Pounds per Square Inch", abbreviated psi. Knowing the pressure and the number of square
inches of area on which it is being exerted, one can readily determine the total force.
(Force in Pounds = Pressure in psi x Area in Sq. In.)
How Pressure is Created:
Pressure results whenever the flow of a fluid is resisted. The resistance may come from (1) a load on an actuator, (2) a control
valve or, (3) a restriction (or orifice) in piping. See Figure 1-3.
PRESSURE CAUSED BY RESTRICTION AND LIMITED BY PRESSURE CONTROL VALVE.
5 gpm
PUMP
0 psi
PUMP
25 psi
PUMP
50 psi
Figure 1-3
When the manual valve is
wide open, all flow is
unrestricted.
There is no pressure in this condition.
As flow is restricted by
closing the manual valve...
... pressure builds up.
With the manual valve
closed...
... the pressure gauge reads the
maximum pressure available.
5