Boiler Transmitter Calibration
Boiler transmitter calibrations are often overlooked and should be performed annually to insure reliable safe boiler operation. With fully metered boilers, transmitters are the feedback for fuel flow and air flow. Any transmitter can develop shift from zero and if used for fuel flow or air flow, they can greatly shift the state of tune.
Having an oxygen trim loop is a plus and one can always tell how far a boiler has gone out of tune by how much the oxygen trim has to compensate to bring it back. To see if your fully metered boiler has shifted from the original tune look no further than the oxygen trim loop and how far it shifts the output from 50%. If the entire operating range with everything in auto has the oxygen trim loop output at or very near 50%, you have a fine running
boiler tune. If the oxygen trim loop output has to swing all the way down or up, it is recommended to have a qualified boiler field service engineer perform a tune or troubleshoot and calibrate the oxygen transmitter.
Transmitters for feed water flow, steam flow, drum pressure and drum level are equally important and performing annual calibrations will help determine potential failures and reliability issues from drift. Having detailed as found and as left reports is vital to keeping a boiler operating safe, economical and reliable.
Oxygen trim function explained:
Typically a boiler control system uses oxygen trim as a multiplier to the air flow. If we have an oxygen trim loop output of 50% we say there is a multiplier of 1 which is no change to the air flow measurement. If we have an oxygen trim loop output of 0% we say there is a multiplier .85 which is a 15% negative change to the air flow measurement. If we have an oxygen trim loop output of 100% we say there is a multiplier 1.15 which is a 15% positive change to the air flow measurement. By lying to the air flow measurement we force the cross limiting to drive the fuel in the appropriate direction to obtain a safe oxygen set point.
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