Many people confuse velocity
and pressure. Think about the basics and what can be happening.
Velocity is
speed and pressure is force.
A pump supplies water at 60 psi and 10 gpm through
a pipe. If a valve on the pipe is slightly opened, a small stream of water shoots
out. The pressure pushing the water out is 60 psi, but the water velocity is
high as water tries to escape through the small cross-sectional area of the
small opening.
The pump is trying to push 10 gpm out the small opening; the
water velocity must be high as the area is small. Less than the full 10 gpm
flow will likely occur because of the restriction of the small opening.
Water
at high velocity will travel some distance when released because of the velocity.
Of course, the full pressure is pushing the water.
When the valve is opened
all the way, the water velocity drops because the cross-sectional area has increased.
Remember that flow is velocity times area. The slower-moving water does not
squirt as far out the larger opening. The full 10 gpm flow escapes. The pump is still
trying to maintain a pressure of 60 psi.
The pipe or valve opening
size, and thus the cross-sectional area of the opening, determines the velocity
of a flow of 10 gpm. If the opening is small enough that 10 gpm cannot flow
through, the pressure will remain high. If the opening is very large and does
not restrict the flow of 10 gpm, then pressure will not develop to 60 psi. The
water just flows out as quickly as the pump can push it in.
For a given flow rate, the
velocity must get higher when the cross-sectional area of the opening becomes
smaller.
Pressure will be maintained if the flow into the pipe remains equal
to or greater than the flow out. If a pipe or hole is so large that no flow
restrictions exist, less pressure develops. The flow is by gravity force. Pressure
is the equivalent feet of head developed in the system. It is the energy pushing
water out of a container.