COMMISSIONING BOILER ALKALI BOIL OUT ,ACID CLEANING PART -V-ABC OF THERMAL POWER PLANT

In our last dispatch I have discussed about the acid cleaning of steam generator with HCL. Further steps are given below after acid cleaning.

DM RINSE 

Re-fill the boiler by running the boiler fill pump with DM water heated to a temperature of 60 deg C – 65 deg C taking care of the operation of the drum vent valves to maintain 0.5 kg/ cm2

 of N2 pressure in the drum. Fill the water to the centre of the gauge glass. Now raise the level in the drum by 100 mm, by back flushing the super heaters. This will also remove, if any trace of acid vapor lodged in the connecting links to the super heaters. Now run the boiler water circulating pump, that was operated earlier for 5 minutes. Drain the boiler under Nitrogen Capping as  earlier. Once the feed links, water walls and economiser draining is completed, close the drain valves and retain Nitrogen atmosphere in the system.

Ammoniated Citric Acid Rinse 

Prepare a solution of citric acid with the required quantity of citric acid monohydrate and liquor ammonia in the chemical mixing tank. Keep the solution under recirculation for better mixing. 

Start filling the boiler simultaneously through the Boiler circulating pump suction header, water wall and the economiser with hot DM water restricted to a temperature of 60 to 65 deg . C. Simultaneously start injecting ammoniated citric acid solution.

Maintain an uniform  strength of 0.2% acid in the water taken into the boiler. Adjust the PH. of the final solution entering the boiler between 3.5 and 4.0. Continue the filling operation, till the level in the upper drum is close to the bottom of the gauge glass. Raise the level in the drum by another 30 to 40 mm by filling through the economiser by closing the valve in boiler circulating pump suction header and continue filling through bottom drum and economiser so as to fill economiser and water wall connecting links. Further bring up the level in the drum to + 100 mm by back flushing the super heaters. Start the same boiler water circulating pump that remained in operation earlier, run it for 5 mts. and shut it down. Collect the sample from CBD sampling provision and check for acid % and iron concentration. When lron concentration attain a steady state value for 2 consecutive readings declare citric acid rinsing is completed. In any case it will not be completed in less than 4 Hrs. or more than 5 Hrs. Close the drum vents and establish nitrogen supply. Drain the boiler under a positive nitrogen pressure.

DEMINERALISED WATER(DM) RINSE 

After draining the citric acid solution, fill the boiler with DM water heated to 60 deg – 65 deg C. Raise the level in the drum up to + 100 mm mark. Operate the boiler water circulating pump for 5 minutes. Stop the pump and drain the boiler under nitrogen capping. Repeat the filling and the dumping operation under Nitrogen Capping till the iron content in the sample collected is less than 50 ppm. It is preferable to lower the iron value to less than 25 ppm. Maintain the nitrogen pressure of 0.5 to 1 kg/ cm2   and maintain the nitrogen atmosphere in the system till the next process of neutralisation is started.

NEUTRALISATION 

This process is employed to neutralize any acid/acidic pocket remaining in the system and also to mildly passivity the acid pickled surface. This prepares the base for achieving an effective final passivation. For this process, the boiler system and the necessary instrumentation should be made ready to make firing feasible. The ‘U’ tube manometer provided for the acid pickling in the drum should be replaced by the gauge glass used for alkali boil out. All the boiler water circulating pumps should be made ready for operation. 

Prepare the neutralizing solution in the chemical mixing tank with approximate quantities of di-sodium phosphate and tri-sodium phosphate  to get a concentration of 0.1% and 0.2% respectively in the boiler. Re circulate the solution in the tank for better mixing.

Start filling the boiler with DM water, with simultaneous injection of phosphate solution, maintaining uniform concentration throughout the filing period. Keep the drum vents open while filling. The boiler filling is to be done as per standard practice. Start the boiler water circulating pumps as per the O & M instructions. Fire the boiler and gradually raise the pressure to 10kg/ cm2 . Follow the precautions as detailed out for alkali boil out. Hold the pressure at 10 kg/ cm2   

  for about 4 hrs. Utilise this opportunity to blow off all the instrument lines that might have come into contact during acid pickling time, for better neutralization of those lines.

After 4 hours, kill the fire, allow the boiler to cool and drain the boiler as per procedure explained earlier and O & M instructions. While draining back flush the super heaters with DM water conditioned with hydrazine and ammonia. The boiler is drained with the drum vents open to the atmosphere. Allow the system to aerate. Prepare the boiler for passivation.

PASSIVATION 

The underlined idea is to form a protective magnetite coating on the steam generating and water touched surfaces that were exposed to the picking process.

Prepare a blended solution of hydrazine and ammonia with calculated quantities in the chemical mixing tank in order to finally achieve a minimum residual amount of 200 ppm hydrazine and a solution PH. of 10.0 when filled in the boiler. 

Fill the boiler with DM water, with simultaneous injection of hydrazine – ammonia mixture. Fill the boiler through bottom and economiser also up to the operating level in the drum gauge glass.

Prepare the boiler for firing as per O&M instructions. Start the boiler water circulating pumps as per O & M instructions and fire the boiler at a slow rate. Raise the drum pressure to 40 kg/ cm2

 and hold it for 24 hrs and maintain a residual hydrazine content of 25 ppm & a solution PH. 10.0 in the feed water being fed into the boiler during the entire passivation operation. Dose, hydrazine – ammonia mixture in the feed water at the suction point of feed pump through L.P dosing system. 

After 24 hours of operation, shut down the boiler and allow it to cool slowly. Keep the boiler water circulating pumps in service as per the O&M instructions. Shut down the pumps when the metal temperature drops to 95 deg C. Back flush the super heaters with conditioned DM water till the drum level raises by 25-30 mm. 

Drain the boiler under atmospheric pressure and aerate the system. 

Open the upper and lower drums for inspection. Manually clean them. Refit the drum internals and restore the boiler and prepare it for regular operation for steam blowing, safety valve floating etc.

NEUTRALISATION OF EFFLUENTS 

Alkali Boil Out & Neutralisation of pickled surface:

Acid pickling 

Effluent is neutralized with caustic soda. PH of disposed effluent may be maintained between 5.5 to 9.0 and fluoride less than 2.0 ppm.

Citric Rinse

Effluent is neutralized with caustic soda & bleaching powder. PH of disposed effluent to be maintained between 5.5 to 9.0.

Passivation

Hydrazine & Ammonia will be neutralized with bleaching powder.

The cleaning activity  should be done with a planning   that the boiler start up could be possible immediately after completion of cleaning process.

PH of the solution is lowered to less than 9.0 by sufficient dilution with raw / fire water.

(continued to part-vi EDTA cleaning)

P.S If  you are looking to earn few legitimate extra money from the comfort of your home then please click on the link given below

LINK 1 - PLEASE CLICK HERE 
LINK 2 - PLEASE CLICK HERE