Which of the following factors determine the number of 2 1/2 in. inlets to supply the required water volume and pressure at the fire department connection?

• A. The required water volume
• B. The pressure at the fire department connection
• C. The hydraulic calculation for the system
• D. All of the above

Answer: (D)

The number of 2 1/2 in. inlets is driven by the system’s ability to deliver the required water flow with the proper pressure at the fire department connection, and that is determined by three interrelated things. First, the required water volume sets how much flow the standpipe must provide overall; if the building’s design calls for a high GPM, more inlet capacity may be needed to meet that demand. Second, the pressure available at the fire department connection sets the starting point for what can be delivered; if the supply pressure is high, fewer inlets may suffice, but if pressure is limited, additional inlets can be necessary to achieve the design flow without dropping pressure below acceptable levels. Third, the hydraulic calculation ties everything together by modeling how water moves through the system—accounting for pipe lengths, diameters, fittings, elevation changes, and valve actions—to determine whether the chosen number of inlets will deliver the target GPM at the required residual pressure at the FDC and remote outlets. Because all three factors influence performance, the correct approach is to consider them together, so the number of inlets is sufficient to meet the design without causing unacceptable pressure loss.