A plumber is sizing the water distribution piping for a residential dwelling. The dwelling has 2 bathrooms, a kitchen, and a laundry room. The total developed length of the main cold water line from the meter to the furthest fixture is 100 feet. The available street main pressure is 60 psi, and the desired minimum residual pressure at the highest fixture is 20 psi. The fixture unit count for the dwelling is 18. Using the Hunter's Curve method, what is the maximum allowable pressure loss due to friction for the entire cold water distribution system?
- A30 psi
- B10 psi
- C20 psi
- D40 psi
Show answer & explanationAnswer & explanation
Correct answer: D. 40 psi
The maximum allowable pressure loss due to friction is calculated by subtracting the desired minimum residual pressure at the highest fixture and any static head loss from the available street main pressure. In this case, there is no mention of static head loss for elevation, so it's 60 psi (available) - 20 psi (residual) = 40 psi. The fixture unit count and developed length are used in conjunction with the Hunter's Curve to determine pipe sizes that stay within this allowable loss.
Why the other options are wrong
- A. 30 psi does not account for the full difference between available and desired residual pressure.
- B. 10 psi is too low based on the given available and residual pressures.
- C. 20 psi is the desired residual pressure, not the allowable pressure loss due to friction.
Allowable Friction Loss
The maximum amount of pressure that can be lost due to friction in a water distribution system while still maintaining adequate residual pressure at the most remote or highest fixture.
- Calculated by (Available Pressure - Residual Pressure - Static Head)
- Crucial for pipe sizing with Hunter's Curve
- Expressed in psi or feet of head
- Excessive friction loss leads to low pressure
Memory trick: Budget your pressure: Start high, lose some, but end right!