3.4 Velocity versus Pressure

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.