Compressor Maximum Operating Limit

We are starting the 6B unit on Gas Fuel & unit trips as soon as it crosses 90% speed. CPRLIM is getting reduced from 6.5 to 4.947 as per the curve & our CPR is coming around 5.3 which calls for the Trip.

We checked the calibration of 96CD transmitters which were absolutely spot on.

Any insights about the issue?
Is your unit equipped with Inlet Bleed Heat, and if so is it working properly? Are the IGVs operating as designed and following reference? If CPR is higher than desired it sounds like devices designed to control airflow through the compressor, IBH, IGV's or compressor bleed valves, are not operating normally. If you have any historical data to compare other normal startups I would look their first to understand what normal is, and see if something now is not normal.

Please get us more information when possible.
IBH operates after 96.4% TNH, but unit trips around 92% TNH. I checked calibration of CPD which are spot on & IGV on the dial matches the command.

Subhash Nemani
Subhash Nemani,

You mentioned CPR.... The atmospheric (barometric) pressure transmitters are critical to the calculation of CPR. Have you verified they are working correctly?

The barometric pressure transmitters are quite often found to be not working correctly. They usually have an open "port" to atmosphere, and insects have very often been found to have built a nest in the tubing/ports and died in there.

Also, the IGV "dial" you referred to, is a pointer on the side of the IGV control ring (around the compressor casing)? If so, that thing is almost NEVER calibrated to the actual angles by the mechanical department (or the contractor) after a maintenance outage when the bellmouth has been disassembled and reassembled. And, it's not all that accurate even from the factory when the unit is new. IGV LVDT calibration and verification is very important, also, to air flow.

Please write back to let us know what you find. You have not mentioned which control system is in use on the turbine....
You just keep believing that.

"Trust by verify." Go out and check the calibration range/scaling of the transmitter. CPR is (CPD + Atmospheric Pressure)/Atmospheric Pressure. So, it's either the CPD transmitters or the atmospheric pressure transmitters, or both.

And, if you're just looking at the value of atmospheric pressure when the unit is at zero speed, how do you know what it's doing when the unit is accelerating?