A section of pipe containing longitudinal baffles or vanes, installed just behind a master stream nozzle or monitor. It reduces the rotational turbulence and agitation in the water caused by pumps and valves. By organizing the water flow into parallel lines, it significantly increases the reach, cohesion, and effectiveness of a solid fire stream.
Heard on the job
If you take the stream straightener off the deluge gun, the water just breaks apart into a useless mist at fifty feet.