A common question when choosing seals for pumps is whether to use single seals or double seals, and when to use a barrier fluid. The answer depends mainly on the level of danger of the medium, emission requirements, cost, and maintenance capability.
Suitable for non-toxic, non-flammable liquids, and small leaks are acceptable. Low cost, simple system. The limitation is that if the seal fails, the medium leaks out immediately, and small continuous leaks may not be accepted with media that have strict emission requirements.
There are two seals with a non-pressurized buffer fluid in between, usually according to Plan 52. The outer seal serves as a backup and the inner seal leakage is collected in a tank. Suitable when an additional safety layer is needed but a small amount of medium entering the buffer fluid is acceptable.
The barrier fluid has a higher pressure than the medium (Plan 53 or 54), so the medium does not leak out but the barrier fluid leaks into the process with a very small amount. This is the choice for toxic, flammable, or extremely low emission media, but requires a more complex auxiliary system and compatible barrier fluid with the product.
Some seals use inert gas as a barrier layer instead of liquid, suitable when avoiding contamination of the barrier fluid into the process or when dry operation seals are needed. Gas schemes such as Plan 72, 74, and 75 are often used for this type.
Double seals do not increase reliability if the auxiliary system is not properly maintained. Please compare requirements with API 682 and contact SNS-VN for consultation.
Compiled from the concept of Arrangement in API 682 and common technical documents.