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Precommissioning Problems
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Target Plate Impacts
Target Plate Impacts
Consulting About Steam Blowing
A few days ago, we were consulted about a very interesting case that I would like to share my opinion as it would be useful for engineers involved with commissioning:
Can carryovers and/or water droplets affect the target plate during steam blowing?
Yes indeed. Carryovers can affect final results, specifically magnetite particles in the outer oxide layer that are not consistently attached to the surface and are driven off the drum.
Solution
This can be solved running the boiler at high pressure for a few days right before steam blowing starts, to allow steel increasing thickness, degree of compaction and hardness of the protective layer.
However, there are other few factors that may contribute to this happening and should be avoided, for instance, intermittent decompressions, chemical instabilities, defective chemical cleaning, insufficient blowdowns, operation with high level in the drum or low steam quality, among other deficiencies. Water droplets can also be a source of impingement and produce false reading of the pipe cleanliness.
For this purpose, it is recommended to place the target upstream temporary desuperheaters, prevent drains from clogging that can overflow driplegs and get dragged by steam, and finally make sure to get rid of all condensation during warmup stage.
FAC Corrosion
FAC Corrosion
Description
When a steel surface is in contact with water under specific chemical and thermodynamic conditions, an oxide protective layer of magnetite (Fe3O4) is developed. Controlling the growth mechanism, the morphology and the solubility of such corrosion product it is possible to minimize the risk of failure and extend the life-time of the plant. The presence of O2 in the water chemistry is well known to be the most harmful corrosion catalyst.
For this purpose, oxide scavenger is generally dosed as protective agent which also contributes to build a film of magnetite. However, this common practice can be quite detrimental in the low temperature regions of the plant where the thermodynamic solubility equilibrium cannot be reached, and magnetite is been formed in the temperature range of maximum dissolution. As a result, single-phase FAC mechanism will lead to general corrosion.
Solution
Solution: keep O2 concentration high to change the Schikorr reaction kinetic to the formation of ferric oxide hydrate (FeOOH) which is stable at low temperature. The following photo shows the frontal baffle plate of a feedwater heater affected by flow-accelerated corrosion