High-Rise Domestic Water Pressure Woes

First, determine the pressure at the 10th floor without PRV. The booster pump discharge is at ground level? Actually, the pump takes suction from the rooftop tank? Wait, the setup says the system is supplied by a rooftop tank and a booster pump. The pump maintains 70 psi at discharge. The elevation of the 10th floor is 100 ft above ground. The rooftop tank is 20 ft above the roof, roof at 120 ft, so tank water level is 140 ft above ground. The pump likely boosts from tank to system. But the pump discharge pressure is given as 70 psi. That pressure is at the pump outlet, which we assume is at ground level. Then pressure at 10th floor = 70 psi - (100 ft / 2.31 ft/psi) = 70 - 43.3 = 26.7 psi. Then the PRV reduces to 50 psi? That would increase pressure, which is impossible. So the PRV must be set to reduce from a higher pressure to 50 psi. But if the available pressure before the PRV is only 26.7 psi, the PRV cannot maintain 50 psi; it will be fully open and the downstream pressure will be about 26.7 psi minus losses. That is above the minimum 20 psi, but flow may be low due to pipe sizing and losses. The low flow at showerheads is likely due to excessive pressure loss in the 1 in. branches and the PRV being wide open, causing insufficient residual pressure at the fixture. Also, the pump may be operating at the end of its curve. The noisy pipes on the 2nd floor are due to high pressure (70 psi - 20 ft/2.31 = 70 - 8.7 = 61.3 psi) before PRV, and the PRV set at 50 psi may be failing due to debris or high differential. The 2nd floor PRV failures suggest that the inlet pressure exceeds the PRV's rated differential. The system design is flawed because the PRV setpoints are too high for lower floors and too low for upper floors. A better design would use a PRV at the bottom to reduce pressure to a manageable level and then rely on booster pump for upper floors, or use different setpoints per floor. The calculation shows that at the 10th floor, the available pressure before PRV is 26.7 psi, which is less than the PRV setpoint of 50 psi, so the PRV is fully open and not regulating. The fixture pressure is then 26.7 psi minus friction loss in the branch and fixture, which may be below the minimum 20 psi, causing low flow. The solution is to adjust the PRV setpoints or resize piping.

The 2nd floor PRVs are failing because the inlet pressure is too high. At the 2nd floor (20 ft above ground), the pressure before the PRV is 70 psi - (20/2.31) = 70 - 8.7 = 61.3 psi. The PRV is set to 50 psi, so it must reduce pressure by 11.3 psi. That is not excessive. However, if the pump discharge is actually higher due to a misadjusted pump or if the static pressure is higher (e.g., pump shutoff head), the inlet pressure could be higher. But the problem states frequent PRV failures, which may be due to debris, water hammer, or thermal expansion. A common cause is that the PRV is undersized or the differential is too high. Actually, the pressure at the 2nd floor before PRV is 61.3 psi, which is below the typical PRV maximum inlet of 300 psi, so that's not it. The failures might be due to the PRV being set too low, causing excessive velocity and wear. Or the PRVs are not rated for the flow. The code requires maximum 80 psi at fixtures, so the PRV set at 50 psi is fine. But the noisy pipes indicate high velocity. A solution is to install a single PRV at the building main to reduce pressure to, say, 60 psi, and then use a booster pump for upper floors, or to use different PRV setpoints: lower floors get lower setpoints (e.g., 30 psi) and upper floors get higher setpoints (e.g., 50 psi) to account for elevation loss. But the code requires minimum 20 psi at top fixture. At the 10th floor, available pressure before PRV is 26.7 psi, so if we set PRV to 30 psi, it won't regulate. So we need to increase pump pressure or upsize piping. A better solution: increase pump discharge to 85 psi, then at 10th floor: 85 - 43.3 = 41.7 psi, set PRV to 40 psi; at 2nd floor: 85 - 8.7 = 76.3 psi, set PRV to 50 psi. That balances. Also, install a pressure-reducing valve at the main to reduce street pressure if needed. The code requires that PRVs be installed in accordance with manufacturer instructions and be accessible for maintenance. So the solution is to recalculate the system, adjust pump pressure, and set PRVs per floor based on actual available pressure, ensuring minimum 20 psi at top fixture and maximum 80 psi at any fixture. Also, install expansion tanks if needed.