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THE FEED WATER-STEAM-CONDENSATE SYSTEM
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THE FEED WATER-STEAM-CONDENSATE SYSTEM
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74CHAPTER SEVEN
7 THE FEED WATER-STEAM-CONDENSATE
SYSTEM
7.1 Introduction
The technical problems of design and operation of the feed water loop of a steam power
plant are conveniently grouped under
1. Water treatment, which is concerned with water quality and the operating
difficulties originating from use of inferior water. Remedies and preventive
treatment.
2. Water heating and
3. Water pressurising and regulation
Water treatment concerns the removal of impurities, which would result into scaling,
corrosion etc if allowed into the boiler and pipe systems. Once the feed water is in the boiler,
it is heated first to saturation temperature, then evaporated at the point of contact with heated
tube surface. In general the steam is free from impurities the water might have carried since
they are left in the boiler water whose concentration increases with time. The point of
evaporation being the tube surface, the impurities may deposit on these surfaces as a scale.
Water heaters are principally installed to promote higher thermal efficiency. Apart from this,
it has three other purposes. First is the fact that the heat which would otherwise be wasted is
diverted into feed water and this represent fuel savings. Secondly, the thermal stress which
can be induced in boiler surfaces by impingement of cold water streams is minimised or
eliminated. and finally heating feedwater increases the steaming capacity per unit area of the
installed boiler capacity.
MPE 571E: POWER PLANT ENGINEERING
, 75
7.2 Effects of contaminated water
Natural water usually contain dissolved salts and gases and some organic and inorganic
material in suspension. The dissolved salts are chiefly the carbonate, sulphate and chlorides
of calcium, sodium and magnesium etc. The suspended matter is usually alumina and silica
in the form of mud or silt or organic, sewage and industrial wastes. The troubles caused by
the feeding of water of undesirable quality are scaling, corrosion, foaming, priming and
embrittlement.
7.2.1 Scaling
Primarily scaling results from the decrease of the solubility of some salts with increase of
temperature. One chemical mechanism used to explain scaling is reaching of chemical
saturation by the water in the boiler, followed by the beginning of precipitation and finally
the sedimentation and formation of a layer of scale on the heating surfaces and loose
precipitate in drum. But more likely, heating surface scale is produced by crystallisation of
scale forming salts from a locally superheated layer of water lying on the heating surface.
Scaling is due mainly to salts of calcium and magnesium. Calcium is the principle offender
especially calcium sulphate.
Scaling may take place in boiler drums or tubes, heater tubes and feedwater piping. Its effects
on the piping system is to choke the flow, requiring an increase of pressure to maintain water
delivery.
There are three types of scaling namely: tightly adherent and tough, adherent and soft and
brittle. Boiler water conditioning often seeks to precipitate the impurities in the boiler as a
soft sludge which can be removed by blowing down. Anti-scaling treatment consists of
removing the scale-forming elements or replacing them with extremely soluble salts.
7.2.2 Corrosion
Corrosion is the destructive conversion of the metal into oxides and salts. It occurs due to an
acid condition of water or oxygen , carbon dioxide or chlorides. The most serious factor in
corrosion is the dissolved oxygen. Corrosion is the most frequent cause of damage to power
boilers. The steam and condensate lines it may attack are often located in such inaccessible
sites that replacement is costly and time consuming.
MPE 571E: POWER PLANT ENGINEERING
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