INTRODUCTION
Water is in fact, a chemical compound containing hydrogen and oxygen,
classified as the "Universal Solvent" and seldom found in nature in
its pure form.
There are three main types of water used aboard ships:
- Fresh or shore water
- Sea and estuarine waters
- Evaporated sea water
All three kinds of water are contained with solids and gases of one type or
another so even though it looks clear it contains appreciable amounts of
impurities.
IMPURITIES IN BOILER feed water
The presence of impurities in boiler feedwater is a constant
source of concern to the operating personnel, because it affects not only the
efficiency of the boiler, but also its safety.
THE PURPOSE OF BOILER feed water TREATMENT
The primary need for feedwater treatment is simply to
eliminate the harmful influence of the impurities which may find their way into
the boilers via the boiler feedwater system.
Without the correct chemical treatment of the water, the following problems
can, and do, arise in the boilers and the feed water system since the steel
surfaces are completely unprotected if not treated.
(A) Scale forming salts present in sea water and harbour
water, which can enter the boiler feed water system by carry-over of salty
moisture with the vapour from the evaporating plant, by salt water leakage from
evaporators into the condensed steam in the evaporator heating coils, by salt
water leaks in condensers, through leaky bottom blow valves in idle boilers.
Scale and deposits can also be found by presence of oil - heavy fuel oil, cargo
oil or lubricating oil - caused by leaks into the steam side of oil heaters and
into steam heating coils in HFO bunker tanks, day- and settling tanks, cargo
tanks, cargo heaters and lubricating oil from bearings of turbines and rotary
pumps.
(B) Corrosion in several forms caused by dissolved oxygen
in the boiler feed water which enters the system through air leaks in those
parts of the system which operate at pressures below atmosphere, such as
condensers, low pressure turbines and pumps. In addition, air is absorbed by
the feed water wherever it is exposed to the atmosphere, as it is through vents
to feed tanks, open feed- and filter tanks and through open drains.
Corrosion in the feed system, feed tanks and condensers can give rise to the
transport of corrosion products of iron oxide and copper or copper oxide into
the boiler which is a collector for such deposits and debris.
(C) Carry-over caused by the impurities brought with the
feed water concentrate in the boiler reaching the concentration where soluble
salts and suspended solids are carried over with the steam.
A carry-over may also occur with an excessive amount of the chemicals used in
the treatment of the boiler feed water due to improper feed water treatment.
EFFECTS OF feed water CONTAMINATION
Sea water leaks result in progressive contamination of boiler water because of
the increasing concentration of salts in the boiler due to the residue of
evaporation. Furthermore, even then boiler feed water is within its allowed
limits of impurities, the concentration of salts within the boiler increases in
proportion to the rate of evaporation (steaming rate) of the boiler.
A typical sample of sea water contains the following salts:
Sodium chloride (NaC1) - 25 600 ppm
Magnesium chloride (MgCI2) - 330 ppm
Magnesium sulphate (MgSO4) - 1 960 ppm
Calcium sulphate (CaSO4) - 1 220 ppm
Calcium carbonate (CaCO3) - 180 ppm
Other salts in varying amounts in sea water near mouths of rivers, harbours,
and bays etc. may also be present.
Sodium chloride (ordinary table salt) is comparatively harmless to boiler
materials. It will, however, cause priming (water carry over), which results in
the building-up of a thick crust in superheater tubes, steam valves, steam
lines and even on turbine blading in cases of excessive and frequent priming.
In the case of superheaters, the heat transfer characteristics of the tubes are
impaired by this salt crust, which will cause "burned-out" tubes.
Magnesium chloride in boiler water breaks down into hydrochloric (muriatic)
acid, which attacks the boiler drum and tube surfaces, causing acid corrosion
which manifests itself by pitting of the surfaces. This acid effect is
controlled by rendering the boiler water slightly alkaline with feed water
treatment.
Magnesium and calcium sulphates precipitate into a hard scale in the hottest
portions of the boiler, i.e. the interior of the tubes nearest the furnace.
This scale has about one-forty-eight of the heat conductivity of steel. When
this scale reaches the thickness of about that of an egg-shell, the water
inside the tube cannot receive and carry away the heat fast enough from the
tube metal to keep its temperature below its fusion temperature, resulting in
the tubes "burning-out". Boiler feed water treatment is used to
prevent the formation of this scale.
Calcium carbonate is comparable to chalk. It is harmless to boiler metals
unless it can concentrate in "dead pockets" in the boiler, in which
case carbonic acid (H2C03) may form, resulting in acid corrosion. This is
controlled by (1) proper boiler design to eliminate "dead pockets"
and (2), the use of feed water treatment to render the boiler water slightly
alkaline.
"Sea water" contamination occurring while an engineering plant is
operating close to beaches, or in harbours and rivers, will introduce silicates
into the boilers. Silicate scales are thin, transparent, brittle and hard.
Because of their transparency they are hard to detect. Avery thin scale can
cause tube failure due to overheating. They are controlled by use of feed water
treatment and by careful distilling plant operation while in such locations.
Oil present in boiler water will cause foaming and moisture carry-over at small
quantities. It will also form heat resisting film, sometimes a carbonized
layer, over the tube or shell surfaces ultimately resulting in tube or plate
material failure due to overheating also at a very thin layer. The oil will
manifest its elf by forming an oily ring inside the water gauge glasses at the
water level.
Oil is controlled primarily by careful inspection of the drain water from fuel
or cargo oil steam heating coils, and in care in the lubrication of machinery
where oil may come in contact with steam or water. The use of feed water
treatment will reduce the foaming effect of oil, but once a boiler has been
contaminated with oil, its water sides must be "boiled out" by filling the boiler with a strong mixture
of fresh water and boiler treatment and then, using steam from another boiler
(via the boiling-out connections on or near the bottom blow fittings), boiling
the mixture for two or three days. Dissolved oxygen in boiler water has become
a serious cause of corrosion in modern boilers.
By stress corrosion, or corrosion fatigue failures, the corrosion attack
plays a great role and is mostly caused by dissolved oxygen in the feed water.
The rate of corrosion attack might then increased rapidly and so much that
serious pittings is encountered in the boiler drum and tube surfaces.
Therefore, the amount of dissolved oxygen present in the boiler feed water is
mainly controlled by heating in Hotwell or cascade tanks by mechanical
deaeration, into the boiler water surface, as well as by chemical scavenge.
Corrosion products enter the boiler in the form of iron oxide, remain suspended
in the water and cause priming and foaming. They shall be removed by bottom and
surface blows. The use of feed water treatment reduces the tendency to prime
and foam.
The use of an excessive amount of feed water treatment might cause excessive
alkalinity of the boiler water as well as high alkalinity concentration will
result and results in "caustic embrittlement", an intercrystalline
cracking of the boiler metal.
Summarizing the above, the contamination of feed water by impurities has the
following effects:
1 Formation of scale on generating and superheater heating surfaces, which
results in:
A. Reduction in the boiler efficiency because of the
decreased rate of heat transfer, and
B. Overheating and burning of tubes resulting in tube
failure.
2 Corrosion of all interior surfaces of the boiler by certain salts and air
(oxygen); and
3 Foaming and priming, which result in moisture carry-over in the steam from
the saturated steam drums to the superheaters (where these are installed and in
use) or direct to the machinery in plants operating on saturated steam.
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