Page 163 - Root Cause Failure Analysis

Mixers and Agitators 151
tions in the viscosity of the products being mixed. As the viscosity increases so will
the brake horsepower demand. Conversely, as the viscosity decreases, so will the
horsepower required to drive the mixer.

INSTALLATION
Installation of propeller-type mixers varies greatly, depending on the specific applica-
tion. Top-entering mixers utilize either a clamp- or flange-type mounting. It is inipor-
tant that the mixer be installed so the propeller or paddle is at a point within the tank,
vessel, or piping that assures proper mixing. Vendor recommendations found in O&M
manuals should be followed to ensure proper operation of the mixer.

Mixers should be mounted on a rigid base that assures level alignment and prevents
lateral movement of the mixer and its drivetrain. While most mixers can be bolted
directly to a base, care must be taken to ensure that the base is rigid and has the struc-
tural capacity to stabilize the mixer.

OPERATING METHODS
There are only three major operating concerns for mixers: setup, incoming-feed rate.
and product viscosity.

Mixer Setup
Both propeller and screw mixers have specific setup requirements. In the case of pro-
pelledpaddle-type mixers, the primary factor is the position of the propellers or pad-
dles within the tank or vessel. Vendor recommendations should he followed to assure
proper operation of the mixer.

If the propellers or paddles are too close to the liquid level. the mixer will create a
vortex that will entrain air and prevent adequate blending or mixing. If the propellers
are set too low, compress vortexing may occur. When this happens, the mixer will cre-
ate a stagnant zone in the area under the rotating assembly. As a result, some of the
product will settle in this zone and proper mixing cannot occur. Setting the mixer too
close to a comer or the side of the mixing vessel also can create a stagnant zone that
will prevent proper blending or mixing of the product.

For screw-type mixers, proper clearance between the rotating element and the mixer
housing must be maintained to vendor specifications. If the clearance is improperly
set, the mixer will bind (i.e., not enough clearance) or fail to blend properly.

Feed Rate
Mixers are designed to handle a relatively narrow band of incoming product flow rate.
Therefore. care must be exercised to ensure that the actual feed rate is maintained