As per HAZOP recommendations in one of our CDUs, the recommendation was to provide TAHH with 2oo3 logic to trip the furnace on High COT. Are there any such trips in any heaters? Is it really necessary to have such a trip ? In case, the trip is provided, what should be my temperature on which the furnace should trip?
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09/05/2016
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Eric Hennings, Technip Stone & Webster, EHennings@technip.com
I agree with the necessity for TAHH shutdown. Another consideration is a small continuous steam injection to each coil to provide some flow and cooling in the event of pump loss. Special care is required to prevent backflow of process into steam condensate.
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22/04/2016
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Jayaraj Jayam, Chennai Petroleum Corporation Limited, njayaraj@gmail.com
TAHH for furnace trip based COT is an essential one. This logic will be useful when furnace coil flow becomes low and the logic based on low coil flow is in bypassed condition and unexpected uncontrolled firing due to malfunction of fuel control valves. These situation will escalate the COT to very high. If human interference takes some time due to various reasons, then the system will take care of the furnace by tripping it, that is by closing the shut off valves of fuel gas, fuel oil supply& return and any other fuel sources to the furnace.
Pl mind that, furnace tubes are the costly spare parts in any heater. Very high COT will lead to cracking of feed stock and this is an unwanted reaction to take place in CDU furnace. This will lead to formation of coke and thereby reducing the run length of the furnace due to early reaching of allowable skin temperature. This reduces the heat transfer efficiency and steam+air decoking is required to clear off the coke which needs shut down of the unit.
Basically all interlock logics are provided to safe guard in event of failure of instruments and due to lack of human interference to normalise the parameters within the allowable deviation and time. As for as operation is concerned, adding interlock means you will get quality support from your maintenance personnel because no one will accept unit shutdown on spurious indications more than once.
Trip value varies based on feed quality and the operating pressure. In general for CDU furnace 390 to 395°C and for VDU furnace 420 to 425°C are the reasonable set value for TAHH.
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20/04/2016
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L K Maheshwari, Essar Oil, laju1984@yahoo.co.in
Yes, I worked in 3 refineries and all heaters had COT high high Temp trip logic. it is really necessary to have this Trip logic for safety of Furnace .
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