All mechanical seals leak; they have to. The leakage rate needs to be sufficient to ensure that there is a lubricating fluid film between the lapped seal faces to minimise friction, heat generation and wear, whilst at the same time being low enough to be acceptable.

There are a number of factors that determine an acceptable level of leakage:

With water, volatile solvents and light hydrocarbons, the leakage is often invisible because it is emitted as vapour. Whereas, fluids that contain dissolved solids, slurries, polymers and heavier hydrocarbons will present visible leakage.

In any mechanical seal application provision must be made for leakage.

Where will the leaked fluid go? Does the mechanical seal have drain ports? Will it be piped to a safe drain or flare? Will it need cleaning away periodically? How will barrier fluid be replenished?