As the age of systems, structures, and components (SCCs) in nuclear power plants increase, so does the level of effort and cost to manage their aging. Half of the nation’s nuclear plants are over 30 years old with essentially all of the remainder older than 20 years. Many stations have undergone significant upgrades to their turbines, pumps, and condensers; each time, increasing operating life and adding efficiency in an ever-challenging power generation market. Within the large engineering design packages for modifying these components, how often have you seen a vibration evaluation of the affected small bore piping? These small bore piping evaluations are being overlooked or improperly analyzed and the consequences, including loss of generation and increasing O&M costs, are trending into the red.
(<2” OD) causing loss of generation occur because high cycle fatigue (HCF) was not identified or analyzed properly in the design phase reference (INPO IER 14-30). When major rotating or piping components are repaired, modified, or replaced, the forcing function and/or system response is likely to change. Although there had been no history of vibration failures prior to modification, the potential for resonance-induced failures increases significantly post-modification. This isn’t a new phenomenon. Early catastrophic failures due to vibration occurred during the early days of plant operation. However, the cost of such a failure has increased dramatically from the view of public perception. Added regulation, and operating expense. Furthermore, resonance induced vibration failures often occur within a fuel cycle, from initiation of a crack to leakage, making early detection increasingly difficult. This means that in order to reduce the number of leaks caused by HCF, a station must become more effective in evaluating the HCF susceptibility of designs and initial testing during operation (Figure 1). Although much of the experience has left or is leaving your mechanical or civil design groups, there are a few tools or key indicators to help you reduce the probability of HCF failures.
In many cases the engineer of choice (EOC) provides the calculations to support an engineering change package. There are plenty of vibration techniques these firms and plant personnel have at their disposal. Hand calculations (i.e., EPRI’s Fatigue Management Handbook), finite element (FE) models, and vibration testing offer great insight when evaluating HCF potential early in the design stage when the cost and risk of a failure is low. However, for small bore line evaluations, small inaccuracies or apparent conservatisms can significantly affect the dynamic response in a model or calculation.
For example, running a piping model with thermally conservative boundary conditions may achieve a bounding thermal model for ASME NB3600 Eq. 11, but can have inaccuracies of more than 20% during a modal analysis – which can be the difference between failure and a full fuel cycle of operation. If the EOC and/ or station is lacking the vibration expertise to properly define the vibration loads and evaluate the responses, experts should be involved to capture these nuances early, before the design makes it into operation. Although models generally provide the earliest indicator of a vibration problem, remember every model is an approximation of the vibration response, with some closer than others. Fortunately, there are easy ways to add confidence to a design model through complementary post-modification testing.
Two different types of testing can be effective in reducing the likelihood of a HCF failure on small bore piping. The first, impact testing, provides accurate insight into system natural frequencies, allowing for a comparison to predicted excitation frequencies. If these align sufficiently (Ω/ω = 0.8 – 1.2, Figure 2), there is an elevated potential for a resonance-induced HCF failure. Now, an FE model can produce a modal analysis with a couple mouse clicks; however, the accuracy of those results can vary widely. Accurate boundary conditions and non-linearity are captured when impact testing as-built designs and when tested properly, they can be counted on for their accuracy. Furthermore, during testing, field adjustments such as temporary support or mass additions can be utilized to tune the small bore piping outside of exclusion zones and quickly evaluate HCF improvements to the design. The +/-20% exclusion zone criteria may not be achievable and additional reasonable assurance might be needed. The ASME NB-3600 Code states, categorically, “the designer shall be responsible, by design and by observation under startup or initial service conditions, for ensuring that vibration of piping systems is within acceptable levels.” This testing during the initial operation of the system/component provides the final insight into HCF failure susceptibility.
Operational testing, by handheld systems or other temporary installations can be used during the early stages of operation to measure the vibration levels of modified or affected small bore piping. These methods do not require permanent installations and often can be installed, acquire data, and removed in a couple shifts capturing the data necessary to evaluate a design. The measurements, along with ASME Operations and Maintenance Guide Part 3 velocity acceptance criteria, provide a quick method for evaluating vibration levels.
There are multiple stages during a design or modification package where the EOC and/or station personnel have an opportunity to evaluate vibration. The earlier this evaluation occurs, the more cost effective design changes are to implement and the lower the likelihood of HCF failure. For this reason, the techniques and testing indicators suggested in this article provide successive tasks, each contributing towards the ultimate goal of reducing the likelihood of an HCF failure to an acceptable level. The execution of a vibration evaluation using this strategy allows for subsequent tasks to be assessed on a line-by-line basis, and executed only to achieve the desired level of reasonable assurance for structural and leakage integrity. For example, portions of small bore piping affected by a modification may only require post modification impact testing and a quick visual assessment of the vibration during initial operation due to their overall lower HCF susceptibility assessed in the design. However, some lines may exhibit a greater HCF susceptibility during the design evaluation and impact testing phases, such that vibration measurements may be needed to justify proceeding with operation. Tailored small bore evaluations for vibration, built upon accurate susceptibilities and risk, have proven effective in driving down loss of generation and O&M costs caused by HCF failures.