Boiler conductivity

NOTE For any specific WT boiler, where high steam purity is required, the BW chemistry parameters should be set as low as necessary to achieve the desired quality. Careful observation of the relationship between steam purity and boiler conductivity, alkalinity, and other parameters quickly establishes the particular control conditions required. 

Optimum boiler conductivity should be maintained using sodium nitrate or sodium sulfite. If sodium sulfite is used, the conductivity should be maintained within the specified range, irrespective of whether the sulfite reserve is excessive. 

This reaction is first conducted on a chromium-promoted iron oxide catalyst in the high temperature shift (HTS) reactor at about 370°C at the inlet. This catalyst is usually in the form of 6 x 6-mm or 9.5 x 9.5-mm tablets, SV about 4000 h . Converted gases are cooled outside of the HTS by producing steam or heating boiler feed water and are sent to the low temperature shift (LTS) converter at about 200—215°C to complete the water gas shift reaction. The LTS catalyst is a copper—zinc oxide catalyst supported on alumina. CO content of the effluent gas is usually 0.1—0.25% on a dry gas basis and has a 14°C approach to equihbrium, ie, an equihbrium temperature 14°C higher than actual, and SV about 4000 h . Operating at as low a temperature as possible is advantageous because of the more favorable equihbrium constants. The product gas from this section contains about 77% H2, 18% CO2, 0.30% CO, and 4.7% CH. ... 

The sulfur dioxide of reaction 1 is cooled in a waste-heat boiler, freed from calcine, and converted to trioxide. The oxidation and conversion to sulfuric acid is conducted in a conventional acid plant (see also Sulfuric acid and sulfur trioxide). 

Refractories. Its low coefficient of expansion, high thermal conductivity, and general chemical and physical stabihty make sihcon carbide a valuable material for refractory use. Suitable apphcations for sihcon carbide refractory shapes include boiler furnace walls, checker bricks, mufflers, kiln furniture, furnace skid rails, trays for zinc purification plants, etc (see Refractories). 

Note 1 B. Kamp [Z. tech. Fhysik, 12, 30 (1931)] skews tke effect of increased porosity in decreasing tkermal conductivity of boiler scale. Partridge [University of Michigan, Eng. Research Bull., 15, 1930] has published a 170-page treatise on Formation and Properties of Boiler Scale. 

As the measurements show, the small heater without an electrical separation (from the boiler) is not detrimental to cathodic protection. However, with the uninsulated built-in Cu heat exchanger without an electrical separation, cathodic protection was not achieved. As expected, the polarization increased with increasing conductivity of the water. It should be pointed out that the Cu tube was tinned and that the tin could act as a weak cathodic component. Apart from the unknown long-term stability of such a coating, the apparent raising of the cathodic polarization resistance of tin is not sufficient to provide cathodic protection with such a large fixture. This applies also to other metal coatings (e.g., nickel). 

A = Surface area ft based on tube ID C = Gas specific heat. Btu/lb°F d = Tube inner diameter, in. k = Gas thermal conductivity, Btu/ft-h°F L = Tube length, ft N = Total number of tubes in boiler Pr = Gas Prandtl number Q = Duty of the boiler. Btu/h... 

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

Conduction, convection, and rachation of heat (from boilers, furnaces, forges, etc.) cause tlie ignition of flanunable liquids and combustibles. 

Heat is transferred from the hot products of combustion to the boiler heating surfaces through the plate and tube walls and to the water by various mechanisms, which involve conduction, radiation and convection. 

The thermal conductivity of an average boiler scale is 2.2 (W/m K) and that of complex silicate scales is 0.2-0.23 (W/m K). Since the furnace peak wall flux can be over 300,000 (W/m ) it may readily be seen that a small thickness of scale can raise the metal temperature into the creep region, resulting in very expensive repairs. 

Water-level controls continuously monitor the level of water in a steam boiler in order to control the flow of feedwater into the boiler and to protect against a low water condition which may expose the heating surfaces with consequent damage. The controls may be either float operated or conductivity probes. 

Maximumtotal alkalinity consistentwith acceptable steam purity. If necessary, the limitation on total alkalinity should override conductance as the control parameter. If make-up is demineralized waterat 600-1000 Ib/in g, boiler wateralkalinity and conductance should be shown in the table for the 1001 -1500 Ib/in g range. NS (not specified) in these cases refers to free sodium- or potassium-hydroxide alkalinity. Some small variable amount of total alkalinity will be present, and measurable with the assumed congruent control or volatile treatment employed at these high-pressure ranges. 

All these methods will require careful monitoring initially to set up and determine the correct rate of blowdown once the plant is operating. In order to take the necessary sample from the boiler the boiler(s) should be fitted with a sample cooler. To automate the continuous blowdown a conductivity-controlled system may be installed. Here a controller continuously compares the boiler water electrical conductivity with a value set in the controller. Depending on whether this is above or below the set rate, it will automatically adjust the blowdown flow rate. 

The run and regeneration cycle on small to medium-size units is normally governed by a multi-port valve, which is the only moving part required. There have been many cases where this has failed and brine was injected directly into the boiler. A conductivity meter on the make-up line would guard against this. 

The raw water silica is 22mg/l as Si02, and therefore becomes a major constituent of the treated water. Silica scale must now be avoided by raising the boiler water pH and letting silica rather than the TDS control the necessary blowdown. Silica scale not only has a tenth of the heat conductivity of calcium carbonate scale but it is glassy, adherent, and extremely resistant to boilercleaning chemicals. 

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... 

Infrared Methods Commercial instrumentation for recording infrared radiation has been available for some years and has been explored by the electrical power industry in the UK for assessing corrosion in boiler tubes at power-station shut-down. An external heal source is played onto the outside of boiler tubes at the same time as cold water is circulated inside the tubes. Hot spots due to poor heat conductivity caused by excessive corrosion product indicated areas of high corrosion. 

Electrode boilers produce hot water or steam (generally saturated steam) by conducting current through the BW. The water provides resistance, which causes heat to be generated when electrical current flows from one electrode to another. As a consequence, the electrical conductivity of the water is a primary factor in the satisfactory operation of these boilers. Other aspects of water treatment control (such as alkalinity levels, oxygen content, and foam control) and maintenance also must be considered if optimum efficiency is to be obtained. 

The electrical conductivity of the water is critical to the correct operation of this type of boiler, and the precise level varies with design and power requirements. However, the conductivity is always relatively low (often specifications require a level of below 15-50 p,S/cm), so demineralized or reverse-osmosis (RO) quality FW is usually specified. 

Electrical conductivity must be strictly controlled, as with the submerged electrode boiler, but the limits are higher (normally around =3,500 jLS/cm maximum). 

Continuous conductivity measurement controlled with the electrode placed in the boiler. This method is not recommended because of potential safety and liability issues. In addition, there are difficulties with cleaning and maintaining the electrode, and the intense heat to which the electrode is constantly subjected may cause failure. FT boiler installations generally provide for the electrode to be placed above the first set of tubes but 4 to 6 inches below the waterline. 

Intermittent conductivity measurement controlled, with the electrode placed in the BD line before the BD valve. In this arrangement the controller program opens the BD valve for a timed period (= 1 minute), sufficient for fresh BW to flow past the electrode and an accurate electrical conductivity measurement to be taken. The BD valve remains open until the conductivity falls below a set point. Again, this arrangement is improved with the use of a cooling coil. It is the most widely employed intermittent BD arrangement for FT boilers. 

As a comparison, Table 4.1 lists the coefficients of thermal conductivity (at room temperature) for some metals employed in heat exchangers, together with some minerals commonly found in boiler deposits. 

Table 4.1 Coefficients of thermal conductivity for some heat-exchanger metals and boiler deposits...

Some measure of control over corrosion also is obtained by limiting the salinity in the boiler (primarily the ions of sodium, chloride, and sulfate). These ions all increase the conductivity of boiler water electrolyte and thus enhance corrosion reaction rates. Also, chloride and sulfate ions affect the passivation process. 

A common problem associated with the operation of ion-exchange water softeners is due to chloride leakage contamination into the FW following regeneration. This is an operational problem caused by inadequate rinsing and usually is detected in the boiler by a periodic and sudden rapid rise in the boiler water s electrical conductivity and a call... 

Where-high purity MU is provided for higher pressure WT boiler plant FW, some form of continuous analyzers for measuring treated water pH and conductivity are almost always installed, as are sodium (Na) ion-selective electrodes for detection of sodium leakage. Automatic online silica analyzers also may be installed, but they measure only reactive (ionizable) silica (Si02), not colloidal or total silica, so caution is required where unforeseen silica leakage may present a problem. 

The problem of color due to tannin can be annoying, although not insurmountable. For small, simple boiler plants, the presence of color is, in fact, a benefit, as the depth of color is more or less related to the inhibitor concentration and the BW conductivity and TDS levels. Where multiblended formulations are employed, if the color is within range, then other parameters are also likely to be acceptable. Also, electrical conductivity is unaffected by color, and tannins do not appreciably add to the measurable TDS of the BW. 

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