Chemical Engineering May 2016 - 51

Part 2
Reliable Operation and
Sealing of Agitators
Mechanical seals, as required by most vessel agitators, are systems sufficiently
complex to warrant a good understanding by engineers and appropriate
training for operators
Bernd Reichert
EKATO
In BrIef
AgITATOR SEAl
SySTEMS cOMpARED
BASIcS OF MEcHAnIcAl
SEAlS
BARRIER FluIDS
MATERIAlS OF
cOnSTRucTIOn
Supply SySTEMS
cOnTInuOuS FlOw
SySTEMS
pRESSuRE
cOMpEnSATORS
T
o ensure safe and reliable agitator
operation, the sealing of the rotating
shaft is of fundamental importance.
Depending on the operating conditions
- such as pressure, temperature and
speed - various sealing principles may be
used. A comparison of their characteristics
with the requirements for mixing shows that
mechanical seal technology offers many
advantages over other sealing methods. In
particular, when hazardous substances are
being mixed or an explosive atmosphere
is present, the use of a mechanical sealing
system is almost mandatory, especially if the
mixing vessel operates at elevated pressure
and temperature.
Agitator seal systems compared
A reliable mechanical design for an agitator
(Figure 1) must take into account the hydraulic
loads on the impellers, which in turn create
the torques and bending moments that
exert mechanical loads on agitator components
such as the shaft, bearing and gearbox.
Secondary loads, such as oscillations,
vibrations and noise emissions also play important
roles.
Shaft seals can be divided into two main
groups: radial and axial seals. The main difference
between these two groups is the direction
in which the contact forces act.
Typical types of radial shaft seals include
radial sealing rings, lip seals and stuffing
boxes. Here, the sealing effect is provided by
radial forces, and the length of the cylindrical
sealing gap is in the axial direction. Although
radial seals are relatively insensitive to axial
displacement, radial shaft deflections lead to
higher sealing forces on one side, which may
cause leakage and accelerate wear.
In contrast, the sealing forces in axial shaft
seals act in the axial direction. This results
in a horizontal sealing surface with a concentric
circular cross-section. Owing to their
design, axial shaft seals are relatively insensitive
to radial shaft deflections and are thus
50
very suitable for agitator applications. Axial
displacements have to be compensated with
elastic elements. Mechanical seals belong to
the group of axial shaft seals. Below, some
examples of each type of seal are discussed
in more detail (see also Figure 2).
Stuffing boxes. Historically, stuffing-box
packings are the oldest type of sealing element.
The term " stuffing box " originates from
early steam ship construction. The passage
for the shaft through the hull was sealed with
oil-soaked rags that were stuffed into the
gap between the shaft tube and the housing.
The first mixing vessels were often equipped
with a stuffing box.
Lip seals. In mixing applications, the working
principle of lip seals can be in either the radial
ChemiCal engineering www.Chemengonline.Com may 2016
figure 1. The forces
on the components of
an agitator - including
the seal - result mainly
from the hydraulic loads
on the impellers
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Chemical Engineering May 2016

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