When designing a new plant, it is important to consider the costs associated with the day-to-day operations of each unit, and not just initial purchase cost (i.e., the cost of the asset). The low initial cost of a unit might be offset by a high maintenance cost or an even higher expected risk cost (i.e., additional costs associated with the chance of failure). When multiple designs are available as possible choices, the comparison of the designs must consider all costs based on each design’s predicted performance throughout the life cycle in order to determine the best design for the particular environment. The following example, based on the analysis provided in API RP 581 1, showcases the ability to compare designs of heat exchanger tube bundles using ReliaSoft’s RBI software. Note that a similar methodology can also be used for other pressurised components.
When designing a heat exchanger, the proper choice for the tube bundle is critical, as any inspection or maintenance of the bundle requires the total shutdown of the heat exchanger. This is unlike the shell, which can be inspected from the outside using various techniques, such as ultrasound.
In this example, the heat exchanger in question will be used to cool one of the process fluids within an oil refinery. The design engineers have three types of tube materials they are considering for the tube bundle. Fortunately for them, there is life data available for each type of material used for this application. They can use this information to generate reliability models for the analysis. The analysis must also take into account that only one shutdown is allowed for tube bundle inspection (and tube plugging) between the startup of the plant and the scheduled maintenance overhaul ten years later. Finally, management has a strict rule that the risk cost associated with the running of the heat exchanger cannot exceed $500,000 at any point.
The first step is to input the life data for each tube bundle type into Weibull++ and perform a life data analysis in order to fit a distribution and obtain a reliability model, as shown next for the carbon steel material.
The engineer then publishes the model, making it available for use in other applications within the Synthesis Platform.
The engineer repeats these steps for each material type, resulting in three published reliability models.
Once the reliability models have been created, the engineer can create the components in the RBI software. He opens the same project within the RBI applicationand creates a system called “Initial Tube Cost Estimates.” He adds a heat exchanger to this system, then adds a HEXTUBE component to it. This first hextube will be used to estimate the initial component (part) cost associated with one of the tube bundle types — in this case, carbon steel.
For this cost, the engineer does not want to take into account the risks and costs involved with a failure during operation (downtime, clean-up, spare part, etc.). The properties that are taken into account at this stage include the material type and the dimensions of the shell and tubes.
The engineer assigns the reliability model created in Weibull++ to the hextube component. He chooses the installation date and the plan date in such a way as to conform to the requirement of being 10 years apart. In order to not take risks and failure costs into account, he keeps the Percentage of Production Loss Due to Bypass, Environmental Cost and Maintenance Cost properties set to 0. The RBI Properties tab looks like this:
To create the components to perform the analysis for the other two material types, he simply copies this component and then changes the name, bundle material and reliability model for each copy. Once he has created all three components, he can view the estimated component cost (initial part cost) for each material in the TOTAL Financial Consequence field on the Results tab, as shown next for the carbon steel hextube.
To introduce the costs involved with a failure (part replacement, downtime, clean-up, etc.) and calculate potential risk, the engineer copies the “Initial Tube Cost Estimates” system and renames it to “Overall Process Risk,” as shown next.
In this new system, he changes the properties previously kept at 0 to reflect the actual costs, as shown next for the carbon steel hextube.
After the engineer has changed the properties for all three hextube components, he checks the recalculated results. The software automatically calculates the date at which the risk reaches the maximum allowed risk (chosen by management) and sets that date as the target date for inspection.
The results for the carbon steel hextube are:
These results indicate that all three material types would require an inspection between the startup date and the scheduled maintenance date. The tube bundle made of carbon steel would exceed the maximum allowed risk of $500,000 (per company policy) even with the inspection (as shown in the Plan Date with Inspection Results > Financial Based Risk field), so that option can be immediately discarded. The risk difference between the 304 and 316L stainless steel is about $73,000; however, the overall cost difference, including component cost and risk, is almost $400,000.
1 American Petroleum Institute. API RP 581, “Risk-Based Inspection Technology,” 2nd edition. September 2008.
Source: Reliability Hotwire, issue 161 Reliasoft