"Backup Reference Temperature Active" Alarm


The Process Alarm you asked about will be active when the backup, or secondary, exhaust temperature control reference value (TTRXS) is less than primary exhaust temperature control reference value (TTRXP). The exhaust temperature reference used to control and protect the unit, TTRX, is always the lesser of the primary, TTRXP, or back-up (secondary), TTRXS, exhaust temperature reference values. The controlling exhaust temperature reference value should always be the primary exhaust temperature reference value when all things are normal: but when the back-up exhaust temperature reference is the lesser of two values then something is amiss (wrong) and the Process Alarm you are asking about will be active--probably in addition to one or more other related Process- and Diagnostic Alarms.

The primary exhaust temperature control reference is CPD-, or CPR-, biased. That means that CPD (and ambient pressure if the unit has CPR-biased exhaust temperature control) is used to calculate the maximum allowable exhaust temperature--which is a way of limiting the temperature of the hot combustion gases leaving the first stage turbine nozzles, which is a way of optimizing parts life as well as power output for the present ambient- and machine operating conditions.

On older machines the back-up exhaust temperature reference was a way of protecting the unit in the event the CPD signal was not available and still allowing the unit to run without a CPD signal. There would be another Process Alarm, LOSS OF COMPRESSOR DISCHARGE BIAS, to alert the operator and the control technician to the loss of the CPD signal.

Newer, more recent units, especially those with DLN combustion systems, cannot be operated without a CPD signal and the ambient pressure signal is also very important, so if the Process Alarm you are asking about is active then it's usually either a problem with the parameters used to calculate the back-up exhaust temperature reference, or a change in the constituents of the fuel being burned, or something is wrong with the fuel control valve LVDT feedback calculation. It could also be the result of improper CPD pressure transmitter calibration and/or ambient pressure transmitter calibration. But, something is not right, and the output of the unit is probably being artificially limited or if the condition is allowed to persist for a long period time while the unit continues to be operated it could experience decreased parts life or even parts failure resulting in lost production and great expense to repair.

To be of any more help it would be necessary to know the complete list of active Process- and Diagnostic Alarms when the Process Alarm you are asking about is active. It would also be helpful to know some details about the machine condition (time since the most recent maintenance outage: time since most recent compressor water wash; exhaust temperature reference values (TTRXP and TTRXS) AND actual turbine exhaust temperature (TTXM))--at a minimum.

Hope this helps!