Packing should be replaced when:

Packing should be replaced when there is heavy leaking because this indicates that the packing is no longer effective at sealing the system. The primary function of packing is to prevent leaks around moving parts, such as valves or pumps, ensuring that fluids do not escape and that the system operates efficiently and safely. When heavy leaking occurs, it suggests that the packing has deteriorated to a point where it cannot maintain the necessary seal, leading to potential operational issues, environmental concerns, and safety hazards.

In contrast, minor leaks or the inability to add more packing (as mentioned in other choices) may not necessitate immediate replacement. Packing can sometimes be adjusted or tightened rather than outright replaced, and routine maintenance can address minor issues. However, heavy leakage is a clear indicator that immediate action is needed to replace packing to restore proper function and prevent further complications.