Low residual pressures are most commonly encountered when using what size supply line during in-line pumping?

Low residual pressures during in-line pumping are most commonly encountered when using a 2.5 or 3-inch supply line due to several factors related to flow dynamics and friction loss.

When a fire department uses smaller diameter supply lines, such as the 1.5 or 2-inch options, they can limit the overall flow of water, but this isn't primarily due to the size of the line impacting pressure in the same way as larger lines. The 2.5 or 3-inch lines, while capable of delivering adequate volumes of water, still face significant friction loss as water moves through them, especially at higher flow rates.

As the diameter of the supply line increases, such as with 3.5 or larger options, the friction loss tends to decrease, allowing for better maintenance of residual pressures. The larger lines can support higher volumes of water with less resistance, which helps keep the pressure stable throughout the system.

Hence, the use of 2.5 or 3-inch connections is a balance – they facilitate a fair flow rate while also encountering substantial friction losses compared to their larger counterparts, leading to low residual pressure when pumping inline. This makes the choice relevant to understanding the hydrodynamics in firefighting operations.