Boiler Tubes Archives

News & Views, Volume 45 | Metallurgical Lab Featured Damage Mechanism – Acid Dewpoint Corrosion in Conventional Fossil Boilers and Combined Cycle HRSGs

January 1, 2019/in Materials Laboratory, News and Views, Wendy Weiss/by Structural Integrity

By: Wendy Weiss

Acid dewpoint corrosion can occur in conventional and HRSG units in locations where temperatures fall below the sulfuric acid dewpoint temperature. This can occur when either the tube metal temperatures are below the acid dewpoint so that condensate forms on the metal surface, or when flue gas temperatures are below the acid dewpoint, so that the condensate will form on fly ash particles.

Mechanism
This type of fire-side damage occurs when sulfur dioxide (SO²) in the flue gas oxidizes to sulfur trioxide (SO³) and the SO³ combines with moisture to form sulfuric acid. If the temperatures are at or below the acid dewpoint, so that the sulfuric acid condenses, then tube metal corrosion occurs. The temperature at which condensate first forms depends on a number of factors, including the partial pressures of SO³ and water vapor in the flue gas, but is usually around 250 to 300°F.

News & Views, Volume 44 | Metallurgical Lab Featured Damage Mechanism – Long-Term Overheating/Creep (LTOC) in Steam-Cooled Boiler Tubes

April 27, 2018/in Materials Laboratory, News and Views, Terry Totemeier/by Structural Integrity

By: Terry Totemeier

Long-term overheating and creep damage are often the damage mechanisms associated with the normal or expected end of life of steam-touched tubes, generally occurring after 100,000 hours or more of service life at elevated temperatures and pressures. Long-term overheating and creep can also occur when the rate or accumulation of creep damage is moderately higher than anticipated by original design. There are a number of possible reasons for this, but in general the problem can be attributed to one of the following: a non-conservative original design, higher-than-anticipated heat absorption, lower-than-anticipated steam flow, or wall loss caused by external wastage.

Mechanism
The mechanism of failure for LTOC is simply the accelerated accumulation of creep damage in the component over a span of time that is well short of the anticipated design life, but sufficiently long that creep is the dominant damage mode. This damage is typically associated with the operation of the tube above the oxidation limit for the material involved. This has two effects, which both contribute to long-term creep failure: reduction in wall thickness due to oxidation loss, and build-up of oxide on the tube internal surface, which insulates the tube from the cooling effect of the steam, leading to increasing tube metal temperatures over time.

News & Views, Volume 43 | Metallurgical Lab: Dissimilar Metal Welds (DMW) in Boiler Tubing

October 5, 2017/in Fossil Power, Materials Laboratory, News and Views, Tony Studer/by Structural Integrity

By: Tony Studer

The need for confirmation: A Case Study

As plants age, the need for inspection for service related damage to ensure unit reliability increases. There are several approaches that plants can take to reduce the risk of premature failures and proactively manage their DMWs. First is metallurgical sampling. Based on temperature profiles across the boiler, operating conditions, and operating history, DMWs can be selected for laboratory analysis. This will provide some insight into possible damage accumulation; however, the better approach, if damage is suspected, is to perform an ultrasonic inspection of the DMWs. This allows inspection of all the DMWs, and only requires access and surface preparation. If indications are detected, then tube sampling should be performed. It is critical to perform a metallurgical analysis of several of the DMWs suspected of containing service damage to confirm that the indications are service related and to help establish the extent of the damage compared to ultrasonic testing results. Typical DMW damage is described in the Featured Damage Mechanism article. The importance of the metallurgical analysis is demonstrated in the three following case studies.

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