October 2024

Maintenance, Reliability and Inspection

Examination of design temperature allowance on creep range piping

This article delves into the effects of providing standard design temperature allowances for piping systems operating in the creep range. The aim of examining this practice is to enhance the understanding of its implications for pipe engineering. 

IP: 3.140.197.140

Process plants, including refineries, petrochemical facilities, fertilizer production plants and thermal power stations, rely on numerous high-temperature fluid systems. These systems may contain piping materials that operate within the creep range specific to each material system. 

The selection of fluid system design temperature typically falls under the purview of the process engineering department. Engineering companies’ process departments sometimes provide a standard allowance above the maximum operating temperature. This temperature, with allowance added, is specified in the line list issued by process engineering as the design temperature. Alongside design pressure, the design temperature serves as a critical input for subsequent piping engineering tasks, such as pipe thickness calculations and thermal pipe stress analysis. 

This article delves into the effects of providing standard design temperature allowances for piping systems operating in the creep range. The aim of examining this practice is to enhance the understanding of its implications for pipe engineering. 

This article explores the approach provided by Appendix V of American Society of Mechanical Engineers (ASME) B31.3 to calculate the utilization factor for the thickness considering an operating case of 520oC and various excursion temperatures.¹  

EVALUATION CASE 

In this evaluation case, offsite (ASME B31.3 scope), high-pressure [high-high pressure (HHP)], very high pressure (VHP) and ultra-high pressure (UHP) steam systems were considered with the following specifications: 

• Pipe size: 16 in. 

• Construction: Seamless 

• Material: ASTM A335 P91 

• Length: 200 m 

• Operating hours: The system is expected to operate for 200,000 hr throughout its lifespan. 

System parameters: 

• Design pressure: The design pressure for the high-pressure steam system is 110 kg/cm²g or 10.79 mega-Pascals (MPag) 

• Normal operating temperature: The system typically operates at 516°C 

• Maximum operating temperature: Process engineering has determined a maximum operating temperature of 520°C 

• High-temperature alarm: The alarm triggers at 524°C 

• The high-excursion case for stress analysis is 530°C. 

A reputable engineering company follows a standard practice of providing an additional 28°C allowance over the maximum operating temperature. Consequently, the company selected a design temperature of 548°C and incorporated it into the line list. 

Based on this design temperature, the pipe engineering department performed material specifications, including material selection, thickness calculations and subsequent stress analysis. By examining this case, the implications of such design temperature allowances on piping systems operating in the creep range were explored. 

EVALUATION APPROACH 

In the context of a 16-in., 110 kg/cm²g high-pressure steam system with a design temperature of 548°C, the piping department initially selected 16-in. seamless P91 piping. However, since this falls within the creep range, the impact of designing the system without providing the standard temperature allowance during selection was explored. The system will also be evaluated for the following alternate cases: 

• 110 kg/cm²g and 516°C  

• 110 kg/cm²g and 520°C  

• 110 kg/cm²g and 524°C  

• 110 kg/cm²g and 530°C. 

Over the total 200,000 hr (22.8 yr of operation), the following temperature scenarios were anticipated: 

• 180,000 hr at the 516°C operating temperature 

• 15,000 hr at the 520°C maximum operating temperature 

• 4,950 hr at the 524°C high-temperature alarm point 

• 50 hr during an assumed high excursion at 530°C. 

This assumed line operation aligns with present-day control loop practices, where the control set point temperature is 516°C. Additionally, fluctuations in the control system may result in some operation outside this set point. While the total failure of the steam generation was not considered, this evaluation approach should represent actual operational parameters. Malfunctioning of the attemperator system is accounted for as high-temperature excursion operation at 530°C. 

EVALUATION METHODOLOGY 

In this analysis, pipe thickness calculations were performed for the following scenarios: 

• Material: ASTM A335 P91 (UNS K90901) 

• Design temperature: 548°C 

• Additional cases: We will calculate thicknesses for the four additional cases: 

• • 110 kg/cm²g and 516°C 

• • 110 kg/cm²g and 520°C 

• • 110 kg/cm²g and 524°C 

• • 110 kg/cm²g and 530°C. 

• P22 material case (520°C): The impact of using ASTM A335 P22 (UNS K21590) material at 520°C was also evaluated. 

Next, the principles outlined in “Appendix V: Allowable variations in high-temperature service” were applied to calculate the usage factor for the thickness if the temperature was 520°C. This involves comparing the calculated thicknesses against the design thickness. 

If the utilization factor remains below 1, with the thickness calculation based on 520°C, it suggests that selecting 110 kg/cm²g and 520°C as the design case for P91 piping thickness would have been sufficient. 

The section below explores the potential benefits that may be realized by adopting a more precise design approach over traditional margins. 

Pipe thickness and utilization factor calculation. In this analysis, the use of P91 material for a high-temperature system was considered. Initially, the design temperature was set at 548°C, resulting in a calculated thickness of 22.50 mm and selected thickness (per ASME B36.10) of 23.83 mm (TABLE 1).

However, an alternative scenario can be explored here: 

A design temperature of 520°C: 

• By assuming a lower design temperature of 520°C, the calculated thickness is reduced to 19.35 mm and a selected thickness as per ASME B36.10 of 20.62 mm 

• By applying the principles from Appendix V of B 31.3, the usage factor for this case was evaluated. 

After considering various temperature excursions, it was determined that the utilization factor remained well below 1 (TABLE 1). This implies that selecting 520°C (or even 516°C) as the design temperature would have been sufficiently conservative. The process engineer could have specified 520°C or 516°C as the design temperature, with the excursion to 530°C reserved for thermal stress analysis purposes, if required. 

Next, attention was focused on P22 material, and similarly, the utilization factor was calculated. When considering 520°C as the design temperature, the resulting thickness was 37.55 mm and the selected thickness as per ASME B36.10 was 40.49 mm. This is substantial but not excessively high compared to what would have been encountered if 548°C had been initially chosen as the design temperature. 

Had a 548°C design temperature been selected for P22 piping, the pipe thickness would have become 49.39 mm and the selected pipe thickness as per ASME B36.10 would have become 50 mm. 

A more precise design temperature approach for time-dependent creep range material decreases the pipe thickness when the original selected material of p91 is used. If an alternate material of P22 is considered, there is about a 20% decrease of thickness if the more precise design temperature is adopted. The different design temperature approaches can have a significant effect on wall thickness and costs (TABLE 2). 

Takeaway. Avoiding excessive thickness due to the traditional design margin approach results in savings on pipe material costs. The following are the potential benefits that may be realized by adopting a more precise design approach over traditional margins: 

• Lowering the margin in design temperature may lead to a more cost-effective materials selection 

• Lower pipe thickness translates to a lighter load on the pipe structure 

• Thinner pipes require less welding, leading to fabrication cost savings 

• Specifying a realistic stress analysis temperature ensures more accurate thermal stress assessments, avoiding unnecessary flexibility 
• Using lower thickness can help optimize nozzle loads for equipment. 

LITERATURE CITED 

¹ ASME, “B31 - Process piping,” 2023, online: https://www.asme.org/codes-standards/find-codes-standards/b31-3-process-piping 

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