News & View, Volume 49 | Piping Fabricated Branch Connections

News & Views, Volume 49 | Piping Fabricated Branch Connections

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.

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

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

By:  Ben Ruchte, Steve Gressler, and Clark McDonaldNews & View, Volume 47 | Surface Preparation – A Pivotal Step in the Inspection Process

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.

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

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

By:  Ben RuchteNews & View, Volume 47 | Metallurgical Lab Case Study- Corrosion Fatigue in WaterWall Tubes Increasingly A Safety Concern as Coal Plants Cycle

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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News & View, Volume 47 | Materials Lab Featured Damage Mechanism- SH:RH Fireside Corrosion in Conventional Coal Fired Boilers

News & Views, Volume 47 | Materials Lab Featured Damage Mechanism: SH/RH Fireside Corrosion in Conventional Coal Fired Boilers

By:  Wendy Weiss

Superheater/reheater fireside corrosion is also known as coal ash corrosion in coal fired units.

News & View, Volume 47 | Materials Lab Featured Damage Mechanism- SH:RH Fireside Corrosion in Conventional Coal Fired Boilers

MECHANISM
Coal ash corrosion generally occurs as the result of the formation of low melting point, liquid phase, alkali-iron trisulfates. During coal combustion, minerals in the coal are exposed to high temperatures, causing release of volatile alkali compounds and sulfur oxides. Coal-ash corrosion occurs when flyash deposits on metal surfaces in the temperature range of 1025 to 1200oF. With time, the volatile alkali compounds and sulfur compounds condense on the flyash and react with it to form complex alkali sulfates such as K3Fe(SO4)3 and Na3Fe(SO4)3 at the metal/deposit interface, which are low melting point compounds. The molten slag fluxes the protective iron oxide covering the tube, exposing the metal beneath to accelerated oxidation.

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