Answer MEP Question 51

Q. Describe how the following conditions are prevented in auxiliary boilers: 

A. Feed contamination by oil from heating coil drains; 

B. Internal corrosion; 

C. Furnace blowback; 

D. Uptake fire;? 

A. Feed contamination by oil from heating coil drains

- As a foremost thing check and find the source of leak and isolate the complete steam coils circuit for the affected tank. 

- Next open the dirty condensate return to the bilge holding tank so that any remaining oil in the line is flushed out to the bilge instead of hot-well. 

- Simultaneously inspect the cascade tank or hot-well, if oil is visible in the last compartment through the inspection glass then it is recommended to check whether a dark oily film contaminates the boiler water level glasses inside. 

- If no oil traces or see it could be early that the boiler may not hay been contaminated yet, in this case the hot-well can be skimmed using the skimming valve by crack opening it and by skimming it. 

- If oil traces or seen on the boiler gauge glass the boiler needs to be drained and inspected and the tubes shall be cleaned adequately with proper cleaning procedure before returning the boiler back in to service. 

- Do not blow down the boiler from the bottom; just scum blowdown several times and increase  the dosing of boiler coagulant chemical. If bottom blowdown the boiler, all the tubes will be totally covered in oil. 

- The boiler should be shut down and pressure released and the venting valve opened. Afterwards, the boiler can be drained slowly until water stops flowing from the loosened, upper manhole door before opening up the manhole for inspection. 

- The oil will now only cover the boiler in the normal water level range and can be manually removed. Hot or cold water high pressure jet equipment together worth oil dispersive additives would be efficient for removal of the oil. 

Preventive action 

- Check and try out the oil in hot-well sensor and alarm, subsequent changeover of the 3-way valve 

to, bilge holding tank if provided. 

- Proper watch keeping procedures to be followed in keeping a constant monitoring of the hot-well inspection glass for any trace of oil. 

- Before opening steam to any tank that has not been in use for some time, please drain the condensate adequately before diverting the condensate to the condensate main to see if there are 

any oil traces found. If oil trades found shut the steam immediately and pressure test the coil for any possible, leak. 

- The feed water / hot well / cascade tanks have to have filters installed, filled with loaf sponges, coconut fibers or similar, which effectively take up small amounts of oil. 

- The filter inserts have to be exchanged regularly according to demand. 

B. Internal Corrosion 

- There are three principal forms of corrosion, direct chemical attack, electro chemical attack and Galvanic corrosion 

- Direct chemical attack mainly occurs in super heaters due to the high metal temperatures involved. It can result in pitting or cracking in tubs bores, or in scaling or flaking on the gas side of tubes. 

- This form of Corrosion also occurs when loss of water circulation causes the metal to overheat in the presence of steam. 

Remedy:- General wastage of the use of the boiler metal due to this form of attack has been virtually eliminated by the use of chemical feed water treatment but isolated pitting can still occur if the treatment is not operated with in the correct limits. 

- It may be found along the water level in the drum. Generally, as the result of poor shut down and storage procedures where the boiler is left partly filled with cold water. Pitting along the roof of the drum can result from condensation hence proper shutting down procedure shall always be practiced. 

- In the lower parts of the boiler it can be due to poor drainage, pools of water remaining in drums and heaters. Hence while draining the boiler prior to opening the manhole door complete draining of water to be ensured, after opening the manhole immediate inspection to check if there is any pool of water present if pool of water is seen then the same need to be removed and the spot shall be dried out quickly. 

Electro chemical corrosion 

- when the boiler water is acidic in nature (pH value less than 7) then hydrogen ions in contact with the metal surface becomes hydrogen atoms by taking an electron from the metal. The resultant metal ion (caused through loss of electrons) combines with the hydroxyl ions in a metallic . hydroxide, hence the metal is corroded. 

- Hydrogen atom may form a polarizing layer upon the metal surface and prevent further corrosion. If, however, dissolved oxygen is present in the water, it will combine with the hydrogen to form water and polarization will occur and corrosion will continue. 

- Also, if the water is acidic enough, the hydrogen can leave the surface of the metal in the form of hydrogen gas, again preventing polarization and continuing corrosion. Hence, the need for the boiler water to be alkaline and with little or no dissolved oxygen content. 

Remedy: -- As a daily routine check the p1-1 value and ensure the pH value is maintained between 10 - 10.5 - Alkalinity control chemical shall be dosed as per recommendations; High alkalinity also leads to caustic embattlement. 

Galvanic corrosion - When two dissimilar metals are present in a saline solution galvanic action takes place, resulting in the corrosion of more base metal. 

Remedy:-- Sacrificial zinc anodes are frequently used to give catholic protection to steel plating scotch boilers.

C. Furnace Blow Back 

- Maintain burning equipment / burner as per PMS. 

- All parameters such as oil pressure, water pressure, steam pressure, air amount etc. to be closely monitored. 

- Regular inspection of the flame - Boiler should be fired and maintained by qualified engineer Adequate pre-ignition purging 

- If flame failure occurs on any other reason boiler trip then do not forget to pre-purge the furnace at-least for 3 min before firing in manual mode. 

- do not bypass any safety feature of the boiler combusting control. 

D. Uptake Fire: 

- Uptake fire generally occurs due to accumulation of soot. 

- Soot mined with fuel oil/ lube oil poses more risk. 

- Regular soot blowing with adequate steam pressure if It is done before purging cycle it is beneficial. 

- Use of soot releasing chemicals. 

- Correct fuel oil temperature and atomizing steam pressure 

- Good fuel treatment 

- Burning equipment is in correct condition. Regular maintenance of burners, atomizers and pilot burners. 

- Correct turn down ratio $\mathrm{=\frac{minimum\ oil\ pressure\ In\ burner}{maximum\ oil\ pressure\ In\ burner } }$  

- Sufficient venting of flue gases before every flash-up

- Calibrated smoke density, alarms, Aux. Boiler exhaust gas temperature alarms.

- Flame Screen and wine mesh at uptake in clean condition.

- Regular water washing and de mucking of exhaust gas side. 

- Regular inspection of flame cone and pattern.