7 MAINTENANCE
PAGE
7.3
7.4 Double Mechanical Seal in "back-to-back" Arrangement
with Buffer Fluid System
For correct operation a double mechanical seal requires a buffer fluid. This fluid is
designed to lubricate the seals and remove the heat caused by friction. It is also apt to
prevent the product handled from entering the sealing gap and it fills the gap
between the product and atmosphere side.
As a buffer fluid, any pure and clean fluid may be used. The fluid, however, should not
corrode with the wetted parts of the seal, and it should not damage the product
handled or harm the environment.
The buffer fluid must:
- be free from solids.
- not tend to form deposits.
- have a fairly high boiling point.
- possess good thermal conductivity.
- not harm the seal elastomers.
Clean soft water meets these requirements quite satisfactorily.
The picture overleaf shows the most common arrangement of a buffer fluid system in
thermosyphon design. The circulation in the system results from the differences in
density in relation to the buffer fluid temperature.
The buffer fluid pressure p
3
should be about 10 %, i.e. 2 bar at least, above the
product pressure p
1
.
The outlet temperature should not exceed 60 °C when water is used as a buffer fluid.
As a rule, the outlet temperature should be about 40 °C below the buffer fluid boiling
point.
issued: 18.05.2011 Revision: 3
text no.
07402-1/1
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