2 replies to this topic

[chemicooool]

-There is a waste heat boiler in our reformer (NG based) plant producing steam at 35 barg.

-The steam drum is mounted on top for natural circulation.

-Regular analysis of steam sample shows high silica content of 0.2mg/l.

-The analysis of Boiler feed water and condensate recycle to deaerator show silica levels belw 0.04mg/l.

- steam drum continuous blow down is kept almost double than recommended. Intermittent blowdown is also done in each shifts.

-Analysis of blow down show silica of 1.2mg/l.

- I can't make out why such high silica content is present in steam sample. Need some advice.

[tarafdar]

I think this is a case of colloidal silica migration to steam drum.Colloidal silica can not be detected by normal Lab. methods.Special methods are required to detect colloidal silica(non-reactive/non-ionic).Your BFW analysis shows only the ionic silica,colloidal silica passes to steam drum undetected.In the steam drum at high pressure & temp. colloidal silica converted to ionic form & only then it is detected.This is the reason of high silica in BBD.

 

Colloidal silica creates problem for water treatment due to its stability as an un-ionized compound,making it difficult to remove using ion- exchange resins & conventional filtration equipments/methods.

 

Silica in water is present mostly in ionic form.But surface water or river water during some months of the year specially during monsoon contains a part of silica as colloidal form.Colloidal silica can not be totally removed by pre-treatment plant(coagulation & clarification).

 

DM plant can do nothing with colloidal silica.Colloidal silica has virtually no ionic character,but has a relatively large size.Colloidal silica can be removed by fine mechanical barriers,such as ultrafiltration.

 

Though your BD is doubled,still steam contains very high silica.It is dangerous.If you use the steam to drive turbines,silica will deposit on turbine blades.Turbine performance will deteriorate & ultimately there  will be vibration.

 

At present you strictly monitor your pre-treatment plant.There is a relation between turbidity & colloidal silica.Control the turbidity. 

 

If every year you face this problem you can think for :

 

             1.Ultrafiltration

             2.Facility in the Lab for colloidal silica measurement.

Edited by tarafdar, 14 June 2014 - 02:21 PM.

[chemicooool]

Dear taradar,

Thank you for this valuable information.

As our plant doesn't have any steam driven turbine. The steam is purely used as heating media and as a utility.

 

Moreover, as this steam is also used in reforming process, does high silica in steam have any effect on reformer catalyst?