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

In continuation to my last blog on alkali boil out and acid cleaning  I will discuss the process of Acid cleaning with HCL ( hydro cholride acid) after completing the above I will discuss the chemical cleaning process  with EDTA . Application of EDTA  is new to INDIA  since last ten years . Lot of apprehension and reservation were shown by customers during its use but now a days it has been followed in every process plant. However I will restrict my discussion on thermal power plant only . At the end advantage of EDTA over HCL cleaning will be addressed.

Note :- It is prudent to follow the OEM’s instruction about ABO & AC cleaning and method to be used in steam generator.

ACID CLEANING

Acid cleaning is carried out by adopting soaking technique, using hydrochloric acid of 5%  + 1%, 0.2%  V/V Rodine 213 Spl. And 0.25% ammonium bi-fluoride. Pickling by acid is followed by plain demineralised water rinse, ammoniated citric rinse (0.2%), plain demineralised water rinse and neutralisation of the pickled surfaces with a mild alkaline solution containing a mixture of di and    tri    sodium    phosphate. The  entire cleaned and neutralised surfaces are passivated with ammoniated – hydrazine  solution at an elevated temperature and pressure. All the draining operations prior to the neutralisation step are to be done under nitrogen capping. The gauge glass used for alkali boil out is to be replaced by a ‘U’ tube manometer.

PROCEDURE:

Back fill the super heaters with demineralised water containing a min. of 200 ppm hydrazine with

solution PH raised to 10.0 using ammonia. Having ensured that the super heaters are filled, fill

the boiler simultaneously through water wall and economizer with plain demineralised water up

to the normal operating  level. Light up the boiler as per the O&M instructions and raise the metal temperature both at the drum and suction manifold to 80 deg. C with boiler circulating pumps in service. Once the temperature is reached, shut down the pumps and drain water from boiler to the waste disposal system. Close all the furnace openings to retain the heat . Mix the required quantities of 30 – 32% Hydrochloric acid and Rodine 213 speical, inhibitor and keep it ready in the acid tanks (2 Nos.). Dissolve required quantity of ammonium bi-fluoride in D.M water and keep it ready in the chemical mixing tank. Transfer the ammonium bi-fluoride solution into the acid tank. Thoroughly mix the inhibited acid and transferred ammonium bifluoride solution by running the chemical injection pump on recirculation for a minimum of one hour, particularly prior to the commencement of chemical injection into the boiler.

Keep all the boiler blow down valves shut and drum vent valves open to facilitate escape of gases generated during pickling process.

Keep the nitrogen capping system ready prior to the commencement of acid filling.

Boiler circulating pump for operation to circulate the chemical solution during soaking/pickling time. Keep a continuous supply of purge water to all the boiler circulating pumps.

Start filling the boiler  through water wall and economiser with heated demineralised water

restricted to 65 deg C – 70 deg C. Simultaneously start injecting the chemical solution already

kept prepared in the acid tanks (mixture of inhibited hydrochloric acid and ammonium bifluoride)

along with the fill water. Suitably control the opening of the control valves to achieve a uniform

concentration of 5% +  1% acid concentration in the water being filled in the boiler. The

regulation of uniform acid concentration is done by very frequent sampling, testing and suitable

adjustment of the opening of the chemical  injection control valve. The temperature of the

chemical mixed water should in no case and at any point of time exceed 65 to 70 deg. C. Stop

filling through suction header of Boiler circulating pump. When the level in the drum is at the

bottom of the gauge glass. But continue filling through the bottom drum and economiser till the

level in the drum touches 100 mm above the centre line of the gauge glass. This will ensure

complete filling of ECO links to the drum and the water wall riser tubes. Do not permit the level

to go out of sight in the gauge glass. Once the filling is completed, start the selected boiler

circulating pump. Immediately collect sample of the filled solution from the boiler after the start

of the circulating pump. Subject the sample to analysis for evaluating acid strength and total iron

concentration.

The boiler circulating pump will be operated for 30 mts. At this stage collect sample at every 5 mts. Interval and analyse for acid strength and iron concentration. At the end of 30 mts. period stop the pump and allow the chemical solution to soak. Continue sample collection and chemical analysis for acid strength and iron concentration till three samples show equilibrium status. If the acid strength and the iron concentration values do not level out and attain equilibrium status, operate the same boiler circulating pump that was in service earlier for 5 mts. at an interval of 30 mets. Regardless of analytical results, the total acid contact time will not be allowed to exceed 6 Hrs. including the time for filling and draining.

Declare the completion of pickling process once the acid strength and iron concentration level out and reach equilibrium. Now, operate the same Boiler circulating pump, that remained in service earlier for 5mts. Stop the pump and drain the unit under Nitrogen Capping, maintaining the nitrogen pressure in the drum around 0.5 to 1 kg/ cm2 . Drain the boiler as fast as possible, maintaining the nitrogen pressure, to waste disposal system.

CHEMICAL  REQUIRED FOR ENTIRE PROCESS – QTY TYPICAL FOR 500MW SET

Na2  HPO4  12 H2O  DISODIUM   PHOSPHATE                       800 Kg.                   1S 566 – 1991

Na3 PO4 12 H2O   TRISODIUM PHOSPHATE                        1500 Kg.                      IS:573:1992

HYDROCHLORIC ACID : 30% MIN                                              70 Tonnes.              IS:265:1993

RODINE : 213 SPECIAL (PROPRIETORY ITEM)                         800 Ltrs.

CITRIC ACID                                                                                 800 Kgs.                    IS:5464:1991

AMMONIA : 25%                                                                        600 Ltrs.                     IS: 799-1990

HYDRAZINE HYDRATE: 80%                                                     750 Ltrs                 IS:12086 – 1992

NITROGEN CYLINDERS: 6.2 mm3  each                               500 Nos.                IS:1747:1991

AMMONIUM BIFLUORIDE ; 97%                            1000 kg.                IS:13119 – 1991

HYDRATED LIME : Grade – A                                              05 Tonnes.   IS:1540:1990 (Part – II)

BLEACHING POWDER : Grade – I                                       06 Tonnes. IS:1065 – 1989

CAUSTIC SODA (IS 252)                                                      30 Tonnes.

I have tried to add IS code so that specification can be obtained and equivalent also can be found in other standards.

Continued to PART-V for DM water rinsing ,citric acid rinsing etc.

 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