Generally, liquid lubricated seals should not be dry run regularly or for prolonged periods. However, some mechanical seals with a carbon face will tolerate marginal lubrication and short, intermittent periods of dry running.
If the temperature at the mechanical seal faces becomes excessive due to insufficient heat dissipation then the liquid film between the faces can start to vapourise and become unstable. In this case the seal face materials will contact excessively and begin to wear rapidly under dry running conditions. Many wet seals that use general purpose carbon will not tolerate this and will ultimately fail. Some mechanical seal designs use grades of face material that are more tolerant to intermittent or short periods of dry running during upset conditions.
Some mechanical seal types are designed to run dry continuously and use seal face materials specifically for this purpose, e.g. some top entry mixers incorporate single mechanical seals where the seal faces affectively seal a nitrogen gas blanket and the seal faces are wetted only by vapours.
Applications in the chemical and pharmaceutical industry require a double mechanical seal that operates on a pressurised dry nitrogen barrier gas. For carbon graphite to dry lubricate effectively then moisture must be present, otherwise it will wear rapidly. However, there are grades of carbon graphite available that are specifically formulated to operate on dry nitrogen with minimum face wear.