E-Boogie
Mechanical
- Nov 26, 2018
- 13
Good morning all,
I am putting together a comprehensive pipe thickness excel tool. Piping codes included: B31.3, B31.4, B31.8, 49 CFR 192, 49 CFR 195. I am struggling a little bit with the differences in required thickness.
B31.3 is very clear, and specifies that minimum thickness (T) must be greater than required minimum thickness (tm) which accounts for manufacturers's minus tolerance as well as the sum of mechanical allowances (eg. Corrosion Allowance). {B31.3, 304.1}
B31.4 is pretty clear as well, and specifies that the nominal wall thickness of straight sections of steel pipe shall be equal to or greater than (tn). Design Factor (F) has been determined with underthickness tolerance in mind and (tn) includes the sum of mechanical allowances (eg. Corrosion Allowance). {B31.4, 403.2}
B31.8 is less clear. As in B31.4, Design factor (F) has been determined with underthickness tolerance in mind. 841.1.1 allows you to calculate the required nominal wall thickness but there is no part of the Code that I am seeing where corrosion allowance is required to be added to this thickness like in B31.3 and B31.4. If you look at Mandatory Appendix F, require thickness (tr) does not include allowance to corrosion or under-thickness. To me it appears that corrosion allowance is not being accounted for. {B31.8, 841.1}
49 CFR 192 uses the same equation as B31.8 to determine nominal thickness of pipe. Design factor (F) matches B31.8, but 49 CFR 192 does not clearly state that the design factor accounts for underthickness tolerance though I assume (although I don't like assuming) that this was the intention. Just like in B31.8, there is no part of the Code that specifies that corrosion allowance should be added to the calculated thickness. {49 CFR 192, 192.105}
49 CFR 195 uses the same equation as B31.4 to determine nominal thickness of pipe. Design factor (F) matches B31.4, but 49 CFR 195 does not clearly state that the design factor accounts for underthickness tolerance though I assume (although I don't like assuming) that this was the intention. Unlike like in B31.4, there is no part of the Code that specifies that corrosion allowance should be added to the calculated thickness. {49 CFR 195, 195.106}
In all codes it seems that underthickness tolerance has been accounted for. Corrosion allowance is clearly accounted for in B31.3 and B31.4. I would think that corrosion should clearly be accounted for in each variation of Code. I believe that even though it is not clearly stated that corrosion allowance must be added to calculated nominal thickness for B31.8, 49 CFR 192, and 49 CFR 195 as good engineering practice it should. BUT looking at mandatory appendix F in B31.8 I get a different impression; it seems as though corrosion should be ignored in reference to require minimum thickness.
Currently I have been adding corrosion allowance to the calculated nominal thickness per code equations.
My question: How should corrosion allowance be accounted for in these 5 piping codes? For B31.8, 49 CFR 192, and 49 CFR 195 should corrosion be added to calculated nominal thickness and taken as the required minimum thickness? Have I been adding inessential thickness to some of my piping and as a result cost?
Thank you for your time and input!
I am putting together a comprehensive pipe thickness excel tool. Piping codes included: B31.3, B31.4, B31.8, 49 CFR 192, 49 CFR 195. I am struggling a little bit with the differences in required thickness.
B31.3 is very clear, and specifies that minimum thickness (T) must be greater than required minimum thickness (tm) which accounts for manufacturers's minus tolerance as well as the sum of mechanical allowances (eg. Corrosion Allowance). {B31.3, 304.1}
Corrosion and underthickness are clearly accounted for
B31.4 is pretty clear as well, and specifies that the nominal wall thickness of straight sections of steel pipe shall be equal to or greater than (tn). Design Factor (F) has been determined with underthickness tolerance in mind and (tn) includes the sum of mechanical allowances (eg. Corrosion Allowance). {B31.4, 403.2}
Corrosion and underthickness are clearly accounted for
B31.8 is less clear. As in B31.4, Design factor (F) has been determined with underthickness tolerance in mind. 841.1.1 allows you to calculate the required nominal wall thickness but there is no part of the Code that I am seeing where corrosion allowance is required to be added to this thickness like in B31.3 and B31.4. If you look at Mandatory Appendix F, require thickness (tr) does not include allowance to corrosion or under-thickness. To me it appears that corrosion allowance is not being accounted for. {B31.8, 841.1}
Underthickness is accounted for through design factor (F)
Corrosion allowance appears to be non-existent. In my mind this should be accounted for and added to the required minimum thickness in order to ensure pipe strength in corroded condition.
49 CFR 192 uses the same equation as B31.8 to determine nominal thickness of pipe. Design factor (F) matches B31.8, but 49 CFR 192 does not clearly state that the design factor accounts for underthickness tolerance though I assume (although I don't like assuming) that this was the intention. Just like in B31.8, there is no part of the Code that specifies that corrosion allowance should be added to the calculated thickness. {49 CFR 192, 192.105}
***Underthickness is accounted for through design factor (F)
Corrosion allowance appears to be non-existent. In my mind this should be accounted for and added to the calculated minimum thickness in order to ensure pipe strength in corroded condition.
49 CFR 195 uses the same equation as B31.4 to determine nominal thickness of pipe. Design factor (F) matches B31.4, but 49 CFR 195 does not clearly state that the design factor accounts for underthickness tolerance though I assume (although I don't like assuming) that this was the intention. Unlike like in B31.4, there is no part of the Code that specifies that corrosion allowance should be added to the calculated thickness. {49 CFR 195, 195.106}
***Underthickness is accounted for through design factor (F)
Corrosion allowance appears to be non-existent. In my mind this should be accounted for and added to the calculated minimum thickness in order to ensure pipe strength in corroded condition.
In all codes it seems that underthickness tolerance has been accounted for. Corrosion allowance is clearly accounted for in B31.3 and B31.4. I would think that corrosion should clearly be accounted for in each variation of Code. I believe that even though it is not clearly stated that corrosion allowance must be added to calculated nominal thickness for B31.8, 49 CFR 192, and 49 CFR 195 as good engineering practice it should. BUT looking at mandatory appendix F in B31.8 I get a different impression; it seems as though corrosion should be ignored in reference to require minimum thickness.
Currently I have been adding corrosion allowance to the calculated nominal thickness per code equations.
My question: How should corrosion allowance be accounted for in these 5 piping codes? For B31.8, 49 CFR 192, and 49 CFR 195 should corrosion be added to calculated nominal thickness and taken as the required minimum thickness? Have I been adding inessential thickness to some of my piping and as a result cost?
Thank you for your time and input!
