Failure Analysis of Boiler Damages.pdf - Sme-gtz.org.vn

TÜV Industrie Service GmbH
TÜV SÜD Group
Westendstrasse 199
80686 München
Germany
Deposition or Scaling may also lead to
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• Localised concentration of impurities within or underneath of deposits at heated
surfaces
• ”On Load Corrosion” with acidic or alkaline attack Photo 10 – 12.
Attention:
– Scaling appears first at spots with high heat transfer and high steam production
– Even non corrosive deposits e.g. Photo 13 - 20
- iron oxides may lead to porous, thermal critical scale,
- silicates as Ca-, Mg, Al-silicates to over-heating
– Boiler operation according to the BFW and BW requirements for some years lead
mostly
to some deposits of iron oxide, which should be preventively removed
Causes and Risks:
∗ Insufficient on-line quality control of make-up water, condensate return or BFW
∗ Measurement of “Acid Conductivity” amplifies detection of ingress of impurities like
cooling
water and some process media but do not indicate ingress of free caustics!
∗ Lack in Inspection of not safety related components like heat exchangers and
condensers
∗ Preventive cleaning during operation or at shut down must be done according to the
specifications
Corrosion on boiler steel (carbon or low alloyed steel) can be mainly avoided, if the BFW
and BW requirements were kept. Deviations may leads to
• Dissolution of the important protective layer, material attack and loss in form of
- Uniform or General Corrosion or - more dangerous - to
- Localised Corrosion (Pitting, On-load corrosion, etc.) Figs. 2 - 4
• Crack formation in specific cases
Attention:
– Uniform or General Corrosion isn’t problem if the material loss is less than 0.1 mm/year
(is easy to achieve with normal protective iron oxide layer)
– Localised corrosion during operation underneath of deposits and within heated gaps as
well
as during stand still in form of oxygen corrosion (pitting formation)
– Crack formation due to high concentration of free caustics in presence of high tensile
stress (Caustic SCC) Photos 3 - 6
Causes and Risks:
∗ Insufficient BFW and BW composition, which do not meet the requirements of the boiler
manufacturer or approved standards
∗ Inadequate preservation during shut down
∗ Localised concentration of acidic acting impurities e. g. cooling water or sea water
ingress
∗ Localised concentration of caustic acting impurities or free caustic underneath deposits
within heated gaps or at heated phase boundaries
TÜV SÜD Industry Services IS-ATW1-MUC, Boiler Failure 2005/11 - Page 2