DRAFT Proceedings of the 16th International Conference on Nuclear Engineering ICONE16 May 11-15, 2008, Orlando, Florida, USA ICONE16-48187 THERMAL CYCLE EVALUATION FOR FEEDWATER HEATER OUT OF SERVICE CONDITION Lindsey L. Dziuba Senior Engineer ILD, Inc. 8867 Highland Rd, #378 Baton Rouge, LA 70808 Robert J. Stakenborghs Engineering Manager ILD, Inc. 8867 Highland Rd, #378 Baton Rouge, LA 70808 ABSTRACT Large power plant secondary side thermal cycles typically include multiple parallel trains of condensate and feedwater heating to improve cycle efficiency. These heaters are typically arranged in multiple identical trains and can contain by-pass lines to accommodate removal from service for on-line maintenance. In a closed system, the heaters are typically horizontal u-tube heat exchangers with various combinations of internal and external drains cooling. Heating fluid is provided from steam extraction and drains flow. Drains flow can be piped either in a forwards (pumped forward) or backwards (cascading drains) direction. All of these possible equipment combinations result in a complex system where the impact of taking single or multiple heaters out of service for on-line maintenance can be difficult to properly determine. This paper presents a description of the various impacts of removing feedwater heaters from service during operation. The impacts can include increased steam flow through the downstream turbine stages, increased extraction flows and drains flows to the remaining heaters, and increased heater tube side flows that can all potentially cause equipment damage. Proper precautions should be taken to protect plant equipment, including reducing plant thermal power output when necessary. This paper provides specific plant examples to illustrate the complexity of the problem and its proper solution. A simplified evaluation methodology is also provided to allow the user to determine the quantitative impact of removing equipment from service. For each particular feedwater heater out of service arrangement, the method uses mass and energy balances around the remaining in-service feedwater heaters to determine the magnitude of the changes in turbine stage, extraction and drain flowrates. The appropriate power level for that condition is determined by applying specific acceptance criteria for the impacted equipment. INTRODUCTION A feedwater/condensate heater (or heaters) may be removed from service in an operating utility power plant for various maintenance activities that may be planned or unplanned. It is desirable to maintain unit power at as high a level as practical during those periods of equipment downtime. Removing a heater or heaters from service impacts other equipment, including the turbine, system pumps, and the tube and shell side of the remaining in service heaters, in ways that may have unintended adverse consequences. Therefore, it is desirable to accurately determine the impact to other plant equipment that results when various combinations of feedwater heaters are removed from service, and in turn, determine the reduced operating thermal power level required to avoid equipment damage. IMPACT OF EQUIPMENT ISOLATION Removing a feedwater heater from service typically involves, as a minimum, isolating all of the shell side inlet and outlets, such as extraction steam, cascading drains, miscellaneous steam inlets, and drain outlets. On some occasions, the entire heater can be removed from service including isolation of the tubeside feedwater/condensate inlet and outlet. This action usually occurs when tube side bypass lines exist in the plant design, or multiple trains of heaters exist such that one train can be isolated. Otherwise, the tubeside of the heater is left in service, although no active heating occurs as a result of the isolation of the shell side. Removing heaters from service changes the thermal balance of the heater string. This change in the thermal balance impacts connecting components by changing extraction and drain conditions. These changes can be extreme in certain cases and are dependent on the heating duty of the heater removed from service and the type of feedwater heating system. 1
If the out of service condition is not properly analyzed, the potential for serious equipment degradation can exist. The various impacts are summarized below. TURBINE IMPACT The removal of a feedwater heater from service impacts the turbine since the extraction normally delivered to that heater now continues through the turbine flow path. This results in more flow through the stages past the extraction point and thus increased stresses on the blades and turbine internals. This is a relatively easy impact to evaluate and guidance is typically provided by the turbine manufacturer in the form of a recommended power level for feedwater heater out of service condition. In cases where the turbine manufacturer has not provided explicit guidance, a reasonable method is to apply the Valve Wide Open (VWO) heat balance. This heat balance typically represents the highest turbine stage steam flow analyzed by the equipment manufacturer. The stage flows for the out of service condition should be computed and reviewed against the VWO heat balance to ensure that the stage flows remain below those listed on the VWO heat balance. If not, power reduction may be required to bring the unit in compliance. IMPACT TO DRAIN VALVES AND PIPING In some instances, the removal of a feedwater heater from service increases the extraction flow delivered to the remaining in service heaters. Higher extraction flow in a heater means increased drain flow that must be removed through either the normal or alternate drain lines. The heater drain line piping and valves may be oversized, and in many cases, they have capabilities of up to 120% of the original design flow. However, this is not always the case and the drain piping and valves should be checked to ensure proper operation under off-normal situations. The maximum flow rates for the drain valves operating in the heater out of service conditions are verified using the standard control valve equations. The evaluation becomes complex simply due to the fact that the flow may be flashing through the drain lines and valves. The drain pipe must be checked for limiting sonic flow at the exit of the pipe to determine flow capacity. Normal drain flow capacities may not be a limiting factor; if levels in the heater increase as a result of reaching the valve and/or piping flow limitations, the emergency drain lines will open and bypass flow to the condenser to prevent high level operation. This will result in a heat rate penalty, but it does not necessarily result in equipment degradation. IMPACT ON OTHER COMPONENTS Removing feedwater heaters from service can also impact other equipment in the balance of plant, such as the main feedwater pumps and heater drain pumps. In those situations where multiple heater trains exist and feedwater flow is diverted to remaining trains with no by-pass, increased tubeside pressure drops may result in diminished NPSHa for the feedwater pumps. Also, in pumped forward systems, changes in drain tank conditions should be 2 reviewed against the operating parameters of the drain pumps and control valves. IMPACT ON OTHER HEATERS There are multiple impacts to the feedwater heaters that result from the removal of heaters from service. All of the impacts are, of course, to those heaters that remain in service. The heaters that remain in service may extract more steam from the turbine due to a decrease in the inlet temperature of the tube side liquid (i.e. feedwater or condensate). This temperature decrease is the result of removing the upstream heater shell from service. In certain instances where a complete train of heaters is removed from service, the tubeside flow is increased to the trains that remain in service. The decrease in tube side inlet temperature or the increase in tube side flow both result in an increase in extraction steam flow to the shell side of the affected heater. This is due to the thermodynamic balance of the heater - additional extraction steam is drawn into the heater due to the higher heat duty tube side condition (i.e. colder fluid or larger mass flow). The increased extraction flows may result in high shell side fluid velocities that cause high pressure drops and potential tube vibration. Shell Side Condensing Section Velocities As part of a typical analysis performed for feedwater heater performance, the manufacturer calculates a condensing section velocity based solely on heater conditions, extraction flow, and shell and tube bundle geometry. The natural frequency of the tubes is also calculated, based on the feedwater heater structural configuration and fluid conditions. A critical fluid velocity is then calculated, which is the fluid velocity that would likely bring about vibration in the tubes based on the tube natural frequency. The actual fluid velocity in the condensing section is then divided by the critical velocity. This ratio is called the critical velocity ratio. Typically, if the critical velocity ratio is less than 0.5, the heater velocity is acceptable. It is important to note that condensing section tube vibration is governed by fluid (i.e. steam) velocity and not mass flow. The shell side heater steam velocity is impacted by both the mass flow rate and the heater pressure. The heater pressure can be approximated as a linear function of power. As power decreases, the heater pressure will decrease causing the steam to expand and the fluid velocity to increase. When a mass flow increase (resulting from an increase in extraction flow due to the heater out of service condition) is coupled with lowering heater pressures due to decreasing power level (resulting from other equipment limitations in the heater out of service condition), the result may be very high condensing section velocities. Thus, care must be used in selecting a proper operating power level for these conditions. Shell Side Drain Cooling Section Velocities Similarly, a check is performed for the shell side drain cooling section. The heater manufacturer calculates drain cooling section velocities based on total drain flow and drain
cooler geometry. This velocity is then compared to several important velocity parameters to predict the propensity for that flow rate to result in unacceptable tube vibrations. This comparison is done by obtaining the ratio of the actual velocity to a so-called critical velocity for that particular physical phenomena, such as vortex shedding. If the ratio is less than a conservative acceptance criteria, typically 0.5, the velocity is acceptable. The drain cooling section velocities are governed by mass flows only since the fluid is in the liquid state, which is nearly invariant in density versus power. Therefore, it is unlikely that vibration in the drain cooler, drain piping, or the control valves will be limiting in the heater out of service condition. Tube Side Velocity and Pressure Drop In those cases where higher feedwater/condensate flows occur, the increased tube side flow results in higher tube velocities and larger tube side pressure drops. These may reach levels that prove to be detrimental to the heater. The pertinent fluid conditions and velocities should be checked for all of the possible operating scenarios and the resulting flow rates compared to acceptance criteria based on the current plant operating conditions. The standard velocity limit established by the Heat Exchange Institute (HEI) for feedwater heater tubes is 10.0 ft/sec. This limit is based on erosion corrosion limits for the heater inlet nozzles and tube sheets. The pressure drop through the tubes is also important and increases with higher velocities. Tube erosion may also be a concern depending on tube material. Many modern heaters use stainless steel as the tube material, which is resistant to erosion in the typical ranges of condensate/feedwater velocity. Typically, limits for feedwater and condensate flow through the heaters can be developed based upon these feedwater flow restrictions. SIMPLIFIED EVALUATION METHOD Extraction Flow Determination The most important factor in determining proper operating power level is the evaluation of condensing section and drain cooling section velocities. Additional extraction steam flow resulting from out of service conditions can lead to harmful tube vibration with possible tube-to-tube impact in both the condensing and drain cooling sections. Thus, it is desirable that a simple methodology be developed to determine the additional extraction flow associated with removing equipment from service. As shown in Figure 1, a typical feedwater heater functions to increase feedwater temperature in the tubeside by extracting energy from the extraction steam and cascading drain on the shell side. These conditions can be expressed mathematically as follows:,, (1) 3 Where: = Feedwater mass flow = Extraction mass flow, lbs/hr = Cascading drain mass flow, lbs/hr, = condensate outlet enthalpy, btu/lbm, = condensate inlet enthalpy, btu/lbm = extraction steam enthalpy, btu/lbm = exiting drains enthalpy, btu/lbm = inlet cascading drains enthalpy, btu/lbm Rearranging this equation in terms of the quantity,, yields the following expression:,, (2) This expression defines the amount of increased energy of the feedwater due to heating by the extraction and cascading drains for any feedwater heater. If the heater shown in Figure 1 was removed from service, this is the amount of heat that must be added by the downstream heater so that the outlet feedwater temperature of that heater is maintained. This increase in heat transfer is driven by the terminal temperature difference of the heater, or TTD. The TTD is defined as the shellside saturation temperature minus the feedwater outlet temperature. Feedwater heaters will extract additional steam from the turbine to maintain TTD to the extent possible. Normally, TTD rises as heat duty increases in the heater, but for this analysis it is acceptable to assume that the heater will maintain a constant TTD. This will yield maximum extraction flows and provide a conservative basis for evaluating shell side conditions. The quantity in the numerator of Equation 2 is called the extraction energy defect, or EED, in BTU/hr: (3) The EED characterizes the amount of energy required to be made up by additional extraction to the downstream heater to maintain a constant feedwater outlet temperature for the same feedwater flow rate. Once the extraction energy defect for the out of service heater is determined, it is applied to those heaters in service that see its effects. The extraction energy defect is divided by the difference in enthalpies of the extraction steam and the drain for the effected heater. This determines how much more mass of extraction steam is required to account for the energy defect. However, due to the changing conditions in the secondary side, the exiting drain condition in the down stream heater also changes. This is a result of a temperature reduction of the incoming feedwater. The drain temperature for the remaining in-service heater is determined by maintaining the drain cooler approach, or DCA, at its design or heat balance value. Drain cooler approach is defined as the difference between the tube side inlet feedwater temperature and the shell side outlet drain temperature. Thus, the heater is capable of extracting additional energy from the incoming extraction steam and cascading drains flows, so that the effect
of the extraction energy defect is somewhat diminished. The additional extraction flow for the downstream heater can be determined as follows:, Where:, = Additional Extraction mass flow, lbs/hr = Extraction Energy Defect, btu/hr = inlet extraction enthalpy, btu/lbm = drains enthalpy computed based on the DCA, btu/lbm The heater removed from service also impacts the upstream heater in a cascading heater system. This is the result of removing the cascaded drains from the downstream out of service heater. When a heater is removed from service, the extraction steam is isolated and the drains that are cascaded to its shell side are typically by-passed to the condenser. This results in no cascaded drains being passed to the upstream heater, in a cascading drains system. The analysis is more complex in a pump forward system. Using Figure 1 and the terms in Equation 1, the amount of energy removed due to the loss of cascading drains, or the Cascade Energy Defect (CED), is as follows: (4) (5) Where: = Cascade Energy Defect, btu/hr = Lost Cascading drain mass flow, lbm/hr = inlet drain enthalpy, btu/lbm = drains enthalpy, btu/lbm Its impact on the upstream heater results in increasing extraction flow to maintain a constant TTD. The amount of additional extraction mass flow as a result of the CED is calculated as follows:, (6) Where: = additional extraction flow, lbs/hr = Cascade Energy Defect, btu/hr = inlet steam enthalpy, btu/lbm = exiting drains enthalpy, btu/lbm In a similar fashion, the extraction to the heaters further upstream are also impacted. This is the result of diminished cascaded drain flow caused by removal of the drains flow in the out of service heater. Some of the drains flow is recouped by the additional extraction steam flow caused by the CED. It does not make up the loss entirely since the heating value of the extraction steam is typically much higher than the cascading drain stream (it takes very little additional extraction steam to make the up the loss of the 4 cascading drains). As a first approximation, the CED can be applied to each upstream heater to determine its additional extraction. More refined calculations can be done by reducing the CED to account for the additional extraction in the drains flow of each successive heater. Note also that winter heat balance conditions should also be considered when determining the effects of out of service heaters on the low pressure heaters. In winter conditions, the low pressure heaters may already be at or near their limit for increased extraction due to the reduced condensate temperature. This is especially important for the lowest pressure heater. Other Considerations Depending on the design features of the feedwater heater system, these analyses can become complex, and there are several situations that require special consideration. A few examples are: 1. Pumped forward systems where fluid mixing occurs 2. When entire trains of heaters are taken out of service and the remaining heaters see significant increase in feed flow 3. Plants with operating data that differs from heat balance conditions 4. Plants with heaters that have significant numbers of tubes plugged or have experienced tube vibration or other internal damage These situations may require additional analytical assumptions and possibly iterative routines to obtain the proper solution. EXAMPLE CALCULATION Using the notion of EED and CED, a simple analysis of proposed out of service conditions for a typical cascading feedwater system is provided. For example, take the heater string shown in Figure 2. Figure 2 shows 4 heaters in series (A, B, C, D), which represents a typical heater string. Heater D would be the lowest pressure heater and Heater A would be the highest pressure. The heaters are backwards cascading with heater D draining to the condenser In Figure 3, Heater B is removed from service by isolating its extraction inlet and its drain outlet. The system would first be evaluated using the 100% power heat balance. EED and CED would be calculated using Equations 3 and 5, respectively. Then the additional extraction in Heater A would be calculated using Equation 4. The additional extraction in heaters C and D would be calculated using Equation 6. A winter (i.e. low condenser backpressure) heat balance should be used to evaluate the full impact to heaters C and D. In situations such as this, typical results are as follows: 1. Extraction to Heater A increases. This is typically the largest impact. 2. Drains flow from Heater A increases commensurate with extraction. This may not be a problem since the drain flow is diverted to the
condenser, but should be checked for choking conditions. 3. Extraction to Heater C increases. This is the result of the loss of two layers of cascaded drains and is typically less than extraction increase for Heater A. 4. Extraction to heater D is also increased, but less than Heater C. 5. The drains flow for Heaters C and D are reduced since the increase in extraction flow is less than the decreased cascaded drains. The resulting extraction flow rates and velocities would be calculated and compared to the 100% power velocity conditions or to feedwater heater manufacturer information for that specific heater. If the velocities exceed acceptable limits by unreasonable amounts, the power level should be reduced. The 75% power heat balance would then be reviewed in the same fashion. Further power reduction levels would be reviewed as required until acceptable velocities are achieved. In practice, all of the equations and iterations can be automated in the form of spreadsheet software (such as Excel) in combination with an add-on steam table lookup program. This allows streamlining of all of the calculations required to evaluate a feedwater heater system. The heat balance data can be entered directly in the spreadsheet and multiple out of service conditions can be easily analyzed. Since the fluid properties can be obtained directly from the steam table add-on, shell side velocities can be accurately calculated for comparison to design conditions. CONCLUSIONS Large power plant secondary side thermal cycles typically include multiple parallel trains of condensate and feedwater heating to improve cycle efficiency. There are many possible equipment combinations that result in complex systems. A feedwater/condensate heater (or heaters) may be removed from service in an operating utility power plant for various maintenance activities that can be either planned or unplanned. It is desirable to maintain unit power at as high a level as practical during those periods of equipment downtime. Removing a heater or heaters from service impacts other equipment in ways that may have adverse consequences if not thoroughly analyzed. The potential impact of removing feedwater heaters from service has been reviewed and described. The removal from service impacts on balance of plant equipment has been discussed. The highest probability for consequential damage that might be over looked is the shellside extraction steam velocity for the heaters that remain in service. A simplified approach has been described, which can easily be used in a spreadsheet application to accurately evaluate the impact on heater shell side conditions. An example calculation methodology has been described. It is believed that thorough review of the conditions associated with the removal from service of a feedwater heater can help prevent unintended adverse consequences. REFERENCES 1. Evaluating and Improving Steam Turbine Performance, By Ken Cotton, Copyright 1993, ISBN 0-9639955-0-2 Figure 1: FEEDWATER HEATER ENERGY BALANCE 5
Figure 2: TYPICAL CASCADING DRAIN SYSTEM 6
Figure 3: EXAMPLE OUT OF SERVICE HEATER WITH CASCADING DRAIN SYSTEM 7