COMMISSIONING BOILER ALKALI BOIL OUT ,ACID /EDTA CLEANING PART -VI-ABC OF THERMAL POWER PLANT

EDTA cleaning of Boiler 

Ammoniated EDTA with initial concentration of 4 +/- 1% is used for chemical cleaning of Boiler now a days instead of HCL acid cleaning.

Following Preliminary cleaning steps are taken before EDTA cleaning take place.

a. Alkali flushing with Tri Sodium phosphate ( 1000 ppm) and di-sodium phosphate (500 ppm) with boiler water temperature measuring at DRUM 140 degree centrigrade.

b. Hot water flushing with water temperature of 140 degree centrigrade.

c. Cold water flushing with completion criteria as phosphate < 10 ppm and pH = 7.0.

Special inhibitor is required for EDTA is already patented by BHEL ( an Indian Power Plant

manufacturer ) for their use.

Process :- EDTA Pickling

Material required:- Ammoniated EDTA ( 4 +/_ 1%) ; Hydrazine - 1000 ppm; inhibitor-

BHEL Max ( 0.15 % w/w) and BHEL Sol ( 0.15 % w/v) ; Ammonia to maintain PH 9.2-9.8.

Completion criteria :- Iron concentration stabilizes in three concurrent values or EDTA

process continuing  for more than 8 hrs.

Treated DM water rinsing :-  Material required Hydrazine 200 ppm  ,Ammonia to

maintain PH 9.2- 9.8.

Material required for EDTA cleaning Process  500 MW                    250 MW  ( Typical)

Tri sodium Phosphate (  Na3(PO4) 12 H2O     400kg                        250 kg

Di sodium Phosphate (Na2H(PO4) 7 H2O       200kg                        125 kg

Ammoniated EDTA ( C6H12N2O8(NH4)4       42000kg                      22000kg

Ammonia (NH3)                                                1500Ltrs                     1000Ltrs

Hydrazine ( N2H4)H2O                                   1500 Ltrs                     1000 Ltrs

BHEL max                                                         700 kg                         350 kg

BHEL Sol                                                          700 Lit                         350 Lit.      

Beside that well furnished chemical laboratory with necessary reagent and instrument 

like  Spectrophotometer, pH meter , Electronic mass balance , pipette ,burette , measuring


cylinder , volumetric flask etc.

Chemical analysis is carried out to monitor and control the chemical parameters

of the chemical cleaning process for ensuring safe and efficient chemical cleaning

of boiler.

Following chemical parameters are monitored during the process:

   Phosphate Concentration

   pH

  Total EDTA Concentration

  Hydrazine Concentration

  Iron Concentration

The chemical analysis to be carried out during various steps of Chemical Cleaning of Boiler

using EDTA.

A Typical sketch for 660 MW super critical set is given below.





 CONTINUED TO PART -VII EDTA CLEANING.

 

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)

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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.

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

During the acid cleaning process, all the permanent boiler instruments are isolated except

providing a temporary pressure gauge of        0 to 5 kg/ cm2 in the drum for monitoring

nitrogen pressure in the drum during nitrogen capping. 

For alkali boil out, neutralizing and passivation processes, essential control and instruments

viz. Protections, interlock, alarms, measurement, annunciation etc. required for  light up

of boiler are made available.  

Boiler is flushed thoroughly including the temporary pipe lines and system. Pre boiler

flushing to be completed for the availability of Boiler Feed Pump ( same will be discussed

during turbine commissioning). 

For chemical cleaning process, the internals of the non-return valve provided in the

economiser recirculation line is removed in order to facilitate fast filling and draining of the

boiler. 

All the boiler auxiliaries are made ready and available for boiler light up. This includes

Auxiliary steam, chemical dozing pumps, tanks, boiler circulating pumps, fans, oil pumps

etc. 

Drum vent lines, Nitrogen capping lines are properly tagged and taken to suitable location

for easy operation. 

Drum internals including primary and secondary separators, orifices at the bottom drum are

to be installed after the chemical cleaning process. The baffle plates inside the bottom

drums are to be installed before the alkali boil out. 

In addition to drum metal temperature 04 number metal temperature to be provided at

the corners of ring header and wired up to control room.  

Instruments are installed as per schemes. 

Various drains from the system at different operations during the process are to

be connected to a common drain leading to neutralizing pit. 

Boiler tube coupons placed at two ends inside the drum and bottom ring header to see the

effectiveness of the cleaning. 

Proper neutralising arrangement is available at site to neutralize and dispose off the effluent

in line with statutory authority of the state.

Chemical dosing system is ready in all respect.

 

MD-BFP is available for regular operation.

ALKALI BOIL OUT

The boiler (steam generating surfaces and economiser) is filled with an alkaline solution

containing 500 ppm of disodium phosphate  and 1000 ppm of Tri sodium phosphate  then

fired to raise the pressure to 40 kg/ cm2, following the specified operational procedures.

During the process, rotation of boiler circulating water pumps should be done as per

operating instructions. 

Procedure

Prepare the boil out chemical solution in hot demineralised water in the chemical mixing

tank. The solution is formulated by dissolving the following proportionate quantities of

chemicals, depending on the volume of the system to be cleaned. 

1.       0.05% Na2  HPO4       12 H2O

2.       0.1  % Na3 PO4         12 H2O 

Hold the solution prepared as above in the tank and homogenise the same by recirculating

the solution by operating chemical injecting pumps. 

Start filling the boiler with demineralised water, with simultaneous blend filling of already

prepared boil out chemical solution. 

Light up the boiler as per O & M instructions and gradually raise the pressure to 40 kg/

cm2 .  Hold the pressure for four hours. Obtain boiler water sample from CBD sampling point

and analyse for alkalinity, phosphate content, dissolved silica and oil content. Prescribe

blow down every 2 Hrs. once, after killing the fire; Duration the blow down shall be for one

minute  Ensure that the drum level does not go out of gauge glass vision. Repeat the

operation of raising the pressure to 40 kg/ cm2

 .

The boil out is declared completed when the oil value is no longer detected or minimum

eight blow downs are completed whichever is later.

Drain the boiler, when it is cooled, as per O & M instructions. Once the draining is

completed, fill the boiler with demineralised water, heated to 80 deg C, through boiler fill

pump. Back flush the super heaters with DM water containing 200 ppm hydrazine and pH

10.0 with NH3  till it overflows into the drum. Drain the boiler to the waste disposal system.

Inspect the lower and upper drum. Manually clean them.

Usual procedure of light up and shut down as described in O&M manual should be followed.

Temporary Piping arrangements and other arrangements at the boiler.

Continued to Part-IV for acid cleaning /edta cleaning procedure.

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