Alkali Boil-out

[mechcolor]

What is alkali boil-out and what is the purpose of doing it?

[whaider]

Before start-up of a new boiler, it is necessary to remove all foreign matters from boiler, steam drum and piping after erection and assembly. Moreover, a new boiler will usually contain grease or oil which must be removed by boiling out . A light film of non-conducting oil could very definitely cause overheating the boiler. To overcome this possibility, boiling out the boiler using a chemical charge is recommended

[mechcolor]

Is it mandatory? What if alkali boil-out not done? Risks involved?

[whaider]

Possible risk is over heating as non-conducting oils and grease are used in manufacturing process and this overheating shall damage the boiler. As boil out is done with chemicals like Na2CO3, NaOH, Na3PO4 which removes any such layers of non-conducting oils/greases.
Your boiler manufacturer will be in a better position to guide you on the importance for yor particular case.

[whaider]

chemical cleaning is also employed as an alternate to boil out using temporary arrangements. there are some specialized companies who offer there services specifically for this.

[sharjeel]

Can you please elaborate a little more, what types of chemicals are used and different steps involved.

[whaider]

Below are the steps involved in the boil out activity. Please contact services for a free advice on your specific case with safety and operational cautions.

Step-1 Preparations

1) When boiling-out, it is most important to keep the water hot and circulating (by natural circulation)

2) Before any operation of rinsing or cleaning is started, it is necessary to remove all instrumentation, PSVs, Silencers etc.

3) Install temporary gaskets (compressed asbestos) on manholes and other flanges connections

4) The internal parts piping, impingement plates will be left in place

5) Following is the list of chemicals that shall be required
3 Kg/m3 water : Calgonite (Product of Hall Lab.)
1 Kg/m3 water : Caustic Soda
0.5Kg/m3 water : wetting agent (a good house hold or commercial detergent).
0.25Kg/m3 water : Sodium nitrate

If above chemicals are not available, following set can also be used
3 Kg/m3 water : Soda ash.
3.2 Kg/m3 water : Caustic soda.
0.50Kg/m3 water : Wetting agent.
0.1 Kg/m3 water : Sodium sulphite (Na2SO3,7H2O) Addition of sodium sulfite is recommended if the water is not deaerated. It can be replaced by Hydrazine.
3 Kg/m3 water : Sodium phosphate (Na3PO4,12H2O)

[whaider]

Step-2 Preliminary Washing & Hot Degreasing

1) Fill-up the system with clean demineralised water until it overflows by the vent nozzle. Drain the entire system and repeat this for some times.

2) Fill the boiler unit to 100mm below normal operating level with demineralised water. Measure the quantity of water used.

3) Feed the following chemicals in cold water (For each cubic meter of water in the system).

3 Kg of Calgonite
0.75 Kg of Caustic soda
0.25 Kg of wetting agent
0.25 Kg of Sodium nitrate

or

3 Kg of Soda Ash
3 Kg of Caustic Soda
0.25 Kg of Wetting agent

4) Gradually heat the contents to 90 Deg.C and keep it for 48 hrs.

5) Fully drain the system

[whaider]

Step-3 Pressurized Boil out

1) Fill the boiler with demineralised water to 100/150 mm below the normal operating level

2) Feed the following chemicals in cold water

3 Kg of Sodium phosphate
0.2 Kg of Caustic soda
0.1 Kg of Sodium sulfite
0.25 Kg of Wetting agent

3) Adjust the water level to the drum centerline. Leave drum vents open

4) Heat slowly the water with hot gases and steam injection. Temperature raising for water will be no more than 50 DegC/hr

5) Close steam drum vents when 1 Kg/sq.cm pressure is reached. (If there is a superheater leave its vent and drain open)

6)Increase the heating and adjust it until the boiler pressure is approximately 15 to 16 Kg/sq.cm. Pressurize & Depressurize the drum a few times. Steam exhaust will be controlled by the vent valves to maintain the recommended pressure for next 48 hours

7) During the desired steaming period, add water slowly to maintain the water level

8) Every 4 hours, using the continuous blowdown line, blowdown until the water falls to the continuous blowdown connection. From this point, use the intermittent or bottom blow off connection to further lower the water level to a point 50 mm above the low level cut off

9) After each blow off, re-fill the boiler to its normal water level.

10) Samples of the blowdown shall be taken at the start of blowdown cycle mentioned in step-8 and also at the end from CBD/IBD points:
Visual inspection for signs of oil or contamination
pH measurement : below 9.5
Chemical analysis

11) When the phosphate content remains unchanged for three blowdown, the boiling out can be considered as achieved.
Samples of the blow-down will be the guide for ending the process.

12) Upon completion of the boil-out, cool the entire system slowly. Drain the unit while still fairly warm, re-fill and drain once more. Please note that the temperature difference between rinsing water and metal surface of steam drum must not exceed 60 C.

13) Inspect the internal areas of the steam drum carefully and remove dismountable panels to ensure that no trace of oil remains, and that all foreign material, dirt, scale, welding spatter and metal chips have been removed from the drums. These heavier particles will be found in the boilers and downcomer pipes and in pocket locations around baffles. Remove them by washing with the high pressure hose through handholes.

14) If no trace of oil can be found, the system is ready for operation. If oil traces are found, repeat the boil-out procedure.

[mechcolor]

This time around you shared a lot more things. Very informative. Thanks.
Can you please share the typical results of blow down sample after alkali boil out?