Posted by: Dorin Daniel Popescu
Thick Wall High Pressure Piping - 02/05/04 09:34 AM
Dear Mr. Ay,
I am currently working at a thick wall high pressure piping system – 8” NPS (219 mm OD / 62 mm wall thk), 10” NPS (273 mm OD / 52 mm wall thk), 12” NPS (324 mm OD / 62 mm wall thk); 360 … 600 bar internal gage pressure; 10000#, 15000# pressure rating; low carbon steel (API 5L X60).
The project specification regarding Piping Stress Analysis requires the piping design&analysis to be accomplished based on ASME Code for Process Pressure Piping B 31.3 and the computer stress analysis to be performed using Caesar II software.
Obviously, the piping stress analysis for the system under discussion has to be developed based on ASME Code B 31.3 / Chapter IX – High Pressure Piping.
The piping material consists of thick wall cylindrical structural components. The k = OD/ID ratio (i.e. Outside Pipe Diameter / Inside Pipe Diameter) values are situated around 1.61 … 1.62, and are significantly higher than the limit/reference of 1.1 … 1.2 that separates shell structures of thick walled revolution bodies.
Because it’s first time when I deal with such a piping system, I’ve performed a few preliminary tests consisting in computer analysis of typical simple problems that involved thick walled high pressure pipe behavior, such as double anchored plane “L” and spatial “double L” loop pipes.
The models have been built and analyzed using both the beam theory approach (i.e. piping stress analysis programs – Caesar II, Pipe Plus) and the general finite element method (three-dimensional model – Ansys Software Package).
My big problem is that comparing the results (especially the ANCHOR REACTION FORCES AND MOMENTS), I’ve seen significant discrepancies between the above approaches. Moreover, Caesar II computer analysis appears to be a LESS CONSERVATIVE APPROACH.
Detailing my comments, it should be noted that for the gravitational (weight) and thermal loadings, beam theory results fit the finite element solution in a satisfactory manner. But when we speak about the PRESSURE LOADING CIRCUMSTANCES, the results are highly different. In fact, the anchor reaction force&moment values obtained by the three-dimensional detailed analysis (Ansys – I’ll name them “Reference Values”), are situated BETWEEN the corresponding values assessed by the beam theory approach, so that Caesar II anchor reaction values are lower than the “Reference Values” (20 … 40 % percentage difference), while Pipe Plus anchor reaction values are higher than the “Reference Values” (20 … 50 % percentage difference).
My analyses revealed that the main cause of the discrepancies between Caesar II results and the detailed three-dimensional finite element solution, is THE CURVE PIPE / ELBOW BEHAVIOR MODEL. For a piping system composed by straight pipes only, Caesar II results fit the corresponding finite element solution in a satisfactory manner. But when the elbows are provided, Caesar II beam theory approach doesn’t match any more the finite element method “Reference Values”.
I have to specify that Caesar II analyses (4.40 and 4.50 versions) have been performed using as ACTIVE the “Bourdon Effect – Translation & Rotation” option. The results obtained using the other possible options (i.e. “No Bourdon Effect” and “Bourdon Effect – Translation Only”) conducted to higher differences between the “Reference Values” and Caesar II solution.
Caesar II Forum contains several topics regarding the high pressure piping stress analysis and the pressure elongation effect problem, but all the previous discussions referred to common thin wall piping systems. I’ve performed a test for this alternative and Caesar II results correspond to the finite element method “Reference Values”. But for the special case of thick wall piping systems, Caesar II results are somehow discouraging.
Now, searching within COADE Caesar II article database, I’ve found an article in Mechanical Engineering News – Volume 17 – December 1993, entitled “Estimation of Nozzle Loads Using Caesar II Software”, that contains some considerations regarding the pressured straight and curve pipe element behavior modeling.
All the formulas involving the internal pressure loading effect correspond to the THIN WALLED SHELL STRUCTURE THEORY. In my opinion, this approach COULD BE the main reason of the above discrepancies.
I am aware of the delicacy/difficulty of this problem. Unfortunately, I am not working now home and I cannot use my own books&materials to study this complex problem more carefully. In addition to the above article, I’ve found on Internet Peng’s article entitled “An Interpretation on Pressure Elongation in Piping Systems”, but again, the theory has been developed based on Thin Walled Shell Theory.
At the end of this topic, two more problems I’d like to submit related to Thick Walled High Pressure Piping stress analysis.
First subject is related to the material allowable stresses as per ASME B 31.3, Chapter IX (“High Pressure Piping”) and appendix/table K-1. Maybe I do not know enough Caesar II software, but I haven’t found any option that allows the user to “tell” to the program to compute the allowable stresses following the rules under discussion (par. K303.3.2). Certainly, user can define his own allowable stresses, but I think it would be useful to provide this tool in the future.
The second subject is related to the Pressure Stress Assessment and Checking. Paragraph K302.3.5.a (Limits of Calculated Stresses Due to Sustained Loads and Displacement Strains – Internal Pressure Stresses) stipulates a different approach than the classical method “slp = p*Di^2 / (Do^2 – Di^2)” and “shp = p*Di / (2*T)”, consisting in the pipe wall thickness checking (par. K304). Obviously, for the thick walled high pressure pipe components, Caesar II calculates accurately only the longitudinal pressure stress (slp); the hoop pressure stress classical calculation [“shp = p*Di / (2*T)”] is not valid any more and, in my opinion, depending on OD/ID ratio value, it should be amended, because, as it is well known, the maximum hoop pressure stress is developed at the inside pipe wall surface.
I would like to know your pertinent opinion regarding these subjects. Certainly, any Forum member remark or comment will be highly appreciated.
Thank you for your time,
Dorin Popescu
Senior Pipe Stress Engineer
Washington Engineers&Constructors Romania
I am currently working at a thick wall high pressure piping system – 8” NPS (219 mm OD / 62 mm wall thk), 10” NPS (273 mm OD / 52 mm wall thk), 12” NPS (324 mm OD / 62 mm wall thk); 360 … 600 bar internal gage pressure; 10000#, 15000# pressure rating; low carbon steel (API 5L X60).
The project specification regarding Piping Stress Analysis requires the piping design&analysis to be accomplished based on ASME Code for Process Pressure Piping B 31.3 and the computer stress analysis to be performed using Caesar II software.
Obviously, the piping stress analysis for the system under discussion has to be developed based on ASME Code B 31.3 / Chapter IX – High Pressure Piping.
The piping material consists of thick wall cylindrical structural components. The k = OD/ID ratio (i.e. Outside Pipe Diameter / Inside Pipe Diameter) values are situated around 1.61 … 1.62, and are significantly higher than the limit/reference of 1.1 … 1.2 that separates shell structures of thick walled revolution bodies.
Because it’s first time when I deal with such a piping system, I’ve performed a few preliminary tests consisting in computer analysis of typical simple problems that involved thick walled high pressure pipe behavior, such as double anchored plane “L” and spatial “double L” loop pipes.
The models have been built and analyzed using both the beam theory approach (i.e. piping stress analysis programs – Caesar II, Pipe Plus) and the general finite element method (three-dimensional model – Ansys Software Package).
My big problem is that comparing the results (especially the ANCHOR REACTION FORCES AND MOMENTS), I’ve seen significant discrepancies between the above approaches. Moreover, Caesar II computer analysis appears to be a LESS CONSERVATIVE APPROACH.
Detailing my comments, it should be noted that for the gravitational (weight) and thermal loadings, beam theory results fit the finite element solution in a satisfactory manner. But when we speak about the PRESSURE LOADING CIRCUMSTANCES, the results are highly different. In fact, the anchor reaction force&moment values obtained by the three-dimensional detailed analysis (Ansys – I’ll name them “Reference Values”), are situated BETWEEN the corresponding values assessed by the beam theory approach, so that Caesar II anchor reaction values are lower than the “Reference Values” (20 … 40 % percentage difference), while Pipe Plus anchor reaction values are higher than the “Reference Values” (20 … 50 % percentage difference).
My analyses revealed that the main cause of the discrepancies between Caesar II results and the detailed three-dimensional finite element solution, is THE CURVE PIPE / ELBOW BEHAVIOR MODEL. For a piping system composed by straight pipes only, Caesar II results fit the corresponding finite element solution in a satisfactory manner. But when the elbows are provided, Caesar II beam theory approach doesn’t match any more the finite element method “Reference Values”.
I have to specify that Caesar II analyses (4.40 and 4.50 versions) have been performed using as ACTIVE the “Bourdon Effect – Translation & Rotation” option. The results obtained using the other possible options (i.e. “No Bourdon Effect” and “Bourdon Effect – Translation Only”) conducted to higher differences between the “Reference Values” and Caesar II solution.
Caesar II Forum contains several topics regarding the high pressure piping stress analysis and the pressure elongation effect problem, but all the previous discussions referred to common thin wall piping systems. I’ve performed a test for this alternative and Caesar II results correspond to the finite element method “Reference Values”. But for the special case of thick wall piping systems, Caesar II results are somehow discouraging.
Now, searching within COADE Caesar II article database, I’ve found an article in Mechanical Engineering News – Volume 17 – December 1993, entitled “Estimation of Nozzle Loads Using Caesar II Software”, that contains some considerations regarding the pressured straight and curve pipe element behavior modeling.
All the formulas involving the internal pressure loading effect correspond to the THIN WALLED SHELL STRUCTURE THEORY. In my opinion, this approach COULD BE the main reason of the above discrepancies.
I am aware of the delicacy/difficulty of this problem. Unfortunately, I am not working now home and I cannot use my own books&materials to study this complex problem more carefully. In addition to the above article, I’ve found on Internet Peng’s article entitled “An Interpretation on Pressure Elongation in Piping Systems”, but again, the theory has been developed based on Thin Walled Shell Theory.
At the end of this topic, two more problems I’d like to submit related to Thick Walled High Pressure Piping stress analysis.
First subject is related to the material allowable stresses as per ASME B 31.3, Chapter IX (“High Pressure Piping”) and appendix/table K-1. Maybe I do not know enough Caesar II software, but I haven’t found any option that allows the user to “tell” to the program to compute the allowable stresses following the rules under discussion (par. K303.3.2). Certainly, user can define his own allowable stresses, but I think it would be useful to provide this tool in the future.
The second subject is related to the Pressure Stress Assessment and Checking. Paragraph K302.3.5.a (Limits of Calculated Stresses Due to Sustained Loads and Displacement Strains – Internal Pressure Stresses) stipulates a different approach than the classical method “slp = p*Di^2 / (Do^2 – Di^2)” and “shp = p*Di / (2*T)”, consisting in the pipe wall thickness checking (par. K304). Obviously, for the thick walled high pressure pipe components, Caesar II calculates accurately only the longitudinal pressure stress (slp); the hoop pressure stress classical calculation [“shp = p*Di / (2*T)”] is not valid any more and, in my opinion, depending on OD/ID ratio value, it should be amended, because, as it is well known, the maximum hoop pressure stress is developed at the inside pipe wall surface.
I would like to know your pertinent opinion regarding these subjects. Certainly, any Forum member remark or comment will be highly appreciated.
Thank you for your time,
Dorin Popescu
Senior Pipe Stress Engineer
Washington Engineers&Constructors Romania