DOUBLE MECHANICAL SEAL:

A double mechanical seal consists of two seals arranged in a series. The inboard, or "primary seal" keeps the product contained within the housing. The outboard, or "secondary seal" prevents the flush liquid from leaking into the atmosphere. 
Double mechanical seals are offered in two arrangements:
Back to back
Two rotating seal rings are arranged facing away from each other. The lubricating film is generated by the barrier fluid. This arrangement is commonly found in the chemical industry. In case of leakage, the barrier liquid penetrates the product. 
Face to face
The spring loaded rotary seal faces are arranged face to face and slide from the opposite direction to one or two the barrier liquid penetrates the product. If the product is considered "hot", the barrier liquid acts as a cooling agent for the mechanical seal. 
Double mechanical seals are commonly used in the following circumstances:
a) When aggressive media is used at high pressures or temperatures.
b) For many polymerizing, sticky media stationary seal parts. This is a popular choice for the food industry, particularly for products which tend to stick in case of leakage.