In a constant pressure relay, at what pressure do the source pumper and all relay apparatus pump water?

In a constant pressure relay system, the designated pressure at which both the source pumper and all relay apparatus operate is typically set at 175 psi (1,206 kPa). This standard pressure ensures that there is adequate water supply and flow for effective firefighting operations over longer distances.

The choice of 175 psi as the operating pressure provides a reliable balance between ensuring sufficient water pressure at the nozzle while managing the capabilities of standard fire pumps, which are designed to operate efficiently at this pressure range. This is especially crucial in relay operations, where multiple pumping units work together to maintain a consistent flow and pressure for suppressing fires, often in scenarios where the water supply must travel long distances.

Establishing the operational pressure at 175 psi promotes effective water distribution and allows for predictable performance from all apparatus involved in the relay. Other pressures, such as 150 psi, 200 psi, or 225 psi, may either be inadequate for the needs of demanding fire situations or exceed the operational capabilities of the fire pumps, leading to potential inefficiencies or risks during operation. Thus, identifying 175 psi as the standard aligns with best practices in fire service for maintaining reliable and effective water supply during firefighting efforts.