TIP 0416-22 ISSUED – 2009 ©2009 TAPPI The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published.
Guidelines for operating and maintenance practices impacting an economizer on a recovery boiler Scope The economizer installed on a recovery boiler is the component responsible for the greatest number of tube leaks that result in lost production for the pulp and paper industry. This technical information paper is based on a comprehensive study initiated by the AF&PA Recovery Boiler Committee (1). A condensation of the report was presented at a TAPPI Conference in 2006 (2). The study determined primary causes of economizer tube damage are stress or fatigue cracks, poor quality welds, internal and external corrosion and thinning, and mechanical design. The knowledge of failure mechanisms was applied to relate operating and maintenance practices that could contribute to failures and then apply these in a review of economizer failure incidents to seek confirmation of practices detrimental to economizer integrity. These guidelines are a summary of those analyses. Definitions A comprehensive description of the economizer and definitions are a part of TIP 0416-21, Guideline specification for procurement of an economizer for a recovery boiler (3). Safety precautions This TIP provides guidelines that can be applied to review and establish the operating and maintenance practices in a manner that will minimize economizer failures. Practices that result should be examined for compliance with the BLRBAC “Recommended Practices” and other guidelines related to safety (4). Synopsis of failures Stresses contributing to cracking problems are generally of three types: thermal stresses, vibratory stresses, and mechanical loadings. Of these, the most insidious and most germane to economizer problems are thermal stresses. Since the economizer is basically a feedwater heater, there is a major difference in metal temperature between the feed end and the discharge end which, because of thermal expansion, can develop stresses. If the temperature profile of the parallel tubes connected to a given header is sufficiently different, differential thermal expansion can lead to stresses tending to flex the header (or bend the tubes) and this stress will be concentrated in the area where the tubes are welded to the header. The temperature profile along any given economizer tube is determined by the flow rate of water within the tube and the rate of heat transfer from the gas side. Thus any operating variables that contribute to differences in water flow rate and heat transfer rates in economizer tubes connected to the same header or otherwise jointly restrained can contribute to thermal stress generation. There are even cases where the water in an economizer bank recirculates with downflow in some of the tubes. Feedwater distribution to and within headers is primarily determined by design factors as discussed elsewhere. The total feedwater flow rate is determined by the drum level control system and can be subject to cycling and surges. Heat transfer rates are primarily determined by local gas temperature, local gas velocities and turbulence, and the degree of fouling on the tube exterior.
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Many cracks are associated with cyclic fatigue caused by tube vibration. The amplitude and frequency of the vibration are critical factors. These in turn are influenced by tube dimensions (particularly length), number and location of restraints, interactions with sootblower jets, gas velocities (especially across tubes) and vortex shedding. Although the buildup of deposits on the tube exterior can act to dampen vibrations, deposits can also contribute to vibration in other areas of the economizer by causing gas channeling. Other mechanical loads contributing to stresses are usually associated with how the economizer elements are supported and are basically design (and possibly maintenance) issues. External corrosion leading to tube thinning and pitting is an important failure mechanism in economizers. Such corrosion commonly involves attack by wet saltcake and/or sulfuric acid. The presence of liquid water plays a key role in accelerating corrosive attack and is normally the most important factor. Sources of moisture include sootblowers, condensation on cold metal surfaces, external sources, and small leaks in economizer tubes. Air infiltration through casing leaks can drop local temperatures below the dew point and cause local condensation. Boiler water washing practices can leave residual moisture and contribute to pitting. Another key factor is the concentration of SO3 in the flue gas, which in turn depends on concentrations of SO2 and O2 in the flue gas. The SO2 concentration depends on furnace operation and black liquor sulfur content. The O2 concentration depends on the amount of excess air used and on the extent of air infiltration through casing leaks and other openings. The waterside environment has a significant effect on economizer integrity. A considerable number of cracks originated and propagated from the waterside and there were also a number of incidents of waterside thinning. The most widespread problem encountered was oxygen pitting; the O2 content of the feedwater is clearly an important factor. Deaerator operation and the use of oxygen scavengers to control residual O2 are very important. The frequency and extent of excursions in feedwater quality is probably more important than steady state levels. pH excursions can have major impact in a short time. Considerations for operation and maintenance practices; recovery boiler opeation External corrosion The three biggest factors influencing external corrosion in the economizer are moisture, SO2 concentration, and O2 concentration. Of these, the most important is moisture. Any source of liquid water is detrimental to economizer life. Moisture entering through sootblowers was discussed in the section above. Another, less frequent source is entry from external sources. Often an important source of water is from a small tube leak. It is a very common experience that a small leak in economizers sprays on and thins an adjacent tube which ruptures. Because of the difficulties in inspection and in repairing small tube leaks and plugging tubes in economizers, it is not uncommon that once an economizer starts experiencing tube leaks that the problem grows and leaks become more frequent because of thinning experienced from the earlier leaks or defective repair welds. If a leak occurs in the economizer, it is critical to inspect the nearby areas for any signs of local wastage and repair it where found. The sulfuric acid dew point plays a major role in external wastage in economizers. Localized corrosion is likely when metal temperatures are lower than the sulfuric acid dew point, which is a function of the concentrations of SO2, O2, and H2O in the flue gases. Increased concentrations of any of these gases, raises the dew point temperature and increases the potential for corrosion. Any metal surface that is below the dew point temperature is subject to wastage. This is most prevalent at the cold end of the economizer where relatively cold feedwater is introduced, and in areas near and on the casing, where external cooling can occur. The temperature of feedwater entering the economizer should be no less than 120ºC (250ºF) and preferably higher. For an operation with elevated SO2 concentrations in the flue gas, the feedwater temperature should be increased to minimize the possibility for acid corrosion. Insulation of the external casing and on pipes and lines entering the economizer should be maintained to minimize external cooling. High concentrations of SO2 in the flue gas (> 200 ppm) clearly aggravate economizer corrosion and it is expedient to try to minimize SO2. This is a function of liquor sulfur content and firing solids content, and furnace operating conditions. Recovery boilers employing “high solids firing” are capable of operating with essentially no SO2 in the
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flue gas and are much less likely to experience external corrosion. Even for those boilers operating with moderate liquor firing solids, it is beneficial to keep SO2 as low as possible. The other important variable is the O2 concentration, which should be kept as low as possible consistent with environmental limits. O2 is determined by the amount of excess air used within the furnace (which should be the minimum needed to keep TRS and other combustible gases within limits) and by the amount of infiltration air. Infiltration air is detrimental from two standpoints; it increases the O2 concentration and lowers local gas temperatures toward the sulfuric acid dew point. It is probably impossible to keep all infiltration air out of the economizer, since negative drafts are highest here, but it can be minimized. Keeping upstream gas passages clean and free from plugging will minimize the amount of negative draft and thus the amount of air infiltration. Openings in the external walls should be patched in a timely manner. Doors that are not tightly closed are a major source of infiltration and should be routinely checked Boiler water wash procedures Pitting as a result of residual moisture following water washing the recovery boiler can be a fairly common problem in economizers included in the study. The most critical areas were those locations where the wash water was unable to drain properly and which stayed wet longest. The BLRBAC Guidelines for Personnel Safety recommend drying out the boiler after water washing to minimize out of service corrosion (5). Economizers are also susceptible to out of service corrosion if residual saltcake in the economizer is exposed to humid air. This usually takes the form of localized pitting in areas where accumulated saltcake traps moisture. The most vulnerable area is at the flue gas outlet in economizers whose design does not allow the saltcake to fall readily into the ash hopper. The solution of problems of this type is to get rid of the saltcake accumulations, by manual washing if necessary. Water washing should never be carried out when the boiler is in service. Startup procedures Boiler manufacturers provide startup procedures and startup curves for bringing the boiler on line and these should be followed. Attempts to speed up the startup can be detrimental to many boiler components, including the economizer. On a cold startup, the boiler goes from a non-boiling condition to a boiling condition and develops voidage in the water walls (swell) as steam generation begins. Feedwater flow will be negligible until levels are established with some steam generation, and then the flow control valve will begin to open. This can lead to relatively cold feedwater flowing into hotter tubes, particularly if flow surges develop. There are effective methods for keeping the economizer full of water during startup to prevent thermal shock when the boiler comes on line. If the economizer is provided with a recirculation line, it should be used to provide some flow through the economizer during startup. Another option is to open the mud drum blowdown line in which case some flow through the economizer is needed in order to maintain drum level. The practice of “burping” the superheater to shorten the time to clear condensate from superheater loops can be detrimental to the economizer. This was suspected of contributing to the multiple failures in Case 173 as described in Appendix 5. The “burping” procedure involves establishing a steam flow and then suddenly opening the superheater vent valve completely. This causes a drop in pressure, an increase in drum level and the feedwater control valve closing to stop water flow to the economizer. When the vent valve is subsequently closed, pressure rises, drum level falls and the feedwater valve opens. This sequence can lead to fluctuations in water flow paths in the cold bank of the economizer and to pronounced temperature fluctuations in the economizer tubes. A bypass valve around the feedwater flow control valve can be used to maintain some flow to the economizer while the superheater is “burped”. It might be noted that a similar set of conditions can develop when drum safety valves open as described below. Shutdown procedures Normal shutdowns that involve reducing fuel loads and burning out the bed with auxiliary fuel are generally not harmful to the economizer. However, some shutdown related procedures have the potential for inducing considerable stresses.
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Emergency shutdowns which involve rapid-draining the boiler induce rapid temperature transients on boiler components including the economizer. BLRBAC Guidelines do not call for an ESP for economizer leaks unless it is located where the water can enter the furnace cavity (6). But, ESPs are frequently enacted for economizer leaks especially if a ruptured tube is involved. BLRBAC does recommend draining the hot section of the economizer during an ESP in order to minimize the possibility of water from the economizer carrying over into the parts of the boiler where water can enter the furnace capacity, but the drain does not have to be as fast as for the other parts of the boiler. ESPs are clearly stressful to the boiler, but by themselves, rarely cause additional leaks to develop. However, if a boiler has developed waterside stress-assisted corrosion cracking problems (SAC), especially at welded attachments, ESPs are likely to aggravate the problem. This includes SAC sites in the economizer. A number of recovery boilers employ so-called chill and blows to help remove deposits from the superheater and generating bank. Exact details of these procedures vary from mill to mill, but they normally involve taking the unit off line for several hours and reducing pressure (which drops metal temperatures to help spall off deposits) and blowing heavily with the sootblowers. This can be an effective way to remove deposits and extend the time between water washes of the boiler, but by its very nature, it stresses the boiler components, including the economizer. It may also require a prolonged startup to get rid of the heavy accumulation of ash which falls to the hearth during the chill and blow. Frequent use of chill and blows was implicated as a possible cause of economizer problems in a number of the cases examined in this study. Drum level / feedwater flow stability Since the thermal stresses on economizer tubes are developed by thermal expansion and contraction in response to the temperature of the water in the tubes, and the water temperature is strongly affected by the water flow rate in each tube, the stability of the feedwater flow rate is very important in minimizing thermal stresses. Boiler startups and shutdowns are particularly susceptible to inducing thermal stresses and frequent boiler ups and downs or major load changes are detrimental to economizer integrity. Typically, recovery boilers are not used as swing boilers and are operated at steady loads, with other power boilers used to handle loads swings and control header pressure. If the recovery boiler is used as a swing boiler, extra care should be taken to watch for indications of thermal stress problems, particularly near the feedwater inlet headers and the economizer exit headers. Boiler trips and hot restarts can generate significant temperature disturbances in economizer tubes. If boiler trips are frequent (more than one or two a month) the reason for the frequent trips should be investigated and the cause corrected. Sometimes frequent trips are caused by improperly installed or calibrated sensors. Low water cutoffs sometimes generate frequent trips. If this is the situation, the possibility of introducing some time delay can help eliminate nuisance trips. However, in no circumstance should the protective function of the trip be defeated. Some recovery boilers operate with relatively unstable combustion and are subject to relatively sudden and severe blackouts that result in a rapid and complete loss of fire. This has many of the same implications as a boiler trip and needs to be dealt with accordingly. The boiler drum level / feedwater flow control system can generate feedwater flow oscillations and other flow disturbances if it is not properly tuned. Many recovery boilers operate with an oscillating drum level and feedwater flow which can generate a large number of stress cycles in a relatively short time. The magnitude of the feedwater flow disturbance determines the stress magnitude. Single-drum boilers are usually less sensitive to these types of disturbances. Frequent popping of drum safety valves can be quite deleterious. The resulting drop in drum pressure will generate swell that can cause a significant drop in feedwater flow. This is followed a little later on by a sharp drop in drum level and a feedwater surge. The disturbance will gradually dampen out depending on the drum level control system. This can especially be a problem in heavily loaded boilers where the pressure drop across the superheater becomes great enough that the drum pressure is close to the limit at which the safeties open.
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Load issues Increasing the load on the boiler does not necessarily increase the temperatures in the economizer, since the feedwater flow rate will be increased proportionately. However, increasing the load may, of itself, increase gas temperatures entering the economizer (as well as the gas flow rate) and may also increase fouling of the superheater and generating bank, which could further increase gas temperature. Higher inlet gas temperatures and velocities will increase heat transfer rates in the economizer and could lead to steam generation within the economizer. This condition, called a steaming economizer, can cause problems in the front end of the economizer. Another situation that can lead to a steaming economizer is the addition of a feedwater heater or a sweetwater condenser for producing attemperation water. Experience shows that the area at the gas inlet to the “hot” bank at the location of fin tube terminations and welding of tubes to headers has a high failure rate. These tubes are particularly susceptible to ‘steaming’. A steaming economizer can lead to local steam blanketing inside and accelerated external wastage or it can set up natural circulation loops within the economizer modules and attendant temperature fluctuations and stresses. Thus the economizer should discharge water to the boiler drum at a temperature sufficiently below saturation to insure that net steam generation cannot occur. It is recommended that the feedwater temperature to the drum normally be at least 28ºC (50ºF) below the saturation temperature of water at the economizer outlet (7). Even if average water temperatures meet this criteria, severe plugging of the superheater or generating bank can lead to gas channeling and the resulting high gas velocities and gas temperatures can lead to localized steam blanketing at the gas inlet to the economizer. Increased boiler load is likely to increase fouling in the superheater or generating bank that can lead to gas channeling, since more liquor is being fired and carryover of ash and burning particles is likely to be higher. The fouling and plugging can lead to distortion of the heat transfer load in the economizer and uneven deposit formation which can generate thermal stresses or the higher local gas velocities may induce tube vibration under conditions where uniform flow would not. External influences Feedwater issues The temperature of feedwater entering the economizer should be no less than 120ºC (250ºF). This temperature avoids most conditions where condensation could occur in downstream equipment. For an operation where there is SO2 in the flue gas, the temperature should be increased to minimize the possibility for acid corrosion, The feedwater temperature must be kept safely above the sulfuric acid dew point temperature to prevent localized corrosion. In order to avoid temperature differences between tubes connected to a common header or other constraint (which can generate thermal stresses) it is essential to have a uniform distribution of feedwater flow and inlet temperature to all of the tubes. This is primarily a design issue. There were quite a few instances where localized turbulence in the feedwater flow caused localized internal wastage due to flow accelerated corrosion that was not detected until the tube thinned to the bursting point. Typically this took place after flow orifices or other obstructions in the line. Misaligned tubes or weld penetrations were capable of generating such turbulence. Boiler water quality The quality of the feedwater has a significant effect on economizer integrity. A considerable number of cracks originated and propagated from the waterside, and thus were influenced by water quality. The most widespread problem encountered was oxygen pitting, so the O2 content of the feedwater is clearly an important factor. Consistent deaerator operation and the use of oxygen scavengers to control residual O2 are clearly important variables. The deaerator should be operated in accordance with the suppliers guidelines. Adequate storage for deaerated water to provide buffering from deaerator upsets is helpful.
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Other boiler water quality parameters are also important. Waterside deposition problems are less common in economizers because of the lack of evaporative concentration, but pH is quite important, since pH excursions can have major impact in a short time. The frequency and extent of excursions in feedwater quality is more important than steady state levels. The powerhouse should have an action plan for dealing with feedwater quality upsets that extends to defining the situations requiring the unit to be shut down. Waterside stress-assisted corrosion (SAC) is a problem that develops over a long time period and the specific waterside quality conditions responsible are difficult to pin down. Experience does show that when SAC is found at one location in the boiler, it also exists at many other locations where stress concentration is present. This is primarily an inspection and maintenance issue, but it can have a significant effect on economizer integrity. Sootblowers Sootblower operation can have a major effect on economizer integrity. It can accelerate local corrosion by directing moisture onto the tube surface. The forces on the tube from sootblowing can generate vibration leading to fatigue cracks or otherwise cause mechanical damage. The ability of the sootblowers to remove deposits influences the heat transfer capacity of the economizer and can contribute to flue gas channeling and thermal stresses from distorted heating profiles. Moisture in sootblowing steam has a well established role in accelerating thinning in sootblower lanes by erosion/corrosion. Every effort should be made to eliminate moisture. Superheat in sootblower supply steam, line insulation, maintenance of condensate traps, lance orientation on retraction, and leaking poppet valves are all factors that influence the amount of moisture in the sootblowing steam. Saturated steam taken directly from the drum is not recommended for sootblowing. The frequency of sootblower use needs to be sufficient to maintain reasonable cleanliness of the economizer tubes, but excessive sootblowing should be avoided, so as to minimize detrimental effects of sootblowing. In a properly operated recovery boiler, dust deposits in the economizer should be soft and easily removed by sootblowers. If hard economizer deposits are encountered, the boiler operation should be reviewed to determine the cause. Sootblowing pressure should not be higher than that needed to maintain reasonable cleanliness. In general blowing pressures in economizers do not need to be as high as in the superheater and generating bank. If sootblowers with conventional nozzles are replaced with high energy nozzles, special attention should be paid in the initial stages of operation to insure that excessive vibration or other movement of the elements is not induced. If it is, sootblower pressures need to be reduced. Stuck sootblowers should be removed from the boiler as quickly as possible. There have been several instances where the steam jet from a sootblower stuck in the boiler blew continuously on the same tube and excited vibrations leading to a rapid failure. If the sootblower cannot be removed from the boiler immediately, it would be desirable to rotate it enough so that the jets are blowing parallel to the tubes rather than across them. Broken nozzles will lead to unbalanced forces on the end of the sootblower lance and can cause it to swing around and vibrate. This can cause the end of the lance to strike economizer elements and do mechanical damage. A sootblower with a broken nozzle should be removed from service and repaired before being put back into service. Repair and maintenance Tube repair procedures Many economizer failures were reportedly caused by weld problems. Frequently these were reported to be due to porosity in welds or inadequate penetration. However, the evidence on which these reports are based was sometimes very fragmentary. Thus “weld problems” is sometimes a euphemism for cause unknown. That said, inadequate welds underlie many of the problems that occur in economizers. A poor weld may lie dormant for many years before it ultimately fails.
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A common feature that was seen on many economizers was a recurrence of leaks in a given area that were directly connected with previous leaks in that area. These might take the form of a leak in a repair weld for a previous leak or a leak in the seal weld of a tube plug. Alternatively, the previous leak may have sprayed on adjacent tubes, causing localized thinning which might eventually lead to another tube leak. These are all manifestations of the difficulties in making effective repairs and doing proper inspections in the economizer where access can be severely limited Economizer leaks occur mainly in the area of the tube-to-header connection especially at the feedwater inlet to the economizer or at the outlet discharging to the steam drum. Some of these are repairable, but more frequently lack of access leaves no alternative but to plug the tube at the inlet and outlet headers. Generally, access to a lower header is more favorable for repair than to an upper header. The exception is the first two rows of tubes near the economizer outlet header where leaks can generally be repaired. Where the defect is to be repaired, it should be ground out to remove all indications and filled with a full penetration weld. If the tube needs to be plugged, it may be necessary to cut a window in the mini-header wall to insert the plug and weld it in place. Sometimes the failure location leaves no alternative except to plug the flow in and out of the miniheaders to take the complete sheet (platen) out of service. When this occurs, the uncooled tubes will serve as a potential location for buildup of hard deposits, and the affected tubes will undergo more thermal expansion than surrounding tubes. In some cases the area of the tube where the leak occurred is cut out and a dutchman welded in. This requires two butt welds to be made. The quality of the butt welds is critical to the reliability of the repair. Since an economizer tube leak normally does not present a threat of water entering the firebox, it is not considered a “critical” tube requiring 100% inspection of all butt welds. The inspection scope for “non-critical” welds must expand if weld quality problems exist. Maintenance practices Maintenance practices have a considerable influence on economizer integrity. These include keeping ties and other restraints to motion intact, sootblower maintenance, maintaining casing integrity, and keeping baffles and other supports intact. Vibration of tubes can cause cracks due to cyclic fatigue, and the degree of vibration is often determined by the extent that the tube motion is restrained. Tubes in a sheet are fixed in position only at the header. Front-to rear alignment is accomplished by fastening together the tips of fins on adjacent tubes at intervals over the height of the bank. This connection is made during fabrication of the individual sheets in the shop. This connection should be designed to last the lifetime of the economizer because it is virtually impossible to access the site for repairs. If the connection fails, accessibility for repair must either be from a central sootblower cavity in the bank or through the baffle wall. Stitch welds are commonly used but are susceptible to failure, A more positive connection is to install a piece of plate on one side of the fins and welded full periphery to the fins on both sides of the gap. Sootblower maintenance is very important. Every effort should be made to eliminate moisture, such as line insulation, maintenance of condensate traps, proper lance orientation on retraction, and fixing leaking poppet valves. Stuck sootblowers should be removed from the boiler as quickly as possible. A sootblower with a broken nozzle should be removed from service and repaired before being put back into service. It is essential to maintain the integrity of the external casing to keep infiltration air out of the economizer. Openings in the external walls should be patched in a timely manner. It is also essential to do whatever is possible to keep baffles and other supports intact. Access limitations can be a problem here. One area often overlooked for inspection is the piping bringing feedwater to the mini-headers or to the larger headers of modules. The seal where the piping passes through the economizer casing or hopper wall should be inspected with the boiler in both the cold and hot position for adequate expansion. Review the distribution piping for connections that are subject to stress and periodically NDT these for cracks.
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Keywords Recovery boilers, Economizers, Maintenance, Operations, Tubes
Literature cited
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2. 3. 4. 5. 6. 7.
Grace, T.M. and Clement, J.L., “Investigation of the Causes of Recovery boiler Economizer Failures and Identification of Means for Preventing Their Occurrence”, AF&PA Recovery Boiler Committee, March 14, 2006 Grace, T.M. and Clement, J.L., “Investigation of the Causes of Recovery boiler Economizer Failures”, Proceedings of the TAPPI Engineering, Pulping and Environmental Conference, Atlanta, GA (2006) “Guideline specification for procurement of an economizer for a recovery boiler”, TAPPI TIP 0416-21, TAPPI Press, Atlanta, GA (2008). www.BLRBAC.org BLRBAC Recommended Guidelines for Personnel Safety Black Liquor Recovery Boilers BLRBAC Recommended Good Practice Emergency Shutdown Procedure (ESP) and Procedure for Testing ESP System for Black Liquor Recovery Boilers STEAM, Its Generation and Use, 40th Edition, The Babcock & Wilcox Company, page 19-4 (1992).
Additional information Effective date of issue: August 24, 2009 Working Group: John L. Clement, Chairman, Clement Consulting Inc. Thomas M. Grace, T.M. Grace Company John D. Andrews, MeadWestvaco Corporation g
