CHEMICAL CLEANING OF BOILER

KARTHICK POOBATHI,

KALYANAM SUDHEER KUMAR

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Purpose of Chemical Cleaning of Pressure

Parts:

To remove dirt, grease, oil, rust etc. which are present

in the pressure parts of boiler.

Advantages of Chemical Cleaning of Boiler:

1) Quality of steam can be maintained at the turbine

inlet.

2) Corrosion of the metal surface of boiler can be

minimized.

3) Under deposit corrosion of metal can be avoided.

4) Heat transfer of the system can be improved.

5) Plant Outage due to boiler tube failures can be

minimized.

Chemical Cleaning Alkali Boil Out

Acid Cleaning

Precaution: During this process normally steam drum

internals are removed and a temporary gauge glass is

fitted .

Alkali Boil Out Process:

1) Tri Sodium Phosphate(TS 0.1%) , Di Sodium

Phosphate (DSP 0.05%), caustic soda and soda ash are

dozed to DM Water.

2) Light up the boiler as per instructions and raise the

temperature of solution up to 130 to 140 deg C by setting

the LDO oil pressure.

3) Boiler level to be monitored and care to be taken to

open the Economizer R.V.

4) At 100 deg C Eco. outlet temp. steaming is started in

the boiler. At 110 deg C Eco. outlet temp. the boiler drum

pressure is 2Kg/sq.cm. Close all the vents.

5) At 5kg/sq.cm trip the boiler by withdrawing all LDO

guns. Keep ID fan & FD fan running slowly. At 110 deg C

open all the vents.

6) At 90 deg C boiler drum temp. open all the drains and

completely drain the solution to specially prepared

disposal tank.

Hot Water Rinse:

1) After the draining the alkali sol. completely fill the boiler with

fresh DM water using ELP from Boiler Filling Tank.

2) Again the light up the boiler and raise the boiler pressure and

temp. to 5 to 6 kg/sq.cm and 130 deg to 140 deg C. Now trip

the boiler and drain the boiler when the temp reaches 100 deg

C.

3) Values of pH & PO4 should be less than 7.5 and 10 ppm

respectively.

ACID CLEANING [EDTA CLEANING]:

What is EDTA?

Ethylene Di- amine Tetra Acetic Acid is a strong chelating agent

having ability to form stable complexes with transition metal

ions present in Boiler such as Iron & Copper.

Advantages of EDTA over HCL Cleaning

Process:

1) DM Water and fuel consumption is less.

2) Shorter Cycle time(5 days).

3) Less Temporary Piping required.

4) Less no. of chemicals required.

5) Effluent Treatment is easy.

6) No usage of Nitrogen.

Preparation of EDTA Solution:

Keep the required amounts of EDTA solution ready in EDTA in

EDTA storage tank. Keep the inhibitors BHEL Max & BHEL Sol

and Hydrazine ready for adding in chemical mixing tank.

Ammoniated EDTA : 40% wt/vol

BHELMAX(inhibitor) : 0.15-0.2% wt/vol

BHELSOL (inhibitor) : 0.15-0.2% wt/vol

Hydrazine Hydrate : 1000 ppm

Ammonia : For setting pH of sol. as 9.5 to 9.8.

EDTA Cleaning Process:

Precaution: Normally acid cleaning is not done to the non

drainable parts of super heater.

1) Thoroughly mix all the above chemicals in the chemical

mixing tank with DM Water and inject this solution to the

bolier upto 3 port level.

2) Light up the boiler by taking LDO guns into service and raise the

boiler drum pressure to 5 to 6kg/sq.cm and 130 to 140 deg C

temperature.

3) Kill the flame & keep the fans running and note down the time

taken for the water to reach 110 deg C.

4) The EDTA Acid reacts with iron oxide of the inner surface of boiler

tube . So the concentration of Iron in the solution increases.

6) The Cleaning process us repeated till the iron concentration

in

consequent samples show equilibrium status.

7) At 90 deg C temperature remove the disposal to the deposit

tank. However EDTA content once after attaining 110 deg C

shall not be allowed more than 8 hours to stay in boiler.

Passivation or Magnetite Layer Formation:

Magnetite is a combination of FeO & FeO3 is protective layer

on the water touched metal surfaces which stops further

corrosion of the base metal.

During the EDTA Cleaning process the removal of deposit

constituents and subsequent passivation of metal surfaces

will occur in a single step.

Treated DM Water Rinsing:

1) Prepare a blended solution of NH3 , Hydrazine and DM

Water with calculated quantities in the dissolving tank in

order to achieve a solution of pH of 9.0 to 9.5 & Hydrazine

content of 50 ppm and to be filled in boiler 100mm above

the normal level.

2) Light up the boiler and raise the boiler water temperature up

to 120 deg C and kill the flame. Allow the boiler to cool

down naturally.

3) When the boiler water temperature comes to 95 deg C drain

the water to the effluent pit and allow the system to cool by

natural aeration.

4) Open the inspection drain manually and clean the drum if

required.

Superheater Back Flushing:

Super Heater is filled with a solution of hydrazine of 50 ppm &

Ammonia of 9.0 to 9.5 pH in DM Water through Super Heater

drain header.

When the solution comes through Superheater openings to the

Boiler Drum, the back flushing is continued for 10 to 30 minutes

and drained.

Conclusion:

After the Alkali Boil Out and Acid Cleaning the Magnetite

Layer is formed on the metal surfaces of water touching

surfaces of boiler thus increasing the boiler efficiency and

decreasing the boiler tube failures.