Steam boilers blowdown

Health and Safety Executive, Guidance Note No. PM 60, Steam boiler blowdown systems, HMSO, London (1987)... 

Feedwater The feedwater for a steam cycle must be purified. The degree of purity depends on the pressure of the boiler. Higher pressure boilers require higher feedwater purity. There is some trade-off between feedwater purity and boiler blowdown rate. However, increasing blowdown rate to compensate for lower feedwater purity is expensive, because blowdown water has been heated to the saturation temperature. Typical feedwater specifications for utihty boilers are given in Table 4. To some extent turbine steam purity requirements determine the feedwater purity requirements. The boiler-water siUca required to maintain adequate steam purity for higher pressure steam turbines is considerably less than the boiler could tolerate if deposition in the boiler were the only issue. 

On a FT boiler, the main blowdown valve is always located at the bottom of the shell towards the rear of the boiler. On a WT boiler, blowdown valves may be located in the side of the top (steam) drum, the bottom (mud) drum, and the lower water-wall headers (sides, front, and rear). 

Flash steam and heat recovery systems are almost never fitted into hydronic heating and LP steam boilers because of the very limited discharge of boiler water as blowdown. A schematic drawing of an FSHR system is shown in Figure 3.2. 

The heat exchanger component of a boiler blowdown, flash steam, and heat recovery system (FSHR). 

Supplementary treated water required to compensate for losses due to steam consumption, blowdown, and leaks in boiler plant steam/water system. 

Gestra (UK) and Highveld Instrumentation (South Africa). Boiler Blowdown and Flash Steam Recovery Systems. Technical and promotional literature. Gestra Aktiengesellschaft, Germany, 1990. 

Hare, Michael D. Automatically Controlled Skimmer Blowdown in Steam Boilers. Industrial Water Treatment, Tall Oaks Publishing, Inc., September/ October 1994. 

As discussed under boiler feedwater treatment, boiler blowdown is required to prevent the build up of solids in the boiler that would otherwise cause fouling and corrosion in the boiler. Carry over of solids from the boiler to the steam system via tiny water droplets should also be avoided. Total dissolved solids (TDS) and silica (SiC>2), as measured by the conductivity of water, are both important to be controlled in the boiler3. Dissolved solids carried over from the boiler will be a problem to all components of the steam system. Silica is a particular problem because of its damaging effect on steam turbines, particularly the low-pressure section of steam turbines where some condensation can occur. Blowdown... 

There are many types of steam boilers, depending on the steam pressure, steam output and fuel type. Blowdown is required to remove the dissolved solids not removed in the boiler feedwater treatment. The efficiency of the boiler depends on its load. 

Improving the energy efficiency to reduce steam demand and hence reduce the wastewater generated by the steam system through boiler blowdown, boiler feedwater treatment and condensate loss (see Chapter 23). 

Increasing condensate return for steam systems to reduce makeup water requirements, reduce aqueous waste from boiler feedwater treatment and boiler blowdown (see Chapter 23). 

Wastewater, which is a basic source of emissions, can be categorized in five ways (1) wastes containing a principal raw material or product (2) by-products produced during reactions, (3) spills, leaks, wash downs, vessel cleanouts, or point overflows, (4) cooling tower and boiler blowdown, steam condensate, water treatment wastes, and general washing water and (5) surface runoff. 

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. 

Table 5 Summary of Priority Pollutants in the Steam Electric Industry Boiler Blowdown... 

Methods 1 and 2 are intended to control the boiler water pH and to precipitate the calcium and magnesium compounds as a flocculenl sludge, so that they can be removed in the boiler blowdown rather than being deposited on heat-transfer surfaces. Method 1 maintains an excess of hydroxide alkalinity. The effects of alkalinity are discussed later under Steam Purity. Method 3 involves the addition of a complex mctal-chelant compound such as ethylenediainine-tetraacetic acid i a.(FDTA) or niirilolriacelic acid (NTA). In Method 4, as ihe name implies, no solid chemicals are added to the boiler or pre-boiler cycle. The pH of the boiler water and condensate cycle is controlled by adding a volatile amine. 

Fire 1.4. Steam boiler and furnace arrangements. [Steam, Babcock and Wilcox, Barberton, OH, 1972, pp. 3.14, 12.2 (Fig. 2), and 25.7 (Fig. 5)]. (a) Natural circulation of water in a two-drum boiler. Upper drum is for steam disengagement the lower one for accumulation and eventual blowdown of sediment, (b) A two-drum boiler. Preheat tubes along the Roor and walls are cormected to heaters that feed into the upper drum, (c) Cross section of a Stirling-type steam boiler with provisions for superheating, air preheating, and flue gas economizing for maximum production of 550,000 Ib/hr of steam at 1575 psia and 900°F. 

The cleared ICB effluent is approximately 98 wt percent water, 0.8 wt percent sodium sulfate, 0.7 wt percent sodium chloride, and traces of agent impurities and degradation products (PMACWA, 2006). The WRS and the brine reduction system (BRS) reclaim water from the biotreatment system effluent and the blowdown from both the cooling tower and the steam boiler. In addition, some process water passes through an RO system to feed the steam boilers and munitions washout system the retentate is fed to the WRS. 

Sulfate m Cad Purge i Cooling Blowdown Steam Boiler ... 

The steam flash drum is a device for steam recovery. Flash steam occurs at the drum where steam condensate or boiler blowdown experiences a drop in pressure causing some of the condensate or boiler blowdown to evaporate forming steam and thus produces steam at the lower pressure (Figure 15.11). For low-pressure condensate, flash steam is negligible and thus it is not worth to recover. However, for medium-and high-pressure condensate, it is important to recover flash steam. 

Steam Losses boiler blowdown, dearation, steam distribution, steam trap and condensate discharge losses. 

For Steam Recovery from Blowdown A flash drum eould be used to recover steam firom boiler blowdown or high-pressure condensate. In the former case, the recovered LP steam (Wp ) comes from the overhead of the drum while the bottom is sent to the sewer ... 

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