The only hazard of a plugged PRV Bonnet Vent is in the case of a bellows design valve. When the PRV actuates, the bellows is compressed which will build up pressure in the bonnet if the Vent is plugged. This pressure exerts a force on the bellows and causes the convolusions to deform and press against one another, effectively gagging the valve.
In addition to balanced bellows valves there are also balanced piston valves that have vents which should not be plugged. These two types of valves are shown in the "Crosby Pressure Relief Valve Engineering Handbook."
Plugging the vent, combined with a leaky bellows or piston seal, will result in an increase of opening pressure if there is any superimposed back pressure.
