If a piece of hot-finished, seamless pipe (per ASTM A530) measures on the low side of OD tolerance (-1/32" under nominal), and the surface texture is required to be "cleaned-up" to 64 AARH and the pipe ends up .010" undersize, is it, therefore, non-compliant? If so, how does the surface texture add to strength?
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Pipe Ratings 1
- Thread starter marklobo
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In order to bring your question on the top again I’ll do some scholar philosophising if you do not mind. And you shouldn’t because being on the top your chances to get a proper (meaning a useful) answer are significantly better.
OD undersized 0.010 inch is not enough data for evaluation. What percent of wall thickness is that? Is that pipe under high pressure in service or there is a significant margin of safety (in which case you do not have to bother)? Are there some significant bending forces in service and again percentage reduction of moment of inertia (meaning the larger nominal OD the smaller influence of 0.010 inch is). Is load constant or fluctuating (exaggerating a little bit but important if Low Cyclic Fatigue (LCF) is a matter of importance).
Surface texture like all nicks, dents, scratches (surface imperfections) adds to stress (stress concentration coefficient) but not so much as those mentioned. The finer the surface the better from stress point of view situation is. Probably some data may be found on the net, or maybe even on some of these numerous eng-tips forums, how much the surface finish contributes to stress coefficient. In particular if dynamic strength is important surface finish is rather influential factor (resistance to LCF increases with better surface finish).
I do recall some “stress rupture” testing (that is a simplified creep testing: measure time to rupture of a test piece under certain tension load and temperature, usually lasing couple of days) where ground test peaces showed significantly better results than turned ones.
OD undersized 0.010 inch is not enough data for evaluation. What percent of wall thickness is that? Is that pipe under high pressure in service or there is a significant margin of safety (in which case you do not have to bother)? Are there some significant bending forces in service and again percentage reduction of moment of inertia (meaning the larger nominal OD the smaller influence of 0.010 inch is). Is load constant or fluctuating (exaggerating a little bit but important if Low Cyclic Fatigue (LCF) is a matter of importance).
Surface texture like all nicks, dents, scratches (surface imperfections) adds to stress (stress concentration coefficient) but not so much as those mentioned. The finer the surface the better from stress point of view situation is. Probably some data may be found on the net, or maybe even on some of these numerous eng-tips forums, how much the surface finish contributes to stress coefficient. In particular if dynamic strength is important surface finish is rather influential factor (resistance to LCF increases with better surface finish).
I do recall some “stress rupture” testing (that is a simplified creep testing: measure time to rupture of a test piece under certain tension load and temperature, usually lasing couple of days) where ground test peaces showed significantly better results than turned ones.
Marklobo:
Cleaning up of the OD and reducting the OD is not a problem if you have done the B31.3 calculations on the final OD (Wall Thickness)to the be sure that the pipe, per code, can still hold the pressure. Normally cyclic considerations would have to be made as balu1 says.
Little consideration is given to the OD imperfections, nicks or guouges in B31.3. If the final finish is near the original manufacturers finish there is no problem. Surface texture cannot add to the strength, this is a function of the material strength, it can only take away by causing stress concentrations. The allowable stress values are conservative allowing for imperfections and surface damage which happens to most all pipe during handling and fabrication.
The pipe will still be "compliant", I assume by this you mean it will meet the requirements of B31.3, if the allowable stress per Table A-1 is not exceeded per the B31.3 pressure calculation for the final wall thickness, given material and temperture.
You say the wall thickness is 1/32 under normal. The manufacturing tollerance is +- 12.5% so you still may be normal depending on the original manfacturing dimensions. The nominal wall thickness is used for the B31.3 calculations unlss you are dealing with a custom made pipe and are taken into consideration by use of a low allowable stress.
Cleaning up of the OD and reducting the OD is not a problem if you have done the B31.3 calculations on the final OD (Wall Thickness)to the be sure that the pipe, per code, can still hold the pressure. Normally cyclic considerations would have to be made as balu1 says.
Little consideration is given to the OD imperfections, nicks or guouges in B31.3. If the final finish is near the original manufacturers finish there is no problem. Surface texture cannot add to the strength, this is a function of the material strength, it can only take away by causing stress concentrations. The allowable stress values are conservative allowing for imperfections and surface damage which happens to most all pipe during handling and fabrication.
The pipe will still be "compliant", I assume by this you mean it will meet the requirements of B31.3, if the allowable stress per Table A-1 is not exceeded per the B31.3 pressure calculation for the final wall thickness, given material and temperture.
You say the wall thickness is 1/32 under normal. The manufacturing tollerance is +- 12.5% so you still may be normal depending on the original manfacturing dimensions. The nominal wall thickness is used for the B31.3 calculations unlss you are dealing with a custom made pipe and are taken into consideration by use of a low allowable stress.
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