Boilers carryover

In addition, even where foaming is not a specific problem in a boiler, carryover may occur, especially in lower pressure boilers with very high TDS (i.e., over 10,000 to 15,000 ppm TDS) because of the collapse of surface bubbles. This leads to BW aerosol generation and entrainment of the spray in steam. Under these circumstances, antifoam agents such as polyamides are useful in preventing these entrainment problems. Furthermore, the antifoaming action of polyamides is often enhanced by protective colloid materials such as tannins, and consequently, formulations containing polyamide emulsions in an alkaline tannin base are available. 

Formation of Airborne Emissions. Airborne emissions are formed from combustion of waste fuels as a function of certain physical and chemical reactions and mechanisms. In grate-fired systems, particulate emissions result from particles being swept through the furnace and boiler in the gaseous combustion products, and from incomplete oxidation of the soHd particles, with consequent char carryover. If pile burning is used, eg, the mass bum units employed for unprocessed MSW, typically only 20—25% of the unbumed soHds and inerts exit the combustion system as flyash. If spreader-stoker technologies are employed, between 75 and 90% of the unbumed soHds and inerts may exit the combustion system in the form of flyash. 

Condensate Polishing. Ion exchange can be used to purify or poHsh returned condensate, removing corrosion products that could cause harmful deposits in boilers. Typically, the contaminants in the condensate system are particulate iron and copper. Low levels of other contaminants may enter the system through condenser and pump seal leaks or carryover of boiler water into the steam. Condensate poHshers filter out the particulates and remove soluble contaminants by ion exchange. 

Steam Purity. Boiler water soHds carried over with steam form deposits in nonreturn valves, superheaters, and turbine stop and control valves. Carryover can contaminate process streams and affect product quaHty. Deposition in superheaters can lead to failure due to overheating and corrosion, as shown in Figure 6. 

Steam can be contaminated with soHds even when carryover is not occurring. Contaminated spray attemperating water, used to control superheated steam temperature at the turbine inlet, can introduce soHds into steam. A heat exchanger coil may be placed in the boiler mud dmm to provide attemperation of the superheated steam. Because the mud dmm is at a higher pressure than superheated steam, contamination will occur if leaks develop in the cod. 

Aikaiinity Bicarbonate (HCOs" ), carbonate (COs , and hydroxyl (OH ), expressed as CaCOs Foaming and carryover of solids with steam embrittlement of boiler steel bicarbonate and carbonate produce CO2 in steam, a source of corrosion Lime and lime-soda softening, acid treatment, hydrogen zeolite softening, demineralization, dealkalization by anion exchange, distillation, degasifying... 

Priming The carryover of particles of water or other fluid in gas flow, mainly in steam boilers. 

In anything but the smallest installations the flow of steam from the boiler into the cold pipes at start-up (while the boiler pressure is still very low) will lead to excessive carry-over of boiler water with the steam. Such carryover may be enough to overload a separator at the boiler take-off point or its steam trap (Figure 22.5). Great care... 

The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

Deposit control is important because porous deposits, under the influence of heat flux, can induce the development of high concentrations of boiler water solutes far above their normally beneficial bulk values with correspondingly increased corrosion rates. This becomes an increasingly important feature with increase in boiler saturation temperature. In addition, deposits can cause overheating owing to loss of heat transfer. Finally, carryover of boiler water solutes, which can be either mechanical or chemical, can lead to consequential corrosion in the circuit, either on-load or off-load. Material so transported can result in corrosion reactions far from its point of origin, with costly penalties. It is therefore preferably dealt with by a policy of prevention rather than cure. 

Solid alkalis Solid alkalis may be used, in principle, for the corrosion control of drum boilers at all pressures but other factors, e.g. carryover or hideout a (reversible disappearance from solution on-load), may preclude them in some cases. However, they are used for feed-line treatment only in lower pressure plant where the boiler has increased tolerance to the higher solids burden which their use entails. Sodium hydroxide or, at very low pressures, sodium carbonate, (which is hydrolysed to the hydroxide at boiler temperatures) have been used, as have potassium and lithium hydroxides and various phosphate mixtures. (For a comparison of various alkalis for this purpose see References.)... 

It is not the intention here to consider in detail the subject of boiler feed-water conditioning and treatment for nuclear plant, but the general principles may be noted. Essentially, the same objectives apply as in fossil-fuelled plant, embodied in the three aims to minimise corrosion, deposition and steam-carryover. Requirements are more stringent in nuclear plant because there is no possibility of repairing tubes which have failed, let alone those which have suffered either deposition or corrosion. Again, certain tubes in nuclear plant have very modest design corrosion allowances so that only minimal loss of thickness from any cause can be tolerated. 

Efficiency is related to turbine loading and the response to changes in operation. Key factors in part-load turbine performance are the operational methods employed to control steam flow to the turbine and the actions taken to reduce boiler cycling, low-load conditions, and boiler water carryover. 

Despite the need for good water treatment, Cl boilers generally have a long life expectancy. However, they can be extremely prone to priming or surging and carryover problems. Any deviation in water chemistry... 

Feedwater regulators are fitted to boilers as a means of controlling the rate of addition of FW. This is necessary to maintain a consistent water level that, in turn, reduces the risks of thermal shock to the boiler, reduces the potential for BW carryover with the steam, and improves boiler efficiency. 

Periodic boiler restarts or frequent load changes that lead to boiler cycling this inevitably produces carryover problems... 

In WT boilers operating at very high firing rates, the risk exists of steam bubbles developing in downcomers. Where this occurs, it causes a temporary halt in the natural steam-water circulation and instantly leads to surging or priming followed by carryover. 

Under these conditions, the tendency is to try to compensate as quickly as possible for the loss of inhibitor and either add treatment directly to the condensate receiver, FW tank, or FW pump. This usually is not a good idea because in many cases the physically compact FT boiler design is insufficiently forgiving and problems of surging (priming) and foaming occur. This typically leads to carryover of BW and also the development of water hammer. 

Lack of BD control High treatment/small boiler Poor design, lead and lag boiler operation scale and ferrous metasilicate FeSi03 Water losses, low inhibitor levels Surging/carryover from high TDS. Excessive TDS in lead boiler. Lack of treatment in lag boiler. 

Higher rates of sludging also take place in the boiler vessel. In turn, this potential fouling problem requires additional maintenance time because more frequent internal surface cleaning, wash-down, and boiler vessel sludge removal usually is required. Carryover of contaminants into the steam also is more likely. 

These factors severely enhance the risks of condensate system corrosion by carbonic acid (resulting from a breakdown of the alkalinity in the boiler water and carbon dioxide [C02] carryover into the steam) and BW carryover. In addition, boiler operation is more difficult because the possible COC is severely limited, there-... 

Additional problems that may cause serious contamination of the treated MU water are those related to carryover and after-precipitation from external treatment processes. Both of these processes may result in the presence of insoluble solids in the various lines, tanks, and valve areas of the pre-boiler section. Some solids may even pass through to the boiler. 

The effect of carryover and after-precipitation is that solids settle out and cause pre-boiler system fouling and result in reduced flow and equipment waterway blockages. Check valves are especially prone to blockage. 

Process leaks of sugars, fats, colloidal materials, pectins, emulsions, and proteins cause stable foams in the boiler, leading to carryover and a further contamination cycle. 

Process contamination. Sticky films (LP boilers) Varnish (HP boilers) Steam discoloration FW pH fluctuations Acid corrosion Stable foams leading to carryover... 

Where particulate matter (in the form of corrosion products of iron oxide) is present in returning condensate, it often contains copper, nickel, and zinc oxides as well. This debris can initiate foaming (through steam bubble nucleation mechanisms) leading to carryover. It certainly contributes to boiler surface deposits, and the Cu usually also leads to copper-induced corrosion of steel. 

The frequency and types of tests employed should, of course, bear some relation to the type of facility being considered. If steam and condensate are not tested, carryover, corrosion, contamination, and other potential problems may be missed, which undoubtedly will have a deleterious impact in other parts of the overall boiler plant. 

Under boiler operating conditions, when steam does not properly and completely separate from BW, both the steam purity and steam quality are compromised. Consequently, to ensure the production of high purity, good quality steam, the risk of carryover must be reduced to a minimum. 

Cause-and-effect analysis reveals that steam purity and quality are both reduced by the degree of carryover taking place in a boiler, and carryover is itself a function of the effectiveness of steam-water separation. In turn, the mechanics of separation are a function of three areas, each with its own variables ... 

Steam flow restrictions. Operating a boiler that has a restricted steam flow due to blockages or deposits in the chevrons, baffles, collecting pipes, splash plates, or pans (perhaps resulting from longterm adverse water chemistry conditions) may cause localized high steam velocities that lead to carryover. 

Gulping Gulping is an intermittent form of BW carryover caused by variable water levels. Some WT boiler designs are prone to gulping, and the carryover passes into superheaters. 

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