CSLB C-36 Plumbing ContractorPlanning and EstimatingHard
A plumbing contractor is asked to provide an estimate for the installation of a new water service line to a commercial building. The existing municipal water main has a static pressure of 80 PSI. The building's plumbing fixtures require a minimum operating pressure of 40 PSI, and the building is 50 feet tall. The contractor must account for pressure loss due to elevation, friction loss in piping, and ensure adequate pressure at the highest fixture. What is the approximate pressure loss due to elevation for the highest fixture in this building?
- A17.3 PSI
- B21.7 PSI
- C8.7 PSI
- D34.6 PSI
Show answer & explanationAnswer & explanation
Correct answer: B. 21.7 PSI
Pressure loss due to elevation is calculated by multiplying the height difference in feet by 0.433 PSI per foot. For a 50-foot building, the pressure loss is 50 feet * 0.433 PSI/foot = 21.65 PSI, which approximates to 21.7 PSI.
Why the other options are wrong
- A. This would be the loss for approximately 40 feet of elevation (40 * 0.433 ≈ 17.3).
- C. This would be the loss for approximately 20 feet of elevation (20 * 0.433 ≈ 8.7).
- D. This would be the loss for approximately 80 feet of elevation (80 * 0.433 ≈ 34.6).
Pressure Loss Due to Elevation
The reduction in water pressure as water flows upwards against gravity in a plumbing system. For every foot of vertical rise, water pressure decreases by approximately 0.433 pounds per square inch (PSI).
- Formula: Pressure Loss (PSI) = Height (feet) × 0.433 PSI/foot.
- Must be accounted for when sizing water supply systems for multi-story buildings.
- A higher building requires a higher initial static pressure or a booster pump to maintain adequate pressure at upper floors.
- This is a 'loss' from the perspective of the system's ability to deliver pressure at higher points.
Memory trick: Up the pipe, pressure dies, 0.433 is the price.