SINGLE MECHANICAL SEAL:

A single mechanical seal consists of two very flat surfaces that are pressed together by a spring and slide against each other. Between these two surfaces is a fluid film generated by the product. This fluid film prevents the mechanical seal from touching the stationary ring. An absence of this fluid film (dry running of the pump) results in frictional heat and ultimate destruction of the mechanical seal.
Mechanical seals tend to leak a vapor from the high pressure side to the low pressure side. This fluid lubricates the seal faces and absorbs the heat generated from the associated friction, which crosses the seal faces as a liquid and vaporizes into the atmosphere. So, it's common practice to use a single mechanical seal if the product poses little to no risk to the environment.