Repacking a valve which is under pressure can cause property damage or personal injury, and
never be done.
This question is often asked of our Customer Service and Engineering groups. Certain manufacturing standards require that gate, globe, and angle valves include a backseat in the design. The function of the backseat does not permit repacking under pressure. Examples of the related standards for valves are: MSS SP70 Cast Iron Gate Valves, MSS SP85 Cast Iron Globe and Angle Valves, and MSS SP80 Bronze Gate, Globe, Angle and Check Valves. Each of these standards calls for a backseat, and in the same paragraph of each standard, they all state that
repacking of valves under pressure is hazardous and not recommended.
In the case of ball valve specifications where there is no requirement for a backseat to be present, the answer still remains - NO! Ball valve standards, almost universally, call for blow-out proof stem retention. This is normally accomplished (for end-entry valves) by retention of an internally inserted stem via a hard shoulder. Examples of the pertinent standards are MSS SP110 Ball Valves and API 608 Metal Ball Valves. That stem retention feature doesn't allow the ability to repack under pressure.
Repacking a valve is not a difficult or lengthy process. Proper preparation, with trained personnel and factory-provided packing made specifically to fit your valve should make the process go very quickly. The service interruption will be minimal - minutes, not hours, of system down-time when done correctly. For all these reasons, the answer from the factory to our original question of can valves be repacked under pressure will always be a resounding NO!