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موسوعة التوربينات والغلايات - Turbines & Boilers

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    Boiler Selection

    Six criteria should be considered when selecting a boiler to meet the
    application needs, The criteria are

     Codes and standards requirements

     Steam or hot water

     Boiler load

     Number of boilers

     Performance considerations

     Special considerations



     Codes and Standards


     There are a number of codes and standards, laws, and regulations covering boilers and related equipment that should be considered when designing a system. Regulatory requirements are dictated by a variety of sources and are all focused primarily on safety. For more information on how the various rules affect boiler selection and operation, you may want to contact your local Cleaver-Brooks authorized representative. Here are some key rules to consider:

     The boiler industry is tightly regulated by the American Society of Mechanical Engineers (ASME) and the ASME Codes, which governs boiler design, inspection, and quality assurance. The boiler's pressure vessel must have an ASME stamp. (Deaerators, economizers, and other pressure vessels must also be ASME stamped).

     The insurance company insuring the facility or boiler may dictate additional requirements. Boiler manufacturers provide special boiler trim according to the requirements of the major insurance companies. Special boiler trim items usually pertain to added safety controls. Some industries, such as food processing, brewing, or pharmaceuticals, may also have additional regulations that have an impact on the boiler and the boiler room.A UL, ULC, cUL, CSA or CGA listing, or Canadian Registration Number (CRN) may be required. State, local, or provincial authorities may require data on the boiler controls or basic design criteria.

     Most areas have established a maximum temperature at which water can be discharged to the sewer. In this case, a blowdown separator aftercooler is required.

     Most state, local or provincial authorities require a permit to install and/or operate a boiler. Additional restrictions may apply in non-attainment areas where air quality does not meet the national ambient air quality standards and emission regulations are more stringent.

     For all new boilers with inputs over 10 MMBtu/hr, U.S. Federal emission standards apply, including permitting and reporting procedures. Limits on fuel sulfur ******* are frequently set at 0.5% maximum.

     A full-time boiler operator may be required. Operator requirement depends on the boiler's size, pressure, heating surface or volume of water. Boilers can be selected which minimize the requirements, either by falling under the requirements and being exempt or with special equipment that gives the operator more freedom in the facility.

     Most states or provinces require an annual boiler inspection. There may be other requirements on piping as well.

     Steam or Hot Water

     Now that you have a general overview of the types of boilers and code and standards requirements, it's time to look at the facility's application in order to see how the boiler will be used. Keep in mind, the primary purpose of the boiler is to supply energy to the facility's operations - for heat, manufacturing process, laundry, kitchen, etc. The nature of the facility's operation will dictate whether a steam or hot water boiler should be used. Hot water is commonly used in heating applications with the boiler supplying water to the system at 180 °F to 220 °F. The operating pressure for hot water heating systems usually is 30 psig to 125 psig. Under these conditions, there is a wide range of hot water boiler products available. If system requirements are for hot water of more than 240 °F, a high temperature water boiler should be considered.

     Steam boilers are designed for low pressure or high pressure applications. Low pressure boilers are limited to 15 psig design, and are typically used for heating applications. High pressure boilers are typically used for process loads and can have an operating pressure of 75 to 700 psig. Most steam boiler systems require saturated steam.

     Steam and hot water boilers are defined according to design pressure and operating pressure. Design pressure is the maximum pressure used in the design of the boiler
    for the purpose of calculating the minimum permissible thickness or physical characteristics of the pressure vessel parts of the boiler. Typically, the safety valves are set at or below design pressure. Operating pressure is the pressure of the boiler at which it normally operates. The operating pressure usually is maintained at a suitable level below the setting of the pressure relieving valve(s) to prevent their frequent opening during normal operation.

     Some steam applications may require superheated steam. It should be noted that superheated steam has a high enthalpy, so there is more energy per pound of steam and higher (drier) steam quality. One example of an application where superheated steam may be required is with a steam turbine. The turbine's blades require very dry steam because the moisture can destroy the blades. When very high pressure or superheated steam is required, an industrial watertube boiler should be selected.

     System Load

     In addition to the system load considerations provided in this section, many excellent reference manuals are available to help further define specific load details and characteristics. For more information, refer to the ABMA Firetube Engineering Guide, the ASHRAE Handbook, or contact your local Cleaver-Brooks authorized representative.

     System load is measured in either Btus or pounds of steam (at a specific pressure and temperature). When discussing the system load, we will include references to both steam and hot water. However, not all situations or criteria apply to both. It would be nearly impossible to size and select a boiler(s) without knowing the system load requirements. Knowing the system load provides the following information:

     The boiler(s) capacity, taken from the maximum system load requirement.

     The boiler(s) turndown, taken from the minimum system load requirement.

     Conditions for maximum efficiency, taken from the average system load requirement.

     Determining the total system load requires an understanding of the type(s) of load in the system. There are three types of loads: heating, process, and combination.

     Heating Load

     A heating load is typically low pressure steam or hot water, and is relatively simple to define because there is not a great deal of instantaneous changes to the load. And, once a heating load is computed, the number can easily be transferred into the equipment size requirements. A heating load is used to maintain building heat. Cooling loads, using steam to run an absorption chiller, also are included when computing a heating load. Characteristics of a heating load include large seasonal variations but small instantaneous demand changes. The boiler should be sized for the worst possible weather conditions, which means that true capacity is rarely reached.

     Process Load

    A process load is usually a high pressure steam load. A process load pertains to manufacturing operations, where heat from steam or hot water is used in the process. A process load is further defined as either continuous or batch. In a continuous load, the demand is fairly constant -
    such as in a heating load. The batch load is characterized by short-term demands. The batch load is a key issue when selecting equipment, because a batch-type process load can have a very large instantaneous demand that can be several times larger than the rating of the boiler. For example, based on its size, a heating coil can consume a large amount of steam simply to fill and pressurize the coil. When designing a boiler room for a process load with instantaneous demand, a more careful boiler selection process should take place.

     Combination Load

    Many facilities have a mixture of loads - different types of process loads and combination of heating and process loads. The information just given on heating and process loads should be taken into consideration when dealing with a combination load.

     Defining Load Variations

    Loads vary and a power plant must be capable of handling the minimum
    load, the maximum load, and any load variations. Boiler selection is often dictated by the variation in load demand, rather than by the total quantity of steam or hot water required. There are three basic types of load variations: seasonal, daily, and instantaneous.

     Seasonal Variations. For a heating system, seasonal variations can mean no demand in the summer, light demand in the fall and spring, and heavy demand in the winter. Manufacturing operations often have seasonal variations, because the demand for production may vary. When selecting boiler equipment, the minimum and maximum load for each season should be determined.

     Daily Variation. Daily variation can occur due to variations in the work hours, or the heat required at various times of the day or weekend. Minimum and maximum seasonal variations mentioned earlier may already reflect these changes if they occur daily. If not, the minimum and maximum daily loads should be included.

     The seasonal and daily variations define the size of the load that the boiler(s) must handle. Seasonal and daily variations also help define the number of boilers and turndown requirements.

     Instantaneous Demand. Instantaneous demand is a sudden peak load change that is usually of short duration. These types of loads are sometimes hidden. Many machines or processes are rated in pounds of steam per hour or Btu/hr as running loads, under balanced operating conditions, and there is no recognition given to "cold startup," "peak" or "pickup loads." The instantaneous load demand is important to consider when selecting a boiler to ensure that these load variations are taken into account. If the instantaneous demand is not included in the system load calculations, the boiler(s) may be undersized.

     System Load Summary

    The load demand matrix shown in Table I1-3 can be used as a work
    sheet in determining the minimum, maximum, and average system loads.

     Load Tracking

    Load tracking is the ability of a boiler to respond to changes in steam or hot water demand. Most often associated with process loads, load tracking focuses on the boiler's ability to supply a constant volume of steam at the required pressure.

     The ability of the boiler to track a variable load depends on the boiler type, burner turndown capability, feedwater valve control, and combustion control design. If the analysis of the load shows highly variable load conditions, a more complex control package may be necessary. This type of control is achieved with sophisticated boiler management systems. For more information on these types of systems contact your local Cleaver-Brooks authorized representative.

     If the application has instantaneous load demands, whereby a large volume of steam is required for a short period of time, a boiler with a large energy storage reserve, such as a firetube, should be considered. If the application dictates large variances in load demand, where the load swings frequently for long periods of time, the best choice is probably a watertube type boiler, because it contains less water and can respond to the variances more rapidly.

     In all cases, operation of the burner should be taken into account in selecting a boiler(s) to meet system demand. The burner will require proper operating controls that can accurately sense the varying demands and be capable of the turndown requirements. The boiler feedwater valve and control design are also critical if load swings are expected.

     Number of Boilers

     Back-UpBoilers

    when selecting the boiler(s), consideration should be given to backup equipment to accommodate future expansion, emergency repairs, and maintenance. There are a number of considerations for a backup boiler.

     Type of Load

     Heating systems and non-critical loads that do not result in a sudden loss
    of production generally have little or no backup. While this is not recommended, it is still common practice. These types of applications rely on the ability to make repairs quickly to reduce downtime. The risk involved in having no backup is a total loss of heat when the boiler is not in service.

     When process or heating loads use multiple boilers during peak times, and one boiler during most other times, the availability of an additional boiler to provide full backup during maximum demand should be considered.

     In applications with critical steam or hot water requirements, laws or codes may dictate a backup. Even if laws or codes do not dictate a backup, there are many cases where the operation cannot tolerate downtime. For example, a hotel uses hot water 24 hours a day, seven
    days a week. During periods of maintenance or in an emergency, a backup boiler is required.



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    مهندس إستشارى / مصطفى الوكيل
    M.E.P. Manager - ITCC Project, Riyadh
    Zuhair Fayez
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  2. [12]
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     Downtime

    Another way to determine whether a backup boiler is a wise decision is to compute the cost of downtime to the owner or the user, as shown in the following three examples:

     A chemical company manufactures dry cell battery compound in a batch process. The process temperature must be maintained within 2 degrees. The boiler shuts down on a flame failure. They have 20 minutes to recover steam or the batch is scrap. The value of the product is $250,000.

     A Midwestern insurance company building has comfort heat supplied by one boiler. There are over 2000 workers in the building. The boiler shuts down due to a failed gas valve. Outside, it's 11 °F. Inside, the temperature continues to drop and, at 1:30 in the afternoon, all 2,000
    workers are sent home.

     A meat processing company makes its entire packaged ham line in a Southern plant. It operates 24 hours a day, every day. A single boiler provides heat for curing, sterilizing, and cleaning. The boiler goes down due to a lack of feedwater. Each hour of steam loss results in four hours of lost production.

     Boiler Turndown

    Boiler turndown is the ratio between full boiler output and the boiler output when operating at low fire. Typical boiler turndown is 4:1. For example, a 400 horsepower boiler, with a 4:1 turndown burner, will modulate down to 100 horsepower before cycling off. The same boiler
    with a 10:1 turndown burner will modulate down to 40 horsepower.

     The ability of the burner to turn down reduces frequent on and off cycling. Fully modulating burners are typically designed to operate down to 25% of rated capacity. At a load that is 20% of the rated capacity, the boiler will turn off and cycle frequently.

     A boiler operating at low load conditions can cycle as frequently as 12 times per hour, or 288 times per day. With each cycle, pre- and postpurge air flow removes heat from the boiler and sends it out the stack. The energy loss can be eliminated by keeping the boiler on at low firing rates. Every time the boiler cycles off, it must go through a specific start-up sequence for safety assurance. It requires about one to two minutes to place the boiler back on line. And, if there's a sudden load demand, the start-up sequence cannot be accelerated. Keeping the boiler on line assures the quickest response to load changes. Frequent cycling also accelerates wear of boiler components. Maintenance increases and, more importantly, the chance of component failure increases.

     As discussed earlier, boiler(s) capacity requirement determined by many different types of load variations in the system. Boiler over-sizing occurs when future expansion and safety factors are added to assure that the boiler is large enough for the application. If the boiler is oversized, the ability of the boiler to handle minimum loads without cycling is reduced. Therefore, capacity and turndown should be considered together for proper boiler selection to meet overall system load requirements.

     Performance Considerations

     Three important considerations pertain to fuels, emissions, and efficiency. All three have important impact on boiler performance, and can affect long-term boiler operating costs.

     Fuels Remember, from an operating perspective, fuel costs typically account for approximately 10% of a facility's total operating budget. Therefore, fuel is an important consideration. Normally, the fuels of choice are natural gas, propane, or light oil. Increasingly stringent emission standards have greatly reduced the use of heavy oil and solid fuels such as coal and wood. Of the fossil fuels, natural gas burns cleanest and leaves fewer residues; therefore less maintenance is required.

     It can be advantageous to supply a boiler with a combination burner that can burn two fuels independently - for example, oil or natural gas. A combination burner allows the customer to take advantage of "peak time" rates, which substantially reduces the costs of a therm of gas when operating "off peak" by merely switching to the back up fuel. Dual fuel capability also is beneficial if the primary fuel supply must be shut down for safety or maintenance reasons.

     Some waste streams can be used as fuel in the boiler. In addition to reducing fuel costs, firing an alternate fuel in a boiler can greatly reduce disposal costs. Waste streams are typically used in combination with standard fuels to ensure safe operation and to provide additional
    operating flexibility.

     Emissions

     Emission standards for boilers have become very stringent in many areas, because of the new clean air regulations. The ability of the boiler to meet emission regulations depends on the type of boiler and burner options.

     Efficiency

     Efficiency is used in the measure of economic performance of any piece of equipment. In the boiler industry, there are four common definitions of efficiency, but only one true measurement. Following are the definitions and how to measure efficiency. Combustion Efficiency Combustion efficiency is the effectiveness of the burner only and relates
    to its ability to completely burn the fuel. The boiler has little bearing on combustion efficiency. A well- designed burner will operate with as little as 15 to 20% excess air, while converting all combustibles in the fuel to thermal energy.

     Thermal Efficiency

    Thermal efficiency is the effectiveness of the heat transfer in a boiler. It does not take into account boiler radiation and convection losses - for example, from the boiler shell, water column piping, etc.

     Boiler Efficiency

    The term "boiler efficiency" is often substituted for combustion or thermal efficiency. True boiler efficiency is the measure of fuel-to-steam efficiency.

     Fuel-to-Steam Efficiency

    Cleaver-Brooks guaranteed boiler efficiencies are fuel-to- steam efficiencies.

     Fuel-to-steam efficiency is the correct definition to use when determining boiler efficiency. Fuel-to-steam efficiency is calculated using either of two methods, as prescribed by the ASME Power Test Code, PTC 4.1. The first method is input-output, which is the ratio of
    Btu output divided by Btu input x 100.

     The second method is heat balance which considers stack temperature and losses, excess air levels, and radiation and convection losses. Therefore, the heat balance calculation for fuel-to-steam efficiency is 100 minus the total percent stack loss and minus the percent radiation and convection losses.

     Stack Temperature and Losses

    Stack temperature is the temperature of the combustion gases (dry and water vapor) leaving the boiler. A well-designed boiler removes as much heat as possible from the combustion gases. Thus, lower stack temperature represents more effective heat transfer and lower heat loss up the stack. The stack temperature reflects the energy that did not transfer from the fuel to steam or hot water. Stack temperature is a visible indicator of boiler efficiency. Any time efficiency is guaranteed, predicted stack temperatures should be verified.

     Stack loss is a measure of the amount of heat carried away by dry flue gases (unused heat) and the moisture loss (product of combustion), based on the fuel analysis of the specific fuel being used, moisture in the combustion air, etc.

     Excess Air

    Excess air provides safe operation above stoichiometric conditions. A burner is typically set up with 15 to 20% excess air. Higher excess air levels result in fuel being used to heat the air instead of transferring it to usable energy, increasing stack losses.

     Radiation and Convection Losses

    Radiation and convection losses will vary with boiler type, size, and operating pressure. The losses are typically considered constant in Btu/hr, but become a larger percentage loss as the firing rate decreases. Boiler design factors that also impact efficiencies of the boiler are heating surface, flue gas passes, and design of the boiler and burner package.


     Heating Surface

    Heating surface is one criterion used when comparing boilers. Boilers with higher heating surface per boiler horsepower tend to be more efficient and operate with less thermal stress. Many packaged boilers are offered with five square feet of heating surface per boiler horsepower as an optimum design for peak efficiency.

     Flue Gas Passes

    The number of passes that the flue gas travels before exiting the boiler is also a good criterion when comparing boilers. As the flue gas travels through the boiler it cools and, therefore, changes volume. Multiple pass boilers increase efficiency because the passes are designed to maximize flue gas velocities as the flue gas cools.

     Integral Boiler/Burner Package

    Ultimately, the performance of the boiler is based on the ability of the burner, the boiler, and the controls to work together. When specifying performance, efficiency, emissions, turndown, capacity, and excess air all must be evaluated together. The efficiency of the boiler is based, in part, on the burner being capable of operating at optimum excess air levels. Burners not properly designed will produce CO or soot at these excess air levels, foul the boiler, and substantially reduce efficiency. In addition to the boiler and burner, the controls included on the boiler (flame safeguard, oxygen trim, etc.) can enhance efficiency and reduce
    overall operating costs for the customer. A true packaged boiler design includes the burner, boiler, and controls as a single, engineered unit.

     Special Considerations

     Replacement Boilers, If the boiler is to be placed in an existing facility, there are a number of considerations:

     Floor space required.

     Total space requirements.

     Access space for maintenance.

     Size and characteristics of the boiler to be replaced, including location of existing piping, the boiler stack and utilities.

     Boiler weight limitations.

     With little or no access to the boiler room, the cast iron boiler and some bent-tube type boilers can be carried into the boiler room in sections or pieces and easily assembled, with no welding required.

     Electric boilers should also be considered, especially since they do not require a stack.

     Vertical firetube boilers have a small floor space requirement

    والله أعلم ،


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    مهندس إستشارى / مصطفى الوكيل
    M.E.P. Manager - ITCC Project, Riyadh
    Zuhair Fayez
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  3. [13]
    مصطفى الوكيل
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    إليكم هذه المعلومات من بعض التقارير الفنية عن شروخ حدثت فى مواسير النار والأفران لبعض الغلايات فى إستراليا ، وستجد أن معظم الأسباب كانت النسبة بين دورات الضغط ودورات درجة الحرارة
    أى PRESSURE AND TEMPERATURE CYCLES


    وسأمدكم إن شاء الله كل يوم لو أردت بمعلومات قيمة عن الغلايات ، لكن إنتبهوا إحتياطات السلامة مهمة جدا فى أعمال الغلايات والبخار وماشابه لأن الخطأ البسيط يؤدي لكارثة لاقدر الله ، إليكم بعض مقتطفات من التقرير
    -----------------------


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    FIRE-TUBE BOILER CRACKING


    Cracking Occurrence


    Virtually all cracks occur at welded joints or at openings

    The root cause is corrosion fatigue with the fatigue cycling being thermally driven. Over 100 boilers in Australia suffered this type of cracking in the 1950 – 1975 period.

    The change to natural gas firing initially accelerated the rate, but it has since fallen. UK inspection data from 2001 showed that 2% of fire-tube boilers inspected had service defects – mainly cracks


    Figure 1 shows a schematic representation of the side and end elevation of a fire-tube boiler.

    The front and rear closure plates and reversal chambers have been omitted for clarity

    Of the eight potential crack locations shown, the major occurrence is furnace tube cracking adjacent to the tube plate weld identified as number 1 in Figures 1 and 2.

    Other locations of the boiler also
    exhibit cracking particularly cracking associated with fire-tubes identified as numbers 3 and 4 in Figures 1 and 2


    Furnace Tube Cracking (location 1)

    Cracking occurs in a highly localized area in the furnace tube on the water-side of the boiler.

    The cracks originate at the root of the furnace tube to tube-plate weld. Cracking can occur at both front and rear ends at any position around the furnace tube plate but cracking is most common at the bottom of the tube plates.

    Cracking generally occurs in the furnace tubes as they are thinner, and hence more highly stressed than the tube plate (see Figure 3).


    The failure mode is corrosion fatigue. Slight corrosion occurs as a result of contact with the water.

    Fatigue arises from thermal cycling and pressure cycling.

    The primary causes of furnace tube cracking are a combination of:-

    High thermal stress generated by large temperature or material thickness differences;

    • Bending stresses due to pressure;

    • Poor weld shape, particularly at the weld root in the lower part of the furnace;

    • High number (over 10,000) of pressure and temperature cycles;

    • Fracture of the protective magnetite layer due to cyclic stresses. Magnetite forms on the furnace tubes and acts as a protective layer but it is brittle and subject to spalling under cyclic stresses.

    Its fracture exposes unprotected surfaces to further corrosion;

    • Un-removed slag from furnace tube to tube plate welds providing corrosion initiation sites.

    The secondary (or service) causes of cracking include:-

    • Rapid firing from cold resulting in high thermal stresses;

    • Over firing, typically when changing to gas firing, resulting in severe cracking at the rear tube plate due to higher temperature differentials;

    • Insulation effect of scale deposits on both surfaces giving increased temperature gradients;

    • Increased boiler pressure and decreased water return temperatures;

    • Untreated feed water leaving deposits that accelerate local corrosion;

    • Incorrect pH of feed water or excessive O2 levels;

    • Reduced circulation and increased temperature differentials due to poor feed water entry;

    • Boilers with low slung furnaces or made from higher strength steels operating at higher stress. For short cracks, the most common type, the resulting failure has generally been leakage.
    For longer cracks, the result can be large scale fracture with a dangerous explosion.

    For short cracks, the most common type, the resulting failure has generally been leakage. For longer cracks, the result can be large scale fracture with a dangerous explosion.

    Shell Cracking (location 2)

    A rare but dangerous occurrence is circumferential shell cracking at the tube plate weld shown at location 2.

    Extensive cracking at this location can cause the tube plate to tear away from the shell in a catastrophic manner.

    This type of cracking is generally limited to highly stressed shell boilers constructed of high strength materials and consequently operating at higher relative stress range.

    Tube End Cracking (location 3)

    This longitudinal cracking of tube ends is sometimes encountered in ERW tubes or tubes with poor ends.

    Tube Plate Ligament Cracking (location 4)

    Ligament cracking has been reported in boilers with high operating temperature differentials up to 400°C. Tube plate cracking typically starts at toes of boiler tube fillet welds and grows across the tube plate ligament from one boiler tube to another.

    Cracking has also occurred from centre-pop marks forming a small notch in the edge of the tube hole with expanded tubes. Ligament cracking is serious. Depending on the age and fracture toughness of the tube plate, material crack extension can occur suddenly by brittle fracture when the boiler cools down to ambient temperature. High local residual stresses can trigger brittle fracture in heavily cold worked and aged steel. This occurred with a unique case at location 5 from a 6 mm deep fatigue crack.

    Other cracking locations (locations 5 to 8)

    Cracking has been reported at all locations depicted in locations 5 through 8 in Figures 1 and 2 , mainly at attachments. Although cracking in these locations is relatively rare they also should be
    subject to examination by the boiler inspector.


    CRACK DETECTION

    Good access is required to visually detect cracks and surfaces should be clean for 50 mm each side of the weld where cracking initiates.

    Visual examination with the aid of lights can detect cracks over 5 mm in length and over 1 mm deep depending on adequate surface cleanliness. Endoscopes and digital cameras can be used to aid
    detection (particularly with low slung boilers), with computers to record information. Magnetic particle testing (MT) and penetrant testing (PT) are more sensitive than visual inspection if the suspected crack area is accessible for examination. Ultrasonic testing (UT) is probably the best method to detect serious cracking.

    INSPECTION INTERVAL AND MONITORING.

    The annual intervals specified in AS/NZS 3788 should be applied in normal circumstances for visual inspection. If the operational circumstances are such that none of the primary and secondary causes mentioned above are applicable the inspection limits can be extended. Conversely frequent rapid firing under harsh conditions requires more frequent NDT especially as boilers age.

    Ultrasonic testing should be carried out within 10 years from the construction date in normal circumstances or more frequently under harsh conditions. Similarly if there are significant changes in
    operating temperature or pressure, ultrasonic testing should be carried out more frequently e.g. after initial 10,000 cycles. The ultrasonic testing program should include a reasonable length of weld at both ends of the furnace and at the top, bottom and sides of the weld circumference at locations 1 and 2. Extra care should be taken with tubes near to stay tubes or near the shell.

    Increased inspection frequency should also be implemented if the furnace was manufactured from steel with Rm>460 MPa, or if the design strength value used is above 110 MPa.

    OPERATING OPTIONS FOLLOWING CRACK DETECTION

    General

    Once cracking has been detected, confirmed and sized, an informed decision is needed on whether to continue operation, repair or scrap. This decision depends on the estimated remaining SAFE life of the cracked part and the desired remaining life of the boiler.

    Fracture Mechanics Analysis

    In order to determine the estimated cycles to failure and the nature of the failure (leak or break) a fracture mechanics assessment may be used. A number of options are available, but the methods
    described in AS/NZS 3788 provide instruction on how to carry out the analysis. The accuracy of the following data is critical:-

    • Crack position, depth, and length around the weld circumference;

    • Physical properties of parent plate – fracture toughness, yield and tensile strength;

    • Parent plate inclusions, laminations and any banding and direction of rolling;

    • Number of anticipated cycles.

    • Developed stress range which is often very difficult to quantify particularly at location 1;

    Only personnel with proven expertise and experience should undertake a fracture mechanics assessment.

    Remaining life assessment

    Practically, a better method of assessment is to use world experience, coupled with the basis of fracture mechanics.

    Experience with early ductile, low strength steels indicates that furnace tube cracking can be tolerated up to the lower of 2 mm and 30% of the furnace tube wall thickness. Operation changes should be implemented to eliminate some of the primary or secondary causes of cracking and de-rating the boiler output may be required. If crack depths are 50% or more through the furnace wall, the boiler should be isolated for repair or replacement.

    Determination of remaining safe life should take into account:-

    • Quality and properties of the steel, direction of rolling, presence of inclusions and age. (Prior to about 1985, most steel in Australia was ingot-poured, with the associated risk of occasional centreline
    inclusions and laminations);

    • Severity of future cycling;

    • Worst crack length and depth;

    • Ratio of furnace tube to tube plate thickness;

    • Age of the boiler (gives an indication of the probable origin and properties of the materials used and the number of cycles experienced);

    • Measures to be taken to reduce further corrosion and thermal cycling;

    • Preparedness, time and available resources to carry out the repair;

    • Feasibility of crack repair;

    • Results of any fracture mechanics analysis;

    • Residual stress and risk of brittle fracture;

    • The measures taken to avoid gas explosions and low water failures. Both can result in severe plastic straining across the crack leading to furnace tube rupture. These measures are essential where cracks over 2 mm deep have been detected.

    • Outcomes of a risk assessment

    • Management responsibility in the event of a failure leading to an explosion. Given that the crack growth mechanism is corrosion fatigue, the number and extent of thermal cycles to which the boiler will experience is the primary issue in determining the length of time the boiler can be operated. In assessing the consequences of failure, it must be determined if the failure mechanism is likely to be a through wall leak of a boiler tube or a catastrophic rupture of the furnace or tube plate, potentially leading to an explosion.

    REPAIR OPTIONS FOR FURNACE TUBE CRACKS

    Local Weld Repair of Furnace Tube

    Local weld repairs have been widely used for the repair of cracks with limited length. Low hydrogen welding processes such as GTAW, MMAW (with EXX16/EXX18 electrodes designed for single sided complete penetration V butt welding) or GMAW. A relatively high preheat should be used, and then no postweld heat treatment is required. The method involves:-

    • Removal of crack and associated damaged material from the inside of the furnace tube;

    • Weld with a AS 3992 qualified welding procedure using low yield strength weld metal;

    • Ultrasonic testing using angle probes to establish quality of repair weld;

    • Dressing the bore of the furnace tube flush by grinding;

    • Hydrostatic pressure testing using warm water (at least 20°C) to full the test pressure, ie. 1.5 times the design pressure.

    Successful repairs rely on competent welders, good weld shape, low hardness, negligible defects and competent NDT technicians and importantly, proof by tests or previous work that the inside root profile is as shown in Figure 4.

    Replacement of One End of Furnace Tube

    This option should be considered where there is extensive cracking at one end and there is good access from inside the furnace. The repair will involve using procedures, inspection and testing practices similar to the original construction.

    Removal and Replacement the Complete Furnace Tube

    This option is only practical when both ends have extensive cracking and there are no stiffening rings to impede removal of the tube.

    Repair of Other Cracks

    Such repairs should be made using the principles in 8.1.

    MEASURES TO PREVENT OR CONTROL CRACKING

    Prevention Measures

    Boiler life is proportional to the number of thermal cycles experienced during operation. With continuous uniform firing and negligible cycling boiler lives 50 or more years are achievable. Thus to prevent cracking it is necessary to establish operating conditions that reduce the severity of cycling as far as possible. The following recommendations apply to all modes of cracking depicted in Figure 1. However the emphasis is on furnace tube cracking.

    Control Measures

    Whilst it may not be feasible to run a boiler continuously to avoid thermal cycling the following control measures will maximise boiler life for the applied operating conditions:-

    • Reduce the risk of low water conditions with reliable low water controls;

    • Do not exceed manufacturer's recommended firing rates and metal temperatures;

    • Reduce risk of excess pressure by checking and correctly maintaining safety valves;

    • Minimise cycling of pressure and temperature;

    • Minimise shock loading - avoid rapid heating and cooling particularly below 800C - preferably use modulated burners and mixing of feed water;

    • Review water treatment to ensure appropriate de-aeration and pH control;

    • Review blow-down procedures and ensure water sediments are flushed out regularly.

    REPORTING AND DOCUMENTATION

    It is necessary to maintain appropriate operating, inspection and maintenance records for boilers so they can be operated, inspected and maintained in a pro-active manner.

    Documenting the number of operating cycles the boiler undergoes together with the severity of those cycles provides the baseline that will ultimately dictate the frequency of inspection and remaining life of the boiler.

    The service records should include:-

    • Inspection history listing the dates and type of inspections undertaken and results received;

    • All repairs including observations, actions taken and the basis for those actions;

    • Correspondence with Regulatory Authorities (where required);

    • An inspection and NDT plan based on operating history and inspection results;

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    International Steam Tables



    International Steam Tables
    Properties of Water and Steam based on the Industrial Formulation
    IAPWS-IF97
    Tables, Algorithms, Diagrams, and CD-ROM Electronic
    of heat cycles, boilers, and steam turbines
    By Wolfgang Wagner, Hans-Joachim Kretzschmar
    * Publisher: Springer
    * Number Of Pages: 392
    * Publication Date: 2008-01
    * ISBN-10 / ASIN: 3540214194
    * ISBN-13 / EAN: 9783540214199

    Book Description:
    Steam tables for practical industrial use are presented which have been calculated using the international standard for the thermodynamic properties of water and steam, the IAPWS-IF97 formulation, and the international standards for transport and other properties. In addition, the complete set of equations of IAPWS-IF97 is presented including all supplementary backward equations adopted by IAPWS between 2001 and 2005 for fast calculations of heat cycles, boilers, and steam turbines. For the first time these steam tables contain the following features:
    * A compact disc (CD) providing an interactive electronic steam table for the calculation of all properties used in the book dependent on pressure and temperature.
    * Formulas to calculate from IAPWS-IF97 arbitrary partial derivatives of the eight most important properties; this is very helpful in non-stationary process modelling.
    * Inclusion of the specific enthalpy and enthalpy differences into the uncertainty values of IPWS-IF97 regarding the most important properties.
    * Pressure-temperature diagrams with isolines of all properties contained in the steam tables and further properties.
    Moreover, a Mollier h-s diagram and a T-s diagram are enclosed as full-colour wall charts in A1-format
    -------------------------------------------------------------------------------------

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    الغلايات البخارية
    عن أبي ذر الغفاري رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : "اتق الله حيثما كنت. وأتبع السيئة الحسنة تمحها، وخالق الناس بخلق حسن" رواه الإمام أحمد والترمذي.

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    جزاك الله خيرا على هذة الروائع " ولا اخفى عليك سرا انى اذهب مباشرة عند دخولى المنتدى الى مواضيعك القيمة " . "مهندس مصطفى وفقكم الله الى ما يحب ويرضى "

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