Page 125 -

The Practical Pumping Handbook ..................

An audible alarm may be designed into the system to alert Operations
of the changed condition.
In a high pressure Barrier fluid system, the higher pressure in the
Barrier System creates a completely new environment for the inboard
seal. Both inboard and outboard seals can now be cooled and lubricated
by the barrier fluid. In this condition, a minute amount of barrier fluid
will migrate into the process through the inboard seal, so the barrier
fluid must be compatible with the process. This condition should also
be alarmed.

To further the drive towards zero emissions, the Seal industry has
developed Gas Barrier sealing which uses an inert gas, such as nitrogen,
to act in place of the Liquid Barrier System.
This requires a special face design on the outer set of faces to ensure its
ability to run without liquid lubrication. In addition to ensuring zero
product leakage, these Gas Seals also guarantee that the pumped
product will not be contaminated by any barrier fluid.
Whether using a liquid or gas barrier system, that system must be
dedicated to the specific seal, and alarmed in such a way that any failure
of the inner set of faces can be immediately recognized for the
appropriate action.

6.5 The seal chamber

Traditionally, the radial clearance between the shaft and stuffing box on
the average process pump was sized to accommodate the 3/8 inch
square section packing. This resulted in a l S/8 inch diameter shaft
running in a 23/8 inch bore stuffing box.
When a mechanical seal was introduced into this area, the minimal
annular space left available was considered inadequate for reliable
operation of the seal and contributed to a high incidence of seal failure.
Consequently, the radial clearance between the shaft and the bore of
the stuffing box was increased to, at least, 7/8 inch. This has proved to
be extremely beneficial in ensuring seal reliability.
By enlarging the bore of the Stuffing Box, the problem identified as
seal rub was eliminated. This is a condition where shaft deflection
causes the mechanical seal to contact the bore of the stuffing box,
resulting in premature seal failure.
Also, the larger volume of the Seal Chamber increased the quantity of
liquid around the mechanical seal. This permits greater heat dissipation
and allows the seal to operate in a cooler environment.

m 104