RECOMMENDED PROCEDURE FOR ALKALI BOIL OUT OF 2 X 165

BHARAT HEAVY ELECTRICALS LIMITED

POWER SECTOR WESTERN REGION

RECOMMENDED PROCEDURE FOR ALKALI BOIL OUT OF 2 X 165 TPH AFBC BOILERS AT JSL, RAIGARH

INTRODUCTION:

The alkali boil out of a boiler is carried out in order to clean the internal surfaces of the water-touched parts from oil, grease, rust, silica and other adhered impurities

SCOPE:

The scope of cleaning during this operation includes a portion of feed line, entire evaporative system of boiler and economizer.

TECHNIQUE:

For carrying out the alkali boil out, 1000 ppm of Trisodium Phosphate and ppm of Disodium Phosphate are used as cleaning agents. The alkali boil out is carried out in one stage at drum pressure of 40 Kg/cm.

PREPARATIONS:

Pre-boiler system should be flushed with alkali solution and 2 nos. BFP should be made ready for regular operation from control room.

All the drum internals including the secondary steam separators are erected in position as per drawings.

Boiler should be made ready in all respects for regular operation upto drum pressure of 40 Kg/cm. For carrying out alkali boil out, external combustor will be utilized for raising bed material temperature of 1st compartment. Further parameters will be raised by coal firing in 1st compartment.

HP/LP Dozing system should be made ready for dozing of chemicals during boil out activity.

Boiler drum level g/g should be commissioned with lighting arrangement.

Feed water and CBD sampling lines should be ready for taking samples.

Required chemicals should be checked for quality and quantity. Arrangements for mixing of chemicals with DM water at drum level should be made. Lab facilities should be ensured for testing of samples.

Expansion constraints should be identified and removed. Expansion trams should be installed to monitor the movement of pressure parts.

Permanent lighting and communication facilities should be ensured at all working locations.

Supply of adequate quantity of DM water should be ensured before start of operation.

First aid box and safety appliances (Gum boots, Hand gloves, Face Masks and Aprons) should be made available.

PROCESS:

Drum manhole door should be opened and water should be filled upto feet below manhole door bottom level. Required quantity of chemicals should be mixed in a separate 200-liter drum and dozed slowly in boiler through manhole door.

After closing manhole door, boiler parameters should be slowly raised upto drum pressure of 40 Kg/cm. Expansion readings should be recorded at 10, 20 & 40 Kg/cm drum pressure and uniformity must be ensured.

Boiler drum pressure will be maintained for 2 hrs. For proper digestion of chemicals. CBD sample should be analysed for phosphate and oil level content.

Put out the coal firing and issue below down through low point drains by opening each drain valve for one minute duration. Ensure that the drum level does not fall below visibility limits. After every blow down, drain sample to be analysed for phosphate and oil content.

If phosphate content drops below half the initial value, further dozing should be done through phosphate dozing pumps.

Make up the drum level and again light up the boiler to raise pressure to 40 Kg/cm for next two hours. The blow down activity is repeated every two hours as per above procedure.

Alkali boil out is declared completed after oil content in sample collected shows less than 5 ppm or till 8 blow are completed, whichever is later. A minimum of 12 hours boil out is a must.

After completion of alkali boil out, Boiler is boxed-up and allowed for natural cooling. When drum pressure reaches 2.0 Kg/cm, open the drum vents. Hot drain the boiler when drum temperature is about 90 deg c.

Fill the boiler upto minimum level of drum and back flush the superheater into the drum, raising level upto normal. Then drain the boiler and analyse drain sample for phosphate. Repeat this activity till phosphate content drops below 25 ppm.

After completion of alkali boil out, drum manhole doors should be opened and checked for any loose debris and the same should be cleaned. Thereafter, boiler is released for acid cleaning operation.

PROCEDURE FOR ACID CLEANING OF 2 X 165 TPH AFBC BOILERS AT JSL, RAIGARH

INTRODUCTION:

Modern power stations demand high purity of steam in order to prevent fouling of the turbine and safe operation of the power plant. High purity of steam depends on the cleanliness of the waterside of the boiler.

Boiler components during manufacture in shop from raw material to final shape and during erection at site undergo both physical and chemical changes. These include rust due to atmospheric corrosion and formation of oxides of iron due to the heat treatment at shop.

Chemical cleaning, apart from imparting cleanliness of waterside surfaces, leaves behind a thin uniform protective layer of magnetic oxides of iron (called magnetite).

SCOPE:

The scope of cleaning includes a portion of the feed line complete evaporative system of the boiler. During pickling the superheater connecting tubes will be plugged suitability from inside the drum and remain filled and pressurized with Ammoniated Hydrazine water confirming to pH 10.0 and Hydrazine content not less than 200 ppm.

TECHNIQUE:

Chemical cleaning of the boiler will be carried out by a pickling solution of 4% inhibited Hydrochloric Acid by circulation method. To intensify the action of Hydrochloric Acid and to dissolve siliceous impurities, Ammonium Bi Fluoride is added to the cleaning solution. To lessen the action of acid on parent metal suitable inhibitor is added to the picking solution. In the final phase of cleaning the freshly pickled surfaces are protected from corrosion by a passivated layer of magnetic formed chemically by the addition of Hydrazine and ammonia. The above layer is reinforced further by the process of second stage passivation, at a later stage, at a drum pressure of 40 Kg/cm.

PREPARATIONS:

Alkali boil out of water-touched parts should be completed as per procedure.

The scheme of connecting the boiler and pipelines including provisional equipment is attached (the provisional equipments are designed to achieve circulation of chemicals in WW/Eco tubes and consist of dissolving tank, circulating pumps (3 nos. of 150 M3/Hr, 12 Kg/cm), connecting pipes and valves.

Arrangements should be made for continuous supply of DM water and heating steam to the dissolving tank (as required for the process) with valves and fittings as per scheme. Steam should be made available at the rate of 10 TPH at 10 Kg/cm, 250C at inlet of dissolving tank. Sufficient storage of DM water must also be ensured before start of process.

Arrangements should be made for nitrogen capping during the process of pickling. Required number of nitrogen cylinders should be made available before start of pickling process.

Representative water wall tube sample should be hung inside the drum as well as dissolving tank.

Inhibitor should be tested before use for its efficiency.

For neutralization of acid, its preferable to dig an earthen pit of 200 M3. Necessary temporary drain lines and raw water lines should be laid accordingly. Arrangement should be made for pumping out of neutralized solution into the plant drain canals.

Fire fighting arrangement, first aid and safety appliances should be kept ready before start of the process. Service water facility should be arranged near tank area for the purpose of getting water bath in case of contact with chemicals.

Before charging chemicals, circulating water pumps must be trial run and system should be ensured leak proof.

Lab facilities including services of chemist must be ensured for analyzing the samples as per the requirement of the process.Remove all the drum internals.

Fixing of down comer orifice

Isolate all the instrument tapping, gauge glasses,

SH drain & fill connections from LPD header should be disconnected/isolated.

PROCESS:

Acid Pickling:

Fill the entire system with DM water. Establish circulation in the system by running circulating pumps. Maintain 3 to 5 Kg/cm pressure in the drum.

Ensure that water flows from the drum vents to the collecting tank and to the mixing tank.

Admit steam into the mixing tank and raise the temperature of circulating water to 60C as measured in return line. Continue circulating for four hours for the purpose of hot flushing and drain the system completely.

Take fresh DM water into the tank and establish circulation as per steps i + ii and raise the temperature of water to 60C.

Cut off steam and add required quantity of inhibitor, with the pumps running to achieve through mixing.

Add required quantity of 30-32% Hydraulic acid in a controlled manner so that acid concentration as measured in the sample from pump discharge does not exceed 6%. This sample is to be collected at intervals of 10 minutes.

Total quantity of acid required to achieve acid concentration of 4% in the circulating solution is to be added in one hour. Simultaneously charge calculated quantity of ammonium bifluoride into the mixing tank by several increments.

Collect sample from circulating solution, at an interval of 15 minutes, at the pump discharge and return line and analyse for acid strength and iron concentration.

Stop circulation once three consecutive samples show almost same values of acid and iron concentration or if the circulation is done for maximum of 4 hours (total acid contact time to be less than 6 hours).

Drain the system under N2 blanking

D. M. Water Rinse

On completion of draining of acidic solution, fill the system with plain D. M. water and establish circulation.

Collect samples from pump discharge and return line and analyse for acid and iron concentration.

Drain the system under nitrogen blanket.

Citric Acid Rinse

On completion of draining, fill the system with DM water and establish circulation.

Raise the temperature to 55C and add 0.2% W/v citric acid monohydrate.

Add liquid ammonia to raise the pH of the solution to 3.5 to 4.0 and circulate for 2 hours.

Collect samples and analyse for pH and iron content, drain the system under nitrogen cover when the values stabilize.

D. M. Water Rinse

On completion of draining rinse the system with plain DM water.

One or two rinsing operations are to be done till the iron content drops down to 50 ppm and acidity is absent.

All the draining operations are to be done under nitrogen blanket.

Neutralization

Fill the system with plain D.M. water; raise the temperature to 85-90C after establishing circulation.

Add required quantity of 0.1% Disodium phosphate Na2 HP04 7H2O and 0.2% Trisodium phosphate Na3 PO4 12H2O and maintain circulation for 6 hours.

Hot drain the system under atmospheric air by keeping the drum vents open.

Passivation

The objective of passivation is to prevent corrosion since the cleaned surfaces are more susceptible for corrosion at a very fast rate. The corrosion is prevented by providing a thin of magnetite (first stage), which gets strengthened in the subsequent firing of the boiler (second stage).

First Stage Passivation

The system is filled with DM water and adjusted for circulation. The temperature of water is raised and maintained at 85C by external steam heating. PH of DM water is raised to 9.5 by adding ammonia and hydrazine is changed to get a connection of 200 ppm. The solution is left to circulate for about 20 hours at a temperature of 85 90C. The hydrazine concentration is maintained at 200 ppm through out the operation.

After 20 hours, the solution is drained hot from the system and left aerated by natural draft.

Inspection of preparatory work for second stage passivation

The drum is inspected thoroughly and all loose debris cleaned manually. Bottom headers hand hole pipe connections are removed and the headers are cleaned with water.

The super heater plugs and down comer orifice plates are removed and drum internals are refitted. SH connections to be normalized .All the provisional equipments are dismantled and original fittings and mounting are installed back. The boiler should be prepared for regular operation, upon completion of hydraulic test at working pressure.

Second Stage Passivation

The chemicals viz. ammonia liquor, raising the pH to 9.5 and hydrazine, to get a concentration of 200 ppm are charged in the operating level in drum and lighted up. The steam pressure in drum is gradually raised to 40 Kg/cm and maintained at that value. Water samples are taken from feed line to find out hydrazine concentration and pH value. The boiler is kept in service for 24 hours maintaining the hydrazine concentration not less than 20-25 ppm in feed water.

At the end of the process, the boiler is boxed up and allowed for natural cooling. At 2 Kg/cm drum pressure, all vents are opened and at about 90C drum temperature, the water in the system is drained and the system is left aerated.

The second stage passivation marks the end of the chemical cleaning process. Thereafter boiler is released for steam blowing operations.

SPECIFICATION OF CHEMICAL AND QUALITYFOR ALKALI BOIL OUT AND ACID CLEANING ACTIVITIESOF2 X 126 TPH AFBC BOILERS AT JSL, RAIGARH(FOR ONE BOILER ONLY)

SL. NO.NAME OF CHEMICALQUALITY

1.Disodium Phosphate Na2HpO47H2O250 Kg.

2.Trisodium Phosphate Na3PO412H2O(IS: 573: 1973)500 Kg.

3.Hydrochloric Acid: 30%(IS: 265: 1976)20 Tons

4.Coronil: 213 Special150 Liters.

5.Citric Acid(IS: 5464: 1970)300 Kg.

6.Ammonia: 25%120 Liters.

7.Hydrazine Hydrate: 80%400 Liters.

8.Nitrogen Cylinders (6.2 m3 each, IS: 1747: 1972)120 Nos.

9.Ammonium Bifluoride600 Kg

10.Hydrated Lime15 Tons

11.Beaching Powder3 Tons

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