What a seal needs to survive:
To provide maximum service life, a mechanical seal requires four things of the fluid in which it operates:
1) The fluid must be clean
2) It must be relatively cool
3) It must provide a stable fluid film between the seal faces
4) It must have good lubricating properties
Change the environment:
If the pumped media does not meet the above conditions, seal engineers look to change the seal’s environment through the use of Seal Support Plans outlined by the API.
Fluids that typically are not conducive to long mechanical life generally fall into one of the following categories:
- Hot liquids that can vaporize as the pressure drops across the seal faces
- Cold liquids that can become very viscous or solidify
- Liquids that crystallize as they contact atmosphere
- Liquids that coke when exposed to atmosphere
- Liquids that leave a film or a deposit across the seal faces due to heat generated between the faces.
Mechanical Seal Environmental Control Ports:
Most cartridge mechanical seals provide ports in the gland to facilitate various API seal plans:
- Single seals typically have three ports: Two on the atmosphere side of the seal faces (quench) and one into the process (flush)
- Double seals normally have 2 ports in the gland, both sets of seal faces are kept cool and well lubricated with a suitable liquid that will provide a stable fluid film between the faces and is compatible with the pumped media. These ports allow fluid to flow into and out of the seal
Buffer/Barrier Media (Double Seals):
- When the pressure of the medium in the double seal is lower than the pressure in the seal chamber (stuffing box), seal engineers refer to it as a buffer fluid.
- When the pressure is higher in the seal than in the seal chamber, it is called a barrier fluid.
The most common buffer/barrier is water. It is readily available and compatible with most pumped media. Oils often are used where water would not be suitable (see Royal Purple FDA 22) (link to RP Page), care should be taken to use a double seal with an efficient pumping device or an external circulating pump as oils are more resistant to flow than water. Finally, an inert gas (typically nitrogen) can be used as a medium. The gas must be supplied at a constant pressure and must be clean and free from moisture. A high quality gas panel, such as AESSEAL® Gas 10 Panel, is required to support gas seals.
AESSEAL® manufactures a complete line of seal support systems, download the Catalog: Systems catalog (LN-UK-SYSPROD-04)
A Few of the Many API Seal Support Plans:
The API seal support plans listed below were selected to illustrate a few common API Plans. For a complete list including all API Plans, please visit the AESSEAL® website