Q & A > Question Details
Radiant tube skin temperatures of CCRU preheter and interheaters has increased and nearing the design tube metal temperature of 625 Deg.C. Whether anyone has faced such issues in CCRU?
What is the solution to bring back the skin temperature to normal? It is to be noted that no decoking facility has been provided since the service is clean.
14/09/2017 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
you need to identify the reasons? whether coke ball formation has occurred in the tube due to hydrogen at that high temperature corroding the mettallurgy and no protection of the metallurgy by the sulfide scale. I hope you are aware of sulfur content of minimum of 0.1 to 0.2 ppm in charge to prevent carbon issue of the metal. The references can be sent to you separately on request
06/09/2017 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
If not all the heaters are constrained on tube skin or bridgewall temperatures, it may be possible to run with a staggered reactor inlet temperature profile without affecting reformate quality if the weighted average inlet temperature (WAIT) and weighted average bed temperature (WABT) for the reactors are maintained.
What method are you using to monitor the tube skin temperatures? Thermocouple measurement, portable infrared optical pyrometer measurement or calculation? I like to use all three methods and monitor trends as well as individual measurements. Skin temperature thermocouples are useful for providing continuous measurements and can trigger automatic alarms on the operator console. Optical pyrometers enable on-line checking of tube skin temperature thermocouples accuracy (at start of run) and can help reveal and correct unbalanced heater firing (throughout the operating cycle). The calculated tube skin temperature is based on the methodology described in API 530 and can be built into an advanced control scheme to control heater fuel firing in normal operation. It can also be used to track and alarm deviations from skin temperature thermocouples.
If your observations are based on tube skin temperature thermocouple measurements and all these are increasing relatively evenly and at a steady rate, the most likely cause of the problem is external oxidation of the radiant tubes (assuming roughly constant process-side operating conditions). This would typically be accompanied by a sustained increase in bridgewall temperature. However, if there has been a step increase in one or more tube skin thermocouples in a localised area of the heater firebox, the most likely explanation is either air ingress to the firebox (requiring increased fired duty to achieve the desired reactor inlet temperatures), flame impingement on the thermocouple lead (lead overheated) or partial failure of the thermocouple to tube attachment weld (thermocouple partially separated from radiant tube which also acts as a heat sink).
On-line removal of oxidation scale from radiant tubes can be attempted by specialist contractors. However, this has limited effectiveness depending on heater configuration, heater dimensions, number and location of peepholes, etc. Off-line removal of oxidation scale can be achieved in order of increasing effectiveness) by sand blasting, power brushing or garnet blasting back to bare metal. These are all temporary solutions as oxidation scale will return within a matter of weeks or months depending on furnace operating conditions and tube metallurgy. Upgrading tube metallurgy to (say) 9 Cr/1 Mo and/or applying a high emissivity ceramic coating to the tube outer surface can significantly increase resistance to oxidation scale formation and will extend the life of the radiant tubes.
Air ingress locations in the firebox can be identified on-line with an optical pyrometer and sealed from the outside using high temperature room temperature vulcanisation (RTV) silicone and aluminium tape. Perished fabric tube seals and corroded firebox casings are common leak locations (also check peepholes, explosion doors and header box doors are closed). For a low pressure CCR Platforming unit the radiant tube inlet and outlet headers are large diameter so a perished fabric seal presents a large area for air ingress via the annular gap between the header OD and header box penetration ID. Flame impingement on a thermocouple sheath or lead can be reduced by trimming the fuel flow to the offending burner and allowing the remaining burners to pick up the missing duty.
05/09/2017 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Getting high end point naphtha or some nitrogen carry through out of the NHT could possibly lead to furnace tube coking as could breakthrough of Si from the NHT if the crude supply has crudes containing antifoam or the NHT processes coker naphtha. The only way to clean the tubes is going to be steam air decoke or pigging if there is coke buildup on the tubes.
There can also be some external oxidation or scale build up on the tubes that raise the apparent skin temperature. In this situation, sometimes injecting walnut hulls into the firebox can knock external scale off of tubes and restore skin temperatures to normal.