Due to continuous flame impingement one of the Vacuum heater tube(9Cr-1Mo)got scaled externally.After removing the scale about 1.5 mm thickness reduction was noticed.original thickness is 6.00 mm.Is it advisable to replace the scaled tube eventhough the retirment thickness is still lower than the design limit.Design tube metal temperature is 610 Deg.C.Tube OD is 114.3 mm x 6.01 AWT.Design pressure is 14.0 kg/sq.cm & FULL Vacuum. Fluid is reduced crude.No other form of corrosion except the localised thinning due to scaling. Request for expert opinion in this regards

Krish Flame impinged heater tube (9 Cr-1 Mo) is susceptible for higher local hardness increase. This can lead to sudden rupture of coil at the affected portions even though you may have sound metal available from discard thickness for this design temp and design pressure ( 14 KSC at 600 deg cent) Replacing the affected portions at the earliest available shut down or in a specifically planned shut down for this heater furnace would minimize risks for you. Risk factors should be weighed - not from available thickness due to external thickness reduction seen (1.5 mm reported by you) - but from possible high hard zone left due to direct flame impingement in 9 Cr 1 Mo material and possibility of sudden rupture of the coil due to this. Since the pressure in the system is not quite high (14 KSC at 600 deg cent) you may have some time or some leverage of time available to replace affected portions in a planned way. Trust this helps you C.V.Srinivasan Nishi Engineers Pvt Ltd India August 2, 2006 E-mail: nishi@vsnl.com >Due to continuous flame impingement one of the Vacuum heater >tube(9Cr-1Mo)got scaled externally.After removing the scale >about 1.5 mm thickness reduction was noticed.original >thickness is 6.00 mm.Is it advisable to replace the scaled >tube eventhough the retirment thickness is still lower than >the design limit.Design tube metal temperature is 610 >Deg.C.Tube OD is 114.3 mm x 6.01 AWT.Design pressure is 14.0 >kg/sq.cm & FULL Vacuum. Fluid is reduced crude.No other form >of corrosion except the localised thinning due to scaling. > >Request for expert opinion in this regards

Krish As expected, high hardness in 9 Cr-1 Mo is a potential source for rapid failure in this heater coil. My suggesion would be : With already hardness close to 300 BHN, hydro test at 50 KSC would add further stress on the coil. If you are considering replacing the affected portions and then carry out HT at 50 KSC for the design temperature conditions then it is OK. Affected portions had already seen 1.5 mm thickness loss. With high hardness (300 BHN) - although your design pressure is low (14 KSC) - the discard thickness may come close to 1.5-1.6 mm for this design temp. Risk evaluation should not be based on discard thickness but with operating with a coil with already high hardness. No strict corelation can be given on life expectancy. If further flame impingement is ruled out, probably you can replace the affected tubes in 6-8 months time. If further flame impingment continues (due to burner configuration or firing pattern changes) you have to immediately shut down and replace the affected coils. Please remember that this heater coil is on vacuum. So this will also accelerate failure with such high hardness. I woudl suggest you to recongize all potential harzards before going by only discard thickness as the resort for replacement criteria for such heat or flame impinged 9 Cr-1 Mo material. Trust this helps you C.V.Srinivasan Nishi Engineers Pvt Ltd India Aug 3, 2006 E-mail: nishi@vsnl.com >Thanks to mr.debasis and mr.srivinivasan.We have measured >the hardness at scaled area and it was 295 BHN.The furnace >is in vacuum service and hydrotest will be carried out after >decoking at 50 kg/sq.cm.We are planning to replace the >affected tubes during shutdown.For my academic purpose,i >would like to how long we can run the heater with the >existing condition.Is there any co-relation between the rate >of scaling and failure if the flame impingment continues for >quite some time. >regards,