24-12-2012, 03:07 PM
Vibration Sensors for Cooling Towers
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Challenges
Cooling towers offer the vibration analyst many
challenges in sensor selection, mounting and
environmental conditions. The motor is
typically out in the open and very easy to access,
but almost always the gear box is the failure
point. Unfortunately for the analyst, the gear
box is normally located above an
environmentally hazardous soup bowl and just
slightly below a rotational guillotine. Is it any
wonder that the motors rarely fail? Technology
and safety can prevail with the installation of
permanent vibration sensors. This improves the
life and reliability of the cooling tower and the
vibration analyst.
Sensors
Accelerometers are typically placed at key
locations on the motor and gear box. Since the
gearbox is the load bearing part of the
mechanical drive train, accelerometers should be
placed on the input and output bearing housings
to measure the vibration levels.
Cables & Connectors
The environmental surroundings of cooling
towers will require connectors rated to IP66.
This level of ingress protection against dirt and
water is ideal. Using the A2A Mil Style
connector, or the B2A seal tight boot will
provide a rugged connection to the sensor that is
protected from the cooling tower environment.
Teflon jacketed cables or armored cable should
be used in conjunction with the connectors to
form a complete cable connection package.
Mounting Hardware
The cooling tower environment almost demands
stud mounted sensors. Other attachment
methods are just not appropriate for this
application. Be sure to use a spot facing tool like
the MH117-1B to simultaneously drill a hole and
machine a flat mounting surface.
Junction Boxes
Having rugged secure connections to the
sensor is a necessity. These same thoughts
should also be applied to the data collection
end of the cables. There are several options
available to keep the interface between the
sensor cables and data collection dry, clean,
and organized. Portable data collection can be
easily completed by interfacing with a MAXX
Box or with a Switch Box. Both of these
options provide convenient connections for the
data collector cable. In permanent monitoring,
a Switch Box or a Signal management Box can
be used. No matter which method is chosen, it
will prevent wiring spaghetti and organize your
data collection points.
Alarms
The alarm levels will depend on the fault
frequency being measured, and the
configuration of the cooling tower. Historical
data collection will provide an important
baseline for performance comparison.
Trending of routine data will indicate shifts in
performance and possible faults developing
that require detailed analysis.
Typical application guidelines and operating
limits for vibration are available from Chapter
10 “Mechanical Components for Cooling
Towers” of the “Cooling Tower Manual”.
These limits are based on the structural design
and components of the cooling tower.