We have all heard the term, “the squeaky wheel gets the grease.” In the case of power plants that squeaky wheel tends to be boiler tube failures. Unfortunately, piping is quiet, until it ISN’T. It goes without saying that high-energy piping systems are essential to the safe and cost-effective operation of power plants. Unfortunately, the likelihood of piping failures increases with the age of the systems involved. The question is not IF high energy piping failures will occur, the question is WHEN will they fail? Catastrophic failures resulting in loss of life have occurred in the utility industry prompting changes in the Code. These changes require mandatory provisions for procedures, records, and inspections for covered piping systems.
Understandably, safety is the primary concern per the requirements set forth by the ASME and those responsible for the production, maintenance, and overall performance within facilities that house high-energy piping systems. In 2007, the ASME published an addendum to the B31.1 “Power Piping” section of the Pressure Piping Code. As a part of this Addendum, Chapter VII – “Operation and Maintenance” [O&M], was added to the Code, which introduces a significant change to the requirements to “covered piping systems” [CPS], formerly referred to as “high-energy piping systems.” A variety of factors played a part of the motivation for this chapters’ addition to the existing Code.
MOTIVATION FOR CHANGE
Of these factors, deregulation of the electric utility industry weighs heavily on the impact of how piping systems have been operated and maintained, resulting in a higher number of piping failures in the industry. The competitive energy market created by deregulation has fostered an environment for reduced maintenance funding, forcing many plant owners and operators to minimize maintenance cost through disregarding the inspection of piping and supports. Additionally, contributing to the lack of continuity of piping inspection and maintenance programs, plants are being bought and sold at an alarming rate. These factors, paired with a significant coincidental piping failure in the industry, prompted the inclusion of the mandatory O&M requirements in Chapter VII for covered piping systems in power plants.
COVERED PIPING SYSTEM REQUIREMENTS
According to Chapter VII, covered piping systems [CPS] for which condition assessments are required to be conducted for electric power generating stations include the following (as a minimum):
- Nominal pipe sizes (NPS) 4 and larger for main steam, hot reheat, cold reheat and boiler feed piping
- NPS 4 and larger piping in other systems that operate above 750°F or above 1025 psi
The operating company may include other piping systems determined (in its judgment) to be hazardous by an engineering evaluation of probability and consequences of failure.
While the Code specifies which systems at a minimum are required to be a part of an O&M program, unfortunately, it does not lay out a specific design of a the program. Leaving many plant owners and operators scratching their heads to develop a program that satisfies the requirements for Code compliance. The chapter essentially says the following: “You shall have a program, and you shall maintain that program.”
So what components make up an effective and compliant CPS Assessment Program? The foundational aspects of the program at a minimum should include unit-specific procedures for operations and maintenance [a procedural plan], functional and retrievable record keeping [a documentation plan], and inspection prioritization with established examination techniques and analysis methods for evaluating fitness for service [an inspection plan].
Because every facility has its particular operational history and conditions, it is essential to consider your facility’s unique conditions to develop a strategic, yet compliant program. A procedural plan should clearly identify personnel responsibilities and reporting guidelines unique to the needs of the plant and ensure personnel safety, equipment maintenance and unit operations. Additionally, the specific procedures should be provided for the operation of piping systems within design limits. Lastly, procedures should then be written to cover the evaluation of specific weld types, piping supports, materials, and address specific damage mechanisms (e.g. creep, fatigue, flow accelerated corrosion, etc.). The procedures should specify the examination techniques as well as required personnel certifications and qualifications.
Chapter VII requires that condition assessment reports and any reference documents (such as O&M procedures as discussed above), drawings, and other reports be maintained and accessible for the life of the plant. For a documentation plan to be viable, these records should be usable and readily available. Records should include as many of the following as possible:
- Design Information including:
- Age of the unit
- Design of the unit
- Hanger loads, setting and types
- Locations of dissimilar welds
- Weld type and location
- Pipe OD and thickness
- Design pressure and temperature
- Pipe spool drawings
- Isometric drawings
- As-built drawings
- Fitting drawings
- Weld detail drawings
- Hanger support detail drawings
- Valve weights
- Original pipe stress analysis required for design acceptance
- Historical Information including:
- Operating pressure and temperature
- Operating hours
- Operating mode, e.g., base-loaded or cyclic
- Number of unit starts and characterization of start type
- Number of operating hours at off-design conditions
- Service Failure/Damage Locations
- Service Failure/Damage Mechanisms
- Operational Changes
- Upset Conditions
- Hanger Walkdowns
- Inspection History
- Failure History
- Stress Analyses
- Recommendations for inspection intervals and scope
To successfully manage critical assets such as high-energy piping, the availability of these types of design and operational documents is necessary to assess and plan an effective program to minimize potential piping failures that may lead to unexpected and severe consequences.
Lastly, Chapter VII requires that an assessment of the condition of all covered piping systems be performed at periodic intervals. These examinations should be performed in accordance with the examination procedures established in the procedural plan as discussed earlier in this article.
Unfortunately, the process associated with developing an inspection plan can be viewed as an incredible obstacle for many utility operators and owners. In many cases, the cookie cutter approach that is often used results in budgetary wastage and improper safety and risk management. Rather, a prioritization process should be followed where the risks with the greatest loss (or impact) and the greatest probability of occurring are handled first, and risks with lower probability of occurrence and lower loss are handled in descending order. Ideally, these risk categories should take into account unit specific considerations such as the system design; the operational, inspection, and repair history; as well as the economic outlook for each system.
Once the risk assignments have been determined, the damage mechanisms of concern for each system should be clearly identified. With respect to the metallurgical degradation, the following damage mechanisms should be considered in the development of the inspection plan and the analysis of the results:
- Creep caused by the combination of high stresses and temperatures
- Local deformation (yielding or straining) caused by severe transient loads
- General metallurgical degradation involving spheroidization, graphitization, decarburization, diffusion, strain aging, temper embrittlement,
- Localized metallurgical degradation caused by improper welding filler metals, welding fluxes, major weld defects,
- Fatigue cracking due to pipe expansion and contraction
- Fatigue cracking due to thermal shocks and vibrations and internal thermal stresses
- Mechanical fatigue cracking due to steam hammer, water hammer, flow induced vibrations, equipment fatigue, fan, and air preheater rotation,
- Overload conditions due to severe water induction (slugging) or similar severe thermal shocks (thermal quenching) incidents
- General corrosion
- Corrosion pitting
- Internal erosion associated with high-velocity steam flow containing water droplets, hard particles, etc.
- Stress corrosion (caustic embrittlement)
The inspection should include some or all of the following inspection techniques:
- Hanger support walkdowns in the hot and cold positions.
- Visual examination (The visual examination should be performed by an experienced engineer familiar with the designed movement of the high-energy piping system. It should include the entire piping system, and identify conditions of sagging, bowing, insulation damage or incorrect pitch.)
- Wet fluorescent magnetic particle examination
- Liquid penetrant examination
- Ultrasonic wall thickness determinations
- Ultrasonic phased array scans
- Ultrasonic time of flight examinations
- Diametric measurements
- In-situ Replication Metallography
- Boat sampling
- Seam verification by macro etching
- Positive material identification
The results of the inspections performed on the covered piping systems should be evaluated to determine the need for a stress analysis. A stress analysis is considered prudent when the hanger support walkdowns reveal that a large number of the pipe supports are not functioning as intended by the designer. Similarly, a stress analysis is considered prudent if the inspections revealed repeated or chronic cracking in specific locations within a covered piping system.
Generally, the safety and low-risk operation of any plant fall within the domain of insurance companies and the regulatory bodies that could impose fines if the operation of the plant violates their guideline. Understandably, insurance companies want to know that their insureds are doing their part to minimize their risk exposure. Many utilities have been able to reduce their insurance premiums by implementing a Code compliant piping program. Additionally, some states, such as California has specific compliance laws in which hefty fines are imposed upon the facilities not operating in accordance with the guidelines outlined in the Code.
Thielsch Engineering, Inc. has spent the past 30 years working with America’s power producers and advanced manufacturers to ensure safety, reliability, and profitability. As James Chiles, Author of Inviting Disaster states, “Failure never happens out of the blue, it propagates from flaws that eventually link up.” The inevitability of failures occurring in the utility industry prompted Thielsch to pair its industry expertise with state of the art technology for data management. The resulting program, 4-SYTE, provides a Code compliant Operation and Maintenance system for covered piping systems. Integrating the requirements of the Code with a user-friendly interactive platform, 4-SYTE gives you the resources you need to track, plan, budget and prevent providing knowledge capture that is usable and readily available.
Success is dependent on effective safety and risk management. By applying a unit specific, targeted inspection plan such as 4-SYTE, utility owners, and operators are not only in compliance with the Code requirements, they are also more likely to succeed in today’s competitive market by increasing the unit’s reliability and availability without sacrificing safety or environmental standards.
For more information on covered piping inspections contact Peter Kennefick at firstname.lastname@example.org or Robert Smoske at Rsmoske@thielsch.com. For more information on the 4-SYTE program contact Pamela Hamblin at email@example.com.