How to Evaluate for the List of Loads in Section VIII, Division 1, Paragraph UG-22
Over the past several years I’ve had several Authorized Inspectors insist on adding a statement to the design report saying that the loads in UG-22 have been considered in the evaluation.
At first, I thought these requests were a bit odd in that the report stated the design met the requirements of Section VIII, Division 1. Paragraph UG-22 is a part of Section VIII, Division 1 so adding this statement was unnecessary, right?
Well, it seems that the AI’s concerns were based on some fabricators not being in compliance with the ASME Code in that they were claiming ignorance with respect to some of the loads listed in Paragraph UG-22.
Paragraph UG-22 starts by stating: “The loadings to be considered in designing a vessel shall include those from: …” and then goes on to list a variety of loads.
The AI’s concern was that some fabricators were not fully aware of how the owner was going to operate the pressure vessel and were inadvertently not considering all of the loads the pressure vessel would be subjected to. This was the result of the fabricators not getting all of the information they needed to properly design the vessel and not asking for it.
Unfortunately, sometimes a fabricator assumes that since the owner hasn’t mentioned some of these loads, then the vessel must not be subjected to them. I’ve personally seen this on more than one occasion when I’ve asked the fabricator if the pressure vessel is to be subjected to cyclic loading only to get a “we don’t know” rather than a definitive “yes” or “no”. Unfortunately, I’ve also seen fabricators get themselves into a bit of a jam when these things come to light after the pressure vessel is already built.
Loads to be Considered
So, what are the loads listed in Paragraph UG-22? To paraphrase UG-22, the loads include:
Internal or external design pressure
Weight of the vessel and its contents
Static reactions from the weight of attached equipment
Loads from the attachment of internals and vessel supports
Cyclic and dynamic loads due to variations in pressure or temperature
Cyclic and dynamic loads from equipment mounted on the vessel or from other mechanical loads
Loads resulting from wind, snow, or seismic activity
Impact loads such as those due to fluid shock
Temperature gradients and differential thermal expansion
Abnormal pressures, such as those caused by deflagration
Test pressure including static head
One thing to note is that the list of loads in UG-22 is not all-inclusive. Other loads not listed in UG-22 may be present that must be considered in the design of the pressure vessel. I personally recommend asking the owner if the vessel is subjected to any of the loads listed in UG-22, or any other loads. I also recommend listing what loads the pressure vessel has and has not been analyzed for in the design report.
Evaluation of Loads
OK, so how do you handle the loads that are applicable?
Designing a vessel for the run-of-the-mill loads such as pressure and weight of the contents is pretty straightforward. Evaluating a pressure vessel for loads from attachments, supports, and from wind, snow, and seismic is usually not too difficult.
Most of the pressure vessel design programs available can easily evaluate the vessel for these types of loads. The design engineer, however, should be well-versed in how these programs evaluate the pressure vessel for these loads, and the limits of applicability of the methods and equations in use.
For example, many of these programs evaluate attachment loads and external nozzle loads using Welding Research Council Bulletin 107 (WRC 107). While this is often acceptable, the stresses in a vessel due to external loads on a nozzle calculated using WRC 107 may be non-conservative when the diameter of the nozzle is large with respect to the diameter of the vessel. Knowing the equations and their underlying assumptions and limitations is essential in ensuring a safe design.
Designing a pressure vessel for the other loads listed in UG-22 usually require a more rigorous analysis method such as finite element analysis (FEA). Unlike designing and evaluating a pressure vessel using the formulas in Section VIII, Division 1, evaluating a pressure vessel using finite element analysis involves categorizing stresses and comparing these stresses to corresponding allowable stress limits. As such, FEA should only be undertaken by an engineer who is experienced in FEA and well-versed in how to evaluate FEA results with respect to the ASME Code.
Cyclic Loads
Cyclic loads, which can lead to failure due to fatigue, are often evaluated using FEA. This is because fatigue life is dictated by local peak stresses which occur at structural discontinuities. Traditional methods of calculating stresses (i.e. hand calculations) are often insufficient in accurately calculating the local stresses at structural discontinuities.
Beyond just calculating local stresses due to cyclic loads and comparing them to an allowable limit, fatigue evaluations entail calculating fatigue damage for each type of cyclic load and then determining the cumulative fatigue damage to confirm the design is acceptable.
Impact Loads
Impact loads, such as those due to fluid shock, are also typically analyzed using FEA. The impact load may be calculated using hand calculations, or if more complex, may entail using a numerical analysis method such as computational fluid dynamics (CFD).
Once the magnitude of the impact load is known, the vessel may be evaluated for the impact loads using hand calculations, such as those contained in WRC 107, or may require FEA depending on the complexity, location, and duration of the loads.
Thermal Loads
Analyzing a pressure vessel for thermal gradients and thermal shock usually require finite element analysis.
While the owner will usually know the temperature and flow rates of fluid entering the pressure vessel, they do not always know the details of the temperature distribution throughout the vessel and its components.
In order to determine the temperature distribution, a finite element thermal analysis (heat transfer analysis) may be performed using the appropriate heat transfer loading parameters (e.g. convective heat transfer film coefficients, heat flux, etc.). Once the temperature distribution is known, then a finite element stress analysis is performed.
Conclusion
In conclusion, evaluating a pressure vessel for the loads listed in UG-22 can be as simple as performing some hand calculations to conducting a complex finite element analysis. The appropriate evaluation method will be dependent on how complex the loading or geometry of the vessel is and the assumptions and limitations of traditional equations and methods. Ultimately, it is the responsibility of the fabricator (and their engineer) to ensure that the pressure vessel has been evaluated for all of the appropriate loads.