Pressure reducing valves (type I) specified herein are intended for installation in air or nitrogen systems where the inlet pressure to the valve is 150 lb/in² gauge (or below) and it is required to maintain a constant (within the accuracy limits specified) lower pressure downstream of the valve. The valve maintains this constant delivered pressure during flow and supply pressure variations by automatically throttling (or shutting off flow) as necessary based on a signal feedback of the downstream pressure.

Back pressure regulating valves (type II) specified herein are intended for installation in low pressure (150 lb/in² gauge or below) air or nitrogen systems for the purpose of maintaining the pressure immediately upstream of the valve within predetermined limits. The valve senses upstream pressure and begins to open when it reaches the set pressure. The valve is fully open when inlet pressure reaches the accumulation pressure and it reseats by the time the inlet pressure has decayed down to the blowdown pressure.

Priority valves (type III) specified herein are a specific application of a back pressure regulation valve (type II) intended for installation in low pressure (150 lb/in² gauge or below) air or nitrogen systems for the purpose of protecting the supply pressure to essential or "priority" services by throttling down or cutting off the flow to all nonessential services if and when necessary. This is accomplished by installing the valve downstream of the essential service take-offs but upstream of the nonessential service take-offs. The valve senses the pressure upstream of it (the essential services pressure) and acts to throttle down (or completely shut off if necessary) the flow through it (that is, the flow to the nonessential services) at any time the sensed pressure drops down to a level which would represent a threat to the continued operation of the essential services.