In the past, difficulty with carry-over and impurities in the steam was frequently encountered (carry-over is the passing of water and impuri-ties to the steam outlet). Efforts were directed to reduce carry-over to a minimum by separating the water from the steam through the
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installation of baffles and the dry pipe. Both measures met with some success. The dry pipe ran the length of the drum, the ends being closed and the upper side of the pipe being drilled with many small holes. The top center of this pipe was connected to the steam outlet.
Steam entering through the series of holes was made to change direc-tion before entering the steam outlet, and in the process the water was separated from the steam. The bottom of the dry pipe contained a drain, which ran below the normal water level in the drum. The dry pipe was installed near the top of the drum so as not to require removal for routine inspection and repairs inside the drum.
The dry pipe proved to be fairly effective for small boilers but unsuited for units operating at high steam capacity. Placing a baffle ahead of the dry pipe offered some slight improvement in steam qual-ity but was still not considered entirely satisfactory.
Modern practice requires high-purity steam for process, for the superheater, and for the turbine. An important contribution to increased boiler capacity and high rating is the fact that the modern boiler is pro-tected by clean, high-quality feedwater. The application of both external and internal feedwater treatment is supplemented by the use of steam scrubbers and separators that are located in the steam drum.
Steam drums are used on recirculating boilers that operate at subcrit-ical pressures.2 The primary purpose of the steam drum is to separate the saturated steam from the steam-water mixture that leaves the heat-transfer surfaces and enters the drum. The steam-free water is recircu-lated within the boiler with the incoming feedwater for further steam generation. The saturated steam is removed from the drum through a series of outlet nozzles, where the steam is used as is or flows to a super-heater for further heating. (By definition, saturated steam is pure steam that is at the temperature that corresponds to the boiling temperature at a particular pressure. For example, saturated steam at a pressure of 500 psia has a temperature of 467°F.) (See Table C.1 of Appendix C.)
The steam drum is also used for the following:
1. To mix the saturated water that remains after steam separation with the incoming feedwater
2. To mix the chemicals that are put into the drum for the purpose of corrosion control and water treatment
3. To purify the steam by removing contaminants and residual moisture 4. To provide the source for a blowdown system where a portion of
the water is rejected as a means of controlling the boiler water chemistry and reducing the solids content
2The term critical pressure is the pressure at which there is no difference between the liquid and vapor states of water; i.e., the density is identical. This occurs at 3206 psia. (See Table C.1 of Appendix C.)
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5. To provide a storage of water to accommodate any rapid changes in the boiler load.
The most important function of the steam drum, however, remains as the separation of steam and water. Separation by natural gravity can be accomplished with a large steam-water surface inside the drum.
This is not the economical choice in today’s design because it results in larger steam drums, and therefore the use of mechanical separation devices is the primary choice for separation of steam and water.
Efficient steam-water separation is of major importance because it produces high-quality steam that is free of moisture. This leads to the following key factors in efficient boiler operation:
1. It prevents the carry-over of water droplets into the superheater, where thermal damage could result.
2. It minimizes the carry-under of steam with the water that leaves the drum, where this residual steam would reduce the circulation effectiveness of the boiler.
3. It prevents the carry-over of solids. Solids are dissolved in the water droplets that may be entrained in the steam if not separated properly. By proper separation, this prevents the formation of deposits in the superheater and ultimately on the turbine blades.
Boiler water often contains contaminants that are primarily in solu-tion. These contaminants come from impurities in makeup water, treatment chemicals, and leaks within the condensate system such as the cooling water. Impurities also occur from the reaction of boiler water and contaminants with the materials of the boiler and of the equip-ment prior to entering the boiler. The steam quality of a power plant depends on proper steam-water separation as well as the feedwater quality, and this is a major consideration to having a plant with high availability and low maintenance costs. Even low levels of solids in the steam can damage the superheater and turbine, causing significant outages, high maintenance costs, and loss of production revenues.
Prior to the development of quality steam-water separators, gravity alone was used for separation. Because the steam drum diameter requirements increased significantly, the use of a single drum became uneconomical, and therefore it became necessary to use multiple smaller drums. Figure 2.11 is an example of this showing three steam drums. Although there are units of this design still in operation, they are no longer common.
The cyclone separators illustrated in cross-sectional elevation in Figs. 2.15 and 2.16 overcome many of the shortcomings previously mentioned for the baffle and dry pipe. Depending on the size of the boiler, there is a single or double row of cyclone steam separators with
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scrubbers running the entire length of the drum. Baffle plates are located above each cyclone, and there is a series of corrugated scrub-ber elements at the entrance to the steam outlet. Water from the scrubber elements drains to a point below the normal water level and is recirculated in the boiler.
Figure 2.15 Single-row arrangement of cyclone steam separators with secondary scrubbers. (Babcock & Wilcox, a McDermott company.)
Figure 2.16 Double-row arrangement of cyclone steam sep-arators for primary steam separation with secondary scrubber elements at top of drum. (Babcock & Wilcox, a McDermott company.)
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In the installation shown in Fig. 2.16, operation is as follows: (1) The steam-water mixture from the risers enter the drum from behind the baffle plate before entering the cyclone; the cyclone is open at top and bottom. (2) Water is thrown to the side of the cyclone by centrifugal force. (3) Additional separation of water and steam occurs in the pas-sage of steam through the baffle plates. (4) On entering the scrubber elements, water is also removed with steam passing to the steam outlet.
Separators of this type can reduce the solids’ carry-over to a very low value depending on the type of feedwater treatment used, the rate of evaporation, and the concentration of solids in the water. The cyclone and scrubber elements are removable for cleaning and inspection and are accessible from manways that are located in the ends of the steam drum.
The combination of cyclone separators and scrubbers provides the means for obtaining steam purity corresponding to less than 1.0 part per million (ppm) solids content under a wide variation of operating con-ditions. This purity is generally adequate in commercial practice; how-ever, the trend to higher pressures and temperatures in steam power plants imposes a severe demand on steam-water separation equipment.