Once-through boilers

Parameter Recirculating boiler Once-through boiler Once-through boiler... 

For once-through boilers, the treatment must be without soHd residues, so all-volatile treatment (AVT) is used. AVT, which is also used in some dmm boiler systems, rehes on the feedwater chemical additives, ammonia and hydrazine, to provide water appropriate to the boiler. Because the additives are volatile, they do not accumulate in the boiler and provide only minimal protection during contaminant ingress. Most plants using AVT have some form of condensate poHsher to remove impurities from the condensate. 

Supercritial boilers use all-volatile treatments, generally consisting of ammonia and hydrazine. Because of the extreme potential for deposit formation and steam contamination, no soHds can be tolerated in supercritical once-through boiler water, including treatment soHds. 

Condensate purification In some boiler systems the condensate returning is retreated using ion exchange to minimise corrosion and deposit accumulation. This particularly applies to once-through boilers (mainly nuclear) where there may be no water/steam separator and perhaps limited facilities for blow-down. Additionally, some high heat-flux boilers (oil-fired) have been fitted with condensate polishing plant (CPP). 

Once-through boilers may be either sub- or super-critical. Sub-critical boilers are clearly at some potential risk of on-load corrosion owing to the presence of the evaporator zone. Measures aimed at avoiding on-load corrosion include—keeping the overall solute concentration low, keeping the ionic balance matched, and maintaining the waterside oxide suitably thin. With super-critical plant, there is no chance of on-load corrosion whilst it is operating in the super-critical mode, as there is no phase boundary. The risk is present, however, when the plant is run in the sub-critical mode, as all super-critical plant must be at times. 

As an example of a standard for feed-water for a once-through boiler BS 2486 gives a maximum conductivity of 0-2/iS/cm at 25°C and pH in the... 

Designs include the once-through boiler, which can operate at sub-critical, critical, or supercritical pressures and at all commercial temperatures, and the radiant boiler, where heat absorption is largely by radiant energy transfer. 

Essentially, except for once-through boilers, steam generation primarily involves two-phase nucleate boiling and convective boiling mechanisms (see Section 1.1). Any deposition at the heat transfer surfaces may disturb the thermal gradient resulting from the initial conduction of heat from the metal surface to the adjacent layer of slower and more laminar flow, inner-wall water and on to the higher velocity and more turbulent flow bulk water. 

Table 12.13 (BS 2486 1997 table 5) Recommended water characteristics of feed water at economizer inlet for once-through boilers...

C) for cast iron and up to 140 °F for marstenitic SS (60 °C). Not suitable for galvanizing, aluminum, or enamels. Formic acid solvents containing the appropriate corrosion inhibitors and iron stabilizers can be very successful in cleaning larger units, such as once-through boilers. 

Half-strength solutions are useful for flushing once-through boilers or... 

To complete the broad picture of what is meant by burn-out, it is useful to consider further the particular burn-out condition which produces tolerable wall temperatures. Such a condition must occur, for example, in any practical, once-through boiler system, where a change from, for example, liquid water at inlet to superheated steam at outlet takes place in a single heated channel. Normal operation of nuclear reactors beyond burn-out has also been contemplated, and Collier et al. (C5) have described successful experiments with irradiated Zircaloy-clad rods operating continuously beyond burn-out... 

Flo. 8. Temperature distribution in a once-through boiler tube [from Schmidt (S3)]. L = 98.5 in., d= 0.197 in., P = 2415 psia, AA = 365 Btu/lb, G=0.44xl06 lb/hr-ft2. Curve 1 water temperature. Curve 2 tube-wall temperature with heat flux of 92 x 103 Btu/hr-ft2. Curve 3 tube-wall temperature with heat flux of 148 x 103 Btu/hr-ft2. Curve 4 tube-wall temperature with heat flux of 221 x 103 Btu/hr-ft2. 

B1. Bagley, R., The application of heat transfer data to the design of once-through boiler furnaces, Proc. Inst. Mech. Engrs. (London) 180, Pt. 3C (1965-1966). 

Few high pressure drum boilers are equipped with the condensate polishing plant illustrated in Fig. 1 but once-through boilers (having no drum to allow the accumulation of impurities in the water phase) normally are. The water is pumped from the condenser back through feed-heaters and into the boiler again. 

Power plant boilers are either of the once-through or dmm-type design. Once-through boilers operate under supercritical conditions and have no wastewater streams directly associated with their operation. Drum-type boilers operate under subcritical conditions where steam generated in the drum-type units is in equilibrium with the boiler water. Boiler water impurities are concentrated in the liquid phase. Boiler blowdown serves to maintain concentrations of dissolved and suspended solids at acceptable levels for boiler operation. The sources of impurities in the blowdown are the intake water, internal corrosion of the boiler, and chemicals added to the boiler. Phosphate is added to the boiler to control solids deposition. 

Satisfactory uperatiun of the once-through boiler and associated turbine requires that the total solids in the feedwater be less than 0.05 ppm. Table 3 lists recommended maximum limits for feedwater contaminants and typical values obtained during operation. 

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