Cleaning of Boilers

Water-side deposits found in boilers vary depending on raw water composition, feed-water treatment, and operating pressure. 

The heaviest deposition occurs in tubes with the highest heat input, an area that may he physically impossible to inspect. A tube section can be taken from the area where deposition is known to be heaviest in order to characterize the deposit. Although various ways (in grams of scale per square foot) have been proposed for determining the need to clean, each case should be individually evaluated. Factors to be considered are the degree of fouhng, the type of service, the reliabihty required, the operating history, and future operation. 

The external fouhng of furnace mbes depends on the nature of the fuel. Oil-burning furnaces usually have significantly more deposit formation and corrosion problems than coal-burning types, while natural-gas-buming furnaces have very few problems. 

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NOTE The view that deposition is an indicator of more widespread problems is also common to cooling water systems. As a result of the complex nature of deposits, their removal by acid or mechanical cleaning of boiler or cooling systems is not always as simple a process as may be portrayed. 

Metal cleaning wastes include wastewater from chemical cleaning of boiler tubes, air preheater washwater, and boiler fireside washwater. 

Hgure 5. Cleaning of boiler tubes in stirred bombs. All Contain 0.25% inhibitor. 

Acid attack may also occur during the acid cleaning of boiler. Caustic gouging of low carbon steel is shown in Fig. 11.19. 

The period of time that a boiler is expected or required to operate before being shut down for cleaning or maintenance. This will vary with the type of boiler, the fuel being used and the operating load on the boiler. 

Reliable contractors should carry out all boiler treatment and cleaning. Boilers are fired pressure vessels and are subject to mandatory insurance inspections. Significant benefits in safe and economic operation, particularly by reducing unnecessary chemical treatment, can be achieved by monitoring the condition of boilers. 

Some manufacturers of this type of boiler claim that water treatment of any kind is not necessary and that no chemicals are required, especially with smaller units below, say, 240 kWh (800 lb/hr). Instead they rely on the on-off expansion-contraction cycles within the vessel to drop scale from the heating elements, or on a daily manual flush of the boiler vessel together with frequent replacement of elements and periodic acid cleaning. (Some improved design models thankfully have an automatic flush system that is actuated upon boiler shutdown.)... 

Hot water generators and LP steam-raising plants of below 15 psig are designed to operate with minimal blowdown and to suffer negligible circulatory losses. As a result, clean, sediment-free water of almost any characteristic nature (e.g., soft and corrosive or hard and scale-forming) is likely to be suitable as a source of boiler water makeup. 

Soot blowers also are used to clean particular boiler tube bundles, such as superheaters and economizers. Steam or compressed air normally is used, and operating practice may require the use of manual boiler control and increased furnace draft or boiler loading during sootblowing periods to avoid the risks of loss of furnace flame (flameout) or small furnace explosions (furnace puff). 

Utility boilers generally require waterside chemical cleaning of all boiler surfaces every 300 to 500 days of operation, and this work may be carried out by specialist contractors. It is regarded as a routine function, irrespective of water chemistry, laboratory involvement, or the quality of FW treatment and water management provided. Chemical cleaning of utility boilers is designed to permit the boilers to operate at peak performance and within knife-edge control parameters. 

Acid cleaning is not a precise process and many variable factors are involved, which, if not fully controlled, may result in acid corrosion of boiler steel. Under some circumstances, even with the use of acid cleaning inhibitors, acid corrosion (i.e., oxidation via hydrogen ions originating from the acid) may take place ... 

Carbon filters find particular application as prefilters for RO and ion-exchange processes in the production of high purity FW. They are also used in clean-steam boilers and other types of steam generators where the steam is ultimately destined for application in food or beverage production, pharmaceuticals, electronics, surgical instrument sterilization, and similar processes. 

Under poor operational conditions, tannin chemistry is a particularly forgiving form of internal treatment because it tolerates FW with significant variations in quality. It is capable of delivering clean, corrosion-free waterside surfaces in many types of boilers, despite low FW temperatures, high oxygen levels, and hardness ingress. It is especially suitable for use in smaller facilities that do not have the benefit of full-time, trained operators, and under on-off, batch process, or permanent low-fire circumstances. 

For phosphate overlay programs, PAA is generally satisfactory, but where good phosphate control is required or for on-line cleaning of fouled boilers using phosphate-cycle, SS/MA or AA/AMPS should be used. These products also perform in the presence of iron. 

Perhaps the most important waterside problems relate to the likelihood of boiler surface deposits and their control. High concentrations of caustic or salines only occur if porous deposits are present. It is much better to remove the cause of deposition problems than to try to manage their effects, and modem iron and silica transport polymers, together with improved cleaning protocols, have done much to limit deposition in large boilers. 

It is important that the tube surfaces be kept clean to avoid the initiation of corrosion. Regular waterside inspections and, if necessary, chemical cleaning of high-pressure equipment is recommended. The level of chloride that may be tolerated in such boilers during steady operation depends on the type of treatment employed. Where all-volatile alkaline treatments (AVT) are used, then the chloride levels should be lower than where nonvolatile alkalis (NVAT), such as sodium hydroxide and sodium phosphate, are used. The value may vary, depending on whether the boiler is coal-fired or oil-fired. 

Cleaning a boiler offline is unlike online cleaning. Ensuring operator safety is essential because the process generally involves using potentially dangerous acids, sometimes at temperatures of 200 to 300 °F. 

Traditionally, a common solution to the problem of matching relatively higher levels of technical support with lower chemical volumes for these smaller customers has been via a one- to three-year, fully inclusive product and services contract. Such a contract will specify the frequency of service visits to be made to the customer s site and the type of work to be carried out. It will also, perhaps, limit the maximum volumes of chemical treatments to be supplied during the contract lifetime, or perhaps designate the amount of chemicals required based on treating a certain annual volume of boiler FW. Contracts may include for the provision of chemical feed and control equipment and for the supply of labor for boiler cleaning, chemical addition, and drum removal services (drumless delivery). Product and services contract prices may some-... 

The use of hydroxyacetic/formic acid in the chemical cleaning of utility boilers is common. It is used in boilers containing austenitic steels because its low chloride content prevents possible chloride stress corrosion cracking of the austenitic-type alloys. It has also found extensive use in the cleaning operations for once-through supercritical boilers. Hydroxyacetic/formic acid has chelation properties and a high iron pick-up capability thus it is used on high iron content systems. It is not effective on hardness scales. 

For ultrafiltration applications, hollow fine fibers have never been seriously considered because of their susceptibility to fouling. If the feed solution is extremely fouling, tubular systems are still used. Recently, however, spiral-wound modules with improved resistance to fouling have been developed these modules are increasingly displacing the more expensive tubular systems. This is particularly the case with clean feed solutions, for example, in the ultrafiltration of boiler feed water or municipal water to make ultrapure water for the electronics industry. Capillary systems are also used in some ultrafiltration applications. 

The combustion system requires cleaning of the boiler tubes after every 30 days of operation. A typical incinerator run is 7 to 17 days. This type of duty cycle is suitable for solid waste disposal at the tire plants, although it would be unacceptable for steady production of electric power. 

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