Double Mechanical seals mean arduous service, an application where leakage cannot be tolerated. It's not just a cost issue but can mean danger for the environment, fire or explosion, or in the case of the shaft seal on a submarine, death by drowning.
There are two configurations for double seals:
Back to Back Seals
This seal configuration sites the seals literally back - to - back. The inner seal is placed with the running faces toward the impeller, the outer seal is facing the gland plate. The space between the seals can be flushed or statically pressurized (or both) and can act as a warning of leakage taking place on the inner seal.
This type of seal has an inherent fault. You need to know about it even though this may not mean that the seal is not suitable for some services, and ideal for a particular application in your plant.
The fault is that the internal seal operates in reverse mode. The chamber between the seals is often pressurized. above the gland pressure. The excess is limited because in this mode the external seal starts to wear faster than the internal seal because the pressure differential becomes greater than the internal seal differential. So greater wear takes place. To cap it all the internal seal face can "blow open" if there a loss of pressure between the two seals. Now consider this, the external seal is wearing faster than the internal seal, it starts to fail and then leaks enough to reduce the pressure between the seal faces, the differential pressure gets above the design limit for a reverse acting seal and the internal seal blows open. The seal unit fails completely.
The internal seal has to move into the stuffing box to accommodate any wear of the faces. The problem is that any dirt that accumulates under the seal at the seal body / shaft interface can lockup the seal head preventing it from moving to accommodate wear. The seal faces wear, the head locks up, the face pressure reduces and a leak develops. At this point the leakage can be detected in the barrier fluid between the two seals, or by a change in pressure. From a maintenance point of view the seal has failed before its useful life is exhausted leading to higher cost than is necessary.
The tandem seal arrangement sites the seal components in a configuration with the seal heads operating normally and not in reverse operating mode.
The seal body / shaft interface seal area is kept clean, not exposed to the dirt in the system. The system pressure is acting on the back of the seal tending to close the seal faces. Additionally, the pressure differential across the seal faces of the internal and external seals ensures that the greatest differential is across the internal seal allowing in effect the external seal to "idle". At the very least the external seal wears more slowly, ensuring that when the internal seal lets go there is a second seal ready to take over, preventing any leakage to atmosphere.