Pressure vessels are weakened when nozzles must be introduced, and reinforcement is required to compensate. Presented here are two methods for calculating the necessary reinforcement
When pressure vessels are penetrated for introducing nozzles, the vessel is weakened and must be reinforced to compensate for the weakening. Pressure-vessel designers generally have two methods at their disposal to calculate the necessary reinforcement: the traditional “area replacement method,” whose origins can be traced back to the ASME Boiler and Pressure Vessel Code (BPVC) [1]; and the “pressure area method,” which has been successfully applied in European countries for many years. Appendix 46 of the ASME BPVC Section VIII – Division 1 now offers the possibility to also apply the pressure area method. In some cases, this approach can lead to a more economical design without compromising structural integrity. This article provides information on two methods for compensating for the weakening of pressure vessels due to nozzle penetration.
Reinforcing nozzle connections
All pressure vessels must be furnished with nozzles and connections to interconnect the pressure vessel with the rest of the process facility. To attach a nozzle in a pressure vessel, it is necessary to make a hole in the shell or head body in question. When making this hole, an area of the vessel is being “taken away.” Therefore, stress paths are going to divert tangentially to the hole, and because of this, the removed area must be “substituted” by adding reinforcement within the limits.
Installing a nozzle in a pressure vessel involves a geometric discontinuity that must be considered in calculations. Consequently, every method that could be implemented to reduce existing stresses is welcome. One of the most used methods consists of removing stress concentrators by means of rounding sharp edges. However, an opening in a shell caused by placing a nozzle on it causes a weakening and consequently a disturbance of the membrane condition. The resulting weakening and disturbance of the membrane condition in the vicinity of the nozzle-shell intersection should be remedied by adding an appropriate amount of reinforcement material. The following reinforcement material options might be considered: introducing a thicker nozzle neck, a thicker shell and a reinforcing pad, or some combination of the two. In principle, there are two methods for determining the required reinforcement to compensate for the weakening of a nozzle in a shell. These are the “pressure-area method” and the “area-replacement method.”
The principle of the pressure area approach is illustrated in Figure 1 for a flush set-on and set-in nozzle configuration with a reinforcing pad on a cylindrical shell. Here, a certain area of the vessel in the region of the opening is multiplied by the design stress and is equated to the cross-sectional area of the vessel in the same region. In turn, this is multiplied by the pressure. This is a simplified limit-load-type approach with the yield stress factored into the design level (yield stress / 1.5).
FIGURE 1. The pressure-area design method for calculating required reinforcement when a nozzzle is introduced to a pressure vessel
The equation to be satisfied is shown in Equation (1):
Pressure (Aps + Apb) = Yield stress (Afs + Afp + Afb + Afw) (1)
The basic principle of the area compensation/replacement method is when the opening is cut in the pressure vessel, an area is removed from the shell and head. It must be reinforced by an equal amount of area near the opening. The area removed should be equal to the area added. This approach is illustrated by Figure 2 for a flush set-on nozzle configuration with a reinforcing pad. Complete details can be obtained from Figure UG-37.1 [1] (Nomenclature and Formulas for Reinforced Openings).
FIGURE 2. The area-replacement method replaces the area of the nozzle cutout with a reinforcing pad
Required area:
A = d ∙ tr ∙ F + 2tn ∙ tr ∙ F (1 – fr1)
Equation to be satisfied for flush nozzle: A1 + A2 + A41 + A5 ≥ A
It should be noted, however, that when calculating the areas, all the corrosion, erosion and manufacturing under-tolerances must be considered. If a reinforcing pad is applied, specific attention should be paid to paragraph UG-37 (g);(h) [1].
The area replacement method is discussed in detail in paragraph UG-37 of ASME BPVC Section VIII-Division 1, while the pressure area method is discussed in a number of European codes, including EN 13445 (EU) [2], PD 5500 (UK) [3], AD 2000 (D) [4], CODAP (France) [5] and RToD (the Netherlands) [6]. In the meantime, ASME BPVC Section VIII-Division 1 has incorporated the Code Case 2695-1 [7] as the mandatory Appendix 46 (Rules for use of Section VIII, Division 2). Appendix 46 refers to Division 2, section 4.5 regarding nozzle design, where the pressure area method is also used. With the recent addition of mandatory Appendix 46 in ASME VIII-1, designers can now take advantage of the modern design rules from ASME VIII-2 [8]. In situations where a nozzle does not pass using traditional ASME VIII-1 rules, designers can use the option of using the more accurate provisions of Appendix 46. Regarding the pressure area method, one should be aware that the boundary limits may differ slightly from each other code or standard.
Comparisons
To gain more insight into the differences between nozzle designs using the “pressure area” and the “area replacement” method, the maximum allowable working pressures (MAWPs) have been calculated in Tables 1, 2 and 3. The tables clearly show the differences in the calculated MAWPs as a result of the respective calculation method applied. The tables also include the results obtained by applying numerical finite element analysis (FEA), which may be assumed to provide the most realistic results. In addition, Tables 1A, 2A and 3A contain the results obtained using NAM specification NSS 12-D-4-05 [9], which is based on numerical (FEA) analysis. The calculated MAWPs are closely related to the maximum allowable stress intensity at the nozzle-shell junction, which typically lies between 1.5 and 3 times the allowable stress.
NAM (Nederlanse Aardolie Maatschappi) is a Dutch exploration and production company that is a joint venture of Shell and ExxonMobil Corp. More information about NAM’s pressure vessel specifications can be found in Ref. 9. NAM’s core business is exploring for and producing oil and gas, both onshore and offshore in the Netherlands.
Table 1 shows both the design basis and the calculation results of flush set-in nozzle configurations without a reinforcing pad on a cylindrical shell.
Observing Tables 1A, 2A and 3A, we can assume from practical experience that if the stress level for the primary local membrane does not exceed 1.5 times Sm (or Sy), there is ample margin available for stresses due to piping reactions acting on the nozzle, provided that the primary plus secondary stress level known as elastic shakedown limit of 3Sm or 2Sy is met.
FIGURE 3. The graph presents the MAWPs related to the data in Table 1
The yield stress values for the materials used in the considered nozzle configurations are as follows:
• Material A 515 Grade 65: Sy = 177 MPa
• Material A 106 Grade B: Sy = 177 MPa
• Material A 105: Sy = 183 MPa
FIGURE 4. The graph presents MAWPs related to Table 2
Impact on design choices
The analysis of MAWPs calculated using the pressure area method and the area replacement method reveals significant differences that impact design choices. The results indicate that while the pressure area method is often perceived as more economically advantageous, this is not consistently reflected in the calculated MAWPs across various nozzle configurations.
Figures 3, 4 and 5 illustrate the variability in MAWP values, with the area replacement method generally yielding lower values compared to the pressure area method. This trend suggests that the traditional ASME design method prioritizes safety, resulting in a more conservative approach that may necessitate additional reinforcement material. The proximity of the pressure area method results to those obtained through finite element analysis (FEA) indicates that this method can provide a more realistic assessment of nozzle performance, although it may not always offer the same level of safety assurance.
The erratic trends observed in the graphical representations highlight the discrepancies between the methodologies, with some configurations showing differences of up to almost 1.5 times greater values depending on the method applied. This variability underscores the importance of selecting the appropriate design methodology based on specific project requirements, including safety, cost, and regulatory compliance.
Furthermore, the economic implications of these findings cannot be overlooked. While the pressure area method may appear to reduce material costs, the potential trade-offs in safety margins must be carefully considered. Designers are encouraged to evaluate the specific needs of their projects and consult with inspection bodies to ensure compliance with industry standards.
FIGURE 5. The graph presents MAWPs related to Table 3
Concluding remarks
This article examined the methodologies available for compensating for the weakening of pressure vessels due to nozzle penetrations, focusing on the differences in maximum allowable internal pressures for various configurations. The analysis demonstrates presented here that while both the area replacement and pressure area methods can yield safe designs, the choice of methodology can lead to substantial differences in calculated MAWPs.
Despite the assumption that the pressure area method is more economically attractive, the results indicate that the area replacement method often provides greater safety margins, albeit at the cost of requiring more reinforcement material. The findings emphasize the necessity for designers to be aware of the implications of their chosen methods and to make informed decisions that align with safety standards and project requirements.
Ultimately, the responsibility lies with the designer to select the appropriate method, ensuring that their choice is well-coordinated with the inspection body or notified authority. The ongoing evolution of design codes and methodologies necessitates a thorough understanding of the available options to achieve optimal safety and efficiency in pressure vessel design.
Edited by Scott Jenkins
References
1. American Society of Mechanical Engineers, ASME Boiler and Pressure Vessel Code (BPVC), Section VIII-Division 1, 2023, USA.
2. British Standards Institution (BSI) EN 13445-3:2021; Unfired pressure vessels – Design (EU).
3. European Standard (CEN), PD 5500: 2024; Specification for unfired pressure vessels (UK), 2024.
4. German Pressure Vessel Association, AD 2000-Merkblätter: 2024; Code of practice for pressure vessels and other pressure equipment. (D)
5. CODAP (Construction Code for Unfired Pressure Vessels), Code Français de Construction des Appareils à Pression, SNCT Publications, France.
6. Regels voor Drukvaten, RToD, Rules for pressure vessels, the Netherlands.
7. American Society of Mechanical Engineers (ASME), ASME Code Case 2695-1; Allowing Section VIII, Division 2 Design Rules to be used for Section VIII, Division 1 (USA).
8. ASME BPVC Section VIII-Division 2: 2023 “Alternative Rules” (USA).
9. Nederlandse Aardolie Maatschappij (NAM), NSS 12-D-4-05 “Pressure Vessels” Supplements to the Rules for Pressure Vessels; August 1998.
Further reading
Stikvoort, W., Pad-Reinforced Nozzles in Pressure Vessels, Chem. Eng., October 2024, pp. 34–36.
Stikvoort, W., Piping Reactions on Pressure-Vessel Nozzles, Chem. Eng., pp. 51–53, July 1986.
Sitkvoort, W., Pressure Vessel Design, Nozzles Piping Reactions, “Encyclopedia of Chemical Processing and Design,” Volume 42 237–244, Marcel Dekker, Inc., CRC Press. N.Y., 1992.
Stikvoort, W., Review of traditional tubular pad-reinforced nozzles versus Long Welding Neck Flange nozzles in pressure vessels, Journal of Research in Mechanical Engineering, Volume 5-Issue 1; pp. 1–7, 2019.
Stikvoort, W., Effect of contributing shell and nozzle length on pressure capacity, American Journal of Engineering Research (AJER) e-ISSN: 2320-0847 p-ISSN:2320-0936, Volume -9, Issue-12, pp: 49–55, 2020.
Stikvoort, W., Overview of different approaches for completing the nozzle design of pressure vessels, International Journal of Engineering Development and Research (IJEDR), 2018, Volume 6, Issue 4 ISSN: 2321-9939.
Stikvoort, W., Effectiveness of reinforcement plates pertaining to pressure equipment” American Journal of Engineering Research (AJER), Volume-10, Issue-8, pp-127–146, 2021.
Kacmarcik, J. and Vukojevic, N., Comparison of design methods for opening in cylindrical shell under internal pressure reinforced by flush (Set-on) nozzles, “Proceedings of the 15th International Research / Expert Conference of Trends in the Development of Machinery and Associated Technology,” Prague, Czech Republic, 2011.
Acknowledgement
The authors would like to express their sincere thanks to Keith Kachelhofer (MacAljon Fabrication / MacAljon Engineering, USA) for his constructive and supportive contributions to this article.
Authors
Walther Stikvoort (Email address: stikvoort@ziggo.nl) is a renowned authority in the field of mechanical and structural integrity of static pressure equipment. He has more than 50 years of experience in pressure vessel and piping design and has developed numerous technical standards and practices to improve the asset integrity of leading operating companies. He is the author of numerous peer-reviewed international journal articles in the field of mechanical and structural integrity. During his career, he was regularly active in developing and teaching courses and training to mechanical engineers in his area of expertise and he was a member of various expertise committees. He is currently active as a consultant on static pressure equipment integrity serving the engineering community.
Farzad Gardaneh (Email address: fa.gardaneh@gmail.com) With over a decade of experience in the oil and gas industry, he specializes in the design, strength calculations, and production of detailed drawings for stationary equipment. In addition to his primary work, he has contributed as a co-author to several published articles. His research primarily focuses on design optimization, where he engages in the exploration of industry standards and comparative analyses to uncover innovative approaches to enhance efficiency and performance.