feed water treatment

Feed water treatment

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.

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 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.

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.