A single mechanical seal is the simplest, preferred solution for many applications. However, if the sealed product is not suitable for lubricating a single seal or if reliability and/or safety need to be improved, then a multiple seal arrangement is required.
A single mechanical seal has one set of seal faces that are pressurised and lubricated by the sealed product. The seal leaks the sealed product to atmosphere.
A tandem mechanical seal incorporates two mechanical seals, arranged in succession, with an unpressurised buffer fluid circulating between them. The product side seal (also called inboard or primary seal) is pressurised and lubricated by the sealed product like a single seal. The atmosphere side seal (also called outboard or secondary seal) is lubricated by the clean buffer fluid. If the primary seal fails then the secondary seal prevents product leakage to atmosphere, effectively adding an additional level of safety and reliability to a single seal.
A double mechanical seal incorporates two mechanicals seals with a pressurised barrier fluid circulating between them. The barrier fluid pressure is always at a higher pressure than the sealed product. Both the inboard and outboard seal faces are pressurised and lubricated by the clean barrier fluid. If the inboard seal fails then the pressurised barrier fluid leaks into the product. If the outboard seal fails then the pressurised barrier fluid leaks to atmosphere. In either case, the sealed product is prevented from leaking to atmosphere. Double seals are used when a high level of safety is required and where the sealed product is too arduous (abrasive, volatile, viscous, hazardous, etc.) to lubricate mechanical seal faces effectively.
A dual mechanical seal is arranged like a tandem seal with the sealed product around the outside of the inboard seal. However, a dual seal has a pressurised barrier fluid circulating between the two seals. Both the inboard and outboard seals are pressurised and lubricated by a clean, stable barrier fluid.