"When inspecting, testing and maintaining fire protection systems, we often are concerned mostly with their ability to perform as designed in the event of a fire or other emergency. Occasionally, however, the work reveals a problem that could have other consequences, including injury or death to the operator. Look closely at the illustration, and you will see a small crack in the upper left-hand elbow of this monitor nozzle assembly. The crack has been circled by the inspector to highlight its location. While the crack's source isn't known, it may be an early sign of metal fatigue that could lead to serious operator injury when the monitor nozzle assembly is pressurized in the event of a fire. Depending upon manufacturer and model, it is not uncommon for monitor nozzle assemblies to operate at 200 pounds per square inch (13.8 bar). Monitor nozzle assemblies, such as this one located at a methanol production and storage facility, often are installed as fixed appliances where large water quantities may be needed early in an event. These devices are installed routinely at bulk fuel facilities, hog fuel piles, lumber yards, liquefied petroleum gas storage facilities, and other high-hazard installations."
Coffee Break Training - Fire Protection Series No. FP-2014-24
U.S. Fire Administration: http://www.usfa.fema.gov/