May 2023

Digital Technologies

Real-time monitoring of creep damage for fired equipment tubes, employing advanced web platforms

Within the petrochemical industry, fired heater tubes and boilers experience high tube metal temperatures (TMTs) for extended periods of time.

Lindblade, M., Ray, C., Lee, H. S., ExxonMobil Technology and Engineering Co.

Within the petrochemical industry, fired heater tubes and boilers experience high tube metal temperatures (TMTs) for extended periods of time. The dominant failure mechanism is often creep damage, which can cause a tube rupture, even when operating under design limits. Since creep is a function of stress, temperature and time, it must be evaluated at regular intervals to predict the equipment’s expected end of life (EOL).

Engineering calculations are performed to estimate the current creep life consumed and to predict the future creep consumption rate based on expected future operating conditions. The objective is to make risk-based decisions on when to proactively replace the heater or boiler tubes, and to determine if the current TMT limit can be extended for future operation to maximize benefit.

Remaining creep life evaluations can be performed using commercially available software or internal tools developed by the end user, such as an Excel spreadsheet. However, these methods require large amounts of data processing and formatting for input to the tools.  Another issue is that this evaluation is typically performed before a planned turnaround, potentially without adequate time to make proper decisions. If operating conditions have changed since the evaluation, then using the results to estimate creep consumption between the time of the evaluation and the turnaround could result in the overestimation of creep life consumed, leading to either a premature re-tubing or leaving margin incentives on the table. Conversely, if the creep consumption rate is underestimated due to inaccurate assumptions or different operating conditions, a premature tube failure could occur.

To effectively address these shortcomings, an advanced web application has been implemented that eliminates the need for manual data processing and iterations of creep calculations. It is possible to monitor the remaining creep life of existing tubes in real time and to predict future creep consumption at various operating scenarios as desired.


While design codes [e.g., American Petroleum Institute (API) 530] address creep rupture, the methodology to evaluate creep life consumption for tubes in fired heaters and boilers once placed in service is detailed in Part 10 of API 579-1/American Society of Mechanical Engineers (ASME) FFS-1. In these procedures, which are widely accepted and used throughout the industry, the user must develop a load history consisting of the tube thicknesses or corrosion rate, TMTs (or infrared scan results), operating pressures, and number of operating cycles. For each segment (nt) of a particular cycle in the load history, a creep damage rate is calculated and summed together to determine the total creep life consumed. This is shown in Eq. 1, and is from API 579 Part 10:

The two common methodologies for calculating remaining creep life, represented by nL in Eq. 1, are MPC Project Omega and the Larson-Miller parameter (LMP). Either of these methodologies can be implemented into the web application tool. Once the total creep life consumed () for a cycle is determined, the user must sum all cycles’ creep damage and verify that the following equation is satisfied, where Dcallow is typically set to 1, unless an alternative value can be justified (Eq. 2):

This process is a summary of the Level 2 assessment in API 579-1 Part 10.

Developing the load history for the calculations is frequently the most time-intensive portion of the analysis. This usually involves reviewing inspection history to determine the corrosion rates, obtaining the measured TMT values (or infrared scan results) over long periods of time, and either obtaining or calculating the operating pressure in the tubes being evaluated.

If there is missing data in the load history, the user must make assumptions and manually insert values based on engineering judgment. Improper assumptions can significantly affect the accuracy of the results, leading to either underestimation or overestimation of creep failure risk. It is also important to check for bad or erroneous data, and to manually change the values as appropriate to increase the accuracy of the analysis.

Once the data for the load history has been formatted and inputted into the calculation tool, the creep life consumed is calculated, as shown in the previous Eqs. 1 and 2. These results can be used to estimate what the future creep life consumed will be if the operating conditions and assumptions are still applicable. However, the tubes will lose thickness due to corrosion or oxidation, and the TMTs can increase due to fouling/coking, resulting in an increased rate of creep life consumption. Therefore, if the tubes are approaching the end of their creep life, calculations may be required on a more frequent basis. This requires updating the load history and pulling more recent temperature and pressure data. The intent of re-running the creep life calculation near EOL is to determine if the tubes can survive an extra turnaround cycle or if replacement is required at the next turnaround. It is also important to note that creep life consumed grows exponentially as corrosion increases and/or TMTs increase.

Online monitoring

Developing the load history for creep life consumption calculations can be very time consuming, as it requires downloading and processing large amounts of operational data. When using commercially available software, the data also must be formatted for entry.

Another limitation in the commercially available software is the lack of flexibility in simulating various future operating scenarios (e.g., pressures, TMT profiles, corrosion rates). Developing and using a manual internal tool may reduce this burden, but the user must still format data from the operational history to input into the creep life evaluation tool. Making this manual internal tool user friendly can be complex and challenging.

To provide real-time creep life monitoring, a C#-based web application has been developed utilizing Microsoft technologies, such as the Blazor app and the Azure cloud. Users input tube properties and corrosion rates, which are calculated or estimated based on inspection history like other creep evaluations. Specific TMT sensor data tags are selected to represent temperature and pressure values over time. The time-series sensor data is stored in a data repository and provided to the program by an application programming interface. With these inputs, an automated tag can be generated to continuously calculate the creep life consumed.

Users can override the measured values from the program to correct erroneous data, and warnings are provided to help users know when it is necessary to correct invalid or inappropriate conditions. As the user changes inputs (FIG. 1), the program recalculates and displays creep life in real time and provides instant feedback for analysis (FIG. 2). Since this tag continuously displays the creep life consumed, engineers can trend the creep rate growth to better predict EOL for the tubes. This allows operators to visually see how fast creep life is consumed when adjusting the firing rate or to determine when tubes start to experience significant fouling or coking.   

FIG. 1. An example of tool inputs for the online monitoring of creep damage.
FIG. 2. Screenshot from the online tool, displaying the creep damage for two different TMTs.

In addition to monitoring creep life in real time, an additional feature of the tool allows the functionality to develop different future TMT and pressure profiles for simulating future operations. These simulations can help provide a planning basis of when to re-tube the heater or boiler. If creep life calculations indicate that the tubes will reach EOL between the next two turnarounds, the TMT targets can be pushed to a higher temperature to squeeze additional margin before the tubes are replaced at the next turnaround. Alternatively, TMT targets could be lowered to extend the life of the tubes to the second turnaround, depending on the economics. In conclusion, heater or boiler operations can be optimized between margin incentives and reduced creep life when modeling future operations.


Numerous benefits for linking measured operating data to a continuous creep life calculation can be realized through a web application program. One benefit is that operators and process engineers can visually monitor how increased firing and higher TMTs affect the rate of creep life consumed in real time. Optimizing equipment operation is feasible, considering increased unit incentives and costs associated with reduced tube life. 

Another advantage is that the average creep consumption rate can be used over certain periods of time to predict when the heater or boiler will require a re-tubing. While manual methods also work, having a real-time tag effectively optimizes when the re-tubing procedure is planned to eliminate the financial risk of prematurely replacing tubes. This tool can considerably help users strike the correct balance of optimizing tube life and minimizing safety/reliability risks prior to the fired heater and boiler tubes’ EOL. HP

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