Trouble description: Inert gas generator blowback during starting. During the starting sequence at pilot

Trouble description: Inert gas generator blowback during starting.

During the starting sequence at pilot burner stage we find there is no visible flame but flame detector somehow detects the flame and keeps the sequence on.

When it is the main burners turn, the flame is unstable, causes strong backfire through the compressor-blower after few seconds of main burner cut-in.

Measures taken:

1. Main burner removed and checked. No visible damage or alteration observed. Cant compare since we do not have spare. (IG was working fantastic when we stopped last time. I doubt it can malfunction suddenly)
2. Pilot burner checked, found good.
3. Fuel pressure 22 bar.
4. Jacket inspection done. Found little oil accumulation.
5. Fuel line filters cleaned.
6. Tried with the blower bypass valve, both in 100% as well as 0%