(By Kamal Hossain, Chief Engineer)
Q. (a) Explain why regular testing of water in auxiliary boiler is advisable
(b) How to take correct sample
(c) Discuss what factors are necessary when measured value differ appreciably from desired value in each test.
(d) Give reasons for the situation when alkalinity is found to have fallen unaccountably.
Answer:
(a) For the well boiler operation, boiler water should be tested regularly and accurately. Then correct chemical treatment should be followed based on the test results for the following reasons.
– To keep the boiler water in alkaline condition
– To precipitate all scale forming , both dissolved and suspended salts in the boiler water , in the forms of removable and non-adherent sludge.
– To remove dissolved oxygen , carbon dioxide and other gases
– To produces substances which will be well below the limits for carry over , foaming and priming..
– To keep their decomposition products not to be harmful to the whole boiler system
– To prevent metal loss due to corrosion
– To prevent scale formation in the system
– To prevent contamination by sea water and by oil
(b) Since highest concentrations are at the water surface , sample should be taken at this point can represent more conditions . Therefore sample drawing line is located in the steam drum , just above the water tubes. In order to get proper results , it should be as far as possible from the chemical feed line. Correct sample must be taken…
– From salinometer cock
– With salinometer gauge
– Allow to flush about 5 min before taking
– Cool down to 25’C before test to prevent flashing which concentrate the sample
– All test apparatus should be cleaned with sample water
– Sample should be tested as soon as possible after drawing
(c) The boiler water should be tested and actions to be taken as follows.
- Excess Phosphate test: Measure content of phosphate salts in the boiler water which can prevent scale, form non-adherent sludge and give alkalinity. Control limit is 20 ~ 40 ppm . If less than 20 ppm dose NaSO4 chemical ( adjunct B ) as per instruction. If excess 40 ppm , take blow down action.
- Phenopthalein and Total Alkalinity Test: Measure content of NaOH , control correct pH value and require alkalinity , to remove hardness salts and precipitate scale forming permanent salts .
– Control limit is 100 ~ 150 ppm. Double for Total Alkalinity test.
If the result is less than 100 ppm , dose the highly concentrate of alkaline chemical NaOH ( Called G.C )
– If the result is excess , action to blow down , because it may cause caustic corrosion and embattlement. - Chloride test: Measure content of NaCL & MgCL to prevent HCL formation in boiler water. Control limit is 240 ppm . Test result should less than limit . If excess, blow down action to be taken.
- Condensate pH value test: Measure to power of hydrogen value in condensate to prevent corrosion. Control limit is 8.3 ~ 8.6.
If the result is less than 8.3, dose SLCC-A corrosion inhibitor chemical increase by 25 %. If more than 8.6 , reduce by 25 %.
(d) When alkalinity is found to have fallen unaccountably in boiler water , this may be caused by
- Due to boiler water forming acidity by contamination of sea water or any chemical reaction in boiler water system
- Due to incorrect strength of reagent used
- Due to direct water losing from boiler, resulting all constituent level reduction.