Chemical Engineering May 2016 - 53

can achieve service lives of a year or more
under appropriate operating conditions.
liquid-lubricated single-acting mechanical
seals can achieve much longer service
lives, where the nature of the product allows
them to be used. Many applications involve
suspended solid particles that - depending
on their hardness and particle-size distribution
- can greatly influence the service
life of the seal rings. These seals are generally
equipped with two seal rings made
from abrasion-resistant silicon carbide (Sic).
However, the use of two hard materials is not
ideal with respect to sliding friction. In this
case, it is usually better to use a softer material
for one of the faces, accepting higher
wear in return for lower friction.
Double-acting mechanical seals. Doubleacting
liquid-lubricated mechanical seals
are the most common type for mixing applications,
where they can be used under
nearly all operating conditions. They are
also available in gas-lubricated variants, in
which a continuous supply of gas into the
seal chamber maintains a seal gap of a few
micro meters, thus preventing wear of the
seal rings. The characteristic feature of a
double-acting mechanical seal is its sealfluid
chamber that can be filled with seal liquid
or gas, thus separating the interior of the
vessel from its surroundings.
Figure 2 shows how the various types of
figure 3. A complete mechanical
seal system typically
includes the seal cartridge
itself, a seal liquid supply tank
and pump, a pressure compensator
with position sensors
to monitor leakage, and
automated isolation valves
Control
system
Mechanical seal
LSLL
LSL
LSH
LSHH
seals discussed above score against process
parameters such as temperature and
pressure, plus broader criteria like cost and
service life. It is obvious that mechanical
seals offer many advantages over the other
types. particularly if hazardous or explosive
materials are being mixed, a mechanical seal
is practically mandatory. A hermetic seal with
a canister in combination with a mechanical
seal is used for applications requiring
the highest safety, such as hydrogenation or
phosgenation reactions.
Storage tank
Solenoid valve
pB
p
pS
~1 μm
Sliding face
figure 4. As the seal fluid cools and lubricates the seal
faces, it creates a hydraulic force that tends to open the gap
Basics of mechanical seals
A mechanical seal system, as shown in Figure
3, has several components. Alongside
the mechanical seal cartridge itself are the
hydraulic components (such as a pressure
compensator), and the rest of the installation,
comprising the pipework, instrumentation
and mountings. Some applications
also include a seal-liquid refilling system. As
a consequence, in most mixing systems,
reliable sealing depends on the complete
mechanical seal system. careful selection
of suitable hydraulics and installation components
is just as important as the design of
the mechanical seal itself.
The function of a mechanical seal is essentially
governed by the mechanisms taking
place in the gap between the rotating and
the stationary seal rings. As Figure 4 shows,
the seal interface can be imagined as a very
narrow annular gap across which the seal
faces are in partial contact. Full solid contact
would be ideal from the perspective of avoiding
leakage. On the other hand, a pure fluid
film - with no solid contact - minimizes frictional
forces, wear and heat generation. The
design of the seal ring must therefore take
into account both aspects, and thus always
represents a compromise. This condition is
known as mixed friction: the seal faces are in
partial contact, yet thanks to lubrication they
also are able to slide over each other.
The physical and chemical processes takSeal
liquid
Pressure compensator
Pump
Short heat trap
52
Seal liquid
ing place within the sealing gap of a mechanical
seal are difficult to describe theoretically.
Some processes, such as blistering
on seal faces, are not yet completely understood
because it is hard to take measurements
at the seal interface. The key variables
influencing the sealing and frictional
characteristics of seal rings are the various
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