What is the average life expectancy of a mechanical seal? Is there a variance between dry-running and wet-running seals?

It is very difficult to determine an average seal life. The life of a mechanical seal depends on many variables. The severity of the service is the primary determining factor. Generally speaking, the higher the temperature, pressure and speed of the agitator, the lower the seal life. The chemicals in the vessel, especially in the vapor space, will also determine seal life. Certain chemicals attack the faces or O-rings causing corrosion. The runout of the agitator is also a very large factor, the general rule of thumb being that the greater the inaccuracy, the lower the seal life. Runout is caused by many factors, such as old or improperly maintained equipment, improper assembly of equipment, and running the agitator at the liquid level in the vessel. In general, the life of a lubricated seal is higher than a dry seal by a factor of two for the same service.