what exactly corrosion allowance means & significance of it..?? I want to make my idea clear about when to add & to subtract it.Can anyone please help me..wat exactly corroded & uncorroded condition means...??????

appreciate could anyone explain deeply to me.

Thanks.

Corrosion allowance is the corrosion added to compensate for the corrosion expected for the lifetime of the vessel. E.g. if a corrosion rate of 0.2 mm/year is expected and the lifetime of the item is 15 years we can calculate a corrosion of 3 mm at the end of the 15 years lifetime. These 3 mm are added to the calculated thk. in order to have the calculated thk for all the 15 years lifetime.
When we have the vessel new, at the end of fabrication, we may calculate the MAWP based upon the total thk. (calculated + corr.allow.). We can perform hydrostatic test at this MAWP (of the vessel uncorroded) multiplied by 1.3, thus making a hydrostatic test at a higher pressure which will stress the item to its full pressure retaining capacity.
Mauro

Mauro, I respectfully disagree on only one point, assuming that this is per ASME Code. The MAWP is to be taken based on the material thickness in the corroded condition; ie: MAWP is based on nominal thickness minus the corrosion allowance. This is per the definition of MAWP (maximum allowable working pressure) given in Mandatory Appendix 3, ASME Section VIII Division 1.

Further, the MAWP is determined based on design temperature, vessel in operating orientation, with design liquid level, etc.

To me it is interesting the the Appendix does not define "MAP" (maximum allowable pressure), a term which is in common usage in vessel analysis.

Correct. I should have written (see UG-99(c)) <A hydrostatic test based on calculated pressure> (instead of: at this MAWP). But I understand you already knew what corrosion allowance means. What was the real scope of your question?
Actually engineering use to play with computer calculations and request a number of differently called and established pressure like, whose common meaning is:
- MAWP: to be stamped on the nameplate
- MAP: means the maximum pressure calculated using the actual thickness used to whih the c.a. has been deducted. Sometimes this MAP is requested to be stamped on nameplate and in this case it becomes the MAWP.
- Sometimes the MAP is required to be established but not to be limited by nozzles compensation, so if it were the case, the compensation should be improved to obtain the pressure which can be obtained by the calculation of stresses in shell and head using their actual thickness (deducted c.a.)
- MAP in hot and corroded: same as the above MAP
- MAP in new and cold (that established in UG-99(c)
Actually there should be one MAWP only, which is the pressure required by the purchaser in his request of offer, to be stamped on the nameplate.
Calculation of minimum thickness should be made by the Manufacturer and selection of minimum thickness to be used, according to calculations.
Mauro

Regards. Mauro

1. MAP is not defined in Appendix 3. It is however defined in part UCS with reference to determining lowest MDMT with out Impact testing.

2. Both MAP and MAWP are to be calculated with nominal thickness excluding CA.

3. MAP is calculated with allowable stresses at MDMT where as MAWP is considering Allowable stresses at designated coincident temperature.

4. MAP may be same as MAWP if allowable stresses are same at both the temperature ie at MDMT and designated coincident temperature

5. MAP in new (uncorroded) condition can be calculated and can be used for hydrotest of a new equipment. This MAP if stamped on name plate (in place of MAWP) may lead to catastrophe if used for hydrotest of equipment after couple of years in service ie in corroded condition.

Regards

Thank u all for your reply