High-pressure boilers

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

Demineralization. Softening alone is insufficient for most high-pressure boiler feed waters and for many process streams, especially those used in the manufacture of electronics equipment. In addition to the removal of hardness, these processes require removal of all dissolved soHds, such as sodium, sihca, alkalinity, and the mineral anions (Cl , and NO ). 

High Pressure Boiler Water Treatment. High pressure boilers usually have feed water composed of demineralized makeup water and a high percentage of condensate returns. Because of these conditions, high pressure boilers are prone to caustic and acid attack. Low pressure boilers that use dernineralized water and condensate as feed water are also susceptible to caustic and acid attack. 

Silica SiOj Results in the formation of scale in boilers and cooling water systems, can produce insoluble scale on turbine blades due to silica vapori2ation in high pressure boilers (usuallu over 600 psi). 

For a high-pressure boiler plant with a high evaporation rate, demineralized feedwater is classified as having an electrical conductivity of less than... 

In addition to varying in costs, the chemical composition of the water provided from the mains supply also varies between the water companies, as may that between independent supplies within each company s area. The current criterion on potable water quality requires it to be wholesome i.e. it should not create a health hazard, with relatively wide limits on particular constituents. The cost of removing these constituents (e.g. calcium, magnesium, chlorides, iron and silica) increases with concentration and variability. This imposes a cost burden on, for example, the semiconductor and electronic component industries and on the operation of high-pressure boilers. Therefore both the potential cost of metered water supply and the chemical composition of the supply waters may influence future decisions on the water company s area in which an industry may wish to locate. 

De-ionized water is required for high-pressure boiler make-up and in many chemical process applications. Where a process has a large-scale steam demand, high-pressure turbine generators are often installed to generate power before providing the process with pass-out steam, thus making the most efficient use of the fuel. If the process does not return the steam as condensate, the... 

High-purity High-pressure boilers De-aeration may be required BS 2486 1978... 

In treated water for high-pressure boilers or where radiation effects are important, as in some nuclear projects, impurities are measured in very small units (e.g. g/litre or p.p. 10 ), but for most purposes it is convenient to express results in mg/litre. In water analysis, determinations (except occasionally for dissolved gases) are made on a weight/volume basis but some analysts still express results in terms of parts per million (p.p.m.). The difference between mg/litre and p.p.m. is small and for practical purposes the two units are interchtmgeable. For some calculations, the use of milli-equivalents per litre or equivalents per million (e.p.m.) has advantages but has not found much application. Hardness, whatever the constituent salts, is usually expressed as p.p.m. CaCOs (see Table 2.10). 

For some applications, notably feed-water treatment for high-pressure boilers, removal of oxygen is essential. For most industrial purposes, however, de-aeration is not applicable, since the water used is in continuous contact with air, from which it would rapidly take up more oxygen. Attention must therefore be given to creating conditions under which oxygen will stifle rather than stimulate corrosion. 

In the absence of sufficient hydrogen, the solubility of magnetite is markedly temperature dependent, which provides an explanation for some of the problems in high-temperature circuits. Most studies in boilers relate to high temperatures. Thus the work of Bloom " and of Potter and Mann has reproduced the types of corrosion found in high-pressure boilers. The relationship between corrosion rates and iron solubility and pH are given in Fig. 17.2. Note that the pH range about the neutral point (relative to 300 C... 

In high-pressure boilers, there are three types of on-load corrosion acidic chloride, neutral chloride/dissolved oxygen, and caustic attack. The first and second (once it becomes established) are brittle and thick-walled and are accompanied by hydrogen damage which can lead to failure within a few hundred hours. Caustic attack tends to produce a gouged appearance of the metal due to extensive wastage. The morphology is fairly characteristic of the failure type. Acidic chloride forms hard, laminated oxide, whilst, with caustic attack, the oxide is often soft, and, as it is easily removed, may be absent. 

With high-pressure boilers operating above about 60 bar, the complete removal of hardness salts from the feed-water by one means or another is... 

Table 17.8 Feed-water standards for high-pressure boilers...

Although this chapter looks primarily at relatively common forms of corrosion, we also explore a number of often complex and esoteric boiler section corrosion mechanisms. Many of these types of corrosion will only ever seriously develop in larger high-temperature or high-pressure boilers, especially those requiring knife-edge control and operating under difficult thermal stress conditions. 

Corrosion processes in the boiler section generate hydrogen—this hydrogen release is regularly measured in very high pressure boilers as it can be indicative of the rate of corrosion taking place... 

Components in water and steam. Volatile treatments only Suitable concentrations for very-high pressure boiler (2000-3000 psi) Suitable concentrations for supercritical boilers (>3203.6 psi)... 

The transport of iron oxides throughout the system is a very noticeable effect resulting from less than fully optimized chemical treatment programs and associated control parameters in high-pressure boilers. 

For high-pressure boilers, dissolved hydrogen (H2) should be limited to < 1 ppb in FW, < 3 ppb in saturated steam, and 10 ppb in superheated steam. 

Hollander, Orin Krager, Richard. Selection Criteria for Amines Used in High-Pressure Boilers. Ultrapure Water, Tall Oaks Publishing, Inc., USA, November 1997. 

Romero, Matilde F. Pdrez, Orlando Navarro, Alfredo. Phosphate Attack as Caustic Corrosion in High-Pressure Boiler Tubes. Material Performance . NACE International, USA, March 1999. 

Steingress, Frederick M., Frost, Harold J. High Pressure Boilers, 2nd edition. American Technical Publishers, Inc., USA, 1994. 

ARMAND, A. A. USAEC translation (AEC-TR-4490 page 19). Hydrodynamics and Hear Transfer during Boiling in High Pressure Boilers. U.S.S.R. Acad, of Sci. (Moscow. 1995). 

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