Chemical Engineering May 2016 - 54

axial forces, which operate in both the
opening and closing directions. As Figure
4 shows, the pressure between the
seal faces pushes the seal rings apart,
whereas the hydraulic pressure on the
rings (Figure 5) pushes them together.
The ratio of these forces governs the
efficiency of the sealing function and
how easily the seal rotates. The closing
forces must be slightly higher than
the opening forces; otherwise, there is
a risk that the gap will open suddenly
and the seal will start to leak.
The ratio between the closing forces
and the opening forces is described
mathematically by the hydraulic balance
ratio K (Figure 5):
(1)
pB
with the assumption of a linear pressure
drop across the sealing interface
(Figure 4), the closing and opening
forces will balance when K = 0.5. In
practice, optimum performance is obtained
when the value of K lies between
0.6 and 0.9.
The hydraulic balance ratio K is also
used to characterize mechanical seals
as unbalanced or balanced. unbalanced
mechanical seals have K > 1, whereas
balanced seals have K < 1. unbalanced
seals are expedient for simple operating
conditions, such as low pressures and
low agitator speeds. Here, the high hydraulic
balance ratio, with closing forces
dominant, provides good sealing efficiency
without thermally overloading the
mechanical seal. In more-difficult operating
conditions, such as high pressures
and high agitator speeds, only balanced
mechanical seals can be used.
So far we have ignored the closing
force contributed by the springs that
form part of every mechanical seal. This
force is generally equivalent to a pressure
of 1-2 bars. This is important at
low operating pressures, but can confidently
be neglected at vessel pressures
above 10 bars. nevertheless, even
high-pressure mechanical seals require
springs to keep them closed while they
are unpressurized.
Barrier fluids
Another essential factor influencing
the function of a mechanical seal is
the choice of barrier fluid. This liquid
has three main functions: lubrication,
cooling and sealing. It must also meet
certain secondary conditions, such as
compatibility with the product and, if
necessary, conformity with the specifications
of the u.S. Food and Drug Administration
(FDA).
Figure 6 compares barrier fluids used
in mixing applications with respect
to their suitability for various tasks. It
is clear that the demands of lubrication
and cooling may conflict. water
cools efficiently, but lubricates poorly,
whereas the reverse is true for mineral
oils and pure glycerin. A mixture of
glycerin and water can be a successful
compromise: the glycerin lubricates,
while the water phase cools. For this
reason, glycerin/water mixtures should
always be used if possible. unfortunately,
not all products tolerate a glycerin
in-leakage of several milliliters per
day, though it is technically possible to
collect the leaked barrier fluid and keep
it away from the product.
Especially when water or organic solvents
are used as barrier fluids, special
Barrier fluid
Water
Mineral oil 20 cSt
Glycerin 100%
Glycerin/water
Synthetic oil
n Very good
n Good
ChemiCal engineering www.Chemengonline.Com may 2016
n Satisfactory
n Poor
n Unsuitable
53
Lubrication
Cooling
A2 =  (r2
2 - r3
2)
pB > pO
r3
r1
A1 =  (r2
r2
ps
2 - r1
2)
figure 5. To ensure reliable operation,
pressure loads on the seal rings create
a closing force that should be somewhat
greater than the opening force
figure 6. For many applications, a
mixture of glycerin and water yields the
preferred balance of properties for the
barrier fluid
Circulation
Product
compatibility
FDA compliance
http://www.Chemengonline.Com
Chemical Engineering May 2016

Table of Contents for the Digital Edition of Chemical Engineering May 2016
