Q & A > Question Details
We have platforming heaters tubes of alloy 2.25 Cr-1Mo (vertical tubes) .
The maximum design skin point temperature 595c while the limiting design metal temperature 650c as per API 530.
What is the maximum temperature we can reach it above the maximum design and below the limiting design to avoid the oxidized of the metal?
 
Answers
12/05/2017 A: Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
The rate of oxidation for a radiant tube of a given metallurgy increases rapidly with tube metal temperature (TMT) and is therefore a function of furnace configuration (single or double fired) and operating conditions (inlet and outlet temperatures and pressures, mass flux, heat flux, burner tip condition, etc). Operating at high TMT can result in excessive oxidation scale formation and creep damage to the tubes. TMT is mainly a function of peak heat flux so a double fired heater (fired on both sides of the tubes) can achieve a higher average heat flux for the same peak heat flux. The maximum allowable TMT for mechanical integrity is based on rupture strength but if the internal process pressure is low enough the rupture strength does not govern the design and creep damage becomes the primary integrity concern. For 2.25 Cr/1.0 Mo tubes, API 530 quotes a limiting design metal temperature of 650 Deg. C (1200 Deg. F) but a typical maximum allowable temperature for maintaining an acceptable rate of oxidation is 607 Deg. C (1125 Deg. F). If the tubes were upgraded to 9.0 Cr/ 1.0 Mo, the API 530 limiting design metal temperature would be 705 Deg. C (1300 Deg. F) and a typical maximum temperature for oxidation control would be 635 Deg. C (1175 Deg. F).
On-line methods of oxidation scale removal (eg. walnut blasting) can be used but are only moderately effective and for a short duration. Off-line oxidation scale removal (eg. light sand blasting or power brushing) methods are more effective but after 6 months or so the same level of scale will have returned. Application of a high emissivity ceramic coating on the external surfaces of the tubes can maintain heat absorption at near to start-of-run conditions for several years provided the tubes are garnet blasted back to white metal before the ceramic layer is sprayed. Furthermore, absence of scale on the ceramic coated tubes makes TMT measurements taken by portable infrared (IR) pyrometer more uniform, thereby improving reliability and interpretation of the readings.
11/05/2017 A: Patrick Bullen, UOP, patrick.bullen@uop.com
Please contact your local UOP service representative or Login to the UOP Customer Portal www.AccessUOP.com to explore our Knowledge Base, Frequently Asked Questions, Tools and more. Not a portal member, click on Registration to create your user credentials.
10/05/2017 A: David Weintz, IGS, david.weintz@integratedglobal.com
Oxidation is inherent to platforming heaters, particularly ones with lower alloy (2.25cr vs. 9cr). The higher the operating temperature the greater the rate of oxidation. This leads to a perpetuating cycle considering the oxide layer insulates the process tubes thereby requiring increased firing rates to maintain the same production level. Unfortunately these increased firing rates accelerate the rate at which the oxide builds up, and on and on it goes.
Application of high emissivity ceramic coatings to the process tubes will stop the oxide growth, improve the heat transfer, extend the life of the tubes, save fuel / increase capacity, and reduce NOX.
10/05/2017 A: David Weintz, IGS, david.weintz@integratedglobal.com
Oxidation is inherent to platforming heaters, particularly ones with lower alloy (2.25cr vs. 9cr). The higher the operating temperature the greater the rate of oxidation. This leads to a perpetuating cycle considering the oxide layer insulates the process tubes thereby requiring increased firing rates to maintain the same production level. Unfortunately these increased firing rates accelerate the rate at which the oxide builds up, and on and on it goes.
The good news is there is a solution. Application of high emissivity ceramic coatings to the process tubes will stop the oxide growth, improve the heat transfer, extend the life of the tubes, save fuel / increase capacity, and reduce NOX.
10/05/2017 A: RUPAM MUKHERJEE, ENGINEERS INDIA LIMITED, rupammukherjee86@gmail.com
The 650 figure in API is mostly related to the metallurgical limit for that MOC. The metal temperature indicated has more to do with that adopted thickness. If the metal is continuously operated over its design figure of 595, the life of the tubes reduces exponentially to the differential temperature. Ultimately, the tube fails much before its design life.