All mechanical seals leak; they have to. The leakage rate needs to be sufficient to ensure that there is a lubricating fluid film between the lapped seal faces to minimise friction, heat generation and wear, whilst at the same time being low enough to be acceptable.
There are a number of factors that determine an acceptable level of leakage:
- What are the hazards and risks of leakage?
- Is there legislation that specifies limits on emission levels?
- What is the operating environment and provision for cleanliness control?
- Is there significant cost associated with lost product or barrier fluid?
- The actual leakage rate depends on the mechanical seal type and diameter, the properties of fluid being sealed, the shaft speed, the sealed pressure, the operating temperature.
With water, volatile solvents and light hydrocarbons, the leakage is often invisible because it is emitted as vapour. Whereas, fluids that contain dissolved solids, slurries, polymers and heavier hydrocarbons will present visible leakage.
In any mechanical seal application provision must be made for leakage.
Where will the leaked fluid go? Does the mechanical seal have drain ports? Will it be piped to a safe drain or flare? Will it need cleaning away periodically? How will barrier fluid be replenished?