Using Oxygen Analyzers As A Predictive Maintenance Tool For Air Heaters0 pages

Application Note

POWER

Using Oxygen Analyzers As A Predictive
Maintenance Tool For Air Heaters
The probe-type In Situ oxygen analyzer has become an indispensable tool for monitoring combustion process efficiency.

New Application
A usual location for mounting an O2 probe in boilers is downstream from the economizer. This location is ideal because it
is close to the boiler, but the flue gas temperatures are usually
below 1000°F (537°C). Some facilities mount one or more
probes downstream from the “air heater”. The air heater exchanges the heat from the flue gas to the fresh air used to fire
the burners. There is always a possibility that a leak can occur
allowing fresh air to infiltrate into the flue gas and reducing
the rate of heat transfer and diminishing boiler efficiency.
Likewise, the leaked air is not available for combustion, reducing the maximum firing rate of the boiler.
This problem is particularly acute in the rotary regenerative
designs, which must use a rotary seal to separate the hot flue
gases from the fresh combustion air (see Figure 1). This type
of air heater operates much like a revolving door. Plates in
each section pick up heat from the hot flue gases, and, as the
unit rotates, transfer this heat to the fresh air being preheated
for combustion.

Figure 1. View from Inside the Duct

For more information:
www.EmersonProcess.com/RAIhome

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The Solution
The Rosemount Analytical Model 6888 O2 Analyzer mounted
downstream from the air heater will tend to read higher than
upstream analyzers, due to some air leakage past the seals.
Over time, the downstream analyzer will read higher and
higher, indicating greater seal wear. This indication of O2 can
be used to adjust heat rate calculations and can become a
useful predictive maintenance tool for establishing a service
interval for the air heater seals (see Figure 2).
For accurate, reliable O2 analysis, Rosemount Analytical offers
the Oxymitter™ In Situ Flue Gas Oxygen Transmitter and
the X-STREAM In Situ Oxygen Transmitter. The user-friendly
design of the Oxymitter™ In Situ Flue Gas Oxygen Transmitter
and the X-STREAM In Situ Oxygen Transmitter probe allows
convenient access to internal probe components for in-house
service. In addition, Rosemount Analytical’s patented electronic cell protection automatically protects the sensor cell’s
electrodes from harmful corrosive gases. And, the HART®
Field Communications Protocol permits all operator functions
to be performed from the control room.

Figure 2. Side View