CHEMICAL CLEANING WITHOUT MISTAKES - Engineering - http://maxpages.com/jpchemical/CHEMICAL_CLEANING

Boilers: Cleaning without mistakes.

Part 1by: John Pitsilos, JPChemical Net Director, Dipl..Chemical Engineer

Why Clean a Boiler?
Historically, different boiler manufacturers, operators, and water treaters have adopted different attitudes toward cleaning boilers. By the mid-1970s, most of the steam electric utility industry and the steam tankers operators acknowledged that essentially all boilers may require cleaning at some point in their operating life. As a rule of thumb, they established criteria for cleaning boilers based upon the number of operating hours and the operating pressure of the boiler.
The higher the operating pressure of the boiler, the more often the cleaning was recommended. The reasons for recommending cleaning of marine, utility, industrial and hot-water boilers and the like are basically the same, If boilers are not clean, heat transfer surfaces are not clean. Dirty heat transfer surfaces adversely affect efficiency, corrosion, and heat transfer. To promote optimum performance with respect to these parameters, it is wise to consider cleaning a boiler when needed. There are many site- or boiler-specific secondary reasons for cleaning boilers. Unwanted deposits might interfere with circulation or flow patterns in some boilers. Unwanted corrosion products might interfere with the normal operation of other boilers (e.g., electrode tracking and resultant cracking in conductivity- type boilers).

Cleaning a boiler should be regarded as normal, just like any other preventative maintenance or repair procedure. How Do You Clean a Boiler?
There are essentially two ways to clean a boiler: mechanically and chemically. Sometimes a combination of mechanical and chemical cleaning can be the most efficient from a time and/or economic point of view. A boiler or parts of a boiler may be pre-cleaned using mechanical techniques, followed by a chemical cleaning operation.
Traditionally the watersides of boilers are chemically cleaned; the firesides are mechanically cleaned followed by one or more chemical cleaning procedures that include rinsing, passivation, and other practices.

When Should a Boiler Be Cleaned?
The best time to clean a boiler is almost never exactly known. The optimum time is just prior to the inception of inefficiency or of unwanted corrosion, deposition, scale formation, or loss of heat transfer. Some companies, particularly operators of higher-pressure boilers, will establish fixed times for cleaning boilers; others will track the operation of boilers by one parameter or another, and set up cleaning time criteria based on this tracking. A combination of methods is probably the best guide. The monitoring of the boiler during operation must be done properly and continuously, however, so as to provide a proper database for estimating when cleaning is needed. Monitoring can be done on-line, and can include tracking of unwanted or feedwater impurities as a function of total quantity of steam generated or of some other related parameter. Boiler feed water iron, copper, and other constituents could be tracked. After a predetermined quantity of contaminants in the feed water enters the boiler waterside, a cleaning may be scheduled. Another method commonly used today is to track deposit weight density at various points on boiler watersides presentation will not go into detail about deposit weight densities because the subject is highly controversial. For now, suffice it to say that the deposit weight density measurement is based on removing one or more samples of boiler tubes and determining the amount of build-up of deposit on the waterside surfaces. If the deposit accumulation exceeds a predetermined number of grams per square meters (milligrams per square decimetre), a boiler cleaning is often scheduled. The manufacturer of each boiler may provide the operator with recommendations for cleaning, based upon specific values of deposit weight densities. What is most important is to realize that deposit weight density must be tracked on a regular basis in each boiler in order to have any meaning. Even when this is done, there may be wide variations in interpretation, accuracy, and precision of the results.

Should a Boiler Be Cleaned On-line or Off-line?Despite the fact that many vendors or of water treatment chemicals recommend chemistries that are claimed to be capable of on-line cleaning of boiler watersides, it is our opinion that a boiler in need of chemical cleaning should be taken off-line.
Up today there is no truly accurate way of following the progress of on-line chemical cleaning of boilers. If the boiler requires cleaning and the cleaning procedure is so slow that its progress can be tracked only from outage to outage, damage is probably being done to the boiler during operation. Alternatively, the boiler is probably being operated under adverse conditions with regard to efficiency and perhaps with regard to corrosion considerations.

New Considerations in the 1990s Safety and Health considerations and environmental considerations must be included during all planning steps and during the actual boiler cleaning operations. These considerations may have major impact on concerns such as the cost of cleaning, the need for manpower during cleaning, the handling of chemicals during cleaning, and the inspection of the boiler.

It is the opinion of this writer that boiler chemical cleaning operations must be properly manned during the entire operation. Properly trained individuals must be available on-site during the entire cleaning. All too often, someone knowledgeable in boiler cleaning enters or leaves the site periodically, or else attempts to remain awake and on-site during a multiple-day cleaning operation. This should never be done. The alternatives are to provide, prior to the start of the cleaning, specific training to those individuals assigned to support the cleaning operations; or to bring in additional properly trained outside help. Clearly, both alternatives will result in increases in the cost of cleaning. However, both will also result in increased boiler reliability, and a decreased probability of the dramatic failures that are sometimes encountered during boiler cleanings.

Once management and all parties have agreed to clean a boiler, both safety and environmental considerations must come first. Otherwise the errors that may result during chemical cleaning could be regulatory in nature, and very expensive and very embarrassing. The true liability for chemical cleaning lies with the owner/operator of the boiler. In truth, only a very small percentage of this liability can be transferred to others, including outside contractors and consultants. Even if this liability is contractually transferred, the legal system typically allows for complaints and cross complaints that may end up increasing costs dramatically and increasing unwanted publicity. With respect to boiler cleaning operations, it is true that "a kilo of prevention is worth a ton of cure.

" At this point part 1 of this article will end, and at the second part issues such as boiler cleaning procedures, and cleaning solvent selection and verification will be tackled. Furthermore, step-by-step cleaning procedures, and issues about Stainless Steels as Source of Miracles Disasters will be discussed. Finally, the subject of interrupted chemical cleanings that lead to extended boiler outages and a conclusion of this article will take place.

Cleaning without mistakes. Part 2by: John Pitsilos, JPChemical Net Director, Dipl..Chemical Engineer

Boiler Cleaning ProceduresIt is not the intent of this article to review the literature or to instruct the reader in how to clean a boiler. There are literally thousands of technical manuscripts on this subject. Additionally, since about 1970 there has been essentially very little new chemistry used in the industrial cleaning of boilers; therefore, each reader has ready access to the technology of boiler cleaning. It is the intent of this article to confirm that proper chemistry has been selected and to discuss this point, and to discuss problems that result from specific errors made during cleaning. Cleaning solvent selection and verification. The materials that typically require cleaning from the waterside surfaces of boilers come, basically, from three different sources. The first is from contamination in the boiler feedwater; the second is from the chemicals added in the treatment of the boiler feedwater; and the final source is from corrosion products that are generated in the preboiler cycle, or in the boiler during operation and during out -of -service time. Many people tend to forget that if a boiler is not properly laid up when out of service, massive corrosion of an unwanted nature can result.
We recently had the opportunity to evaluate approximately 50 different solvents that enter chemical cleaning operations of boilers under different conditions. Obviously, some of these are very exotic and do not see much common service duty. However, even consideration of the commonly used mineral acids, organic acids, synthetic chelants, inhibitors, and surfactants requires careful solvent selection and verification. This process is made more complex by the fact that different sections of boilers may be constructed of different materials. Preboiler cycle components, including feedwater heaters, pump, condensers, and the like, may be constructed of different materials than the primary boiler. Because these components are connected to the boiler via the flow of boiler makeup water, boiler water and boiler feedwater, proper solvent selection can often be complicated. Proper solvent selection is also hard to do if there are inaccuracies in the analysis of deposit/scale matrix that requires cleaning.

The first problem with the selection of solvents rests in deposit/scale sampling. The typical method of sampling is to cut a tube section from the highest heat input zone of the boiler or from the area requiring chemical cleaning and to subject this section to a series of beaker tests. Basically the tube sample is put in an inhibited cleaning solvent on a magnetic stirrer in a simple circulating test loop. The laboratory technician or chemist watches the sample to see that the unwanted material dissolves in the solvent and that the resulting corrosion is not excessive. The reader should realize that during chemical cleaning operations even with proper inhibition, the rates of corrosion by cleaning solvents to common boiler materials of construction range from about 100 to about 100.000 times faster than those encountered during normal operation. Typical corrosion rates of standard carbon steel used in industrial boiler construction tend to be on the order of 300 mils per year during proper chemical cleaning. At this corrosion rate, the total metal loss of carbon steel during a typical chemical cleaning is about 0.3 mils (0.008 millimeters). This is a very tolerable rate of corrosion be cause the chemical cleaning operation does not last for an extended period of time.
Obviously, the corrosion that is experienced will vary with the chemistry of the solvent, the type of metals, the stress in the metals, the temperature used during cleaning, the degree of exclusion of unwanted air and other gases, and the velocity used in cleaning. Additionally, redeposition of suspended or dissolved matter during cleaning will have a dramatic effect on the resulting corrosivity. Because it is relatively expensive to cut a tube out of a boiler in order to obtain samples of scale, this procedure, is often neglected or bypassed. We recommend an alternative, a compromise between bringing a boiler off-line twice, and skipping the proper determination of the deposit/scale matrix for purposes of solvent selection and verification. This alternative involves allowing a little extra time in the planning of the cleaning operation for bringing the solvent on-site. The boiler can be initially brought off-line and an appropriate tube can be cut out of the boiler. Laboratory testing for solvent selection is a relatively rapid procedure that can be accomplished from between a few hours and perhaps 2 days. The cost of the testing and solvent selection is relatively small compared either to the cost of the chemical cleaning or to the cost of an error that could result. Care must be taken to ensure that the analyzed scale is representative of the material that must be removed during boiler cleaning. This becomes particularly complex when multiple layers of different scales are present in a boiler or when different components of the deposit/scale matrix demand different chemical solvents. Often, boilers used in the marine, pulp and paper, hydrocarbon processing, refinery, and food-processing industries are subject to intrusions of unwanted organic material. If these materials become part of deposits and/or scale in boilers, they typically must be removed by oxidation before solvent cleaning can proceed. Depending upon how these materials are bound in the deposit and/or scale, removal can be a very difficult task. Sometimes partial removal occurs, leading to dramatic post cleaning failures. Wet chemical and analytical techniques are typically not totally adequate for characterizing the scale. While these techniques are relatively standard easy to use, inexpensive and well known. They suffer from interference when applied in a complex chemical environment. We recommend that wet chemical analyses be verified using instrumental techniques. The use of multiple analytical procedures serves as a verification of analytical accuracy if each technique yields the same analytical information. Additionally, where deposits and/or scales are crystalline in nature it is wise to verify the chemistry by subjecting the crystalline matrix to X-ray diffraction crystalline matrix to X-ray diffraction analysis. Under most conditions, it is not wise to have scale analyzed using energy-dispersive X-ray spectroscopy in a scanning electron microscope because the electron microscope typically analyzes a very small area or raster. If the area is not representative, gross errors in analysis may result. If analytical errors occur the resulting solvent selection may be inappropriate. Is scale/deposit matrix analysis and solvent selection really this important?

Absolutely! Particular care needs to be used when mixed metallurgy is present in boiler water systems, such as when ordinary carbon steel are combined with alloy steels, stainless steels, and/or copper-bearing alloys. It is under these conditions that the greatest errors are known to occur. Most boiler operators do not have the facilities to properly analyze their scale/ deposit matrix and to make an informed decision on solvent selection. On the other hand, some do have the facilities and the required knowledge and experience. Whatever the case, we emphasize that extreme care should be used at this point. Please remember that precleaning chemicals, inhibitors, surfactants, rinses, and passivating chemical choices must dovetail with proper deposit/scale analysis and the resulting choice of solvent(s)

Step-by-step cleaning procedures.
The abstract of this article lists several different items that are important in the preplanning and/or planning of chemical cleaning operations, including the execution of such. Dramatic problems are known to occur when any of these steps are short-circuited or not taken seriously. When operators of boilers attempt to return boilers to steam-generating service in a shortened period of time, errors almost always occur. Each of the essential, prioritized step-by-step cleaning procedures is carefully detailed in the technical literature. The best sources of this literature are chemical cleaning service companies and boiler manufacturers. Additionally, specific chemical information dealing with selection, application, corrosion, and disposal has been generated by the primary manufacturers of the basic chemicals that are used in boiler cleanings. All of this information is readily accessible.
One of the best sources is the bibliography on chemical cleaning prepared by NACE International Technical Practices Committee T-3M. The remainder of this article will examine typical errors that occur when several of the steps are not properly followed. Too long, too hot, and/or too fast.
One of the most common errors encountered during cleaning, assuming that the proper chemistry has been selected, occurs when the cleaning solutions, rinse solutions, and/or passivating solutions contact the boiler metal surfaces for an excessive period of time, at an excessive temperature, or with an excessive velocity. As a rule of thumb, more is not better when applied to chemical cleaning. Leaving a cleaning solution in a boiler for an extended period of time typically will not increase the amount of scale removal, scale removal often occurs during fairly narrow bands of chemical concentrations. Keeping the chemical in the boiler at concentration levels outside of this narrow band will not enhance scale removal and may enhance redeposition of deposit/scale components and/or corrosion.
Often, heating a solution will increase its rate of dissolution of a deposit/scale component. However, there are temperature limits to which a boiler cleaner must adhere. Generally speaking, over-heating leads to unwanted corrosion and to less-than-complete cleaning.

Excessive velocity during chemical cleaning often leads to erosion corrosion. Generalized corrosion can proceed at accelerated rates if velocity is excessive. Velocity threshold values for erosion corrosion are often greatly reduced in boiler materials of construction at elevated temperatures and in the presence of solvents.
Conversely, an insufficient velocity can result in improper chemical cleaning because a low velocity may not allow fresh chemical cleaning solvent to contact deposit/scale for purposes of promoting dissolution. Stainless Steels - Source of Miracles and DisastersWhere chemically sensitive stainless steels are present in boilers care must be taken to ensure that environmental cracking or specialized metal dissolution resulting from chemical concentration does not occur. While the stainless steel is used to alloy boiler components so as to allow these to operate at higher temperatures without losing structural properties, holes in the stainless steel obviously will not allow proper performance of the boilers. Such holes will result if the stainless steel is affected by problems such as chloride stress corrosion cracking or pitting to failure. These problems are commonly known to occur during boiler cleanings when proper preplanning and execution have not been exercised. Proper planning where stainless steels are involved includes the proper selection of rinse water qualities. Typically an effort is made to save money. If rinse water quality is not sufficient to protect the stainless steel from unwanted corrosion or if the boiler is not properly filled so that unwanted chemistry is flushed from stainless steel surfaces, poor results will often occur. Interrupted Chemical Cleanings Lead to Extended Boiler Outages

Two basic errors commonly made during chemical cleaning operations lead to interruptions of chemical cleaning procedures. These interruptions typically Lead to, among other problems, disastrous results in terms of costs and extensive downtime. Chemical inventory during cleaning operations is often based on estimations made from extrapolating that chemistry required to remove the deposits/scale from a tube sample. This extrapolation should best be based upon the laboratory data that is determined prior to beginning the cleaning of the boiler. Nonrepresentative samples or other errors could result in underestimating chemical inventory requirements. Additionally it is possible that equipment such as

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