Does anyone know what hazards are associated with having the relief valve bonnet vent plugged and it is the type of relief that requires no plug?

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.

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