One of the Heater in Diesel HydroDesulphurisation unit has been designed for the design tube metal temperature of 446 Deg.C.The operating parameters are as below:- Tube fluid medium: Hydrocarbon+Hydrogen+H2S Design pressure: 72 kg/sq.cm(elastic & rupture) Material: 6"/7.11 mm thick SS347H Temperature: Inlet-308 Deg.C / Outlet - 352 Deg.C We are planning to increase the thru'put 10 % more than the design value.In doing so we are getting the skin temperature in the range of 475-525 Deg.C more than the design limit. Is it safe to operate with high skin temperature continuously more than design tube metal temperature?If we operate continuously in the above range,what problem we can expect in the long run? regards,

Krish Your proposal to operate at 475-525 deg cent for your 6" NB - Heater Tubes - with 7.11 mm design thickness will result in operation of heater with reduced safety factor. Designer had provided just 7.11 mm thickness which is just adequate for this design pressure ( 72 KSI) at 446 deg cent. You have to recognize with increased thru'put (10% over design pressure and design temp - 475-525 deg cent), your safety factor in S.S. 347 (6"NB pipe) is reduced equally or by a factor of 20% easily. If the tube had already a service life of > 40,000-50,000 hrs, the safety factor will get further reduced due to associated effects of permanent metallurgical transformation and creep related damages. You may have to commute or do a computerized tomographic evaluation of this also while evaluating the risks and the reduced safety factor to be considered for future operation. Also SSS 347 in the range of 475-525 deg cent is susceptible for IGSCC if H2S content is more in Diesel Desulphurizer. Further to this, additional creep related damages have to be evaluated if the heater tube had already seen a service life of > 40,000 hrs at design temp 446 deg cent. for design pressure 72 KSI. After this evaluation, recognizing that continuous operation will be with reduced safety factor, you can decide on risks. If the thru put planned is for a short period of < 8000 hr or so, then for future you have to take into account the reduced safety factor for evaluation of the heater tube for further operation. Trust this helps you to evaluate all risks and operate your tube if thru put increase is of immediate importance for your plant operation. C.V.Srinivasan Nishi Engineers Pvt Ltd India Oct 20, 2006 E-mail: nishi@vsnl.com >One of the Heater in Diesel HydroDesulphurisation unit has >been designed for the design tube metal temperature of 446 >Deg.C.The operating parameters are as below:- > >Tube fluid medium: Hydrocarbon+Hydrogen+H2S >Design pressure: 72 kg/sq.cm(elastic & rupture) >Material: 6"/7.11 mm thick SS347H >Temperature: Inlet-308 Deg.C / Outlet - 352 Deg.C > >We are planning to increase the thru'put 10 % more than the >design value.In doing so we are getting the skin temperature >in the range of 475-525 Deg.C more than the design limit. Is >it safe to operate with high skin temperature continuously >more than design tube metal temperature?If we operate >continuously in the above range,what problem we can expect >in the long run? > >regards,

if you use calorised pipes of the same base material, you can avoid the allowance in wall thickness for corrosion and increase the haet transfer rate with lower wall thickness and almost no corrosion. We can calorise the pipes for you