As has been stated, an Intake Relief Valve (IRV) is an NFPA required component designed to relieve excess pressure from the intake side of the pump. Current NFPA standards require an IRV to the exterior of any 3" or larger intake valve, and if you have any 3" or larger non-valved intakes you must also have one on the intake manifold of the pump (NFPA 1901, page 1901-52, sections 16.6.6 and 16.6.7). This means you could have as many as three or four IRVs depending on how your apparatus is set up.
The true purposes of the IRV are to reduce the likelihood of a burst suction line due to water hammer and to create a pressure differential between intake and discharge pressures. This is because discharge pressure control systems (Discharge Relief Valves and Electronic Pressure Governors) cannot lower your discharge pressure any lower than the intake pressure, and in most cases only down to about 20 psi or so above your intake pressure.
In general your IRV should be set to about 5-10 psi above your average hydrant pressure. Common settings for these systems are in the 80-125 psi range. Setting an IRV at high pressures (especially any pressure that is above your discharge pressure) defeats the purpose of the system and is not recommended.
Some systems are set using a pilot valve, others are set by adjusting an external nut that increases or decreases tension on a spring-loaded piston. Pilot operated systems are easily adjusted by the engineer while non-piloted valves will require tools to change the pressure setting. Regardless of the control mechanism the valve should be tested and exercised on a monthly basis as part of operational checks on the pump and related equipment. Failure to perform this maintenance is likely to lead to water leaks and/or vacuum leaks from the IRV.
Here is the recommended maintenance procedure for the Waterous system:
1. Clean the pilot valve strainer once per month, or as needed. Remove
the screen using a 24MM wrench. Clean and reinstall.
2. Remove the throttle screw from the pilot valve once per month and
clean it. If the bore is plugged, it can be flushed out with water.
3. Test the intake relief valve system once a month with a pressurized
water source capable of supplying continuous flow at 50 to 100 psi.
a) Set the pilot valve slightly above the source pressure and apply
this pressure to the pump intake. Relief valve should remain closed.
b) Reduce pilot valve setting to slightly below source pressure. Relief
valve should open, ‘‘dumping’’ a large volume of water.
c) Reset pilot valve above source pressure. Relief valve should4. Perform pump dry vacuum test each month.
a) Close discharge and drain valves and similar openings.
b) Operate priming device to create a vacuum of about 22 in. Hg in
pump, then stop primer and engine.
c) Watch pressure gauge, if vacuum drops more than 10 in. Hg in 10
minutes, listen for air leaks at the main valve outlet and pilot valve
drain port and drain nipples.
Probably way more info than you wanted to know, but it's never bad to have more knowledge.