Guard against failure of main- and reheat-steam piping

High-energy pipe (HEP) is under sustained stress, and stress shortens material life—that’s a given. Owner/operators need a way to judge when an HEP system is approaching the end of its life so the weakened piping can be repaired or replaced. Experts recommend having a formal HEP inspection/monitoring program in place before damage occurs.

The program should measure the HEP system’s exposure and vulnerabilities to predict when and where failure may occur. With that information, staff can create a plan and a budget to monitor the system and respond to threats as they are identified.

Several data systems capable of supporting your inspection/monitoring program are commercially available. No matter which one is selected, Matt Freeman of Structural Integrity Associates Inc told CCJ’s Consulting Editor Tom Armistead, the most important thing is to base the HEP program on verified data regarding the age, location, and nature of the various welds, elbows, and other vulnerable points in the piping system.

Structural Integrity (SI) recommends a “building-blocks” approach to program development based on the verified data and the operators’ knowledge of the HEP system—one that predicts the onset of wear-out and identifies items to minimize the impact of early and random failures. An initial prediction of wear-out may be approximate, but as the predicted date of wear-out draws nearer, it can be refined and focused through a combination of non-destructive examination (NDE) and appropriate lifetime calculations, to identify repairs consistent with failure mitigation.

The foundation for the program is data, so getting a comprehensive inventory of HEP system components is the first priority. This information is culled from drawings, fabrication records, inspections, repairs, replacements, and other facts to provide an accurate picture of the system’s configuration, history, and needs. Engineers then can evaluate the system to estimate when it can be expected to wear out and define the urgency at which wide-scale inspection, more precise analysis, component replacement, or remediation should occur.

To set priorities for system maintenance and safety, operators must draw on their industry experience, the data, observations during walkdowns, and other available information, and then balance these factors with an assessment of risk that includes stress analysis to identify locations where damage is most likely to occur.

For example, creep in the pipe redistributes the stress from what it was in the original installation, and that can dramatically affect the stress in other locations in the system. Taking all the known factors, including the plant’s tolerance for risk, into account, personnel can create an inspection plan.

Periodic, multi-disciplined inspections provide a clear picture of the piping system and allow early detection of potential problems. Inspection intervals are guided by stress analysis, lifetime assessments, and the sensitivity of the inspection method. A relatively insensitive method—such as conventional shear wave—can detect macrocracks in piping, but if none is detected, a shorter inspection interval will likely result. Important to keep in mind is that undetected microcracks could propagate to a macrocrack in a relatively short period, possibly a year or so.

If a more sensitive technique—such as linear phased array or time-of-flight diffraction is used (Sidebar 1)—then microcracks can be detected. Hence, in combination with other life-assessment calculations, a longer interval can be specified before re-inspection. Advanced nondestructive inspection technologies—such as focused annular phased array—are able to detect incipient damage. They too may allow longer intervals before subsequent re-inspection—depending on other factors from life-assessment calculations.

The more highly refined inspection technologies give greater confidence in the life prediction and setting re-inspection interval, but they require additional inspection time and more skilled operators to collect and analyze data.

 1. Combine NDE options for best inspection results

Advanced ultrasonic techniques, mentioned earlier and profiled below, are replacing radiography; they are less invasive and do not require clearance of personnel. Plus, radiography is best suited to detection of large flaws—such as those that occur during fabrication—rather than micro-scale flaws that occur as a result of service damage.

But keep in mind that none of the advanced ultrasonic techniques described above can be applied universally; each has its own strengths and limitations. For example, to evaluate seam-weld damage TOFD and LPA are routinely used for general scanning of the weld volume, with APA used to provide higher fidelity data at key locations along the seam.

Linear phased array is popular, but it cannot routinely detect microcracking and is quite sensitive to the orientation of the cracking relative to the ultrasonic beam. LPA can scan long lengths of pipe, as the transducer is moved along the pipe, with good volumetric coverage. Scan results can be used to detect and characterize fabrication- and service-induced flaws in welds and base metal.

LPA also can identify locations for supplemental testing by one or more of the methods that follow, or potentially by surface replication, to detect early-stage creep. It electronically sweeps a beam of ultrasound through a series of angles, or can create a different beam angles where required, allowing for good volumetric coverage at the transducer location.

Time-of-flight diffraction is less sensitive to the orientation of damage, and has detection capability similar to that of LPA. TOFD can be used to rapidly scan full lengths of seam welds and provide comprehensive volumetric coverage. It also can detect and characterize fabrication and base-metal flaws, define macroscopic creep damage, and identify areas for supplemental testing—for example, with linear or annular phased array (APA). Supplemental testing is good because different methods produce a variety of views and insights on the damage.

Focused annular phased array ultrasonic imaging is highly sensitive and capable of detecting both micro- and macro-damage, but can only be applied over a limited region. APA provides distinct enhancements over conventional pulse-echo and TOFD inspections, electronically shifting the focal spot to different depths. As a result, its focused beam is highly sensitive, with improved resolution for flaw characterization. However, it cannot steer and vary focal depth as LPA does.

APA is often time-consuming because the small spot size dictates fine pulse increments and scan raster spacing, hence its use as a complementary technique to obtain higher-resolution data.

Stress analysis and detailed engineering analysis are the keys to effective management of your HEP system. Confronted by an extensive system with potential for catastrophic failure, operators may be tempted to lavish resources on inspection. However, the goal of good asset management is “to get the optimum use out of the piping system without having to pour exorbitant amounts of dollars into it,” says SI Senior Associate Scott Rau. Stress and engineering analyses support that goal, allowing the operator to predict the remaining useful life of the system.

A stress analysis requires extensive system information: a dimensioned isometric drawing, a map of both field and shop welds, detail drawings and walkdown data of supports, spool-piece drawings, and terminal-point thermal displacements. Augmenting this is information on system loads: the weight, internal steam or water pressure, and thermal expansion, which is the primary contributor to the stresses that govern component life.

As a system goes from cold to hot, the pipe will expand significantly, stressing the supports in addition to the thermal stresses on the pipes. “The thermal-expansion load conditions are a prime mover in trying to predict the life your system. It’s where all the uncertainty comes in,” notes Rau.

Pipe supports must hold the weight of the system while allowing thermal expansion with minimal restriction. Thermal expansion may cause pipe lengths to grow by 12 to 24 in. To accurately model the piping system, the loads and displacement constraints of the supports must be accurately simulated using the data of their settings, and readings recorded in periodic walk-downs, to detect trends and alerts to maintenance needs. Computer modeling is essential for accurate assessment. Several commercial software packages are available, but only a few can simulate what is really happening: creep (Sidebar 2).

2. Major damage mechanisms affecting high-energy piping

Creep, a progressive damage mechanism that develops over time, is caused by the sustained application of stress at high temperature (more than 800F).

Fatigue, a progressive damage mechanism that develops over time, is caused by repetitive and fluctuating thermal or mechanical loading.

Creep-fatigue is the interaction of creep and fatigue mechanisms. It can reduce life to 20% of that predicted independently.

Corrosion-fatigue, or corrosion-assisted fatigue, describes cracking initiated by fatigue and crack growth accelerated by corrosion and oxidation.

Flow-accelerated corrosion refers to the thinning of steam/water-cycle components, such as HRSG headers and tubes, caused by dissolution of the protective oxide layer under certain chemical and flow conditions.

Creep. With long-term exposure to elevated temperatures (more than 25,000 hours), permanent deformation—growth—of the pipe will occur. “The material will actually permanently deform with exposure to stress and to elevated temperatures,” continues Rau. Bends and sweeps originally installed at 90 degrees won’t be 90 degrees anymore; pipe lengths will increase.

“You’re going to have more pipe out there after you’ve operated that system for an extended period of time.” When doing lifetime calculations, stress differences of 10-12% have “significant impact on what life predictions are,” he emphasizes. Doing good stress work is extremely important to ensure the accuracy of life predictions.

For predictions of high-temperature component life, creep is the most critical aspect. Alloy steel will void over time, so creep cavitation is an obvious means for qualitative condition assessment and life prediction.

The creep strength of creep-strength-enhanced ferritic (CSEF) steels—such as P91 and P92—is significantly greater than that of low-alloy steel, so the pipe wall can be thinner and more flexible, allowing fewer supports. CSEF steel is an advancement over low-alloy steel, but it has some vulnerabilities of its own.

Perhaps the most significant is the potential for degraded strength, more similar to that of P22 steel, if heat treatments are not performed properly. In this so-called “soft” condition, P91 can be very susceptible to premature failure. Even if the heat treatment is correct, CSEF steels are also sensitive to impurity elements, which can influence long-term creep behavior.

Structural Integrity recommends obtaining a full chemical analysis to determine all levels of the minor elements in this grade of pipe. For another, early detection of creep damage in CSEF steels is difficult; ultrasonic techniques reliably detect creep damage in CSEF steels only at the microcracking stage, which often occurs late in life. The bottom line: CSEF steel piping has advantages over low-alloy steel, but requires greater care and oversight in manufacturing, installation, and life management.

Fatigue, like creep is a progressive damage mechanism. It results from repetitive and fluctuating thermal and mechanical loading while creep develops over time because of the sustained application of stress at high temperature. Interacting as creep-fatigue, they can reduce pipe life significantly more than what would be predicted of them operating independently.

Corrosion is another major threat to pipe integrity. Corrosion-assisted fatigue accelerates cracking initiated by fatigue with corrosion and oxidation. Certain chemical and flow conditions in feedwater piping can dissolve the protective oxide layer in carbon-steel pipe in a damage mechanism known as flow-accelerated corrosion.

Dissolution of the magnetite layer on the surface of the steel is the principal aggravating factor in making FAC a major damage mechanism for feedwater piping, which is less exposed to damage from the mechanisms mentioned above because of its lower operating temperature.  

Fatigue is the greatest single threat to cold-reheat piping, while high temperatures and pressures make hot-reheat and main steam pipes most susceptible to creep (Sidebar 3).

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