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.
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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.
Seales on Seals
by: Tommy Seales
A: An end-face mechanical seal is a device used on a rotating shaft to seal fluids. It consists of two flat faces that are installed perpendicular to the shaft. One of the faces is mounted stationary to the seal chamber or housing. The other face rotates with the shaft to provide the primary seal. Axial mechanical force and fluid pressure maintain the contact between the wearable seal face materials.
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A: A cartridge-mounted, end-face mechanical seal (cartridge seal) is a completely self-contained unit that consists of the sealing components, a gland, sleeve, and hardware that allows the unit to be pre-assembled and preset. This feature eases installation and maintenance on rotating equipment on which axial adjustments are required.
A: Component, end-face mechanical seals (a component seal) consist of a separate rotating member and stationary seat that mount in a gland or housing. Since they are not preset, installation and maintenance are generally more difficult requiring experienced technicians to properly install and adjust them.
A: Air seals are non-contacting, pneumatic devices engineered for sealing rotating shafts. They protect against product loss, emissions, and contamination by using small amounts of air or inert gas that is throttled to create positive pressure and an effective seal.
A: Rotating equipment used in processing gases, liquids, and slurries are the primary applications where end-face mechanical seals can be cost effectively considered and applied. Some types of rotating equipment that are not equipped with a seal chamber or stuffing box for installing a gland require that a component seal be used.
Cartridge-mounted, end-face seals were primarily introduced for installation on American National Standards Institute centrifugal pumps on which axial shaft adjustments can be made. The simplicity and ease of installation and maintenance they offer makes cartridge-mounted single and dual units a primary consideration for all types of rotating equipment that are equipped with a stuffing box or seal chamber in which a gland can be installed.
Air seals are primarily installed on rotating equipment used for moving or mixing powders and bulk solids.
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