Boiler operating point

The formation of complex silicate scales takes place at high temperature (usually they are only found in boilers operating at over 300 psig) and high heat-flux density points in the boiler section. The presence of complex silicates such as analcite and acmite may indicate steam blanketing problems. 

What is clear is that chelant corrosion takes place only when a number of unfavorable boiler operating factors are collectively present and that the starting point for corrosion is an overfeed of chelant product. 

Roller Water The steam purity limits define boiler-water limits because the steam cannot be purified once it leaves the boiler. For a once-through boiler, the boiler water must have the same specifications as the steam. A recirculating boiler is a still, and there can be considerable purification of the steam as it boils and is separated from the water in the steam dmm. The process of separation is not perfect, however, and some water is entrained in the steam. This water, called mechanical carryover, contains impurities in the same proportions as the boiler water, and its contribution to steam impurity is in those proportions. Typical mechanical carryover is less than 0.25% and often less than 0.1%, but operating conditions in the boiler can affect the mechanical carryover. In addition to mechanical carryover, chemicals can be carried into the steam because of solubility. This is called vaporous carryover. Total carryover is the sum of mechanical and vaporous carryover. The boiler-water specification must be such that the total carryover conforms to the steam purity requirements. For salts, such as sodium phosphate and sodium chloride, vaporous carryover is not a significant problem below approximately 15 MPa (2175 psia). As boiler pressures approach the critical point, vaporous carryover increases rapidly. Above 15 MPa (150 bar), boiler solids concentrations must be carefully controlled to minimize vaporous carryover. Most boilers operating over 18 MPa (180 bar) use all volatile treatment to prevent deposition of salts in turbines. Boiler-water limits for utility boiler are Us ted in Table 2. Recommendations from American Boiler Manufacturers Association (ABMA) for boiler-water limits for drum-type boilers and associated steam purity for watertube boilers are listed in Table 3. 

Weather controller An outdoor sensor is setting the operating point for the boiler temperature, along a curve (adjust at installation) that answers to layout and type of heafing sysfem. Sometimes the systems are combined with thermostatic radiator valves or separate electronic systems with or without optimizer. 

Many chemical industries have boilers for supply of steam— they are either fired by fuels like coal/oil or recover heat from hot process gases. It will be useful to familiarise aU plant operating persons with the important points for smooth and safe boiler operations. 

There are two kinds of startup schemes currently used in FPPs [1]. One is the constant pressure startup scheme, in which the boiler operates at constant supercritical pressure after the coolant is pressurized to this point. The other is the sliding pressure startup scheme, in which the boiler operates with variable pressures and the pressure increases with the generation output. 

In most utibty boilers, steam pressure regulation is achieved by the throttling of turbine control values where steam generated by the boiler is admitted into the steam turbine. Some modem steam generators have been designed to operate at pressures above the critical point where the phase change between Hquid and vapor does not occur. 

Solvent Effects m Extractive Distillation In the distillation of ideal or nonazeotropic mixtures, the component with the lowest pure-component boihng point is always recovered primarily in the distillate, while the highest boiler is recovered primarily in the bottoms. The situation is not as straightforward for an extractive-distillation operation. With some solvents, the component with the lower pure-component boiling point wih be recovered in the distillate as in ordinaiy distillation. For another solvent, the expected order is reversed, and the component with the higher pure-component boiling point wih be... 

The heat transfer oil control valve was leaking. Unknown to the operators, the boiler temperature rose from 75°C to 143°C, the boiling point of the contents. Finally, bumping in the boiler caused about 0.2 ton of liquid to be discharged through the vent. 

There are two possible causes. The first could be incorrect control of water treatment and blowdown. This can result in excessive levels of suspended solids in the boiler water, organic matter in the boiler water or high alkalinity. The second can be mechanical. If the boiler is operated below its designed working pressure it will increase the efflux velocity of the steam leaving the water surface area to a point where it may lift the water surface and drop the water level. It is important therefore to give due consideration to the steam load required from the boiler. 

Most steam generating plants operate below the critical pressure of water, and the boiling process therefore involves two-phase, nucleate boiling within the boiler water. At its critical pressure of 3,208.2 pounds per square inch absolute (psia), however, the boiling point of water is 374.15 C (705.47 °F), the latent heat of vaporization declines to zero, and steam bubble formation stops (despite the continued application of heat), to be replaced by a smooth transition of water directly to single-phase gaseous steam. 

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