3rd Class Power Engineering - B1 - Part 2
Why boil out refractory? - ANS To remove any oil, grease, or other contaminating materials from the internal surfaces of the waterside of the boiler.
Steps to boil out refractory? - ANS Add the following for each 45000L of boiler water content:
-...
3rd Class Power Engineering - B1 - Part 2
Why boil out refractory? - ANS To remove any oil, grease, or other contaminating
materials from the internal surfaces of the waterside of the boiler.
Steps to boil out refractory? - ANS Add the following for each 45000L of boiler water
content:
- 18kg of trisodium phosphate
- 2.25 kg of caustic soda
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- 2.25 kg of sodium silicate
- 22.5 kg of soda ash
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Pumped as a solution with a chemical feed pump or through a steam drum manhole.
Raise pressure to 1/2 design working pressure.
Operate blow off valves for 2-4 hours to clear sludge and sediment.
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Boil out is complete when samples from the blow off show that the water is clear.
Boiler layup: Dry storage - ANS Preferable for boilers that are out of service for an
extended period of time or where freezing temperatures are expected.
IS
Small flame is used to evaporate any water left.
Moisture content is measured until the content is water free.
A moisture absorbing material, such as silica gel, should be placed on trays to absorb
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any free moisture in the air.
Manholes should then closed and all connections on the boiler should be tightly
blanked.
If available use an inert gas to provide positive pressure at approx. 35kPa.
Warning signs posted noting that boiler is stored under N2 pressure.
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Boiler layup: Wet Storage - ANS Used for boiler placed in stand-by condition.
2 methods: Boiler completely filled or boiler partially filled.
Completely filled:
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Clean, empty boiler is closed and filled with softened, deaerated feedwater.
While filling, chemicals are added to prevent corrosion including an oxygen scavenger
and alkalinity control. Excess of O2 scavenger should be maintained and PH should be
kept at 10.0.
Partially filled:
Fill boiler to normal working level. Space above the water is filled with an inert gas (N2)
at 35kPa. In this way water will not be exposed to O2 and corrosion will be prevented.
TBC - ANS
, Cause of boiler scale? - ANS Primary cause is presence of undesirable minerals in
the boiler water. Calcium and magnesium are the most common. Others are iron,
copper, aluminum and silica.
Effects of boiler scale? - ANS Scale restricts heat transfer, causing metal to overheat.
Scale traps water beneath it, encouraging corrosion.
Scale sloughs off and causes flow restrictions.
2 internal methods of controlling boiler scale? - ANS Use phosphates to precipitate
A
the scale forming solids and remove via blowdown. Other method involves using
chelates to react with scale forming solids and prevents them from dropping out of the
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solution with boiler water.
Keys to phosphate treatment? - ANS Typically uses orthophosphates (monosodium,
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disodium, trisodium phosphate).
Success depends on alkalinity remaining above 9.5 pH and preferably between 11 and
12 pH.
Phosphate reacts and forms to create impurities known as suspended solids.
IS
Susp. solids are then removed via blowdown.
Keys to chelate treatment? - ANS Uses weak organic acids called chelants to hold
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scale forming impurities in solution. Most common are EDTA and NTA.
Principle by which chelants act is called solubilisation.
What is corrosion? - ANS The loss of metal due to chemical interaction between the
metal, the water and certain impurities in the water.
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What are the two main areas of concern for corrosion in a boiler system? - ANS
Primary area is the inside of the boiler itself. Secondary area is the condensate system.
What is a desirable corrosion activity? - ANS The formation of a protective magnetite
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layer on the boiler surface. Formed from a natural reaction with water and mild steel.
Reacts to a thickness of about 2.7mm.
What are the 4 proponents of corrosion? - ANS 1. Low boiler water pH (acid
corrosion)
2. High boiler water pH (caustic corrosion)
3. Oxygen corrosion
4. CO2 corrosion
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