A gravity storage tank will supply the top floor of a building. The highest fixture outlet served needs 20 pounds per square inch of flowing pressure. Take these values as given: friction loss between the tank and that outlet is 5 pounds per square inch at design flow, and each foot of elevation is worth 0.433 pound per square inch. How far above that outlet must the water level in the tank stand?
- AAbout 46 feet.
This covers the fixture requirement but drops the 5 pounds of friction loss, so the far outlet would fall short at design flow.
- About 58 feet.
- CAbout 25 feet.
This treats 1 pound per square inch as 1 foot of water, which understates the required height by more than half.
- DAbout 11 feet.
This multiplies by 0.433 instead of dividing by it, the most common direction error in head conversions.
Why B is correct
Head has to cover both the fixture requirement and the friction between tank and fixture, and converting psi to feet means dividing by 0.433, which is the same as multiplying by 2.31 feet per pound.
What this question is testing
Whether the candidate can convert a pressure requirement into elevation in the correct direction and include friction in the requirement.
On the job
Gravity tanks still serve the upper floors of older mid-rise buildings, and the top floor is always the one that complains. When a renovation adds a shower or a flushometer up there, run this check before promising performance, and remember the usable head shrinks as the tank drains, so design from the low water level rather than the overflow.
Memory technique
Every 2.31 feet of standing water buys you 1 pound.
Exam tip
Feet to pounds, multiply by 0.433. Pounds to feet, divide by 0.433 or multiply by 2.31.
Where to look it up
Gravity tank and booster provisions are in IPC Chapter 6, and design pressure criteria are in Section 604. UPC covers gravity tanks separately, so verify the details against your adopted code.