the corroison rate is more likely a function of temperature, pH, O2 content, and water velocity .
If the corrosion is occurring at the inlet tubesheet- to -tube connections, then it is associated with water velocity. A HP heater is designed + fabricated to minimize cost, and this includes minimizing the shell OD which implies a smaller dia tubesheet and reduced number of tubes. This finally leads to a higher waterside velocity since there are fewer tubes to carry the water flow. The erosion-corrosion occurs at the vena contracta ( flow contraction)that occurs about 0.3 diameters downstream of the tube inlet - there wil be a higher velcotiy there plus erosive eddies .
Thanks for the reply,
My company use 70-30 Cu-Ni tube. with an average water velocity about 2.01 m/s. Is it still water velocity is the main source of the corrosion in inlet tube connection? Or maybe the steam itself who also contribute the corrosion rate by its amonia (from hydrazin injection)? because in our HP feedwater, the corrosion is happen mainly in outlet tube connection.
If steam is generated in the outlet tube then erosion corrosion would occur. The only way I can imagine this is:
a) steam extraction to the heater is highly superheated, and directly impinges the outlet tubes in the desuperheater zone of the heater.
b)excessive heating of a few tubes leads to those tubes heating up to saturation . This is unusual, but can be possible at part loads if one is also operating the HP evaporator at variable pressure with load. If the low load HP pressure is 500 psig , the saturation temp is only 467 F, and if the extraction steam temp is at 750F, one can postulate the outlet tube section of some tubes exceeding satured steam enthalpy.
c)The exit fluid enthalpy of thefew overheated tube will have a larger specific volume than the average tube, so the amount of water which flows to those few tubes will be reduced below the average flow, leading to a greater entahlpy unbalance at the tube outlet.
d) The outlet 2-phase velocity is high enough to cause erosion -corrosion.
