Double mechanical seals are sealing systems composed of two mechanical seals mounted together to ensure a higher level of safety compared to a single seal. They are selected when the process fluid is dangerous, abrasive, non-lubricating, very hot/cold. In these cases, the double seal allows an intermediate liquid to be interposed between the process and the atmosphere to enable correct lubrication and cooling of the contact surfaces, obtaining a system of extremely reliable sealing even in critical conditions.
There are two main classes of double seals:
- Non-pressurized: the auxiliary liquid (buffer fluid) is kept at a lower pressure than the process fluid. In this case, any process contamination is avoided, but seal leakage is not dispersed into the environment.
- Pressurized: the auxiliary liquid (barrier fluid) is kept at a pressure higher than the process fluid, guaranteeing zero emissions into the atmosphere, as well as lubrication and clean cooling using clean and controlled liquid.
Why use two seals instead of one?
A single seal may not be sufficient when:
- the fluid is dangerous, toxic, flammable; in this case, physiological leaks outwards could cause damage or risks to the environment or operators;
- the fluid does not lubricate the seal surfaces well;
- the process fluid has solids present that could abrade the seal.
Double seals can be installed in various configurations (arrangements), each with different peculiarities, and are selected based on operating conditions.