4.2 Review of Pressure

In this review each concept will be discussed as it would apply to a real irrigation system.  Study this carefully.  Refer back to earlier section for specific topics.

Pressure can be created by elevation such as a water tower.  It is the weight of water in a column that creates pressure, the force that pushes water through the irrigation system.  A column of water 100 feet tall creates 100 ft x 0.433 pound per square inch (psi) per foot of height = 43.3 psi.  Note the units of measure show that ft x psi/ft = psi.  Pressure can be created mechanically by a pump that pushes the water into the irrigation system.

In the irrigation system in the exercise (look ahead to the illustration) that follows, the pump is pushing water into the pipeline, if all valves are closed the water can go nowhere and the pressure will be the maximum that the pump can create.  It is a closed system like a water tank.  If the pump can create 70 psi, then the water will be at 70 psi in all parts of the tank.

In a pipeline with no outlets, the water will flow in until the pipeline is full, coming to a static pressure of 70 psi.  The dynamic situation will be discussed under friction loss.