2. Steam Plant The steam plant comprises many different systems incl uding the pre-boiler units, the HRSGs, the condenser and condensate system, and a range of auxil iaries. it is beyond the scope of this paper to discuss al i the chal lenges and shortcomings present in this area of the plant, but below are several that occur very frequently and deserve advice. t Flow-assisted corrosion One of the most talked about problems i n the lndependent Power (IPP) i ndustry in the last decade is flow-assisted corrosion (FAC) (a.k.a. erosion-corrosion). in heat recovery steam generators, FAC occurs predominantly in steam separation equipment in drum-type units, in steam generating tubes and in boiler feed water piping. Although there have been many fail ures, predominantly i n the USA, the problem is less common than, for i nstance, corrosion fatigue due to cycli ng. Today's IPP pl ants are built under strong pressure to reduce project capital cost and maximise the return on i nvestment. ünce in operation, owners ai m for high efficiency targets. Although operating practices have a strong i mpact on fai lure probability when carbon steels are used, a key step i n avoidi ng FAC is maki ng a proper material s selection i n the origi nal design. Higher alloys containing l evels of chromium above 1.5% (e.g. T11, T22, and 400 series of stai nless steel ) greatly lower the l i kel ihood of FAC. Cycle chemistry has erroneously shouldered a lot of the bl ame for the FAC phenomenon, and the author cautions strongly agai nst misleadi ng i nformation i n this regard. t Sampling and Monitoring of Steam and Water A very common and critical shortcomi ng l ies in the sampl ing and monitori ng of steam and water. it is after al l ironic that so l ittle thought is given to a process meant to "audit" the very same working fluids that drive critical , expensive machi nery. Sampl i ng panels seldom meet even the most basic of requirements. Meani ngful sampl ing needs to take place isokinetical ly, but this very rarely occurs. Sample flow rates are al sa far too low to be representative of system conditions. it is common to find either no superheated steam sampli ng at all or a sampl ing protocol that is far too remote from the point of extraction. Duri ng a recent start-up, 1 observed a pri mitive sampl ing scheme for superheated steam util ising a copper coil and a bucket. Six months into operations, the makeshift device was sti l i being used. t Cascading blowdown i n the quest tor the perfect planı heat rate, even tiny thermal efficiency gains are welcome. New HRSG designs i ncorporate cascading blowdown schemes between the various boiler loops. This innovation, however, often leads to serious cycle chemistry control problems. The concept should be careful ly weighed in future projects since the 1Figııre 3. Sample panel. potential small savings in thermal energy and water do not stack up against the inherent risks to steam planı equi pment due to upset chemistry conditions. t Condensate Contamination Condensate contamination is of particular concern in cogeneration plants. it must be considered when eval uating treatment options.Not al l treatment equi pment removes al l contami nants. Specific contamination in formation must be obtai ned from the steam host, in order to understand how to properly treat the condensate stream. Care must be laken i n the waste permi! appl ication to ensure that there is a provision for the regeneration waste from the demineral iser/pol isher to contai n contami nants and process chemicals. Table 1 lists some common types of condensate treatment equi pment and their contaminant removal abil ity. Table 1 - Condensate Polisher Application Guidelines. Deep Sodium Cellulose Magnetic Bed Pre-coat Contaminant Fonn Source Zeolite Precoat Filter Mixed Polisher Bed Fe and Cu lnsoluble Corrosion in Yes Yes Yes Yes Yes oxide 11Crud'1 condensate system lron .or Soluble Condensate Yes No No Yes Yes Hardness corrosion, makeup contamination TOS Soluble Cooling water No No No Yes Yes (1) in-leakage, (1) process contamination Oil/Organic lnsoluble Process, No (2) Yes No No(2) Yes (2) coolers Organic (3) Soluble Process No No No No No (1) Tlıese ııııits are desigııedpriıııarily asfi/ters. Becaııse oftlıe large aıııoııııt ofwater beiııg treated, tlıey wi/1 qııickly exlıııııst ifexposed to lıiglı TDS. (2) Oil aııd certaiıı orgaııics are reıııoved byfoıı/iııg tlıe ion exclıaııge media. Activated carboıı lıas beeıı ıısed bııt resıılts are geııeraly 111ısatisfactory. Eaclı poteııtial contanıiııaııt mııst be tested. 411 ENERJi OÜNYASI EKiM 2006 75
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