From my personal experience, there has never been anything more perplexing for new—and even experienced—drivers than understanding how to set a fire truck's mechanical relief valve. It’s important to use these devices regularly to assure that they remain in good working order.
There are only two items that control pressure on your hoselines: the throttle and the discharge gates. The relief valve is a very important safety feature that protects those working on or around hoselines from “over pressurization.” Too much pressure can prevent proper hoseline handling, including inadvertent movement when using master streams.
While newer pressure governors have become the accepted standard on newer fire apparatus, chances are you will still have to deal with mechanical relief valves on front-line fire apparatus or spare fleet vehicles.
When setting the relief valve to your preference during morning checkout, it needs to be set with water flowing. This can be accomplished by flowing water through a booster or trash line back into the top of the tank or through a nozzle on a discharge gate. You will then need to refill the tank if not using a hydrant.
Often drivers open their tank refill/recirculating valve to set the relief valve. This is not recommended because you are pressurizing the water tank on a closed system, which can affect the baffles or other components attached to the tank. Also, when at a fire, the relief valve can only be set to the highest pressure line.
When at a working fire, the relief valve should stay off/closed/green and momentarily turn on/open/amber. When more pressure occurs from another line being shut down for advancing, or from pressure differentials in the domestic systems or supply lines, it should then momentarily open.
Many times I have seen drivers at working fires with the truck engine RPMs screaming way more than necessary, and the relief valve is amber. They probably were initially pumping from their tank to supply their fire lines. Afterward, their water supply was established from a pressurized water source. Once they opened the valve to supply themselves and refill their tank, the extra surge of pressure made the relief valve open (which is what it is supposed to do). The driver did not take the extra step to lower the RPMs (throttle) until the relief valve closed (green) and adjust the throttle to previous desired pressure.
Remember, this is a mechanical device—not electronic—so it doesn’t respond as fast or as accurately. The electronic pressure governor automatically lowers engine RPMs with pressure changes; the mechanical relief does not. There is no need to have your vehicle’s engine working harder than necessary.
Operating the relief valve
When training drivers, here are suggestions and techniques I used for proper relief valve operation:
This article was written for & published by FireApparatusMagazine.com.