CSEF High Energy Piping Inspections
Power plant engineers have long understood that periodic inspections of the High Energy Piping systems in their plants are as essential to the safety and reliability of their plants as any item on their list of maintenance tasks. In those cases where the piping system was fabricated using one of the “traditional” low alloy steels, such as Grade 22, an inspection approach has been developed that has proven effective in the course of many system-wide assessments. This approach focuses on two areas: first, the identification of locations of abnormally high stress in the system, particularly those that developed early in the operating cycle; and, second, the detection and sizing flaws in welds or piping that escaped detection during installation. This establishes a baseline condition for the system and through effective monitoring thereafter unexpected and potentially catastrophic failures can be avoided, thereby safeguarding the well-being of plant workers and maintaining plant operational reliability.
With the advent of piping systems fabricated using one of the Creep Strength-Enhanced Ferritic (CSEF) steels, such as Grades 91 or 92, these inspection programs have become both more critical and more complex in their execution in the effort to insure worker safety and maintain plant reliability. The reason for this is the fact that, unlike the traditional alloys, the elevated temperature properties of the CSEF steels can be substantially degraded by improper processing during either fabrication or erection. The result can be that in the as-installed condition the material will not be capable of supporting the Code allowable stresses, so that, depending on the specific circumstances of the deficiency and the actual conditions of operation, the service life of the component may be reduced by as much as a factor of 10 compared to a component that had been processed properly. This is particularly an issue for piping systems in which rigorous process control was not maintained during all phases of fabrication and erection of the system. That inadequate process control was all too often the norm during the fabrication and installation of many of these systems has been demonstrated by the results of numerous inspection programs that have been conducted on Grade 91 piping systems in US plants, with deficient material being detected in more than 90% of the Grade 91 piping systems that have been inspected to date.
From an inspection standpoint, the concern over the condition of these CSEF materials is further heightened by the fact that, in some cases, the degraded condition of the material resists detection by the standard inspection techniques, such as hardness testing and metallographic replication. This puts added emphasis on the experience and knowledge of the engineer conducting the assessment, since in such cases it is not one single piece of evidence that will identify the deficient condition, but the cumulative weight of a number of apparently disparate pieces of evidence whose connection will only be obvious to a highly experienced observer.
Problems that can be considered unique to piping systems involving the CSEF steels include: (1) poorly designed weld joints in which the influence of the so-called Type IV region of the weld Heat-Affected Zone was not properly accounted for by the designer; (2) the presence of narrow strips of degraded, or “soft” material, in some cases running the full length of sections of pipe, as well as random “soft” spots randomly positioned along the length of a pipe section, an elbow, or a fitting – typically due to improper cooling of the piece during the normalizing heat treatment; (3) degraded material in the immediate vicinity of major seam welds, resulting from poor control of temperature during field PWHT; (4) complete segments of the piping system, particularly elbows and fittings, supplied in a “dead soft” condition as a result of lack of control of heat treating processes in the shop; (5) “hard” base material in the immediate vicinity of welds – a condition that, again, is the result of poor control of PWHT temperature and that can only be corrected by re-normalizing and re-tempering – or by replacement; (6) stress-corrosion cracking in un-tempered or under-tempered material – often occurring due to exposure of under-tempered material to moisture and air-borne contaminants following completion of a weld but prior to PWHT: (7) seemingly “normal” material, as characterized by field hardness testing or metallographic replication, but in which the critical sub-structure has been compromised by some processing deficiency, leading to a significant loss in creep strength with no obvious outward sign of the problem. These various deficiencies have led to a number of premature failures, such as the complete separation of end caps from headers and through-wall cracking in a piping-to-stop valve weld joint after less than 5000 hours of operation (see attached photographs). More importantly, a large number of failures have been averted due to the fact that operators, alerted to the high level of risk associated with improperly processed Grade 91 material, have implemented comprehensive baseline inspection programs that have succeeded in detecting many of these deficiencies before failures could occur.
Fortunately, once a comprehensive baseline inspection of a Grade 91 piping system has been conducted, and when all deficient material has been identified and the appropriate remedial actions taken, then the inspection protocol reverts to the standard practice adopted for piping systems fabricated from the traditional alloys.
Separation of a Header End Cap from the Body of a Grade 91 Header As a Result of Accelerated Cracking at the Outer Edge of a Weld Heat-Affected Zone
Example of Cracking at a Grade 91 Piping to Stop Valve Weld – Failure Occurred in Less Than 5000 Hours