High-Energy Piping: Where to Inspect?Victor Rodgers | March 24, 2016
Fossil power plant high energy piping (HEP) systems transport steam to rotate the turbines that drive electric generators. Main steam, hot reheat and cold reheat piping are subjected both to elevated pressures and temperatures. The continued accumulation of creep degradation, fatigue damage or the two in combination act as primary failure mechanisms to destroy piping integrity.
Although most piping was designed to operate for 30-40 years, inferior weld quality or design, erratic operating conditions or substandard metallurgy of the piping introduces untimely issues. While the probability of premature failure is low, accidents continue to occur among newer installations. The larger threats are with older piping, seam-welded piping, P91 material and piping among units heavily cycled.
The potential for catastrophic failure, injury to people and property and lengthy unplanned outages necessitates power plant engineers to perform meaningful condition assessments. Non-destructive examinations (NDE) are used to inspect piping to identify failure mechanisms, assess risks and estimate remaining life.
Power plant engineers have long understood that HEP inspections are essential to the safety and reliability of their plants. If money were no object, engineers would inspect the entirety of each pipe upon construction and at regular intervals. Unfortunately, the costs associated with erecting scaffolding to access every weld, stripping insulation (or abating asbestos), performing NDE and reinstalling insulation from the boiler to the turbine can exceed $1 million for each HEP line.
Instead of inspecting 100% of the welds within a single outage, conventional practice tends to accomplish this over a 10–15-year period. Utilities seek to establish this level of “baseline” knowledge for each system by focusing on two areas: 1) identifying locations of abnormally high stress, particularly those that developed early in the operating cycle, and, 2) detecting and sizing weld or parent metal flaws that escaped detection during installation.
Along with information obtained from a stress analyses and hanger walk-downs, each weld is inspected over the course of several outage cycles. Baseline inspections identify surprises such as accelerated creep cracking or errors upon construction like incorrect filler material. The data gathered from these inspections helps engineers in targeting higher-risk welds to be re-inspected at certain intervals. This approach has served the industry well for many years, especially among the “traditional” low alloy steels, such as Grade 22 piping.
Risk based inspection (RBI) is the process of developing an inspection plan based on knowledge of the risk of failure of equipment. It considers the safety of personnel along with the risk of forced outages and prolonged shutdowns. “Active” and “potential” damage mechanisms are quantified to assess and rank failure probability and consequence. Rankings are then used to optimize inspection intervals based on site-acceptable risk levels and operating limits.
Linked to Probabilistic Risk Assessment (PRA), RBI emerged in the 1980’s from the nuclear power industry and quickly spread using guidance from standards applicable to different situations such as API 580/581 for oil, gas and petro-chemical scenarios, DNV-RP F116 for subsea pipeline and ASME PCC-3 for fixed pressure-containing equipment. (Visit the IHS Standards Library for more information on these standards.)
Electric power generating companies face an ever-increasing challenge in managing asset integrity. Operational excellence must be achieved and asset performance maximized while costs are minimized and high safety standards maintained. RBI is a valid and respected approach, not to mention convenient for today’s budgetary constraints as it uses inspection resources. The method ensures that power plant personnel are complying with current safety regulations and also enables them to make inspection decisions informed by greater information and expertise, thereby saving time and money.
Among piping systems, it’s common to assume a large percentage of the overall risk is concentrated within a relatively small number of welds while a large percentage of welds may pose minimal risk. The welds having higher risk will require more attention in an RBI plan but the associated increased inspection costs should be offset by reducing (or even eliminating) weld inspections that pose minimal risk.
Therein lies the problem, however. RBI can provide a false sense of confidence insofar as some piping locations are perceived to have negligible risk. This may lead the uninformed to believe the piping never needs to be inspected, especially when budgets are tight. Frequently, engineers overemphasize locations whose risk can easily be quantified and entered into an RBI matrix. Locations that rank the highest as a result of a stress analysis are inspected, often at the expense of locations that may suffer from more ominous failure mechanisms such as undocumented welds, inferior weld material, cyclic loading, P91 vulnerabilities or transient events such as water hammer.
Given economic pressures, some plant operators are foregoing baseline inspections altogether in favor of full-on risk-based inspection. This proves attractive in the sense that inspection dollars target only the locations of quantifiable risk. The problem is that some risks are very difficult to quantify. For example, consider these scenarios:
- Who would have ever guessed the Grade 22 piping was constructed with carbon filler welds?
- Why didn’t the drawings specify that this section of piping was seamed?
- Somebody said this pipe shook violently on startup last year.
- We can’t find any fabrication or construction documentation for this P91 pipe.
Frequently, risks first present themselves as surprises, or as failures.
The design and construction code for HEP systems is ASME B31.1. Non-mandatory Appendix V of ASME B31.1 refers to “continued examination” to be conducted “at intervals based upon the results of the initial inspection, but not to exceed 5 years.” The practice in ASME B31.1 fossil power plants is to periodically inspect high-risk systems such as main steam and hot reheat.
B31.1 explicitly refers to initial inspections as a precursor to any continued examination. Although RBI plays a critical role in today’s inspection regimens relating to HEP, initial baseline inspections are indeed necessary. A baseline condition for a piping system must be established to provide valuable information complementing RBI in its endeavor to provide effective monitoring such that unexpected and potentially catastrophic failures can be avoided.