“If I perform a stress analysis on my critical piping systems, isn’t that enough to determine my highest risk locations for inspections?
I cannot tell you the number of occasions over the last 18+ years I have been asked this seemingly simple, yet many times confusing question. You see, as engineers operating in the environment of high stress and high-temperature components, we tend to fall back to our educational experience to provide answers to our questions. In school, when we wanted to know the highest stress area of a component, we would perform a stress analyses to provide that answer to us. However, when dealing with piping systems that have been in operation for many hours and many cycles, we must take other factors into account when determining the highest priority areas for inspection.
Foundationally, a piping stress analysis is typically performed during the initial design and construction of a pipeline system. This analysis is conducted to determine the location and type of hanger supports that will be used to support the piping system over the course of its operational life. Over time, the piping system can be subjected to off design conditions such as a water hammer event, temperature excursions or unit trips. These events can induce stresses not calculated by the original design intentions. Additionally, we must consider that high-temperature piping systems may experience a relaxation of thermal stresses as a result of local yielding or creep. The amount of this relaxation is dependent upon the magnitude of the initial stress levels, the operating temperatures, stress duration and piping material. Observable symptoms of significant stress relaxation in steam piping include spring hangers which exceed their travel ranges, pipe sag, water pockets formation, damage to branch lines, and damage to insulation.
This is one of the reasons why Thielsch Engineering emphasizes the importance of regular and periodic hanger walkdown in both the hot and cold conditions. We believe that hanger walkdowns can lend insight into the health of your critical piping systems. That being said, if the results of your hanger walkdowns reveal that the hangers all appear in good condition, operating in the design range and moving in the correct direction, you have a general sense that system is being supported as designed and therefore a stress analysis is not the wisest choice for your budgetary considerations.
A risk analysis, on the other hand, offers a more strategic approach to determining high risk areas to focus inspections for those piping systems that appear to be operating in an acceptable range for hanger support, but have other unit specific considerations. Nondestructive testing is necessary and recommended for all critical piping systems based on the number of hours and cycles a system has experienced over the course of its operational life. Determining the areas where funding should be focused to mitigate risk and maximize maintenance dollars effectively is always a challenge. For this reason, Thielsch Engineering developed their proprietary 80-point risk matrix utilized in their 4-SYTE Asset Management Risk Analysis.
Utilizing the same budgetary dollars that would be expended on an assumption based stress analysis, Thielsch Engineering provides an in-depth analytical review of your system’s design, operation and inspection history within their risk matrix. Included in this systematic review is the comparison of the “design” conditions against the “as is” reported conditions. If off design conditions exist, the matrix employs a categorical stress value to areas most likely affected by the circumstance. Additionally, weld types, location, and materials are considered by the matrix, and risk values are assigned categorically for priority. Lastly, and in some cases, more importantly, a full engineering review is conducted on the inspection and repair history of the system. Results of previous inspections are studied and compared to industry standards for lifetime deterioration and damage mechanism propagation rates. These factors are added into the matrix to calculate a final risk value determination for each location along the piping system. The values are then broken down into prioritized tiers for inspection.
In making the important decisions of where to focus your budgetary funding to maximize your resources and minimize your risk, consideration must be given to the health of your piping system. A stress analysis is deemed prudent and necessary for systems that are no longer operating as designed and hanger adjustments are required to bring the system back into an operationally supported scenario. A risk analysis, however, is a wise use of funding for systems that have multi-faceted risk considerations. These systems include those that are exposed to cycling operation, use exotic materials, have either a lack of inspection history or inspection history that has revealed damage and/or have consumed a moderate to substantial amount of hours of operational life.