What is the procedure of boiler alkali bailout, Boiler preservation and why it is necessary ?

The main aim of alkali bailout is Alkali Boil Out to remove loose muck including Iron Oxide, oil, grease and debris. It also ensure dry-out of the new refractory poured in burner-mouth or Air-heater area in the boiler.
Pre-checklist of alkali boil out:-
1) Ensure mechanically remove as much oil and grease as possible and boiler is completely flushed with fresh water from drums and headers before alkaline boiling out. Check all drum internals for tightness of bolted parts.
2) Ensure there is no loose materials, nuts, bolts, washers, etc. should be left inside the drum and All boiler instruments, leads and sampling line should be isolated except drum sampling and direct reading level gauge glass (better use temporary gauge glass) in drum..
3)Ensure the steam drum internals should be kept loose inside the drum prior to boil-out.
4) Ensure secondary Steam Separators (Scrubbers) can be kept outside the drum as chemicals can stick inside the corrugated surfaces which would be very difficult to clean afterwards. These parts can be mechanically cleaned before fitting into the drum after Alkali Boil Out.
5) Ensure ?all instrument impulse pipes connected to Drum except those minimum required during Boiler Pressure-raising should be isolated during the boil-out.
6) Ensure hydro test is completed before alkali.
7) that all temporary anchors made in the boiler during erection has been removed and boiler is free to expand.
8) Ensure all expansion reading indicator are erected properly.
9) Before introducing chemical to the boiler, particular care should be taken to eliminate possible leaks in the pressure parts and adequate precautions should be taken to protect personnel in the event leaks occur. 
Chemicals to be used:-
Generally the chemical is added 3 kg per ton of water in boiler. For example if the water holding capacity of boiler is 150 ton then chemical used is 150x3=450 kg. Here water holding of super heater is not considered or deducted from boiler water holding, because super heater is not filled with chemical.
i) Crystalline Tri-sodium Phosphate  and  sodium carbonate . In addition to the above chemical, a wetting agent (like TEEPOL or equivalent) may be added @ of 100 ml per ton of water holding capacity of the boiler. Final strength 1000 ppm phosphate and 500 ppm carbonate.

Procedure of alkali:-
Before starting Alkali Boil Out, the boiler has to be flushed with DM water as follows:
The boiler is filled through economizer or filling line using boiler filling pump up to the bottom of the manhole door of the drum. After filling, the water is drained partially by opening all the furnace bottom header drain valves for a few minutes. This process is to be repeated one or two times till the water collected from drains is reasonably clear. If the super heater back-filling arrangement is available, these can be filled with water for flushing till clear water starts coming to Steam Drum through the saturated pipes. Refill up the boiler with the DM water up-to bottom most portion of the steam-drum gauge
glass. Mix the required quantity of chemicals in the separate mixing drum, mix with water and transfer in small increment to boiler drum through open manhole door. After adding the chemical to boiler, close the drum manhole door. Light-up the boiler and start heating with slow-firing to raise the temperature at a very slow rate (1o C per minute for change in saturated steam temperature). As this firing will also be utilized for drying of the burner-mouth refractory, so slow-firing will help in proper dry-out of the refractory.
             The firing will be maintained throughout the day & boiler pressure will be raised up to 15 kg/cm2 (g) on the first day, maintained throughout the day and boxed up in the evening for overnight cooling. During pressure-raising, furnace bottom header-drains will be operated one at a time at every two-hour interval and flushed for a minute to remove the deposited sludge
utilizing the pressure of boiler. Samples should be taken at random for one or two drain samples to check the pH, alkalinity and conductivity. Traces of oil can be checked in the final stage. Drum-level to be monitored continuously using the Drum-level gauge or one level transmitter put in service for checking from control room. As these instruments (once put in service) will handle the high-concentration of alkaline water, so on completion of the Alkali boil out
process, the impulse pipes need to be flushed thoroughly during rinsing operation.
      On 2nd day morning, the drum level to be normalized by taking D, M. water and again lighted up in a similar-fashion like 1st day. Gradually, the boiler-pressure to be raised to 30 kg/cm2 (g) and maintained throughout the day. Intermittent flushing/sampling to be done like first day. Flushing by opening the bottom-header drains should be done up to 10 kg/cm2 pressure in boiler.
         On 3rd day after blow down normal drum level is restored boiler is lighted up and pressure is raised to 40 kg/cm2 and held for 2 hours. Samples drawn for analysis of pH, alkalinity , oil and phosphate. Again fire is killed and low point sequential blow down for 30 sec is given. This process is repeated for 8 to 9 times or till the oil content is nill (less than 0.5 ppm). If chemical concentration falls below half of the initial concentration, excess chemicals is injected to restore initial concentration. When test result shown no further repeat is required the boiler is allowed to cool down. and at 2-3 kg/cm2 pressure the entire system is drained through all the bottom drains keeping the vent open.

Necessity of boiler preservation –
When the boiler is kept standby for long period it is very necessary to keep  boiler in preservation to avoid  internal corrosion of tube. If water is left in boiler without properly treating for oxygen removal, can lead to corrosion. The attacks  on metals take place due to improper pH level too. Corrosion of the steam/water circuit, at ambient temperature is the result of
oxidization of metal by combined effect of water, oxygen and lowering of pH from absorption of CO2 and SO4.
Types of preservation:-
There are generally two types of preservation done as,
1) Dry preservation.
2) Wet preservation under hydraulic pressure.
1) Dry preservation:-

     The dry preservation method is effective provided the boiler is completely drainable. If Super heater/economizer sections are completely drainable then this method can be practicable. The plant which will remain unopened during outage can be completely filled with N2
gas and a little over pressure is maintained to prevent O2 ingress. The problem with N2 preservation is that if there is any leakage than that has to repaired. N2 being odorless, colorless and chemically inactive gas; it is very difficult to detect N2 leakage.
2) Wet preservation:-  

      Here in this preservation the plant item is completely filled with D M Water containing 200 ppm hydrazine ammonia is added to raise the pH to 10. chloride should be below 2 ppm. Keep proper pre-mixing of the chemicals with the water to insure a uniform mixture entering the boiler It is necessary to back fill the boiler from Superheater section and ensure the water overflows from Superheater to the steam drum. The water should be filled from the lowest point to ensure the air is properly released from boiler. The boiler pressure should be maintained  5 kg/cm2 that means above atmospheric pressure to prevent ingress of air. Every  day pressure should be monitored. Sample may be collected and checked periodically and if needed, add chemicals

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