Air compressors with one cylinder typically have one pressure relief valve (PRV).
Air compressors with twin cylinders, those with two pumping cylinders, will typically have two PRV’s. Why?
It is common to have a PRV located near the pressure switch, one that is plumbed into the line coming from the tank to the pressure switch. This PRV oversees the pressure in the tank. Should the compressor not shut off when it is supposed to, the pressure in the tank may rise to catastrophic levels. Long before a tank might burst the PRV will react to the too high pressure, open, and vent air pressure, even while the compressor pump still runs.
On a two cylinder, two stage air compressor, air is compressed from the first cylinder through a line into the second cylinder, before the air is further compressed and driven down into the compressor tank.
Both pump cylinders have intake valves and pressure valves, to help keep the flow of air in a uni-direction, from the outside through the first cylinder, into the second, and into the tank.
Let us surmise for a second that something negative happens to the valve system in the second cylinder, and for some reason air can be pumped into it, but the air cannot be pumped further along and into the tank.
The first cylinder is pumping away, driving compressed air towards the second cylinder, the second cylinder has become, effectively, a blockade, and since the first cylinder continues to pump, air in the line between the two cylinders can quickly exceed safe levels.
To ensure that an air pressure crisis does not occur if the secondary cylinder fails, there will be a PRV in the line somewhere between the two cylinders that will blow off once the pressure in the line reaches the pressure setting of the relief valve.
A potentially dangerous situation will have been averted.
Test your compressor PRV’s regularly to help ensure that they will work when they are supposed to.