There is a class of economizers that are designed to condense the flue gases and/or have the water in direct contact with flue gases. I have called them 'Flue Gas Condensers'. Stack Economizers and Condensers should be considered as an efficiency measure when large amounts of make-up water are used (ie: not all condensate is returned to the boiler or large amounts of live steam is used in the process so there is no condensate to return) or there is a simultaneous need for large volumes of hot water.
The application difference between an economizer and condenser is that economizers are primarily used to heat a smaller volume of water to a high temperature for boiler feed water, and condenser units heat a larger volume of water to a lower temperature. Condensers can be more efficient because they can have a lower outlet exhaust temperature and take advantage of the energy in condensed flue gasses (the Latent Heat of Vaporization).
For more information see Flue Gas Condensers
Origin of the Problem
The most common source of corrosion in boiler systems is dissolved gas: oxygen, carbon dioxide and ammonia. Of these, oxygen is the most aggressive. The importance of eliminating oxygen as a source of pitting and iron deposition cannot be over-emphasized. Even small concentrations of this gas can cause serious corrosion problems.
Makeup water introduces appreciable amounts of oxygen into the system. Oxygen can also enter the feed water system from the condensate return system. Possible return line sources are direct air-leakage on the suction side of pumps, systems under vacuum, the breathing action of closed condensate receiving tanks, open condensate receiving tanks and leakage of nondeaerated water used for condensate pump seal and/or quench water. With all of these sources, good housekeeping is an essential part of the preventive program.
One of the most serious aspects of oxygen corrosion is that it occurs as pitting. This type of corrosion can produce failures even though only a relatively small amount of metal has been lost and the overall corrosion rate is relatively low. The degree of oxygen attack depends on the concentration of dissolved oxygen, the pH and the temperature of the water.
The influence of temperature on the corrosivity of dissolved oxygen is particularly important in closed heaters and economizers where the water temperature increases rapidly. Elevated temperature in itself does not cause corrosion. Small concentrations of oxygen at elevated temperatures do cause severe problems. This temperature rise provides the driving force that accelerates the reaction so that even small quantities of dissolved oxygen can cause serious corrosion.
The Corrosion Process
Localized attack on metal can result in a forced shutdown. The prevention of a forced shutdown is the true aim of corrosion control.
Because boiler systems are constructed primarily of carbon steel and the heat transfer medium is water, the potential for corrosion is high. Iron is carried into the boiler in various forms of chemical composition and physical state. Most of the iron found in the boiler enters as iron oxide or hydroxide. Any soluble iron in the feed water is converted to the insoluble hydroxide when exposed to the high alkalinity and temperature in the boiler.
These iron compounds are divided roughly into two types, red iron oxide (Fe2O3) and black magnetic oxide (Fe3O4). The red oxide (hematite) is formed under oxidizing conditions that exist, for example, in the condensate system or in a boiler that is out of service. The black oxides (magnetite) are formed under reducing conditions that typically exist in an operating boiler.
External treatment, as the term is applied to water prepared for use as boiler feed water, usually refers to the chemical and mechanical treatment of the water source. The goal is to improve the quality of this source prior to its use as boiler feed water, external to the operating boiler itself. Such external treatment normally includes:
Any or all of these approaches can be used in feed water or boiler water preparation.
Even after the best and most appropriate external treatment of the water source, boiler feed water (including return condensate) still contains impurities that could adversely affect boiler operation. Internal boiler water treatment is then applied to minimize the potential problems and to avoid any catastrophic failure, regardless of external treatment malfunction.
Feed Water Preparation
The basic assumption with regard to the quality of feed water is that calcium and magnesium hardness, migratory iron, migratory copper, colloidal silica and other contaminants have been reduced to a minimum, consistent with boiler design and operation parameters.
Once feed water quality has been optimized with regard to soluble and particulate contaminants, the next problem is corrosive gases. Dissolved oxygen and dissolved carbon dioxide are among the principal causes of corrosion in the boiler and pre-boiler systems. The deposition of these metallic oxides in the boiler is frequently more troublesome than the actual damage caused by the corrosion. Deposition is not only harmful in itself, but it offers an opening for further corrosion mechanisms as well.
Contaminant products in the feed water cycle up and concentrate in the boiler. As a result, deposition takes place on internal surfaces, particularly in high heat transfer areas, where it can be least tolerated. Metallic deposits act as insulators, which can cause local overheating and failure. Deposits can also restrict boiler water circulation. Reduced circulation can contribute to overheating, film boiling and accelerated deposition.
The best way to start to control pre-boiler corrosion and ultimate deposition in the boiler is to eliminate the contaminants from the feed water. Consequently, this section deals principally with the removal of oxygen, the impact of trace amounts of contaminants remaining in the feed water, and heat exchange impact.
Feed water is defined as follows:
Feed water (FW) = Makeup water (MW) + Return condensate (RC)
The above equation is a mass balance (pounds or kilograms).
Deaeration (Mechanical and Chemical)
Mechanical and chemical deaeration is an integral part of modern boiler water protection and control. Deaeration, coupled with other aspects of external treatment, provides the best and highest quality feed water for boiler use.
Simply speaking, the purposes of deaeration are:
1. To remove oxygen, carbon dioxide and other noncondensable gases from feed water
2. To heat the incoming makeup water and return condensate to an optimum temp
3. Minimizing solubility of the undesirable gases
4. Providing the highest temperature water for injection to the boiler
For more information see Water Treatment
Mechanical deaeration is the first step in eliminating oxygen and other corrosive gases from the feed water. Free carbon dioxide is also removed by deaeration, while combined carbon dioxide is released with the steam in the boiler and subsequently dissolves in the condensate. This can cause additional corrosion problems.
Because dissolved oxygen is a constant threat to boiler tube integrity, our discussion on the deaerator will be aimed at reducing the oxygen content of the feed water. The two major types of deaerators are the tray type and the spray type. In both cases, the major portion of gas removal is accomplished by spraying cold makeup water into a steam environment.
Tray Type Deaerating Heaters
Tray-type deaerating heaters release dissolved gases in the incoming water by reducing it to a fine spray as it cascades over several rows of trays. The steam that makes intimate contact with the water droplets then scrubs the dissolved gases by its counter-current flow. The steam heats the water to within 3-5 º F of the steam saturation temperature and it should remove all but the very last traces of oxygen. The deaerated water then falls to the storage space below, where a steam blanket protects it from recontamination.
Nozzles and trays should be inspected regularly to insure that they are free of deposits and are in their proper position.
Spray-Type Deaerating Heaters
Spray-type deaerating heaters work on the same general philosophy as the tray-type, but differ in their operation. Spring-loaded nozzles located in the top of the unit spray the water into a steam atmosphere that heats it. Simply stated, the steam heats the water, and at the elevated temperature the solubility of oxygen is extremely low and most of the dissolved gases are removed from the system by venting. The spray will reduce the dissolved oxygen content to 20-50 ppb, while the scrubber or trays further reduce the oxygen content to approximately 7 ppb or less.
During normal operation, the vent valve must be open to maintain a continuous plume of vented vapors and steam at least 18 inches long. If this valve is throttled too much, air and nonconclensable gases will accumulate in the deaerator. This is known as air blanketing and can be remedied by increasing the vent rate.
For optimum oxygen removal, the water in the storage section must be heated to within 5 º F of the temperature of the steam at saturation conditions. From inlet to outlet, the water is deaerated in less than 10 seconds.
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Deaerators and Economizers
Where economizers are installed, good deaerating heater operation is essential. Because oxygen pitting is the most common cause of economizer tube failure, this vital part of the boiler must be protected with an oxygen scavenger, usually catalyzed sodium sulfite. In order to insure complete corrosion protection of the economizer, it is common practice to maintain a sulfite residual of 5-10 ppm in the feed water and, if necessary, feed sufficient caustic soda or neutralizing amine to increase the feed water pH to between 8.0 and 9.0.
Below 900 psi excess sulfite (up to 200 ppm) in the boiler will not be harmful. To maintain blowdown rates, the conductivity can then be raised to compensate for the extra solids due to the presence of the higher level of sulfite in the boiler water. This added consideration (in protecting the economizer) is aimed at preventing a pitting failure. Make the application of an oxygen scavenger, such as catalyzed sulfite, a standard recommendation in all of your boiler treatment programs.
The foregoing discussion shows the importance of proper deaeration of boiler feed water in order to prevent oxygen corrosion. Complete oxygen removal cannot be attained by mechanical deaeration alone. Equipment manufacturers state that a properly operated deaerating heater can mechanically reduce the dissolved oxygen concentrations in the feed water to 0.005 cc per liter (7 ppb) and 0 free carbon dioxide. Typically, plant oxygen levels vary from 3 to 50 ppb. Traces of dissolved oxygen remaining in the feed water can then be chemically removed with the oxygen scavenger.
The main purpose of blowdown is to maintain the solids content of the boiler water within prescribed limits. This would be under normal steaming conditions. However, in the event contamination is introduced in the boiler, high continuous and manual blowdown rates are used to reduce the contamination as quickly as possible.
Because each boiler and plant operation is different, maximum levels should be determined on an individual basis.
By definition, bottom blowdown is intermittent and designed to remove sludge from the areas of the boiler where it settles. The frequency of bottom blowdown is a function of experience and plant operation. Bottom blowdown can be accomplished manually or electronically using automatic blowdown controllers.
Frequently used in conjunction with manual blowdown, continuous blowdown constantly removes concentrated water from the boiler. By design, it is in the area of highest boiler water concentration. This point is determined by the design of the boiler and is generally the area of greatest steam release.
Continuous blowdown allows for excellent control over boiler water solids. In addition, it can remove significant levels of suspended solids. Another advantage is that the continuous blowdown can be passed through heat recovery equipment.
Blowdown Control Summary
Proper boiler blowdown control in conjunction with proper internal boiler water treatment will provide the desired results for a boiler water program. Many modern devices can automate boiler blowdown, thereby increasing the overall efficiency of the unit.
Ion exchange systems range from light commercial water softeners and filters to specially designed industrial equipment. Also known as deionizations (DI) systems. These systems are considered high-end where the highest quality of water treatment is needed, such as with steam turbines.
For more information see Water Softener
Reverse Osmosis (RO) systems are available for tap water, brackish water or seawater. These systems are considered high-end where the highest quality of water treatment is needed, such as with steam turbines.
For more information see Reverse Osmosis
7800 N. 113th Street
Milwaukee, WI 53224
Web site: www.cleaver-brooks.com
300 Pine Street
Ferrysburg, MI 49409
Web site: www.johnstonboiler.com
4213 North Temple City Blvd.
El Monte, CA 91734
Web site: www.claytonindustries.com
MIURA Boiler Company
8 Copernicus Boulevard
N3P 1Y4 Canada
Go to the Miura Boiler web site at www.miuraboiler.com
Gasmaster Industries Incorporated
#5 - 15050 54A Avenue
Surrey, British Columbia Canada, V3S 5X7
Web site www.gasmaster-ind.com
P.O. Box 35258
Tulsa, OK 74153
Web site: www.e-techinc.com
Cannon Boiler Works, Inc.
510 Constitution Blvd.
New Kensington, PA 15068
Web site: www.cannonboilerworks.com
Sidel USA Systems
PO Box 1868
Atascadero, CA 93423
Telephone: 805-462-1250 or 800-668-5003
Web site: www.sidelsystems.com
Sources: Bob Fegan 8/01; Johnston Boiler web site, 8/01; Armstrong International web site, 9/01; N.E.M. Business Solutions, "An Introduction to Steam Boilers and Steam Raising" at www.cip.ukcentre.com/steam.htm 9/01; 3-10-03; economizer picture from www.e-techninc.com 9/01; rev 1/2004; rev 2-2005; flue gas condenser photo from Sidel Systems USA 3/2005; Boiler type descriptions from DOE 'Improving Steam System Performance - a Sourcebook for Industry' Oct.2004; diagram of superheater system from Spirax Sarco Learning web site 3/2005; rev 3/2005; rev with Miura Compact/Modular, GasMaster Condensing Boilers 2-2007;