We are often "taken-back" when we see an "application of the theory" type of problem. We are all aware that a 2.31 ft column of water creates a 1 psi (one pound per square inch) pressure under it. But HOW can we use this information? What if the PIPE is under pressure? Does the pressure in the pipe act the same way? (Yes, it does.) We will use the following example to illustrate our response.

EXAMPLE: Terry is building a 2 story home on the coast. The house pad is at an elevation 75 ft above the ocean level. The second story bathroom fixtures are 16 feet above the house pad. Terry will accept a minimum of 30 psi in water pressure on the second floor, when the tank is just at "empty." Ignoring line losses, what is the minimum tank pad elevation? Here is the answer. Please try working the problem before looking at the solution. (Ans: 160.3 ft)

We first draw a picture to see just what we need to do, and fill in all of the available information that we have:

We see that the second floor is 16 feet above the pad, so the elevation at the faucets in the second floor would be:
75 ft + 16 ft = 91 foot elevation of the faucets

The floor of the tank, the tank base, would need to be "30 psi" above the faucets. We would need to convert then, the 30 psi into feet to help find the tank pad elevation:

"height of tank pad above second story faucets"

(How do we end up with "ft" units? The "psi" units cancel out....multiplying by a number and then dividing by that same number, yields "one.")

We add the elevation of the faucets to the additional 30 psi equivalent elevation to get the water tank pad elevation:
( 91 ft faucet elev ) + ( 69.3 ft height required to produce 30 psi) = 160.3 ft tank pad elevation. Our answer is illustrated in the graphic below:

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