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Chemical Technology • April 2015
6
WirelessHART
automates cooling tower
operations
C
ooling towers are heat removal devices used to
transfer process waste heat to the atmosphere. They
vary in size and in the amount of instrumentation to
monitor process variables. Accurate, reliablemeasurements
are critical in calculating cooling tower efficiency, and are im-
portant for controlling blow-down and makeup flows, as well
as the pH of the water to minimize fouling of the equipment.
Cooling tower instrumentation in many refineries is often
old, with many measuring devices out of service (Figure 1).
Measurements are difficult because the process environment
is corrosive to wiring, mainly due to chemical vapours. As a
result, these areas can be poorly instrumented and poorly
controlled. Consequently, control and monitoring are poor,
operations are inefficient, and the towers require a great deal
of maintenance and manual operator interaction.
This article discusses how to use WirelessHART instru-
mentation and asset management software to automate
cooling towers by obtaining the information needed for more
reliable and efficient operation.
Cooling tower problems
Large fans generating air flow are the principle heat removal
devices in cooling towers. Typically, each process area has
a cooling tower, and each tower has six to 12 cells with one
or two cooling fans in each cell. These fans are expensive
and monitoring is critical to prevent failure. At one refinery,
it costs an average of $1,6 million per fan in maintenance
and repair fees when a fan runs to failure.
Refineries naturally do not want the fans to fail, but
they also do not want to over-maintain them, as each time
maintenance is done on a fan, the entire cell in the cooling
tower is shut down.
Themost common leading indicator of failure in a cooling
tower fan is high vibration of the motor (Figure 2). Fan failure
decreases the cooling capacity and efficiency of the tower,
and emergency shutdowns due to cooling tower damage can
last 4-8 hours, causing a significant loss in revenue.
These fan failures also cause an increase in water
consumption, which leads to an increase in the quantity of
chemical needed by the cooling tower. Chemical dosing is
often provided by an external company, but they frequently
do not have the necessary data about the behaviour of each
fan or the pH and conductivity measurements, so they can
only apply chemicals in relation to water consumption.
Cooling towers are a very tough environment with chemi-
cal vapours highly corrosive to wiring, and wired instruments
require frequent maintenance. As a result, operators spend a
good deal of their time manually gathering process informa-
tion. At one refinery, operators perform three rounds per day,
or 1 095 rounds per year, which calculates to 8 760 hours
annually, as rounds take a long time. This is not only time-
consuming and unsafe for operators, but also often results
in poor readings.
Although excessive maintenance, fan replacements and
chemical costs are significant, the biggest problems most
refineries face are shutdowns because of equipment failures.
by Nikki Bishop and Jason Sprayberry, Emerson Process Management
Wireless instrumentation and an asset
management system reduce chemical,
maintenance, lost production and repair
costs.