P. R. Gajapathy, Fluor Daniel, Manila, Philippines; A. IBARRA, Fluor Corp., Batangas, Philippines; and R. SANKARAN, Fluor Daniel, Gurgaon, India
In the current market scenario, all projects are schedule-driven and highly dependent on total installed cost. To help meet crucial deadlines, industry professionals utilize various technologies and high-end software, and industry must continuously innovate and seek new ideas to find alternative solutions to challenges. In this article, the authors detail a case study to show how low-temperature carbon-steel (LTCS) material can be used instead of stainless-steel materials, while adhering to all code regulations. This pathway can provide significant savings on time and money.
In a refinery, pipes can experience extreme cold temperatures. One solution is to use stainless-steel pipes, although it is not always required.
In this study, a scenario is described where a vessel with a pressure relief valve to protect the system against overpressure experienced a blowdown process condition to reduce the contaminants and remove hazardous inventory from the vessel. The blowdown of process equipment may result in significantly reduced temperatures—ranging from –37°C to –82°C—in the equipment, piping and through the flare system, which may lead to pipe brittleness and high thermal stress. The piping material used from the vessel to the inlet of the pressure safety valve (PSV) was stainless steel [American Society of Mechanical Engineers (ASME) A312-TP316L], while the discharge up to the flare header—including the bypass—was LTCS (ASME A333-6). ASME B31.3, “Process Piping,” says that stainless steel can withstand low temperatures to –254°C, while temperatures below –46°C in LTCS may lead to failure.
ASME B31.3 paragraph 323.2.2 provides a method for reducing the minimum temperature of LTCS even further for carbon-steel materials. For temperatures below –46°C, chart 323.2.2B in ASME B31.3 can be used to determine the further reduction in the minimum temperature, and whether a material can be used below its rated minimum temperature without impact testing.
This system was analyzed using proprietary pipe softwarea to check whether carbon steel (particularly A333-6) is still acceptable. The solution for this study was to revise the piping configuration and provide support and flexibility to reduce stress, as per code requirements.
Study parameters and goals. While carbon steel is one of the strongest materials on Earth, it is still susceptible to the effects of cold temperatures. Exposure to low temperatures can make carbon steel more brittle and prone to failure. However, ASME B31.3 provides a way to check if LTCS pipe can be used below the rated temperature without impact testing.
The following example is from an upstream project the authors’ company worked on. During design development, as per requirements of the original equipment, the material for the piping up to the inlet of the PSV was classified as stainless steel and LTCS on the tailpipe. The equipment being protected by the PSVs was subjected to a blowdown scenario where the temperature reached –82°C. As per ASME B31.3, stainless steels can withstand temperatures as low as –254°C, whereas carbon steel’s low-temperature thresholds range from –29°C (A106-B) to –46°C (A333-6). In the authors’ company’s project, the carbon steel used had a rating of A333-B (i.e., –46°C temperature threshold).
The primary intention of this study was to analyze the lowest temperature carbon steel can withstand before having to change to stainless steel. This study also showed how pipes react to blowdown conditions from temperatures ranging from –37.6°C to –82.1°C. The authors also plotted the stress ratio and temperature reduction using the chart and diagram in paragraph 323.2.2B of ASME B31.3 without impact testing.
Specifications and standards. FIGS. 1–3 are references used to evaluate low temperatures for carbon steel—all figures are excerpts from ASME B31.3. As per point (g) in Table 323.2.2 of ASME B31.3, impact testing is not required if the designed minimum temperature is −29°C or warmer than −104°C, the stress ratio defined in FIG. 4 does not exceed 0.3, and the following conditions apply:
If these points are not met, stainless steel should be used. The stress ratio is defined as the maximum of:
Interpretation of the data. The layout of the materials used in the system is shown in FIG. 5. In this system, the vessel experienced negative temperatures during blowdown. Process data of the system shown in FIG. 5 is detailed in TABLE 1. As the process indicated, during a blowdown scenario, PSVs are on stand-by mode while the bypass is open.
Within the proprietary pipe softwarea, the authors combined sustained and displacement stresses. Upon obtaining the results, the authors added an additional user code combination to the combined sustained and expansion stresses (FIG. 6). Note: The ambient temperature was 0°C, and the code used was ASME B31.3 2020 edition.
Worst-case scenario: Thermal case 8 (–82.17°C at bypass temperature downstream). FIG. 7 shows an illustration of Thermal Case 8. The orange-colored pipe reached temperatures lower than 82°C. FIG. 8 shows the stresses for Case 8—this system has 104.5-N/mm2 stress [(sustained + expansion (sus + exp)] at the tee connection. This illustrated that the highest stress occurs at the fitting or tee connection, which may lead to failure in the future.
This method was applied in all cases. Therefore, the results were plotted using paragraph 323.2.2B of ASME B31.3 to check whether carbon steel A333-6 can withstand the scenarios laid out in all cases, including up to the extremes in Case 8—temperature extremes at the globe valve inlet (–80.8827°C) and outlet (–82.1707°C). The results for all eight cases are shown in TABLE 2.
Using the data in TABLE 2, the authors plotted the stress calculated in Cases 1–8 (FIG. 9). The data was plotted using the temperature reduction vs. the ratio of (sus + exp) stress/allowed (sustained maximum). Note: Stress ratios shown in FIG. 9 were calculated using the combined longitudinal stress due to pressure, dead weight and displacement strain (from ambient to design minimum temperature) given for the process condition—refer to Table 323.2.2B (FIG. 10) for quick identification based on the reduction in exemption temperatures and stress ratio calculation.
In Cases 5–8, the lines intersect outside of the graph. This means that the piping will fail in those cases. Routing cannot be implemented, and an immediate solution is required. The authors focused on Case 8, which is the worst-case scenario regarding low temperatures. To address the issue, the stress ratio must be lowered to 0.48 (dash line) or have combined stresses of 66.2 N/mm2 or lower.
Solution. Several methods can be used to solve this issue for the given blowdown scenario resulting in extreme low temperatures that will be experienced by the entire system.
Reduce stress. Reducing stress is a good option. Considering the requirement of B31.3, the stress ratio should not exceed the ratio of 0.3. Two methods can reduce stress in the bypass part: adding flexibility and/or adding support to the bypass.
For this study, the authors used the option of adding flexibility vs. adding support to the bypass, as the latter may affect the system during the PSV operating case. Additional support on the bypass discharge elbow to reduce sustained stresses will work, as well (FIG. 11). The data for this scenario is detailed in TABLE 3 and the results are plotted in FIG. 12.
This simulation addressed sustained force due to gravity or from contraction, which helped lower the combined stresses to 28.3 MPa. As shown by the green line in FIG. 12, the additional support on the elbow significantly worked. There are cases where adding support may not be possible. If additional support cannot be included, adding additional flexibility for pipe contraction due to negative temperatures is a better option. Out of several routing options, some preferable pathways fall within the required combined stresses of 66.2 N/mm2. The first optional routing pathway (Option 1) is shown in FIG. 13. The combined sustained and expansion stress for Option 1 was 65.4 N/mm2 for Case 8—the combined sustained and expansion stress varied from 54.4 N/mm2–65.4 N/mm2 in Cases 1–8.
For Case 8, a larger loop resulted in lower stresses. However, as the loop progresses, a support is required to compensate for sagging. To fix this, a second option (Option 2) was simulated (FIG. 14). In this scenario, the combined sustained and expansion stress was 34 N/mm2—the combined sustained and expansion stress varied from 30.7 N/mm2–34 N/mm2 in Cases 1–8. The stress ratios of both routing options are shown in FIG. 15.
Takeaway. This work shows that it is possible to use A333-6 carbon steel at extreme low temperatures if the stresses are within the ranges detailed in 323.2.2B of ASME B31.3. This practice can be used when trying to utilize materials outside of their intended parameters. Although the simpler solution is to use stainless steel as per the operating temperature, using LTCS in place of stainless steel is a value addition, especially in terms of cost. HP
ACKNOWLEGMENTS
The authors are grateful for the Professional Publications and Presentation Program (P4) committee of Fluor Daniel Corp. for its continued support. The authors are also thankful to Fluor Corp. for promoting value engineering in its projects and recognizing engineers who have provided valuable ideas. Finally, the authors are also indebted to all their colleagues and friends at Fluor Corp. for their encouragement, guidance and support while writing this article.
NOTE
Pramodh Raj Kumar Gajapathy works with Fluor Daniel Inc. in the Philippines as a Senior Design Engineer. He is focused on stress engineering, pipe supports and piping design. He is involved in numerous projects for major clients around the world in both onshore and offshore applications.
Alben Ibarra is a Piping Stress Engineer for Fluor Daniel Inc. in the Philippines. With more than a decade of experience in piping engineering, he has worked extensively in the energy solutions and oil and gas industries. His proficiency lies in conducting flexibility analyses aligned with the code B31.3 standards, pipe rack layout and design, designing engineered items and support, and quality assessment of various stress systems.
Ranjith Sankaran is a Lead Piping Stress Engineer with Fluor Daniel India Pvt. Ltd. He has more than 18 yr of extensive work experience and has been involved in many critical projects for major clients worldwide. He has robust experience in both ASME and EN codes.