Boilers, drum once-through

Steam-Generator Circulation System Circulation systems for utility application are generally classified as natural circulation and forced or pump-assisted circulation in drum-type boilers, and as once-through now in subcritical- and supercriticaf-pressure boilers. The... 

Few high pressure drum boilers are equipped with the condensate polishing plant illustrated in Fig. 1 but once-through boilers (having no drum to allow the accumulation of impurities in the water phase) normally are. The water is pumped from the condenser back through feed-heaters and into the boiler again. 

In a once-through system, the feedwater entering the unit absorbs heat until it is completely converted to steam. The total mass flow through the waterwall tubes equals the feedwater flow and, during normal operation, the total steam flow. As only steam leaves the boiler, there is no need for a steam drum. 

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. 

The raw water for drum-type boilers operating above 2000 psi (136 atm) drum pressure and for once-through units should be prepared by passing water through a mixed-bed demineralizer as a final step before adding to the cycle. 

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. 

The application of evaporators, in particular vertical tube falling film vapor compression evaporators, to treat PW is increasing, in particular in SAGD applications. The system provides a clean water stream that can be used by a once-through steam generators (OTSG) or drum boilers for stream production. Evaporator blowdown is disposed either... 

Similar or repeat design It is quite possible that different units are set up by a company with a similar (or even the same) design. One company may have two or three different types of (say) 500 MW plants at different locations, or in the same location one is a drum-type boiler and the other is a once-through boiler. In all such cases HAZOP by difference, that is, detailed comparison of the two systems may be helpful. However, the team must be well aware of variations between the two systems. 

The once-through heat recovery steam generator design is ideally matched to very high temperature and pressure, well into the supercritical range. Moreover this type of boiler is structurally simpler than a conventional one, since no drum is required. A specific mathematical model has been developed. Thermodynamic model has been implemented to suit very high pressure (up to 240 bar), sub- and supercritical steam properties. We illustrate the model use with a 180 bar once-through boiler (0TB). 

In the present work we propose a mathematical model for the simulation and design of the once-through boiler. It is not possible to use empirical equations used for the simulation of each part of the traditional boiler. General equations have to be used for each tube of the boiler. Moreover there is a more significant evolution of the water/steam flow pattern type due to the complete water vaporization inside the tubes (in a conventional boiler, the circulation flow is adjusted to reach a vapor fraction between 20% and 40% in the tubes and the vapor is separated in the drum). 

The operation of a once-through boiler is more readily analyzed because it is dynamically continuous, not broken in half by a drum. Feedwater is pumped into the tubes at one end and superheated steam withdrawn at the other. There is no recirculation. There is also no liquid level to measure. In subcritical boilers, there is a transition from liquid to vapor somewhere in the tubes, but exactly where is of little consequence. In... 

How is steam temperature controlled in a drum boiler What feature of a once-through boiler enables manipulation of feedwater flow to control temperature ... 

Steam temperature in a drum boiler is controlled principally with spray. The once-through boiler has one less controlled variable (liqnid level), which frees a manipulated variable (feedwater flow) for temperature control. 

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