The machine in question experienced a trip on low atomising air pressure ratio trip (LAAPRTRP_ALM) during ignition.

The trip is only armed when the machine has fired ( L28FDX) at this point if the ratio between the absolute atomising air pressure (AAP4) and Absolute CPD (CPDABS) is less than 1.05 for 3 seconds the trip is initiated.

After investigation and comparisons to previous mark 6 projects it seems the OEM has reduced the buffer time from 8 seconds to 3 seconds? Still under review but wondering if anyone has similar logic to this?
This logic was implemented under a til to prevent an over speed should liquid fuel be present in the atomising air manifold, hence the reason for maintaining a greater pressure than the CPD.

 

Peter Clarke,

If you have just made this application code modification in response to a TIL, comparing this to other similar units might not be an apples-to-apples comparison.

You said the "protection" logic is enabled at flame detection. That's when AA pressure is pretty low, especially during firing which occurs at low speed (I presume the Main AA Compressor is Acc. Gear-driven so output is low). Also, during firing on Liquid Fuel, the Booster AA Compressor is running, and the output pressure is notoriously difficult to measure (given the size of the AA piping and nozzle passages).

I believe you also said the logic is looking at an absolute pressure ratio, which means the ambient/barometric pressure transmitter(s) are being used in the calculation. So, they have to be working properly (and they don't always).

You should be working with GE to sort this. Frame 9Es now fall under the purview of the Belfort Bunch of GE. They are known for needlessly complicating a simple need, taking a long time admit there is a problem, then over-engineering a solution.

 

We have this logic impllemented on 2 of our new units. Never had a problem. AA is low at firing speed. Check you transmitter calibrations. What is your AAPR with 88AB running

 

Hello CSA,
Thanks for the reply.

I may have mislead you on the TIL. The software was installed during commissioning 3 years ago under (TIL 1642). More recently GE have made changes to this software including reducing the timer to 3 seconds.

We are opening up communication with GE on this but as you have pointed out they are notoriously slow in their responses so I thought I would give it a run here.

It does look like the 3 transmitters measuring the Atomizing air pressure aap4a,b,c are more than likely the culprit with them reading about 1.41,1.51 & 1.42 psi with a ratio of 1.042 at ignition.

I have found some commissioning run data that shows the same transmitters reading approx 2.4 psi at ignition with a ratio of 1.12.

Another thing to note is the range of these transmitters -14.5 to 507.6 psi trying to measure 1-2 psi???

There is diff pressure transmitter (96AD-1) across the acc driven compressor(CA1) which shows this compressor not contributing at all at this low speed of 450 RPM, so the pressure is reliant upon the booster compressor alone, only when the unit starts to accelerate does CA1 contribute.

We have had successful starts since but the logic is right on the edge most times coming within a second of another trip.

 

Hello,
AAPR at ignition is always around 1.05 to 1.07.

Would be interested to know your timer constant (KAAPRTRP_TD) 3 seconds or 8 seconds?

Your probably right with the transmitter cal, our transmitters are ranged -14.5 to 507.6 psi are yours similar?