Hydraulic Sizing for a Booster Pump and Backflow Assembly
An engineer is designing a water service for a three-story building. The municipal main provides 55 psi static pressure at the property line. The service line is 150 feet of 2-inch copper, and the building requires 40 psi residual at the top floor fixture. The design includes a reduced pressure zone assembly with a published friction loss of 7 psi at the design flow of 90 gpm. The engineer calculates friction loss in the service line at 9 psi and elevation loss at 30 feet. A contractor proposes deleting the backflow assembly to 'save 7 psi' and meet the residual requirement.
1. Calculate the available residual pressure at the top floor with the assembly installed and determine whether the design meets the 40 psi requirement.
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Work from the static pressure and subtract each loss:
- Static pressure at property line: 55 psi
- Elevation loss: 30 ft × 0.433 psi/ft = 13.0 psi
- Service line friction loss: 9 psi
- Backflow assembly loss: 7 psi
Residual = 55 − 13.0 − 9 − 7 = 26 psi.
The design does not meet the 40 psi requirement; it is 14 psi short. Note that the assembly loss is a real hydraulic cost, not a paperwork item. The shortfall must be solved by design changes: increasing service line diameter to reduce friction, raising the supply pressure with a booster pump, or reducing the required residual through fixture selection. Deleting the assembly is not an option because it is required for the hazard.
2. Evaluate the contractor's proposal to delete the assembly and propose at least two compliant alternatives.
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The proposal is non-compliant. The backflow assembly is required because the building's use creates a cross-connection hazard; removing it to gain pressure violates the cross-connection control requirement and shifts risk to the public supply. The 7 psi is a design constraint, not a negotiable item.
Compliant alternatives:
- Upsize the service line. Moving from 2-inch to 3-inch copper reduces friction loss substantially, recovering several psi without touching the assembly.
- Install a booster pump downstream of the assembly. This raises pressure to the building while keeping protection in place. The pump must be sized for the design flow and coordinated with the assembly's pressure requirements.
- Relocate or reselect the assembly. A different approved assembly with lower friction loss at 90 gpm may be available, provided it meets the hazard requirement.
- Reduce demand. Lower-flow fixtures can reduce the design flow and therefore friction loss.
The engineer should document the calculation and reject the deletion in writing.