A plumber is designing a potable water distribution system for a new three-story apartment building. The building will have a constant supply pressure of 60 PSI at the entry point. What is the approximate maximum height above the entry point that a fixture can be reliably supplied with water before considering additional pressure boosting, assuming a minimum residual pressure of 20 PSI is required at the fixture and accounting for typical friction losses?
- A92 feet
- B46 feet
- C69 feet
- D115 feet
Show answer & explanationAnswer & explanation
Correct answer: B. 46 feet
The available pressure for elevation and friction loss is 60 PSI (supply) - 20 PSI (required residual) = 40 PSI. Converting PSI to feet of head (1 PSI ≈ 2.31 feet), we get 40 PSI * 2.31 ft/PSI = 92.4 feet. Accounting for typical friction losses, which can be significant in a multi-story building (often estimated at 1/2 to 1/3 of the available pressure for elevation in such calculations), a reasonable estimate would be to take about half of this for elevation, placing it around 46 feet. This ensures a margin for friction losses.
Why the other options are wrong
- A. This option only accounts for the pressure drop due to elevation, without considering friction losses, which are significant.
- C. This option is too high, as it does not adequately account for friction losses.
- D. This option is significantly too high and would result in insufficient pressure at the fixture.
Water Pressure for Elevation
Water pressure is required to overcome the force of gravity to elevate water to higher fixtures, in addition to overcoming friction losses within the piping system and maintaining a minimum residual pressure at the fixture.
- 1 PSI of pressure can lift a column of water approximately 2.31 feet.
- Residual pressure (typically 8-20 PSI) must be maintained at the fixture.
- Friction losses increase with pipe length, diameter, and flow rate.
Memory trick: Pressure Pushes Pipes Up, but Friction Fights.