Chemical Engineering September 2014 - 63

Figure 2. Shown here is range of traditional, risky, external and internal component seal designs. These are still found in hygienic
applications. Internal components are wetted by process fluid, shown in orange. Arrows point to locations where buffer
fluid is injected
Figure 2: A range of traditional, risky, external/internal component seal designs. These
are still found in hygienic applications. Internal components are wetted by
process fluid, shown in orange. Arrows point to locations where buffer fluid is
injected. (Source: AESSEAL plc, Rotherham, UK and Rockford, TN)
Product wetted
components
Figure 3. Shown here is a cartridge seal design for use in hygienic applications.
Buffer fluid is introduced into the space between the product-wetted seal on the left,
and the external seal on the right side
device should protect the process
fluid from being contaminated by
the fluid environment. It should be
easy to clean the sealing region to
reduce the risk of microorganism
growth. It is intuitively evident that
equipment that is difficult to clean
will result in an inferior processed
product. Flawed product quality
may result in a rejected product
batch, incur additional reprocessing
cost, force product recalls and
represent a real danger for human
consumption or use.
Within the three equipment
Product-wetted
components
are surrounded
by orange color
fluid. A dual-seal arrangement is
intended to reduce the possibility
of cross-contamination and issues
arising from process fluid traveling
across the seal fluid film of the inboard
seal faces.
EHEDG also stipulates that all
groups or categories, Class I and
Aseptic equipment requirements
detail the use of an external seal to
create a buffer region (seal cavity).
This buffer region can be continually
flushed with an appropriate
unavoidable gaps must be sited at
the non-process fluid side of the
seal and the components in contact
with the process fluid must be
smooth and free of crevices. Again,
this guideline is intended to reduce
the potential for bacteria and
microorganism growth. It would
serve no-one's best interests to have
process fluids stick to any part of
a pump. Contamination risks are
even greater when changing batch
composition, or with time elapsing
between successive batches.
Reality versus best practice
Informal surveys show that many
seals presently found in the food,
beverage and pharmaceutical
industries are not representing
best-available designs. Single-coil
spring " component seal " designs
of the 1970s and 1980s (Figure 1)
are still dominant and in common
use. Note that Figure 1 depicts an
old-style dual (inboard versus external
" face to face " ) component
mechanical seal used in certain
hygienic applications.
However, on the process side, the
fluid (shown in orange in Figure 1)
is in direct contact with multiple
seal crevices and cavities. It will
be impractical, if not impossible,
to thoroughly clean these regions.
As an aside, practically every bestpractice
seal-design guideline has
been ignored in this particular geometry.
Regrettably, this is not an
isolated issue.
Figure 2 illustrates another
family of similarly unsuitable or
outdated designs. All three are internal/external
component-seal designs
with buffer fluid. These seals
find extensive use in the food-andbeverage
industry. But these and
similar designs have significant
bacteria and microorganism growth
potential around the components
wetted by the process fluid. While
the issue is entirely design-related,
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