Project Description

It is now three weeks since the commencement of your role as a Mechanical Asset Engineer, and you   are ready to embark on your first integrity assessment project in the plant. The integrity inspector calls for an urgent meeting to which you are invited. During the meeting you learn that the skirt’s thickness  of a vertical pressure vessel (V2-1979) has decreased substantially. The pressure vessel (V2-1979) is    located at the beginning of the production line, and its overhaul comes with a huge financial loss that the management team is attempting to avoid. The V2-1979 vessel consists of a 4.2 meter cylindrical support (skirt), two ellipsoidal ends, and a 30m shell. Figures 4, 5, and 6 show the full vessel supported with skirt, the shell with ellipsoidal heads without the skirt, and the shell cross section, respectively. Figures 7, 8 and 9 depict the 3D view of skirt with refractory bricks and manhole, along with a cross-sectional view. A complete CAD drawing is available at the end of Part 1 of the assignment to assist you in developing the model. The skirt, which acts as a supporting structure, is typically attached to the bottom of the vessel through a circumferential weld. In our case, the joint weld efficiency is 100%, ensuring that this connection provides stability and evenly distributes the  load.

The integrity team engaged a contractor to perform. a 3D laser scan of the entire circumference of the skirt for the first meter from the ground. The results are displayed in Figures 1, 2, and 3. For this purpose, the refractory bricks, which provide fireproofing protection, are removed both internally and externally from the skirt so that the actual carbon steel plate can be accessed for thickness measurement.

Think of the skirt as a thin cylindrical plate covered by refractory bricks for fire isolation. The issue is that the thickness of the actual carbon steel plate beneath these bricks is decreasing, necessitating the   removal of the refractory bricks to take accurate thickness measurements. The laser scan method offers a more robust thickness measurement compared to traditional UT techniques; however, it is still prone to inaccuracies. According to the contractor who conducted the measurement, a tolerance of 1 mm must be considered when using the data. As shown, these 3D thickness surveys include contour plots on the right-hand side, indicating the measured thickness. The thickness of several red zones has  been marked for reference. To gain a better understanding and analysis of the data, one can map each   zone with the contour to understand the corrosion pattern more comprehensively.

Wind and Seismic Loading, The vessel is exposed to the wind force as well as the seismic loading. However, according to AS1210-2010, their cumulative effect must not be considered. In other words, between seismic loading and wind force only one with a greater value should be taken as the bending moment. You have access to an old wind and seismic load calculation performed by a former contractor using PV-Elite software. According to this report, the wind force and seismic load produce bending moments with magnitudes of 3133 [KN.m] and 4008 [KN.m] around the skirt, respectively.  However, the problem with this data is that it cannot be verified due to the contractor’s refusal to supply a detailed calculation report, and you have only access to the final results of their calculation. Thus, it is up to you whether to trust these bending moment data (provided by the former contractor) or to recalculate the wind and seismic bending moments from scratch. Should you decide to recalculate these external bending moments due to wind and seismic forcing, refer to the relevant Australian Standards (AS-NZS 1170.2-2002 & AS-NZS 1170.4-2002 ) uploaded on LMS.

Alternatively, you can use the guideline document (also uploaded on LMS) that provides a different method for computing wind & seismic load based on the Pressure Vessel Handbook (Megyesy, 2001b).

About V2-1979, Originally designed in 1977, fully fabricated in 1978, and installed onsite by 1979. The vessel contains a mixture of Liquefied Petroleum Gas (LPG) with a density around 490kg/m3.

According to the available drawing of V2-1979, the nominal thickness of the skirt is 16mm. However, one of the most experienced engineers of the plant who had been involved in the manufacturing of the vessel tells you that the nominal thickness is around 24mm. While the original thickness at the time of fabrication remains unclear, the current thickness measurements ofthe skirt are available (as shown in Figure 1, 2, and 3.)

Part A

Questions that you need to answer in your theoretical analysis.

[a] Determine all the stresses that are acting on the shell and its support (the skirt). Draw a free body diagram for all components of the vessel.

[b] Calculate all the stresses identified in the previous part. Tabulate your final answers in SI units.

[c] Make a comparison between the hoop and longitudinal stress at different sections of the vessel. Explain which one is greater in each section. Why?

[d] Suppose the vessel is empty. Redo part a to c. Assume the hydro-test condition (the vessel is filled with water). Redo part a to c. Compare and contrast the results. What is the main difference?

[e] What is the minimum acceptable thickness of the skirt under the operating condition of the vessel? Assuming no significant change in the operating condition, how the skirt’s thickness will change in the next 15 years. Plot the thickness reduction trend as a function of time (year). Show all the steps clearly, and state any assumptions made.

[f] What is the minimum acceptable thickness of the shell, top and bottom heads? Plot the cumulative  hoop and longitudinal stress as a function of thickness for each component and explain the position of the minimum thickness point on the graphs, what do they physically mean?

[g] As a result of wind, the V2-1979 vessel develops vibration. What is the maximum allowable period of vibration? Is the vessel safe from a vibration point of view? (Hint: Use the guideline document on LMS to find the relevant formula).

[h] Your senior suggests that this pressure vessel was not stress-relieved during manufacturing. How does this impact your analysis? How can you confirm if this claim is true? What changes would you  make in your stress analysis, and what is the path forward? (2 marks)

[i] If you were to introduce thermal stresses, how would that change your theoretical analysis?

[j] Suppose the initial thickness of the skirt is 12mm. How would that change the remnant life of the vessel?

[k] List any relevant sources of error that could have impacted your calculation.

Part B

The full configuration of a horizontal pressure vessel with hemispherical end caps is shown in Figures 10, 11, 12 and 13. All dimensions are provided in the figures, except the wall thickness, which is 20 mm. All components are made of steel with yield strength, σy = 620 MPa, Young’s modulus E =  210GPa, Poisson’s ratio, µ = 0.29, and density, ρ = 7800kg/m3 are welded onto the cylindrical pressure vessel. 1. (Analytical Study) Questions that you need to answer in your theoretical analysis:

[a] Determine the yield pressure if the pressure vessel is considered to be thin walled.

[b] If it is not considered thin-walled, determine the yield pressure. Is the difference between this and  the yield pressure determined using the thin-walled assumption (part 1) significant? [c] As pressure is increased beyond the initial yield point, more and more of the wall of the pressure vessel will yield. Determine the pressure required to yield the vessel to 5mm.

[d] Assuming the steel becomes perfectly plastic after yield, determine the residual stress in the pressure vessel walls.

[e] A thick-walled pressure vessel of similar dimensions to that considered here has been sent to the surface of the planet Venus as a container for automated expedition equipment. Sufficient time has   elapsed such that the vessel has expanded due to the high temperatures on Venus. The vessel is now exactly the same dimensions as that given in the figure below. (Ignore any stresses due to thermal expansion and any other temperature-related effects). A valve is then opened and the pressure inside   the vessel equalizes with the atmospheric pressure on Venus. Calculate the hoop stress in the walls of the pressure vessel and clearly state your assumptions and/or relevant data sources.