Flow And Pressure Drop; Fluid Seeks A Level - Emerson Daniel 700 Series Technical Manual

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

Flow and Pressure Drop

Whenever a liquid is flowing, there must be a condition of unbalanced force to cause motion. Therefore, when a fluid flows
through a constant-diameter pipe, the pressure will always be slightly lower downstream than to any point upstream. The
difference in pressure or pressure drop is required to overcome friction in the line.
Figure 1-7 illustrates pressure drop due to friction. The succeeding pressure drops, from maximum pressure to zero pressure, are
shown as differences in head in succeeding vertical pipes.

Fluid Seeks A Level

When there is no pressure difference on a liquid (no flow) it is distributed equally in the pipes as shown in Figure 1-7. If the
pressure changes the liquid levels rise until the weight is sufficient to make up the difference in pressure. The difference in height
(head) in the case of oil is one foot per 0.4 psi. Thus, it can be seen that additional pressure difference will be required to cause
a liquid to flow up a pipe or to lift the fluid, since the force due to the weight of the liquid must be overcome. In circuit design,
naturally, the pressure required to move the oil mass and to overcome friction must be added to the pressure needed to move the
load. In most applications, good design minimizes these pressure "drops" to the point where they become almost negligible.
Pressure is maximum here
because of the head height
of liquid.
P1
Friction in the pipe drops pressure
from maximum to zero.
FRICTION IN PIPES RESULTS IN A PRESSURE DROP
P2 P3
P4
Figure 1-7
9
The lower level
of liquid in these pipes is
a measure of reduced
pressure at points
downstream from the
source.
(P1 minus P5) equals
maximum differential
pressure available.
P5
Pressure is zero here as the liquid
flows out unrestricted.