Pressure-Reducing Valve Setting for a Four-Story Hotel Conversion
A four-story hotel is being converted from an office building. The municipal main provides 92 psi static pressure at the street. The building's water service is 2-inch Type L copper, and the developed length to the top-floor fixture is 240 feet. The highest fixture is 46 feet above the street main. The plumbing engineer proposes a direct-acting pressure-reducing valve (PRV) at the basement entry set to 55 psi, with no booster pump. The fixture demand is 38 fixture units; the service and meter losses are 9 psi at peak demand, and the building's distribution friction loss to the top floor is 22 psi. The local authority requires a minimum residual pressure of 15 psi at the highest fixture and a maximum static pressure of 80 psi at any fixture. There is no room for a booster pump without relocating the electrical room.
1. Using the stated pressures and losses, calculate the residual pressure at the highest fixture with the PRV set at 55 psi. Does the design comply with the 15 psi minimum?
Read the worked answer
The calculation must account for elevation loss and friction loss downstream of the PRV. Elevation loss is 46 ft × 0.433 psi/ft = 19.9 psi. With the PRV set at 55 psi, the static pressure at the top fixture before flow is 55 − 19.9 = 35.1 psi. At peak demand, subtract the distribution friction loss of 22 psi: 35.1 − 22 = 13.1 psi residual. That is below the required 15 psi, so the design fails the minimum residual pressure criterion. The service and meter losses of 9 psi occur upstream of the PRV and do not further reduce the downstream residual, but they do explain why the street pressure cannot be used directly. The inspector should reject the 55 psi setting and require a higher PRV setting, a larger distribution main to reduce friction, or a booster pump if the PRV cannot be raised enough.
2. What is the minimum PRV setting that satisfies the 15 psi residual requirement, and what is the maximum setting allowed by the 80 psi fixture limit? Is the resulting range feasible?
Read the worked answer
Work backward from the top fixture. Required residual = 15 psi. Add elevation loss 19.9 psi and distribution friction 22 psi: 15 + 19.9 + 22 = 56.9 psi. Therefore the PRV must be set to at least 57 psi. The 80 psi maximum static pressure applies at fixtures near the PRV, where elevation loss is negligible. The PRV setting is the downstream static pressure at the basement, so the setting cannot exceed 80 psi. The feasible range is 57 psi to 80 psi. A setting of 60 psi, for example, gives 60 − 19.9 − 22 = 18.1 psi at the top fixture and 60 psi static at the basement fixtures, which satisfies both criteria. The range is feasible, but the inspector should also verify that the PRV is rated for the inlet pressure and that the reduced setting does not create excessively low pressure at upper floors during low-flow conditions.
3. If the PRV fails open, what pressures occur at the highest and lowest fixtures, and what code concern does that create?
Read the worked answer
With the PRV failed open, the street pressure of 92 psi reaches the building. At the highest fixture, subtract elevation loss only: 92 − 19.9 = 72.1 psi static. At the lowest fixtures, the pressure is essentially the street pressure, 92 psi. The highest fixture remains below 80 psi, but the lower fixtures exceed the 80 psi maximum and may also exceed the pressure ratings of fixtures, hose bibbs, and appliances. The code concern is that a single PRV failure can expose the lower portions of the system to excessive pressure. The inspector should require a PRV with a documented failure mode, a monitored relief or bypass arrangement acceptable to the authority, or another approved means to limit downstream pressure. A simple check valve is not a substitute for pressure control.