Boiler feedwater is the water which is supplied to a boiler. The feed water is put into the steam drum from a feed pump. In the steam drum the feed water is then turned into steam from the heat. After the steam is used, it is then dumped to the main condenser. From the condenser, it is then pumped to the deaerated feed tank. From this tank it then goes back to the steam drum to complete its cycle. The feedwater is never open to the atmosphere. This cycle is known as a closed system or Rankine cycle.
During the early development of boilers, water treatment was not so much of an issue, as temperatures and pressures were so low that high amounts of scale and rust would not form to such a significant extent, especially if the boiler was “blown down”. It was general practice to install zinc plates and/or alkaline chemicals to reduce corrosion within the boiler. Many tests had been performed to determine the cause (and possible protection) from corrosion in boilers using distilled water, various chemicals, and sacrificial metals. [1] Silver nitrate can be added to feedwater samples to detect contamination by seawater. Use of lime for alkalinity control was mentioned as early as 1900, and was used by the French and British Navies until about 1935. [2] In modern boilers, treatment of feedwater is critical, as problems result from using untreated water in extreme pressure and temperature environments. This includes lower efficiency in terms of heat transfer, overheating, damage, and costly cleaning.
Water has a higher heat capacity than most other substances. This quality makes it an ideal raw material for boiler operations. Boilers are part of a closed system as compared to open systems in a gas turbine. The closed system that is used is the Rankine cycle. This means that the water is recirculated throughout the system and is never in contact with the atmosphere. The water is reused and needs to be treated to continue efficient operations. Boiler water must be treated in order to be proficient in producing steam. Boiler water is treated to prevent scaling, corrosion, foaming, and priming. Chemicals are put into boiler water through the chemical feed tank to keep the water within chemical range. These chemicals are mostly oxygen scavengers and phosphates. The boiler water also has frequent blowdowns in order to keep the chloride content down. The boiler operations also include bottom blows in order to get rid of solids. Scale is precipitated impurities out of the water and then forms on heat transfer surfaces. This is a problem because scale does not transfer heat very well and causes the tubes to fail by getting too hot. Corrosion is caused by oxygen in the water. The oxygen causes the metal to oxidize which lowers the melting point of the metal. Foaming and priming are aused when the boiler water does not have the correct amount of chemicals and there are suspended solids in the water which carry over in the dry pipe. The dry pipe is where the steam and water mixtures are separated.
Boiler water treatment is used to control alkalinity, prevent scaling, correct pH, and to control conductivity. [3] The boiler water needs to be alkaline and not acidic, so that it does not ruin the tubes. There can be too much conductivity in the feed water when there are too many dissolved solids. These correct treatments can be controlled by efficient operator and use of treatment chemicals. The main objectives to treat and condition boiler water is to exchange heat without scaling, protect against scaling, and produce high quality steam. The treatment of boiler water can be put into two parts. These are internal treatment and external treatment. (Sendelbach, p. 131) [4] The internal treatment is for boiler feed water and external treatment is for make-up feed water and the condensate part of the system. Internal treatment protects against feed water hardness by preventing precipitating of scale on the boiler tubes. This treatment also protects against concentrations of dissolved and suspended solids in the feed water without priming or foaming. These treatment chemicals also help with the alkalinity of the feed water making it more of a base to help protect against boiler corrosion. The correct alkalinity is protected by adding phosphates. These phosphates precipitate the solids to the bottom of the boiler drum. At the bottom of the boiler drum there is a bottom blow to remove these solids. These chemicals also include anti-scaling agents, oxygen scavengers, and anti-foaming agents. Sludge can also be treated by two approaches. These are by coagulation and dispersion. When there is a high amount of sludge content it is better to coagulate the sludge to form large particles in order to just use the bottom blow to remove them from the feed water. When there is a low amount of sludge content it is better to use dispersants because it disperses the sludge throughout the feed water so sludge does not form.
Oxygen and carbon dioxide are removed from the feed water by deaeration. Deaeration can be accomplished by using deaerator heaters, vacuum deaerators, mechanical pumps, and steam-jet ejectors. In deaerating heaters, steam sprays incoming feed water and carries away the dissolved gases. The deaerators also store hot feed water which is ready to be used in the boiler. This means of mechanical deaeration is used with chemical oxygen scavenging agents to increase efficiency. (Sendelbach, p. 129) [4] Deaerating heaters can be classified in two groups: spray types and tray types. With tray type heaters the incoming water is sprayed into steam atmosphere to reach saturation temperature. When the saturation temperature is reached most of the oxygen and non-condensable gases are released. There are seals that prevent the recontamination of the water in the spray section. The water then falls to the storage tank below. The non-condensables and oxygen are then vented to the atmosphere. The components of the tray type deaerating heater are a shell, spray nozzles, a direct contact vent condenser, tray stacks, and protective interchamber walls. The spray type deaerator is similar to the tray type deaerator. The water is sprayed into a steam atmosphere and most of the oxygen and non-condensables are released to the steam. The water then falls to the steam scrubber where the slight pressure loss causes the water to flash a little bit which also aids the removal of oxygen and non-condensables. The water then overflows to the storage tank. The gases are then vented to the atmosphere. With vacuum deaeration a vacuum is applied to the system and water is then brought to its saturation temperature. The water is sprayed into the tank like the spray and tray deaerators. The oxygen and non-condensables are vented to the atmosphere. (Sendelbach, p. 130)
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The feedwater must be specially treated to avoid problems in the boiler and downstream systems. Untreated boiler feed water can cause corrosion and fouling.
Corrosive compounds, especially O2 and CO2 must be removed, usually by use of a deaerator. Residual amounts can be removed chemically, by use of oxygen scavengers. Additionally, feed water is typically alkalized to a pH of 9.0 or higher, to reduce oxidation and to support the formation of a stable layer of magnetite on the water-side surface of the boiler, protecting the material underneath from further corrosion. This is usually done by dosing alkaline agents into the feed water, such as sodium hydroxide (caustic soda) or ammonia. Corrosion in boilers is due to the presence of dissolved oxygen, dissolved carbon dioxide, or dissolved salts.
Deposits reduce the heat transfer in the boiler, reduce the flow rate and eventually block boiler tubes. Any non-volatile salts and minerals that will remain when the feedwater is evaporated must be removed, because they will become concentrated in the liquid phase and require excessive "blow-down" (draining) to prevent the formation of solid precipitates. Even worse are minerals that form scale. Therefore, the make-up water added to replace any losses of feedwater must be demineralized/deionized water, unless a purge valve is used to remove dissolved minerals.
Steam locomotives usually do not have condensers so the feedwater is not recycled and water consumption is high. The use of deionized water would be prohibitively expensive so other types of water treatment are used. Chemicals employed typically include sodium carbonate, sodium bisulfite, tannin, phosphate and an anti-foaming agent. [6]
Treatment systems have included:
A boiler is a closed vessel in which fluid is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.
The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat source and heat sink. The Rankine cycle is named after William John Macquorn Rankine, a Scottish polymath professor at Glasgow University.
In the oil industry, waterflooding or water injection is where water is injected into the oil reservoir, to maintain the pressure, or to drive oil towards the wells, and thereby increase production. Water injection wells may be located on- and offshore, to increase oil recovery from an existing reservoir.
A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system. This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feedwater is introduced back into the steam cycle.
A thermal power station, also known as a thermal power plant, is a type of power station in which the heat energy generated from various fuel sources is converted to electrical energy. The heat from the source is converted into mechanical energy using a thermodynamic power cycle. The most common cycle involves a working fluid heated and boiled under high pressure in a pressure vessel to produce high-pressure steam. This high pressure-steam is then directed to a turbine, where it rotates the turbine's blades. The rotating turbine is mechanically connected to an electric generator which converts rotary motion into electricity. Fuels such as natural gas or oil can also be burnt directly in gas turbines, skipping the steam generation step. These plants can be of the open cycle or the more efficient combined cycle type.
A deaerator is a device that is used for the removal of dissolved gases like oxygen from a liquid.
A surface condenser is a water-cooled shell and tube heat exchanger installed to condense exhaust steam from a steam turbine in thermal power stations. These condensers are heat exchangers which convert steam from its gaseous to its liquid state at a pressure below atmospheric pressure. Where cooling water is in short supply, an air-cooled condenser is often used. An air-cooled condenser is however, significantly more expensive and cannot achieve as low a steam turbine exhaust pressure as a water-cooled surface condenser.
Chemical fouling inhibitors are products that are mixtures of fouling and corrosion inhibitors use in boiler feedwater treatment. Several of these products use aliphatic polyamines to coat the surface of pipes.
A condensate pump is a specific type of pump used to pump the condensate (water) produced in an HVAC, refrigeration, condensing boiler furnace, or steam system.
There are many uses of water in industry and, in most cases, the used water also needs treatment to render it fit for re-use or disposal. Raw water entering an industrial plant often needs treatment to meet tight quality specifications to be of use in specific industrial processes. Industrial water treatment encompasses all these aspects which include industrial wastewater treatment, boiler water treatment and cooling water treatment.
Boiler water is liquid water within a boiler, or in associated piping, pumps and other equipment, that is intended for evaporation into steam. The term may also be applied to raw water intended for use in boilers, treated boiler feedwater, steam condensate being returned to a boiler, or boiler blowdown being removed from a boiler.
A steam generator is a type of boiler used to produce steam for climate control and potable water heating in railroad passenger cars. The output of a railroad steam generator is low-pressure, saturated steam that is passed through a system of pipes and conduits throughout the length of the train.
A condensate polisher is a device used to filter water condensed from steam as part of the steam cycle, for example in a conventional or nuclear power plant. It is frequently filled with tiny polymer resin beads which are used to remove or exchange ions so that the purity of the condensate is maintained at or near that of distilled water.
A deaerating feed tank (DFT), often found in steam plants that propel ships, is located after the main condensate pump and before the main feed booster pump. It has these three purposes:
The purpose of the Kirchweger condenser was to preheat feedwater in a steam locomotive using the exhaust steam from the engine. It was invented in 1850 by Heinrich Kirchweger. A similar system was developed by Rohrbeck.
Boilers for generating steam or hot water have been designed in countless shapes, sizes and configurations. An extensive terminology has evolved to describe their common features. This glossary provides definitions for these terms.
An evaporator, distiller or distilling apparatus is a piece of ship's equipment used to produce fresh drinking water from sea water by distillation. As fresh water is bulky, may spoil in storage, and is an essential supply for any long voyage, the ability to produce more fresh water in mid-ocean is important for any ship.
Boiler blowdown is water intentionally wasted from a boiler to avoid concentration of impurities during continuing evaporation of steam. The water is blown out of the boiler with some force by steam pressure within the boiler. Bottom blowdown used with early boilers caused abrupt downward adjustment of boiler water level and was customarily expelled downward to avoid the safety hazard of showering hot water on nearby individuals.
Steam and water analysis system (SWAS) is a system dedicated to the analysis of steam or water. In power stations, it is usually used to analyze boiler steam and water to ensure the water used to generate electricity is clean from impurities which can cause corrosion to any metallic surface, such as in boiler and turbine.
Oxygenated treatment (OT) is a technique used to reduce corrosion in a boiler and its associated feedwater system in flow-through boilers.