Book - School of Science and Technology

230 Pumps and other auxiliary equipmentTable 8.2Water treatment additives for low pressure steam and hot water systemsTreatmentProcess/additiveEliminationDissolved oxygen Hydrazine N 2 H 4Sodium hydroxideNaOHSodium sulphate Na 2 SO 4InhibitionCorrosionScaleNeutralising CO 2pH controlSodium compoundsbenzoateborate Na 2 B 4 O 7nitrite NaNO 3phosphatesilicateTanninsLigninsOrganic polymersPhosphatesPhosphonatesFilming aminesAlkalisNeutralising aminesSodium hydroxideNa 2 HPONaOHNoteThis list is for information only; specialist advice should be sought for all water treatment problems.feed water to steam systems has been well known and well explored for many years and itis not necessary to deal with that aspect of the matter here, other than by reference toTable 8.2 which summarises the additive processes commonly used.As far as water systems are concerned, a radical change in the availability of equipmentand consequently in design practice has taken place during the last 25 or more years.Boilers of solid construction with wide waterways, whether cast iron sectional or weldedsteel shell-type, have given way generally to higher-efficiency units of smaller size manufacturedto designs meeting quite different criteria. Similarly, heat emitting equipment isno longer made in the British Isles in substantially proportioned cast iron but in lightgauge pressed-steel sheet or aluminium. Lastly, the various new-type components are nowvery often piped together with small-bore light-gauge copper, rather than with sensiblythick mild steel pipes. The former are commonly made by a process which leaves acarbonaceous film on the internal surfaces which may lead to accelerated deteriorationwhen these are exposed to certain hard waters.Ideally, a water system should be filled with a stable, non-corrosive, non-scaling water,softened or de-mineralised as may be appropriate. No engineer who has seen a swimmingpool filled by water straight from a mains supply, without first being passed through thefiltration plant as is good practice, will remain under any illusion as to the condition ofwater from such a source! During the construction of the heating system, mill scale andfoundry sand will have accompanied material deliveries and welding beads, cement,plaster and other site debris will have been introduced: not all of this will have beenremoved by any flushing process.The mixture of diverse materials noted above, when brought into contact with mainswater, lays the foundation for ultimate disaster if no positive remedial action is taken.Dependent upon the composition of the particular water supply, corrosive or scale