Feedwater boiler is an important part of boiler operation. The feed water is fed into the steam drum from the feed pump. In the steam drum, the feed water then turns into steam from heat. After steam is used, it is discharged to the main condenser. From the condenser it is then pumped into the deaeration feed tank. From this tank then return to the steam drum to complete the cycle. The bait water never opens into the atmosphere. This cycle is known as a closed system or Rankine cycle.
Video Boiler feedwater
History of feed water processing
During the initial development of the boiler, water treatment is not so much a problem, because the temperature and pressure are so low that the scale and the high rust will not form up to a high amount, especially if the boiler is cleaned and/or "blown". It is a common practice, to install zinc plates and/or alkaline chemicals to reduce corrosion inside the boiler. Many tests have been conducted to try to determine the cause and possible protection of corrosion in the boiler using distilled water, various chemicals, and sacrificial metals. Silver nitrate can be added to the feed water sample to detect contamination by sea water. The use of lime for alkalinity control was mentioned in the early 1900s, and was used by the French and British Navy up to about 1935. However, in modern boilers, boiler feedwater treatment is essential, as many problems can result from the use of untreated water in extreme pressure and temperature environment; this includes lower efficiency in terms of heat transfer, overheating, damage, and high cleaning costs.
Maps Boiler feedwater
Boiler feed water characteristics
Water has a higher heat capacity than most other substances. This quality makes it an ideal feedstock for boiler operations. The boiler is part of a closed system compared to an open system in a gas turbine. Closed system used is Rankine cycle. This means that water is circulated back to the entire system and never in contact with the atmosphere. Water is reused and needs to be treated to continue efficient operation. Boiler water should be treated to be proficient to produce steam. Boiler water is treated to prevent scaling, corrosion, foaming, and priming. Chemicals are fed into boiler water through chemical feed tanks to keep water in the chemical range. These chemicals are mostly oxygen and phosphate scavengers. Water boilers also often experience blowdown to keep chloride levels down. The boiler operation also includes a bottom blow to get rid of the solids. The scale is dirt that is deposited out of water and then formed on the heat transfer surface. This is a problem because the scale does not transfer heat well and causes the tube to fail with overheating. Corrosion is caused by oxygen in the water. Oxygen causes the metal to oxidize which lowers the melting point of the metal. Foaming and priming are caused when boiler water does not have the right amount of chemicals and there is a suspended solid in the water carried in the dry pipe. Dry pipe is where the mixture of steam and water is separated.
Boiler feed water treatment
Boiler water treatment is used to control the alkalinity, prevent scaling, correct pH, and to control the conductivity. Boiler water should be alkaline and not acidic, so as not to damage the pipe. There is too much conductivity in the feed water when there are too many dissolved solids. This correct treatment can be controlled by efficient operators and the use of maintenance chemicals. The main purpose for treating and conditioning boiler water is to exchange heat without scaling, protect against scaling, and produce high-quality steam. Boiler water treatment can be put into two parts. It is an internal care and external treatment. (Sendelbach, p.Ã, 131) The internal treatment for boiler feed water and external treatment is for the feed water of make-up and the condensate portion of the system. The internal treatment protects against the hardness of the feed water by preventing scale precipitation on the boiler tube. This treatment also protects against the concentration of dissolved solids and suspended in the feed water without priming or bubbly. These chemical treatments also help with the alkalinity of the feed water so that more than one base helps to protect against boiler corrosion. The correct alkalinity is protected by adding phosphate. This phosphate deposits solids to the bottom of the boiler drum. At the bottom of the boiler drum there is a bottom punch to remove this solid. These chemicals also include anti-scaling agents, oxygen scavengers, and anti-foaming agents. Sludge can also be handled by two approaches. This is by coagulation and dispersion. When there is a high mud content, it is better to coagulate the sediment to form large particles to use only a lower punch to remove it from the feed water. When there is a low mud content, it is better to use dispersants because it spreads mud throughout the feed water so that the mud does not form.
Deaerated water feed
Oxygen and Carbon Dioxide are removed from the feed water through deeration. Deaeration can be achieved by using a deaerator heater, vacuum deaerator, mechanical pump, and steam-jet ejector. In deaerating the spray steam heater enters the feed water and carries the dissolved gases. The deaerators also store hot water feeds that are ready to use in the boiler. This means mechanical deaeration is also used with chemical oxygen cleaning agents to improve efficiency. (Sendelbach, p.Ã, 129) Deaerating heaters can be classified into two groups. Two deaerating heaters are a type of spray and a tray type. With the type of tray deaerating heaters incoming water is sprayed into the vapor atmosphere to reach the saturation temperature. When the saturation temperature reaches most of the oxygen and the non-condensable gas is released. There is a seal that prevents water contamination in the spray. The water then falls into the storage tank below. Non-condensate and oxygen are then released into the atmosphere. Tray deaerator type heater components are shell, spray nozzle, direct contact vent condenser, tray rack, and protective interchamber wall. The type of deaerater spray is very similar to the tray type deaerater. Water is sprayed into the vapor atmosphere and most of the oxygen and non-condenser are released to the vapor. Water then falls onto a steam scrubber where a slight loss of pressure causes water to blink slightly which also helps remove oxygen and non-condensables. The water then overflows into the storage tank. The gases are then released into the atmosphere. With vacuum deaeration, the vacuum is applied to the system and water is then brought to its saturation temperature. Water is sprayed into the tank like sprays and barriers. Oxygen and non-condensate are released into the atmosphere. (Sendelbach, p.Ã, 130)
Conditioning
Bait water should be specially treated to avoid problems in boilers and downstream systems. Unprocessed boiler feed water can cause corrosion and fouling.
Boiler corrosion
Corrosive compounds, especially O 2 and CO 2 should be removed, usually by using a deaerator. The amount of residue can be removed chemically, using an oxygen scavenger. In addition, the feed water is usually alkalized to pH 9.0 or higher, to reduce oxidation and to support the formation of a magnetite stable layer on the water-side surface of the boiler, protecting the under material from further corrosion. This is usually done by dosing alkaline materials into the feed water, such as sodium hydroxide (caustic soda) or ammonia. Corrosion in the boiler is due to the presence of dissolved oxygen, dissolved carbon dioxide, or dissolved salt.
Fouling
Deposit reduces heat transfer in the boiler, reduces flow rate and ultimately blocks the boiler tube. Any non-volatile salts and minerals that will remain when the feed water is evaporated must be removed, as they will be concentrated in the liquid phase and require excessive "blow-down" (drying) to prevent the formation of solid deposits. Worse is the mineral that makes up the scale. Therefore, the added make-up water to replace any loss of feed water should be demineralized/deionized water, unless the cleaning valve is used to remove the dissolved minerals.
Corruption of Caustic Voltage
Priming and foaming
Locomotive boiler
Steam locomotives usually have no condenser so the feed water is not recycled and water consumption is high. The use of deionized water will be very expensive so other types of water treatment are used. Chemicals used typically include sodium carbonate, sodium bisulfite, tannins, phosphates and anti-foam agents.
The maintenance system includes:
- Alfloc, developed by British Railways and Imperial Chemical Industries
- Integral Armand Traitement (TIA), developed by Louis Armand
- Porta Treatment, developed by Livio Dante Porta
See also
- Boiler feed water pump
- Evaporator
- Helamin
References
- Shun'an, C., Qing, Z., & amp; Zhixin, Z. (2008). A study on the effect of the concentration of chloride ions on the corrosion behavior of carbon steels in the chemistry of high temperature boiler water phosphates. Anti-Corrosion Methods and Materials, 55 (1), 15-19.
- Sendelbach, M. (1988). Boiler water treatment: Why, what and how. Chemical Engineering, 95 (11), 127.
- Boiler feed water characteristics. (n.d.). Retrieved 21 March 2015, from http://www.lenntech.com/applications/process/boiler/boiler-feedwater-characteristics.htm
External links
- Water Boiler Feed System Configuration
Source of the article : Wikipedia
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