Process Engineering Beginner's Guide

CHAPTER EIGHTHYDRAULICS

Have you ever wondered what a water tower is for? I used to think it was to store water. But if that was the water tower’s only function, why have the water tank elevated 100 feet above grade? Why not just locate the tank on the ground?

The reason that the water tank sits 100 ft. above the ground is to maintain a constant pressure in a town’s water supply. If I fill a glass tube 28 inches or 2.31 feet high with 60°F water, and place a pressure gauge at the base, I will read a pressure of one psig. Or, I can say, 2.31 feet of water exerts a head pressure of one psig. If the water tower’s tank is 100 feet above my house, then:

• 100 feet ÷ 2.31 = 43.3 psig

That’s the water pressure that I’ll have in my home in New Orleans. I’ll use this pressure to water my vegetable garden. The water flows through my garden hose at a pressure of 43.3 psig. When I say this, I’m ignoring frictional losses; I’m assuming head losses due to the roughness inside the hose and the city water piping system is zero. At the end of the hose I have a brass nozzle. The pressure of the water coming out of this nozzle is less than 43.3 psig. The pressure of the water at this point is zero psig (or 14.7 psia at sea level in New Orleans).

What has happened to the energy represented by the 43.3 psig of pressure? In reality, some of it is converted to friction. But I have decided to assume frictional losses are zero. Therefore, the 43.3 psi reduction in water pressure has been converted into: ...