Advanced Waterworks Mathematics, 2019a

UNIT 8
8.1 PRESSURE
Pressure is the amount of force that is “pushing” on a specific unit area. What does this mean?
When you turn on your water faucet or shower you feel the water flowing out, but why is it
flowing out? Water flows through pipes and out of faucets because it is under pressure. It
could be that a pump is turned on in which case the pump and motor are providing the
pressure. More commonly, the pressure is being provided by water being stored at a higher
elevation. This is why you see water tanks on top of hills.
Figure 8.1 35
Pressures are usually expressed as pounds per square inch (psi), but they can be expressed as
pounds per square foot or pounds per square yard as well. The key is that the force is
expressed per unit area.
Typically, water operators will measure pressures with gauges and express the unit answer as
psig. The “g” is this case represents gauge. However, it is also common to express pressure in
feet. Feet represent the height of the water in relation to the location that the pressure is
being measured.
There are two commonly used factors to convert from feet to psi and vice versa. For every foot
in elevation change there is a 0.433 change in psi. Conversely, for every psi change there is a
2.31 foot in elevation change.
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Photo used with permission of SCV Water
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