On mains with high fire flows, why restrict hydraulically efficient discharges to one engine per hydrant? The double steamer allows two large diameter supply lines to be run from one hydrant to two separate pumping engines at the scene, or for two separate pumping engines to pump directly from a single hydrant.

Some of these hydrants flow 3,000 GPM or more. Why would someone want to lay a supply line several hundred feet from another hydrant when the closest hydrant can supply more than one engine?

The weakness with "traditional" hydrants having a single pumper outlet and one or more hose outlets is that the second pumping engine has to take water from a smaller discharge opening. In a major fire, as the residual pressure in the hydrant (and/or main) starts to drop, that second engine may experience a loss of adequate supply due to the smaller hydrant discharge opening. There are ways to compensate for this problem (see the feature entitled "Achieving Maximum flow from Hydrants") but by far the simplest is to have the most efficient openings on the hydrant to begin with.