Dry-run in GTG Waste Heat Recovery , ISO 21905

[Takitomo Hifumi]

My apologies if this the not the right thread for this topic 1st
Greeting,
We currently have a Waste Heat Recovery Unit (WHRU) System to recover heat from a Gas Turbine Generator (GTG). Equipped with a multi louver and sealing air blower.
Current our client want to continue dry run the WHRU, keep the GTG running except for major scenario like tube leak, then we have to shutdown the GTG. This is due to the GTG of course being of main source of energy in whole system.
However our higher up want to to also shutdown the GTG whenever isolate Heat Transfer Oil.
I have read through ISO 21905 prat 9.3.2.2, it only mention case of tube bundle have to be designed for dry-run in case of unable to shutdown GTG AND not 100% fully isolate Heat Transfer Oil. Reason for not fully isolate even with damper + sealing blower is unclear to me, but suggested due to backflow or radiation. There are confirmation from MNF that the tube bundle is designed for GTG temperature / dry-run. This is a bit conflict of why our boss want to shutdown the GTG, but the reason is unknown for me at least.
Therefore i would love there is any ideas/ opinion on whether to keep the GTG running when isolate is better, if there is any big advantage for turning off the GTG when isolate the Heat Trasnfer Oil , safety reason included

 

[Dhurjati Sen]

The Gas Turbine (GTG) and the Heat Recovery Steam Generator (WHRU) are connected through duct. When WHRU fails, the gas turbine exhaust is diverted to the intermediate stack through diverter damper. During normal operation, the flue gas passes through the guillotine damper, then the WHRU and goes out through the main stack.

 

[georgeverghese]

If temp control on hot oil is by operating the GTG exhaust air feed to WHRU damper in tandem with WHRU bypass damper, then you wouldnt have to shutdown the GTG also in case the hot oil system is tripped and not flowing.

However, there is still a probability that, in the event the HO system has stopped and GTG is running, the feed air multilouvre damper may not have closed 100%, and is leaking hot exhaust into WHRU - for this case, I can see a reason to trip the GTG also ( ie if HO system tripped, and you have TSHH for hot oil leaving the WHRU, then trip the GTG also). This will prevent the stationary hot oil inventory in the WHRU tubes from coking up due to excessive temp.

WHRU in GT service is notorious for failure of tube to tubesheet joints due to wide operating and cycling temp range of tube metal (thermal fatigue). Also poor mechanical design and poor fabrication of WHRU can also lead to hot oil leak due to tube - tubesheet joint failure.

WHRU should be design for dry run in any case, and also for the widest temp differential between tube bundle and WRU shellside.

How do you intend to detect hot oil leak at WHRU during plant operation if this occurs ?