fluids at rest or in motion, while inertia and
friction are dynamic factors that apply only to
fluids in motion. The mathematical sum of
gravity, applied force, and atmospheric pressure
is the static pressure obtained at any one point
in a fluid at any given time. Static pressure exists
in addition to any dynamic factors that may also
be present at the same time.
Remember, Pascals law states that a pressure
set up in a fluid acts equally in all directions and
at right angles to the containing surfaces. This
covers the situation only for fluids at rest or
practically at rest. It is true only for the factors
making up static head. Obviously, when velocity
becomes a factor it must have a direction, and
as previously explained, the force related to the
velocity must also have a direction, so that
Pascals law alone does not apply to the dynamic
factors of fluid power.
The dynamic factors of inertia and friction are
related to the static factors. Velocity head and
friction head are obtained at the expense of static
head. However, a portion of the velocity head can
always be reconverted to static head. Force, which
can be produced by pressure or head when dealing
with fluids, is necessary to start a body moving
if it is at rest, and is present in some form when
the motion of the body is arrested; therefore,
whenever a fluid is given velocity, some part of
its original static head is used to impart this
velocity, which then exists as velocity head.
Consider the system illustrated in figure 2-18.
Chamber A is under pressure and is connected by
a tube to chamber B, which is also under pressure.
The pressure in chamber A is static pressure of
100 psi. The pressure at some point (X) along the
connecting tube consists of a velocity pressure of
Figure 2-18.Relation of static and dynamic factors
10 psi exerted in a direction parallel to the line
of flow, plus the unused static pressure of 90 psi,
which still obeys Pascals law and operates equally
in all directions. As the fluid enters chamber B
it is slowed down, and its velocity is changed back
to pressure. The force required to absorb its
inertia equals the force required to start the fluid
moving originally, so that the static pressure in
chamber B is equal to that in chamber A.
This situation (fig. 2-18) disregards friction;
therefore, it would not be encountered in actual
practice. Force or head is also required to
overcome friction but, unlike inertia effect, this
force cannot be recovered again, although the
energy represented still exists somewhere as heat.
Therefore, in an actual system the pressure in
chamber B would be less than in chamber A by
the amount of pressure used in overcoming
friction along the way.
At all points in a system the static pressure is
always the original static pressure, less any velocity
head at the point in question and less the friction
head consumed in reaching that point. Since both
the velocity head and the friction head represent
energy that came from the original static head,
and since energy cannot be destroyed, the sum of
the static head, the velocity head, and the friction
head at any point in the system must add up to
the original static head. This is known as
Bernoulli's principle, which states: For the
horizontal flow of fluid through a tube, the sum
of the pressure and the kinetic energy per unit
volume of the fluid is constant. This principle
governs the relations of the static and dynamic
factors concerning fluids, while Pascals law states
the manner in which the static factors behave
when taken by themselves.
Fluid power equipment is designed to reduce
friction to the lowest possible level. Volume and
velocity of flow are made the subject of careful
study. The proper fluid for the system is chosen.
Clean, smooth pipe of the best dimensions for the
particular conditions is used, and it is installed
along as direct a route as possible. Sharp bends
and sudden changes in cross-sectional areas are
avoided. Valves, gauges, and other components
are designed to interrupt flow as little as possible.
Careful thought is given to the size and shape of
the openings. The systems are designed so they