Materials Lab Archives

News & Views, Volume 49 | Materials Lab Featured Damage Mechanism: Soot Blower Erosion

April 20, 2021/in Combined Cycle, Fossil Power, Heat Recovery Steam Generator (HRSG), Materials Laboratory, News and Views, Wendy Weiss/by Structural Integrity

By: Wendy Weiss

Soot blower erosion (SBE) is caused by mechanical removal of tube material due to the impingement on the tube wall of particles entrained in the “wet” blower steam. As the erosion becomes more severe, the tube wall thickness is reduced and eventually internal pressure causes the tube rupture.

Mechanism

SBE is due to the loss of tube material caused by the impingement of ash particles entrained in the blowing steam on the tube OD surface. In addition to the direct loss of material by the mechanical erosion, SBE also removes the protective fireside oxide. (Where the erosion only affects the protective oxide layer on the fireside surface, the damage is more properly characterized as erosion-corrosion.) Due to the parabolic nature of the oxidation process, the fireside oxidation rate of the freshly exposed metal is increased. The rate of damage caused by the steam is related to the velocity and physical properties of the ash, the velocity of the particles and the approach or impact angle. While the damage sustained by the tube is a function of its resistance to erosion, its composition, and its operating temperature, the properties of the impinging particles are more influential in determining the rate of wall loss.

News & Views, Volume 49 | Piping Fabricated Branch Connections

April 20, 2021/in Ben Ruchte, Combined Cycle, Fossil Power, Heat Recovery Steam Generator (HRSG), Materials Lab, News and Views/by Structural Integrity

By: Ben Ruchte

Fabricated branch connections represent a common industry issue in combined cycle plants. Many are vulnerable to early damage development and have experienced failures. Despite these challenges, a well-engineered approach exists to ensure that the baseline condition is fully documented and a life management plan is put in place to help reduce the overall risk to personnel and to help improve plant reliability.

Fabricated branch connections between large bore pipes (including headers and manifolds) are often fabricated with a reinforced branch commonly in the form of a “catalogue” (standard size) fitting, such as an ‘o-let’. These are more prevalent in today’s combined cycle environment as compared to conventional units that used forged blocks or nozzles rather than welded-on, integrally reinforced pipe fittings. The fittings are typically thicker than the pipes in which they are installed to provide compensating reinforcement for the piping run penetration. Full reinforcement is often not achieved as the current Code requirements place all of the reinforcement on the branch side of the weld joint. As a result, higher sustained stresses are generated and, particularly in the case of creep strength enhanced ferritic (CSEF) steels, early formation creep cracking in the weld heat-affected zone (HAZ) can occur (known as Type IV damage – see Figure 1). The well documented challenges of incorrect heat treatment of the o-let weld can also add to the likelihood of damage in CSEF components. Damage is therefore most likely to occur in fabricated branches that operate with temperatures in the creep range.

News & Views, Volume 48 | Metallurgical Lab Case Study – Grade 91 Elbows Cracked Before Installation

September 30, 2020/in Materials Laboratory, News and Views, Terry Totemeier, Wendy Weiss/by Structural Integrity

By: Wendy Weiss and Terry Totemeier

Structural Integrity (SI) personnel visited a power plant construction site to examine four Grade 91 elbows (ASTM A234-WP91 20-inch OD Sch. 60) that were found to contain axially oriented surface indications. The elbows had not yet been installed. The indications were initially noticed during magnetic particle testing (MT) after one end of an elbow was field welded to a straight section and post weld heat treated (PWHT). Subsequently, three additional similarly welded elbows were inspected and indications were found at both the welded (inlet) and open (outlet) ends of three elbows. The elbow with the most significant indications was selected for SI’s on-site examinations. Figure 1 shows the inlet and outlet ends of the selected elbow.

News & View, Volume 47 | Surface Preparation – A Pivotal Step in the Inspection Process

March 3, 2020/in Ben Ruchte, Clark McDonald, Combined Cycle, Fossil Power, Heat Recovery Steam Generator (HRSG), Materials Lab, Materials Laboratory, News and Views, Non-Destructive Evaluation, Steve Gressler/by Structural Integrity

By: Ben Ruchte, Steve Gressler, and Clark McDonald

Properly inspecting plant piping and components for service damage is an integral part of proper asset management. High energy systems constructed in accordance with ASME codes require appropriate inspections that are based on established industry practices, such as implementation of complimentary and non-destructive examination (NDE) methods that are best suited for detecting the types of damage expected within the system. In any instance where NDE is used to target service damage, it is desirable to perform high quality inspections while at the same time optimizing inspection efficiency in light of the need to return the unit to service. This concept is universally applicable to high energy piping, tubing, headers, valves, turbines, and various other power and industrial systems and components.

News & Views, Volume 47 | Metallurgical Lab Case Study: Corrosion Fatigue in WaterWall Tubes Increasingly A Safety Concern as Coal Plants Cycle

March 3, 2020/in Ben Ruchte, Fossil Power, Materials Lab, Materials Laboratory, News and Views/by Structural Integrity

By: Ben Ruchte

It is well known that conventional coal-fired utility boilers are cycling more today than they ever have. As these units have shifted to more of an ‘on-call’ demand they experience many more cycles (start-ups and shutdowns, and/or significant load swings) making other damage mechanisms such as fatigue or other related mechanisms a concern.

The most recent short-term energy outlook provided by the U.S. Energy Information Administration (EIA) indicates the share of electricity generation from coal will average 25% in 2019 and 23% in 2020, down from 27% in 2018. While the industry shifts towards new construction of flexible operating units, some of the safety issues that have been prevalent in the past are fading from memory. The inherent risks of aging seam-welded failures and waterwall tube cold-side corrosion fatigue failures are a case in point. It is well known that conventional coal-fired utility boilers are cycling more today than they ever have. As these units have shifted to more of an ‘on-call’ demand they experience many more cycles (start-ups and shutdowns, and/or significant load swings) making other damage mechanisms such as fatigue or other related mechanisms a concern. The following case study highlights this point by investigating a cold-side waterwall failure that experienced Corrosion Fatigue. While this failure did not lead to any injuries, it must be stressed that the potential for injuries is significant if the failure occurs on the cold-side of the tubes (towards the furnace wall).

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