If you are searching for a steam energy solution for your business, you likely have come across the confusing differentiation between a steam generator and a steam boiler. The most basic understanding of these two systems is that they both produce energy through steam, however, they accomplish this in fundamentally different ways. These differences affect everything about each system, including their size, operation, and most importantly, their application.
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Because steam boilers and steam generators are intended to be used in entirely different situations, it’s essential for a business to understand the discrepancies between them. Understanding the design of these two systems will help in choosing which is right for meeting your energy production needs.
What is a Steam Boiler?
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Steam boilers are typically larger pressure vessels capable of providing energy to industrial operations. They accomplish this by boiling water at subcritical pressures with intricate fuel systems. In some jurisdictions, the high pressure and high steam output capability of steam boilers requires a fully certified and licensed operator to be on-site in order to be operated. There are two common designs for a steam boiler: firetube and watertube.
In a firetube design, a steam boiler is a pressure vessel comprised of a large shell in which the hot gases of combustion pass through one or more boiler tubes connected to the front and rear boiler sheet. The most common firetube type is the scotch marine firetube boiler, which utilizes a large furnace tube and many smaller boiler tubes. The hot gases from the combustion process pass through the tubes transferring the heat to the surrounding water. This process develops the high temperature required to boil the water and begin the steaming process.
The watertube boiler actually inverts this design. The water flows through the smaller diameter boiler tubes while the combustion gases travel around them to transfer heat to the water. The boiler tubes in the watertube design carry the heated water inside the tubes between the lower drum (mud drum) and upper drums (steam drum), with the generated steam accumulating in the upper drum. Heat is generated in the furnace area and transferred to the water through two main zones, the furnace zone and the convection zone, while the hot gases are moving across the tubes and out the exhaust.
What is a Steam Generator?
Compared to steam boilers, steam generators have less steel built into their design, utilizing a single tube coil instead of many smaller diameter boiler tubes. A specialized feedwater pump is used to continuously pump the feedwater through the coil. A steam generator uses a once through, forced flow, design to convert the incoming water to steam during the single pass through the water coil. As water passes through the coil, the heat is transferred from the hot gases which causes the water to convert to steam. The generator design does not utilize a steam drum where the steam of a boiler has a disengagement area from the water in so the use of a steam/water separator to achieve 99.5% steam quality is required.
Because generators do not utilize a large pressure vessel as in a firetube, they are often smaller and quicker to start up, making them ideal for quick on demand situations. However, this comes at the cost of energy production, as generators have low turn-down rates, and are therefore less capable of providing steam during periods of variable demand.
Advantages of a Steam Boiler
When it comes to deciding which of these systems is right for your business, the key consideration is simple: output and demand. While steam generators are quicker to start up, they are limited in size as their maximum capacity falls short of a typical watertube boiler. Steam boilers, on the other hand, are designed for long-term industrial use. The larger capacities of a steam and hot water boiler are powerful enough to support large-scale, high demand critical operations that see swings in steam demand. The water stored in the steam boilers allows for a much better response to changing steam demand from the plant than the single through design of a steam generator.
It is also worth noting that steam boilers are more adaptable to the needs of a business. Depending on the construction of your building or its energy usage, steam boilers can be packaged and designed to meet any of your demands. If your business is in need of a custom industrial boiler solution, contact Powerhouse today!
The inverted U-tube bundle of a Combustion Engineering steam generator.
Steam generators are heat exchangers used to convert water into steam from heat produced in a nuclear reactor core. They are used in pressurized water reactors (PWR) between the primary and secondary coolant loops.
In typical PWR designs, the primary coolant is high-purity water, kept under high pressure so it cannot boil. Quick heal product key activation. This primary coolant is pumped through the reactor core where it absorbs heat from the fuel rods. It then passes through the steam generator, where it transfers its heat (via conduction through metal) to lower-pressure water which is allowed to boil.
Purpose[edit]
Unlike PWRs, Boiling water reactors (BWR) do not use steam generators. The primary coolant is allowed to boil directly in the reactor core, and the steam is simply passed through a steam turbine. While theoretically simple, this has a downside for maintenance. While passing through the core, primary coolant water is subjected to high neutron flux. This activates oxygen and dissolved nitrogen in the water. The major reaction[1] is: an atom of oxygen-16 absorbs 1 neutron and emits 1 proton, becoming nitrogen-16. Nitrogen-16 has a 7-second half-life and produces a gamma ray when it decays back to oxygen-16. The 7-second half-life is long enough for the water to circulate out of the reactor. In a BWR, this means that the water may be in the steam turbine when it releases its gamma rays. Although no long-lived radioisotopes are produced by this reaction, the gamma radiation means that humans cannot be present in a BWR's turbine hall during reactor operation and for a short time afterwards.
By contrast, in a PWR, the steam generator separates the activated primary coolant water from the secondary coolant which passes through the steam turbine. Thus, humans can freely access a PWR's turbines and other steam plant components during operation. This reduces maintenance cost and improves up-time.
Description[edit]
Vertical recirculating-type steam generator (typical of Westinghouse and Combustion Engineering-designed reactors) and components.
In commercial power plants, there are two to four steam generators per reactor; each steam generator can measure up to 70 feet (21 m) in height and weigh as much as 800 tons. Each steam generator can contain anywhere from 3,000 to 16,000 tubes, each about .75 inches (19 mm) in diameter. The coolant (treated water), which is maintained at high pressure to prevent boiling, is pumped through the nuclear reactor core. Heat transfer takes place between the reactor core and the circulating water and the coolant is then pumped through the primary tube side of the steam generator by coolant pumps before returning to the reactor core. This is referred to as the primary loop.
That water flowing through the steam generator boils water on the shell side (which is kept at a lower pressure than the primary side) to produce steam. This is referred to as the secondary loop. The secondary-side steam is delivered to the turbines to make electricity. The steam is subsequently condensed via cooled water from a tertiary loop and returned to the steam generator to be heated once again. The tertiary cooling water may be recirculated to cooling towers where it sheds waste heat before returning to condense more steam. Once-through tertiary cooling may otherwise be provided by a river, lake, or ocean. This primary, secondary, tertiary cooling scheme is the basis of the pressurized water reactor, which is the most common nuclear power plant design worldwide.
In other types of reactors, such as the pressurised heavy water reactors of the CANDU design, the primary fluid is heavy water. Liquid metal cooled reactors such as the Russian BN-600 reactor use a liquid metal, such as sodium, as the primary coolant. These also use heat exchangers between primary metal coolant and the secondary water coolant, and thus their secondary and tertiary cooling is similar to a PWR.
A steam generator's heat-exchange tubes have an important safety role, because they separate radioactive and non-radioactive fluid systems. (The primary coolant becomes briefly radioactive from its exposure to the core, and also has trace amounts of longer-lived radioactive isotopes dissolved in it, such as dissolved atoms of iron from pipes.) Because the primary coolant is at higher pressure, a ruptured heat-exchange tube would cause primary coolant to leak into the secondary loop. Typically this would require the plant to shutdown for repair. To avoid such primary-secondary leaks, steam generator tubes are periodically inspected by eddy-current testing, and individual tubes can be plugged to remove them from operation.[2] As with many nuclear components, mechanical engineers determine the inspection frequency using the known rates of corrosion and crack propagation in the material. https://labelspowerup.weebly.com/halo-ce-cd-key-generator.html. If an inspection finds that a tube wall is thin enough that it might corrode through before the next scheduled inspection, the tube is plugged. (Plugging a tube is typically easier than attempting to repair it. Word 2003 professional product key generator. There are many small heat-exchange tubes, and steam generators are designed with excess tubes to allow some to be plugged.)
Entire steam generators are often replaced in plant mid-life, which is a major undertaking. Most U.S. PWR plants have had steam generators replaced.[2] https://labelspowerup.weebly.com/microsoft-office-2013-serial-key-generator-keygen-rar.html.
History[edit]
The nuclear powered steam generator started as a power plant for the first nuclear submarine, the USS Nautilus (SSN-571). It was designed and built by the Westinghouse power company for the submarine; from there the company started its development and research of nuclear-powered steam generators.[3] Once peaceful nuclear reactors were legalized for use as power plants, power corporations jumped at the opportunity to utilize the growing development of nuclear powered steam generators. Westinghouse built one of the first nuclear power plants, the Yankee Rowe nuclear power station (NPS), which also used a nuclear powered steam generator, in 1960. This power plant had a one hundred MWe (mega watt electric) output. By comparison, some modern plants have over 1100 MWe output. Eventually, other international companies such as Babcock & Wilcox and Combustion Engineering began their own programs for research and development of the nuclear power steam generator.
Types[edit]
This Babcock & Wilcox nuclear steam generator moved in a special train (restricted to 20 mph) via the Penn Central Railroad and Southern Railway from Barberton, Ohio to a Duke Energy site in Oconee, S.C. This generator weights 1,140,000 lbs and is a record shipment for the Railroad at that time (1970).
Westinghouse and Combustion Engineering designs have vertical U-tubes with inverted tubes for the primary water. Canadian, Japanese, French, and German PWR suppliers use the vertical configuration as well. Russian VVER reactor designs use horizontal steam generators, which have the tubes mounted horizontally. Babcock & Wilcox plants (e.g., Three Mile Island) have smaller steam generators that force water through the top of the OTSGs (once-through steam generators; counter-flow to the feedwater) and out the bottom to be recirculated by the reactor coolant pumps. The horizontal design has proven to be less susceptible to degradation than the vertical U-tube design.
Materials and construction[edit]
The materials that make up the turbine and pipes of a nuclear powered steam generator are specially made and specifically designed to withstand the heat and radiation of the reactor. The water tubes also have to be able to resist corrosion from water for an extended period of time. The pipes that are used in American reactors are made of Inconel, either Alloy 600 or Alloy 690. Alloy 690 is made with extra chromium and most facilities heat treat the metal to make it better able to resist heat and corrosion. The high nickel content in Alloy 600 and Alloy 690 make them well suited for resisting acids and high degrees of stress and temperature.
Degradation[edit]
The annealed, or heat treated, Alloy 600 was prone to tube denting and thinning due to water chemistry. Plants that used the Alloy 600 in their water tubes therefore had to install new water chemistry controllers and change the chemicals they put in the water. Due to this, pipe thinning has been taken care of, but on rare occasions, tube denting still occurs, causing leaks and ruptures. The only way to prevent this is regular maintenance and check-ups, but this forces the reactor to shut down. In some cases, plants replaced their Alloy 600 tubes with Alloy 690 tubes and a few plants were shut down. To prevent future problems, manufacturers of steam turbines for nuclear power plants have improved their fabrication techniques and used other materials, such as stainless steel, to prevent tube denting.[4]
Typical operating conditions[edit]
Steam generators in a 'typical' PWR in the USA have the following operating conditions:
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Tube material[edit]
Various high-performance alloys and superalloys have been used for steam generator tubing, including type 316 stainless steel, Alloy 400, Alloy 600MA (mill annealed), Alloy 600TT (thermally treated), Alloy 690TT, and Alloy 800Mod.
Steam Key CodesSee also[edit]References[edit]
External links[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Steam_generator_(nuclear_power)&oldid=926881031'
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