Debasis Heavy walled vessels undergo higher gradient thermal transients than thinner walled vessels or vessel components. Internal pressure (Hoop stress) at the ID may be partly tensile and partly compressive. Purpose of subjecting heavy walled vessel or vessel components, as a standard operating procedure, is to do gradual heating and gradual cooling. This is to avoid higher gradient of thermal transient stresses ( which will be mostly tensile in nature). Also heavy walled vessels undergo low-cycle fatigue from drastic or sudden heating and cooling and a greater thermal transient stresses. Thermal stresses are difficult to predict and may act as additional stress raiser in heavy walled components (tensile stress). Gradual heating and gradual slow cooling will act as lowered stress (both tensile and compressive). It is well known that Compressive stresses aid in delay of "on -set" in stress related failures in heavy walled vessel components especially vessels in service of syn gas, hydro cracker units etc. Actually internal ID pressurization is mostly tensile in nature. Gradual heating and cooling will avoid rapid tensile stress build-up in heavy walled vessel compoents. Rapid tensile stress will act as catalyst for both low cycle fatigue and greater thermal stress in these vessels. Most heavy wall vessel plant operation procedures should follow slow heating and slow cooling to avoid high thermal transient gradation even if there is necessity to have a crash shut-down due to plant emergencies. A re-start should take care of adverse effects due to rapid heating for heavy wall vessel components. Plants which adopt a rapid heating after emergency shut down would be introducing indeterminate thermal transient stresses (tensile in nature) at their own cost. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India Oct 10, 2007 E mail: nishi@vsnl.com >Residual compressive stresses are introduced at the ID face >of a thick-walled Vessel by application and release of >internal Pressure in a defined manner that cause Yielding of >the metal at/near the ID or Bore. This is heard to be done >to improve fatigue perfromances. > >If any one having any operating experience pl. share ? What >is the Pressuring procedure ? >Pl. share. >regards