We are facing issue of high conductivity in our steam drum though the BFW conductivity on spec. We have been increasing CBD & also frequency of IBD is increased, but conductivity is still more than spec?Jan-2022
James Robinson, Suez, firstname.lastname@example.org
The experience you are having is very common for plants with sodium zeolite softened make up water. This is because sodium bicarbonate and to a lesser extent sodium carbonate in the feedwater decompose in the boiler releasing carbon dioxide into the steam and forming sodium hydroxide in the boiler water. The conductivity of 1 ppm of sodium hydroxide is much greater than 1 ppm of sodium salts such as sodium chloride, sodium sulfate, sodium carbonate, sodium phosphate, etc. Consequently, systems with sodium zeolite softened make up water normally control boiler blowdown based on a "neutralized conductivity" measurement.
The neutralized conductivity is measured by adding enough organic acid to change a phenolphthalein treated boiler sample clear and then measuring the conductivity. Since this would not be practical for your online blowdown controller I suggest that you develop the relationship between neutralized conductivity and the conductivity of your boiler water (usually 0.6 to 0.8) and set your automatic blowdown limits accordingly.
Nasir Hussain, Pak Arab, email@example.com
Also check the conductivity of recycle condensate from the process units. Further, check any leakage if DMN is preheated in any process exchanger.
raj dhamelia, HPCL, firstname.lastname@example.org
If the sudden increase in steam conductivity has been noticed to have occured recently then check for any upset in steam drum level which may have caused damage to steam drum internals (steam purifiers/demisters) which would increase the mechanical carry over in the steam. Also, check the steam drum level indication for correctness. Reconfirm from guage glass.
Steam purity (saturated steam from steam drum ) should be checked for abnormal quantity of silica, sodium,calcium ions using gravimetric analysis or otherwise to confirm that mechanical carry over in the steam causing higher conductitvity. Checking the sodium balance using BFW, CBD and steam purity can help estimate % carryover in steam.
Conductivity can increase due to addition of pH boosting chemicals. Hence , abnormal presence of inorganic ions (Sodium, calcium etc) needs to be checked to know if it is cause of concern (high conductivity) or not.
Also, check if the boiler steam drum pressure is being operated close to design or not. Operating at pressure much lower than design can increase carry over in steam due to high velocity in separators.
Also, get the conductivity analyzer re-calibrated once just to rule out measurement error.