Boiling Out

Boiling Out

Boiling out of the boiler and the steam/feed water system shall be done:
1. before initial start up
2. in case of oil in the boilers water side
3. in case of heavy corrosion.
There are two methods of cleaning:
- alkaline boiling
- acid cleaning.

Boiling out before the boiler is placed in service to remove any foreign products

The unit should be filled through the feed connections to the normal working level with water of the same quality as that of the feed water to be used for regular service.
Care should be taken to use only properly treated and deaerated water, so that optimum conditions will be in force from the start in limiting the formation of scale, corrosion and other difficulties caused by improperly conditioned water.
The chemicals used for boiling out are completely dissolved in water before they enter the unit. The dissolved chemicals are pumped into the boiler through the chemical feed connection or, as a second choice, the chemical solution may be poured through the manhole after bringing the water level up to about half way between the bottom of the drum and the manhole.
With all the water in the unit at the concentration noted below, a good cleaning job can be done with any one of the four following combinations of chemicals:

Trisodium phosphate (Na3PO412H2O)
Caustic soda (NaOH)

2500 ppm
2500 ppm
Trisodium phosphate (Na3PO412H2O)
Caustic soda (NaOH)

5000 ppm
500 ppm
Trisodium phosphate (Na3PO412H2O)
Caustic soda (NaOH)
Soda ash (Na2CO3)

1500 ppm
1500 ppm
1500 ppm
Caustic soda (NaOH)
Soda ash (Na2CO3)
3000 ppm
3000 ppm

Start firing slowly. The steam pressure for boiling out is 3.5-4 bar minimum. Once vapour is formed, the pressure should be reached slowly and at a minimum and consistent rate of firing.
The boiler and each piece of the system, should be blown down at least once every 8 hours during boiling out. The total amount of water blown down should be equivalent of a difference in level of one half gage glass, divided equally between all main blow downs. If necessary, water should be added between the blow downs of individual valves to hold the level within safe limits.

At the end of blow down, the water should be brought up to the normal working level and replenished with chemicals to the proper concentration.

Experience indicates that 36 to 48 hours should be sufficient. The procedure should be continued until examination of water samples from blow down lines indicate that no foreign products are present.

This procedure is to be used when rust or mill scale is known to exist to a high degree in the boiler. Normally, acid cleaning will not be required on new boilers.

The procedures given below outlines one method which can be used on marine boilers. Other methods are possible, such as cleaning with citric acid. Acid cleaning by any method should always be performed under the guidance of an acid cleaning firm or specialist.

Chemical cleaning temporary piping should be completed.
Temporary thermocouples should be installed so that temperatures can be monitored at the following locations:
1. Steam drum (if not permanently installed).
2. Near the centre of the furnace wall.
3. One loser furnace wall header.

Fill the boiler with clear, filtered, preferably softened water.
Fire the boiler at a very low rate until boiler metal temperatures (as indicated by thermocouples) are in the range of 60-90°C.

Acid dosing:
The chemical solvent used in this step is composed of the following:
5 % hydrochloric acid (HCI)
0.5 % ammonium biflouride (NH4F .HF)
0.1-0.2% inhibitor

Adjust water temperature to 65°C with steam while running to waste.
Begin injection of inhibited concentrated acid solvent (5 % HC1) to boiler.

Adjust steam supply to maintain a dilute acid solution of 65°C at a thermowell downstream of the steam addition point.
Continue to fill boiler until the level of acid solution is at the top of the glass. Stop fill.
Wait a few minutes and check to see if level is stable.
Let boiler soak for six (6) hours with acid solution. During this period, flush chemical cleaning header to waste.
At the completion of the soak period, close drum vents and drain boiler and economizer under 0.2 - 0.3 bar nitrogen pressure to waste. During the drain, collect a composite sample of the spent acid. At the completion of the drain, check for complete drainage of the acid solution.

Open drum vents and fill the boiler and economizer with clear, filtered, preferably softened water with a pH of less than 8.0 until a level is indicated in the temporary gage glass.
Close drum vents and drain the boiler and economizer under nitrogen pressure as above to waste.
Open drum vents and operate the boiler feed pump to flush the boiler.
Continue to fill the boiler through chemical cleaning header with water as indicated above until water is discharged at the drum vents.
Close drum vents and drain the steam drum under nitrogen to its midpoint.

Close drum vents and drain boiler under nitrogen to waste. Check the pH of the rinse water. If the pH is less than 5.0, another rinse (fill and drain) is required. If pH is greater than 5.0 proceed to neutralize.
Remove the temporary gage glass and prepare the unit for operation.

Open drum vents and through the chemical cleaning header, pump into the boiler the dissolved concentration sodium carbonate which should be equivalent to 0.5-1.0% sodium carbonate when boiler is filled to normal firing level.
Operate boiler feed pump and continue to fill to normal firing level.
Fire the boiler until a minimum of 7 bar is reached.
Hold the 7 bar minimum pressure with intermittent firing for four hours.
At the completion of the boil-out period, cool the unit to 1 bar and drain (nitrogen pressure not required).

While the unit is still hot, blend the concentrated sodium nitrite solution with condensate to obtain 0.5 percent sodium nitrite and fill the boiler and economizer to ¾ the of steam drum.
After the unit has soaked for approximately one (1) hour, drain the boiler and economizer to waste (nitrogen pressure not required).

When the boiler has cooled down, inspect the steam drum and remove scale or piles of iron oxide present. Be sure that none of the foreign material is allowed to enter downcomers or connecting tubes.
Open the lower wall header and flush.
Renew all boiler hand hole fittings installed.
It might be well to point out the following precautions:
1. Under no circumstances should the boiler be fired while the acid is in the unit.
2. Vents should be opened when the inhibited acid is in the unit. Hydrogen gas is evolved through the reaction of the acid and steel. Avoid open flames or using electrical equipment that might produce a spark until it is certain that the hydrogen gas has been dispersed.
3. Neutralizing boil-out chemicais should be stored in a dry place prior to use.
4. Have adequate protective clothing for men who are to handle and mix chemicals. The protective clothing should include goggles, rubber gloves, aprons etc.