Caring for Fire Apparatus Mechanical Relief Valves

By Frank R. Myers

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:

  1. Each driver sets the relief valve to his preferred pressure—my suggestion is 150 psi. Let’s say our first line’s pressure is 125 psi and we are pumping from the water tank. We then turn the actuator knob on (on/off switch), open the gate, start flowing water, raise the throttle up to 125 psi on that line’s discharge gauge. If the relief valve was set at 150 psi from before, the indicator light should be green (closed). 
  2. Turn the larger silver knob to the left, in half-turn increments, with a slight pause in between. Each half turn represents approximately 10 psi (8-12 psi range). Once the relief valve opens (amber), turn it back to the right in half-turn increments with a slight pause in between until it closes (green). We are now set.
  3. Let’s say another hoseline is pulled off the truck. We have calculated this line’s pressure to be 190 psi. Based on the previous relief valve setting at 125 psi, we would then need approximately seven half turns. Give it one to two extra half turns (eight to nine) prior to raising the pressure/throttle. This way, you will be able to reach your desired pressure of 190 psi (relief valve remains green) without the relief valve opening (amber) and limiting your pressure. If the open/amber light comes on prior to reaching your desired pressure, lower the RPMs to allow the relief valve to close and release some of the pressure/tension on the internal mechanism. Then give it a few extra turns to the right, raise the RPMs again to your desired pressure and follow the procedure in number 2 for setting. 
  4. Don’t set the relief valve right at the exact desired pressure because any slight pressure differential will set it off. My suggestion is to set it 10 to 20 psi over the desired pressure. This allows it to not continually activate. A safety margin of no more than 20 psi (or your established SOP) over or under the desired pressure will prevent injuries.
  5. On occasion, the higher-pressure lines will burst if multiple lines are flowing. The driver should shut that line down immediately to prevent injury. Chances are the other lines have been gated down. When this happens, it will activate the relief valve to the open setting (amber). Look at the master discharge gauge. Lower the throttle/psi to the next highest pressure line, open the gate for that line fully, adjust the other gates to their calculated pressures and set the relief valve as described above.
  6. The relief valves usually set (go from open to closed) with two to three half turns. Once the relief valve is set, check it (raise the RPM/pressure) to make sure it activates at no more than the desired safety margin. Each relief valve has its own idiosyncrasy—some are very responsive, some are sluggish, and some require four to five turns. It’s key to drill-drill-drill to exercise relief valves and to understand exactly how your truck operates. Always keep in mind that your crew’s safety is priority one. Seeing the relief valve opening and closing is a good thing. It means that our crews are still moving and alive because they are opening and closing the lines to advance.

FRANK R. MYERS is a retired lieutenant with the City of Miami (FL) Fire Rescue, where he served for 32 years. Before his retirement, he served at the Training Center for six years as the driver engineer instructor. He works as a consultant for, a technology service that helps fire departments across the country automate their apparatus, equipment, and inventory checks.

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