There are three main things that can cause a mechanical seal to leak, and identifying the right one is the starting point for any effective repair.
The most common cause is face failure. The two seal faces, one stationary and one rotating, rely on a thin fluid film for lubrication. When that film breaks down due to heat, contamination, misalignment, or dry running, the faces make contact, wear unevenly, and lose their ability to seal.
Secondary seal failure is the next source. O-rings, gaskets, and flexible graphite elements can degrade due to chemical attack, temperature excursion, or physical damage during installation. When a secondary seal fails, the leak path bypasses the faces entirely.
In dual-seal arrangements, barrier pressure imbalance is the third cause. If the differential pressure between the barrier fluid and the process fluid climbs too high, the faces can be forced open, which leads to excessive barrier fluid consumption and potential process contamination.