1. TITLE: H.P. Feed-Water Heater Performance
2. ABSTRACT: Facing to the paradoxical situation of increasing the efficiency of power plant at lesser cost, it is an urgent necessity to improve the performances of the accessories including that of the feed-water heaters. Hence, immediate implementation of measures to achieve higher performances of heaters & their analysis is of utmost importance. The present work outlines the salient features of feed-water heaters, HP Heaters in particular & the probable causes of the discrepancy between the actual performance & design performance along with methods to minimize losses.
3. OBJECTIVE: The objectives of analysis of the performance high pressure feed water heater are: 1. Prior to an outage, to provide information to determine whether corrective action is required to maintain optimum feed-water heater performance & provide guidance in determining materials, tools & equipment, workers, cost estimates & scheduling. 2. Following an outage, provide information to allow evaluation of the effect of the work on the feed water heater. 3. During normal operation, provide information to allow identification of abnormal changes in heater performance & provide information to assist in identifying the source of the change. 4. During normal operation, provide information to assist in optimizing the operation of the heater. 5. During normal operation, provide information to allow accounting for the contribution of the heater performance deficiencies on unit heat rate & capacity.
4. SCOPE: The present project envisages the necessity of High Pressure feed-water heaters & extends the analysis for the selection of heater type, its construction & design aspect. The performance parameters & the proper functioning of the heaters depend on the construction, tube material selection, arrangement, location & this has been discussed briefly. Taking into view the constraints, the scope can be extended for further technical analysis in selection of the above parameters. Further, the same concept has been simultaneously extended for designing of Low Pressure heaters as they are of similar type (non-mixing type heaters).
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5. INTRODUCTION: In modern thermal power plants the feed-water heaters play a vital role by way of increasing the average temperature of heat addition and hence improving the cycle efficiency. In order to prevent thermal shock caused by cold feed-water and improve unit efficiency, feed-water heaters are used to heat the incoming feed-water prior to flowing through the economizer section of a boiler. For example, by increasing the feed-water temperature by ten degrees, the unit efficiency increases by one percent. Good performance of these heaters is thus crucial for the overall performance of the plant. Present work aims at performance prediction of HP feed-water heater of horizontal configuration, which is the most commonly used configuration in present day power plants. The three zones in an HP heater viz. de-superheating zone, condensing zone and drain cooling zone can be modeled separately and simulation of the three is used to theoretically predict the heat transfer rate. While Delaware method is conventionally used for de-superheating and drain cooling zones, more detailed simulation is carried out for condensing zone for which Delaware method is strictly not applicable. The control volume approach accounts for the variation in heat transfer coefficient and fluid properties along the flow and hence is expected to be more accurate than a model using a uniform heat transfer coefficient in the entire condensing zone.
Fig.1. Different zones of Shell & tube type heat exchanger
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The feed-water heaters increase the plant efficiency by making the process regenerative. This is accomplished by using extraction steam pulled from the high, intermediate and low-pressure sections of the steam turbine. The condensate from each heater has a considerable amount of energy and is also used to heat the incoming feedwater. Figure 1 shows a cross section of a shell and tube heat exchanger that utilizes both steam and the successive heater drains. A cascading heater arrangement is the most common type of heater arrangement found in a power plant. These rely on extraction steam that is at or very near saturated conditions to heat the incoming feed-water. The condensate formed from the heating action is still at saturated conditions and is then sent to the next heater. A control valve is used to maintain heater level and will cause the draining condensate to flash, which increases the efficiency of heat transfer. In a power plant, there are two sets of feed-water heaters. Low-pressure heaters use extraction steam pulled from the low-pressure turbine and are located after the condensate pumps. The condensate is routed through the low-pressure heaters into an open heater called a deaerator. In the deaerator, the heating action drives off gases that can become corrosive to piping and associated equipment at elevated temperatures. The deaerator provides the suction for the main boiler feed-pumps, which sends the feedwater through a series of high pressure heaters prior to the economizer that, are fed by extraction steam from the high and intermediate pressure turbines. Figure 2 shows the high pressure and low-pressure heater arrangement.
Fig.2. Arrangement of feed-water heaters in a Power Plant 3
Feed Water heaters are used to achieve thermodynamic gain by bleeding steam from the turbine & heating the incoming feed water, thereby, tending to make the cycle a regenerative one. The steam in turn gets condensed in the shell of the heaters raising the temperature of the feed water & drains are cascaded from heater to heater (higher to lower pressure). With increased thermal rating of the power plants, Feed-Water Heaters are subjected to very high pressure & temperature, especially; the high pressure feed water heaters, which are located just after the Boiler Feed Pump. The severe operating conditions, demands proper selection of Feed-Water Heaters, which have high degree of equipment reliability.
6. SELECTION OF FEED WATER HEATER TYPE: The design of Feed-Water Heater has developed considerably since the first generation of the power station, but the basic type of heaters remained to be: • Surface type • Direct contact type The condensation mechanism, occurring in the direct contact type heaters, is the same as in surface contact type, although, the cooling medium is not separated by a metal wall from the mixture of heating vapour as in the later type. These heaters have got a certain specific advantages over the surface type heaters. These are1. Higher thermal efficiency, since there is no metal barrier to the heat transfer between steam & water 2. Low cost & maintenance But, direct contact type feed-water heaters require larger number of condensate pumps & larger space due to their bigger size compared to surface type heaters. Due to the above reason, they are not commonly used in power plants. The only direct contact type heater used in the feed heating line is Deaerator. There are two types of surface type feed water heaters used in thermal power plants. One is a U-tube type or straight tube heater & another is a coiled tube type heater. The coiled tube type HP heaters are used for 210MW & lesser capacity power stations. They have helical coils tubes, which are welded to the feed-water inlet & outlet headers. Main advantage of this type is that, there is no tube plate, thereby, ensuring excellent thermal transient capability. But, these heaters require complicated maintenance practices. U-tube, shell & tube type heaters with tube plates are extensively being used for HP Heaters & LP Heaters. These heaters can be mounted either vertically or horizontally. When the heaters are double pass, U-tubes are used & for single pass straight tubes are used. These heaters have the following advantages(i) Requires less space, as these heaters are compact.
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(ii) Maintenance is easy because tubes can be plugged without dismantling the heater. (iii) Wide choice of tube materials. (iv) High reliability. Considering the above advantages, U-tube shell & tube type HP Heaters, LP Heaters & Drain Cooler are selected for TSTPP, Stage-II, 4x500 MW units.
7. DESIGN CRITERIA: The Feed-Water Heaters shall be designed as per design code-HEI standard (USA) for closed Feed-Water Heaters & Indian Boiler Regulation (IBR). The design criteria are discussed below:
7.1. DESIGN PRESSURE: The shell design pressure of the heater, taking extraction from Cold-Reheat (CRH) line shall be same as the CRH piping design pressure. The shell design pressure of all other heaters shall be that corresponding to safety relief valve set pressure + 10% extra margin on the pressure, for safety precaution. The safety relief valve set pressure shall be computed based on 7% blow-down pressure (i.e., the pressure at which the valve closes) & shall not be, in any case, less than the maximum operating pressure. The maximum operating pressure is determined basing on the worst operating conditions. For all LP Heaters & Drain Cooler the minimum shell thickness shall be 10mm, excluding the corrosion allowance. The shell side of the heaters shall be designed for full vacuum also, to prevent them from imploding. The tube side & water-box design pressure of HP Heaters shall be the maximum design pressure of the inter-connecting Boiler Feed Pump (BFP) discharge line or the maximum pressure to which feed water is subjected to under worst operating condition. The tube side & water box design pressure of LP Heaters shall be the Condensate Extraction Pump (CEP) shut-off head under 3% over frequency operation.
7.2. DESIGN TEMPERATURE: The design temperature of the shell skirt, shell barrel & tube side of HP & LP Heaters shall be based on HEI Standard for closed feed water heaters. While applying the above design criteria, as per HEI Standard, the normal operating conditions to be considered is 500 MW output with 3% make-up & back pressure at design cooling water inlet temperature. The maximum operating pressure shall be decided considering operating conditions as brought out above for design pressure. In all heaters which would be exposed to superheated steam, irrespective of whether a de-superheating section is provided or not, the margin on design temperature for tubes as per HEI shall be applied & the tubes (in all the zones) shall be designed to this temperature. In the case of LPH-1 mounted in the condenser neck, the design temperature shall be same as the condenser design temperature. The design temperature of water-box of the heaters shall be the 5
maximum temperature of the feed water/main condensate leaving the heater rounded off to the next higher 50C.
7.3. TUBE VELOCITY & TUBE DIAMETER: The velocity of the feed water shall be restricted to 3.05 m/s under all operating conditions. However, the use of SS304 tubes permits that tube velocities may be maintained as high as economically feasible in order to preclude stagnation including localized pitting. The velocity of 3.05 m/s is chosen to optimize the pump power requirement, entry erosion & stagnation induced localized pitting. Minimum size of tubes shall be 15.875 mm O.D. Average wall thickness of the tube shall not be less than 20 BWG (0.89) after bending. The tube thickness shall be increased to compensate for tube wall thinking on the inner rows of the tube bundle.
7.4. FOULING RESISTANCE: Fouling resistance for the tube side & shell side shall be taken as per HEI.
8. OPTIMUM DEGREE OF REGENERATION:
Fig.3. Optimization of regenerative feed-water heater (T-s diagram of Rankine Cycle)
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Fig.4. Rankine Cycle as shown in Mollier diagram Complete Carnotization of Rankine Cycle is not possible with finite number of heaters. If there is one feed water heater used, ‘m’ kg of steam is extracted from the turbine for each kg of steam entering it to the heat the feed water heater from state 5 to state 6 (Fig.3.) so that by energy balance, m(h2-h6) = (1-m)(h6-h5) Or m = h6-h5 = h6-h4 h2-h5 h2-h4 Therefore, the thermal efficiency of the cycle is η = 1 - (1-m)(h3-h4) = 1 - [1- (h6 –h4) ](h3-h4) = 1- (h2-h6)(h3-h4) (h1-h6) __(h2 -h4)_______ (h2-h4) (h1-h6) (h1-h6) Following Haywood[1], Horlock[3] & Salisbury[4] it may be approximately assumed that turbine expansion line follows a path on the diagram such that (h- h f) = constant = β, where h is the local enthalpy on the expansion line at a given pressure, & hf is the enthalpy of saturated water at that pressure. Therefore, as seen from the Fig.3., h1-h8 = h2-h6= h3- h4 = β = constant. Let the enthalpy rise of feed water in the heater is γ, which is equal to (h6-h4). Now, h2-h4 = h2-h6+h6-h4 = β+γ If the total enthalpy rise of feed water is equal to α = h8-h4, then, h1-h6 = h1-h8+h8-h4+h4-h6 = β+α-γ Therefore, equation can be written in the form η = 1β2_____ [(β+γ)(α+β-γ)]
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Here, α & β are fixed & γ is a variable. So, there is an optimum value of γ for which η is maximum. On differentiation, dη = β2[(α+β-γ)-(β+γ)] =0 dγ Or, γ = α/2. The cycle efficiency is maximum when total enthalpy rise of feed water (h8-h4) from the condenser temperature to the boiler saturation temperature is divided equally between the feed water heater & the economizer (i.e.,h8-h6 = h6-h4) in a single bleed cycle. So, the temperature rise of feed water in the heater is, Δt = ½(tboiler saturation – tcondenser) & the corresponding cycle efficiency is η = 1β2 = 1β2 = α2 + 4αβ__ [(β+α/2)(α+β-α/2)] (β+α/2)2 (α+2β)2 For a non-regenerative cycle, η0 = 1 - (h3-h4) (h1-h4) Now, h3-h4 = β & h1-h4 = h1-h8+h8-h4 = β+α η0 = 1β = α__ (α+β) (α+β) The efficiency gain due to regeneration, Δη = η- η0 = α2 + 4αβ – _ α = α2β____ (α+2β)2 (α+β) (α+β)(α+2β)2 This indicates that Δη is a positive quantity, justifying the fact that due to regeneration the cycle efficiency increases.
Fig.5. Heater train of a steam power plant (γ = α/2) 8
In the heater train, the feed water enters the economizer section of the boiler at state F (Fig.5.), where feed water is heated to the saturation temperature (G) at the boiler pressure. Assuming the economizer also as a feed water heater (where feed water is heated by the outgoing flue gases, instead of by the bled turbine steam, the total enthalpy rise (hG-h3) or temperature rise from the condenser to the boiler saturation temperature is dividing equally among the feed water heaters for maximum gain in efficiency. The enthalpy rise per heater (including the economizer) is thus, Δhper heater = hG –h3 (n+1) where ‘n’ is number of heaters & 1 stands for the economizer. Therefore, the total enthalpy rise of the feed water for ‘n’ heaters by regenerative feed heating is Δhtotal = n (hG-h3) (n+1) Thus, the total temperature rise of feed water, Δtfw, due to regeneration for the maximum cycle efficiency is given by Δtfw = n ΔtOA (n+1) where the overall temperature difference is given by ΔtOA = boiler saturation temperature-condenser temperature More is the number of heater, more is the total temperature rise of the feed water, Δtfw , by regeneration, less become the heat addition to the water in the boiler, more becomes the mean temperature of heat addition, & more is the cycle efficiency. From Eq. () If n=0, Δtfw0 = 0 If n=1, Δtfw1 = ½ ΔtOA If n=2, Δtfw2 = ⅔ ΔtOA If n=3, Δtfw3 = ¾ ΔtOA If n=4, Δtfw3 = 4/5 ΔtOA & so on. By the use of first heater, the gain is Δtfw1 - Δtfw0 = ½ ΔtOA By the use of second heater, the gain is Δtfw2 - Δtfw1 = ⅔ ΔtOA - ½ ΔtOA = 1/6 ΔtOA By the use of third heater, the gain is Δtfw3 - Δtfw2 = ¾ ΔtOA - ⅔ ΔtOA = 1/12 ΔtOA By the use of fourth heater, the gain is Δtfw4 - Δtfw3 = 4/5 ΔtOA - ¾ ΔtOA = 1/20 ΔtOA & so on. Since the gain in cycle efficiency is proportional to the gain in feed water temperature, the efficiency law follows the law of diminishing return with the increase in number of heaters. In fact, the greatest increment in efficiency is brought by addition of the first heater. The increments for each additional heater successively diminish (Fig.6.). The number of heaters is fixed up by heat balance of the whole plant where it is found that cost of adding another heater does not justify the saving in the heat supply Q 1 or the marginal increase in cycle efficiency. An increase in feed water temperature tfw reduces the heat absorption in the outgoing flue gases from the economizer & may cause a reduction in boiler efficiency. The number of heater & hence, the degree of regeneration thus get optimized.
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Fig.6. Variation of cycle efficiency with increase in no. of feed-water heaters
9. PERFORMANCE: The heater shall perform satisfactorily under turbine throttle valve wide open condition, 3% make-up without any over pressure, 100% TMCR load & part load operation of the plant. Beside the satisfactory operation throughout the load range, heaters shall also be capable of operation without any problem whatever so under all abnormal conditions, i.e., HP Heaters bypassed, LP Heaters bypassed at full load; HP-LP bypass operation full load at minimum cooling water temperature, etc.
10. CONSTRUCTIONAL FEATURES: The feed water heaters shall be all welded construction, i.e., shell to tube plate joint, tube-to-tube sheet joint, tube plate to channel joint are welded. The feed water inlet & outlet lines, extraction steam line & condensate extraction lines are also welded to channel & shell, respectively. All welded heaters offers freedom from leakage, even at high pressure & temperature. All the critical areas get safeguarded against the leakages. These heaters are also able to withstand the transient conditions more efficiently than any other type of design. The maintenance, inspection & cleaning of all welded feed water heater is easy.
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10.1. TUBE MATERIAL: Both copper alloys & non-ferrous alloys are used for the LP Heaters & HP Heaters tubes. Copper alloys are used extensively in the LP Heaters tubes. These alloys have got excellent thermal conductivity but on the other hand these alloys have problems of copper carry over & ammonia attack, which may require a complex boiler cleaning after short intervals. Copper alloys are also affecter by ammonium sulphide & oxygen in the feed water, which can result in failures due to stress corrosion cracking, exfoliation, dezincification & denikelification. To avoid all the above problems, the stainless steel tubes are invariably used for LP Heaters. Stainless steel is unaffected at all operating conditions, except that, it is susceptible to chloride induced stress corrosion. Most common materials used for HP Heaters are carbon steel, stainless steel & monel metal. The carbon steel is the least expensive. It has good heat transfer properties & high allowable stresses. But, carbon steel can corrode rapidly if the pH value falls below 8.5. The carbon steel tubes are also susceptible to the entry erosion near the tube plates. Moreover, it is highly susceptible to oxidization when wet system is opened during shutdown. Of all the alloys, the carbon steel is the most likely to sustain erosion damage in the de-superheating zone as well as in the drain sub-cooling zone due to the bad level control problem. Type 304 stainless steel mitigates all the above problems although it is costlier than the carbon steel tubes. SS-304 type stainless steel has got an excellent corrosion resistance property & is unaffected at nearly all operating conditions. Hence, welded type 304 stainless steel tubes, although costlier, are used quiet extensively for the HP Heaters. When stainless steel tubes are used for HP Heaters & LP Heaters, condensate & feed water lines becomes completely ferrous & so, it is no longer necessary to maintain different pH of the water on pre-deaeration & post-deaeration sections. Thus, pH control becomes easier. A pH of 9.2-9.4, right from the Condensate Polishing Unit outlet to the economizer inlet, will be adequate to take care of corrosion in the total condensate feed water line. Due to the above considerations, welded/seamless tubes of stainless steel type 304 are selected for both LP & HP Heaters.
10.2. TUBE TO TUBE PLATE JOINT: The low pressure heater tubes shall be roller expanded to the tube sheet. The HP Heater tubes shall be welded to tube sheet & then roller expanded. The tubes are passed through the tube plate, fillet welded at their ends & then, roller expanded in the tubes with care to avoid stressing in the weld. The expansion will help in preventing tube vibration, which could cause failure of welding. It also gives strength to the joints.
10.3. WATER BOXES: The full access bolted type or self-sealing type of water boxes shall be used for Drain Cooler & LP Heater. The shape of the water box can be cylindrical or 11
hemispherical. It should be designed to reduce entry losses & entry attack on the tube ends. The flow inside the water box should be smooth without any water hammer. When full access bolted type of water boxes are used for HP Heaters, prevention of leakage at the cover joint is a major problem. With rapid feed water temperature changes, joint pressure can be released, since the studs are not in contact with water & cool more slowly than the tube plates & cover. A careful assessment of the tension required in the studs is necessary to avoid leakage during the transient conditions. To avoid all such problems, self-sealing type of water boxes with elliptical manhole shall be held shut by the presence of water in the heater. As there is no bolting used to keep the water box leak-proof, the problems of unsymmetrical sealing pressure & leakage of gasket does not exist. The water boxes for the HP Heaters shall be elliptical or hemispherical head design. The water boxes for HP Heaters & LP Heaters shall have sufficient straight barrel length to provide access for the tube ends. The pass partition plate of the heaters shall be bolted type to facilitate removal & access to the tube sheet.
10.4. TUBE SHEET: The tube sheet of LP Heater & Drain Cooler shall be of carbon steel & shall be welded to water box & shell. The tube sheet of the HP Heater shall be overlaid with stainless steel with a minimum thickness of of 6.35mm. The overlay is necessary as the HP Heater tubes are welded to the tube sheet. Where joints are not to be welded, tube hold shall be grooved. Grooves are not required for welded tube-to-tube sheet joints.
10.5. SHELL: For HP & LP Heaters all welded constructions shall be used. This design eliminates flanges in the critical areas & reduces the possibility of leakage. It is light in weight & has a smooth unobstructed shape that simplifies the application of insulation. In installations where there are wide variation in operating conditions the advantages of all the welded shell is particularly striking when a joint is subjected to a large temperature difference, such as between de-superheating zone & the drain cooling zone, the joint will be flexed by difference in expansion between two zones. With all welded shall, this is not a problem. But, with a gasket joint, frequent cycling may lead to leakage.
10.6. TUBE VIBRATION: Several tube failures have occurred in feed water heater in & adjacent to desuperheating zone. In many instances, the failures can be attributed to tube vibration in which the tube appears to have exhibited & suffered corrosion damage with adjacent tubes or worn circumferentially at baffle holes. While designing a heater a careful consideration shall be given to the baffle pitching, tube hole clearances, steam velocity, etc., to minimize the tube vibration. The heater shall be checked for any undue vibration that can damage the heater tubes. 12
11. CONSTRUCTION: • • •
The three arrangements generally used in construction of feed water heaters are: The head-up vertical arrangement (wherein the head of the feed water heater is located on the top). The head-down vertical arrangement (wherein the head of the feed water heater is located at the base). The horizontal design. In all the above three types of heaters, it is desirable to minimize the diameter of the heater & to achieve the required heating surface by adding length. This approach helps in minimizing the thickness of the forged tube sheet & shell wall thickness. The functional requirement of the feed water heater viz., condensation, de-superheating & drain cooling have different design requirement on each type of feed water heater. The nature of these designs consideration & their impact is discussed below.
11.1. CONDENSATION: The condensation operation in a feed water heater does not call for any significantly different design consideration on the above arrangements. Consequently, this is not a major factor in deciding the type of a feed water heater.
11.2. DE-SUPERHEATING: The de-superheating section does not present any serious design problem with “head-up” vertical heaters & horizontal heaters. However, if the de-superheater is installed in a “head-down” vertical heater, there is a potential problem associated with the draining of the condensate, specifically, during the operation of the feed water, to drain towards the base of the heater. In order to prevent its entry into the de-superheating section, which may also be located at the base because, that is, where the hot feed water exist, a baffle is installed at the top of the de-superheating section for shedding condensate into de-superheating area. However, tube holes, which pass through & are intended to accommodate the passage & feed water tubes, cannot be gas tight, as they must allow for expansion of heater. Consequently, a certain amount of steam passes through these clearances & where this steam has considerable turbulence, it has a tendency to erode. To avoid erosion, the two extreme ends baffles of de-superheating & drain cooling zone shall be sealed properly to ensure leakage against steam & condensate in the condensing zone.
11.3. DRAIN COOLING: Installation of a drain cooler into a vertical feed water heater poses practical problems. But, in horizontal type heaters, the design of drain cooling portion is relatively simpler. In head-up design, it is necessary to duct the drains from the base of the feed water heater, which is farthest away from the head, up through the length of the feed 13
water heater to the drain outlet location just below the heads. The consequence of this design is that, a high-pressure differential is required between the shell pressure & the drain cooler outlet. This pressure differential is required to lift the condensate from the bottom of the shell, up through the drain cooler to drain outlet. At part load operations the drain accumulates in the base of the feed water heaters until there is sufficient pressure differential to deliver the condensate out of the drain. A design feature, which may be implemented to avoid this flooding of the condensate section, is to install an auxiliary drain at the base of the heater, which would open at low loads & allow the condensate to leave without passing through the drain section. In head-down arrangement the drain cooler section is located at the base of feed water heater. If no de-superheater is installed as with the low-pressure heater, a substantial section of the potential condensing tube is removed by flooding of the drain cooling section. Utilizing a separate drain cooler can eliminate this problem, although it must be noted that, this approach compromises the saving associated with the reduced floor space requirement of the vertical heaters.
12. LOCATION: The location of the vertical feed water heater must consider the need to service the heater for maintenance. Because of their height, possibly as high as 12 m, it is necessary to locate the feed water heater below the operating floor. Otherwise, units cannot be handled with the turbine hall crane & must be serviced through a hole located in the turbine-building roof. Hence, it is impossible to stagger the elevation of the heater in such a manner as to assist the cascading mode of drain operation. This potential problem should only occur with the lower pressure feed water heaters, as the shell pressure differential between the high pressure feed water heaters & the deaerator should be sufficient to implement cascading drain operation through most of the plant operation range. Horizontal feed water heaters, however, may be staggered in their elevation so as to assist cascading to very low plant loads.
13. OPERATION: 13.1. PERFORMANCE: The elevation of the vertical low pressure feed water heater shall be set so as to assist in cascading mode of drain operation. Because the shell pressure differentials between the low pressure feed water heaters may become extremely low, particularly at reduced plant load operation, it may become necessary to utilize alternate means of draining feed water heater other than cascading. These are as follows: • Direct the drain to the condenser at part load but it will affect the part load heat rate. • Use a separate drain cooler. Compromise has to be done with space advantage. • Use heater drain pump. This will result in additional rotary equipment & also increase the maintenance requirements. • Use of a flash tank. This will increase capital cost & occupy additional space.
13.2. CONTROL: 14
In a horizontal heater installation, the surface capacitance is adequate to allow sufficient ‘dwell time’ for low-level control. Therefore, level controller can be designed so as to provide a stable & high-resolution signal to the control valve, thus allowing the valve to position itself without hunting. In a vertical heater installation, however the surface capacitance of the drain fluid is much reduced, resulting in potentially rapid fluctuation of level during unit load changes. In order to compensate for this rapid flow changes through the heaters, the controller must be such as to dampen the signal to the control valve. As a consequence of this requirement, the valve setting may at a given time be inappropriate for the water level in the heater. As a result, the pressure drop through the valve under a given set of operating circumstances may be inadequate to prevent flashing in the drain lines of the valve or conversely, may result in a temporarily high level in the feed water heater.
14. ARRANGEMENT OF HEATERS: The heaters can be arranged either vertically or horizontally. The principal advantage associated with the vertical type feed water heaters, is the considerable reduction in the floor space compared with that required for horizontal feed water heaters. This reduction in the floor space can be achieved only if the considerations do not dictate the installation of equivalent floor space. Specifically, it is necessary to either relocate the deaerator so that the auxiliary bay can be eliminated or the space within the auxiliary bay must be utilized for other purpose. Horizontal high-pressure heaters are usually placed in the operating floor & low pressure feed water heaters are placed in the mezzanine. The controlling parameters for the determination of permissible elevation are again the shell pressure shown on the heat diagram. The low-pressure heater shell pressures are adequate to support the heaters located on the mezzanine floor & the lowest pressure heater on condenser neck. The placement of the low-pressure heater in the mezzanine floor subsequently allows the location of the high-pressure heaters on the operating floor. It allows more convenient access & removal procedures if maintenance of these heaters is required. The above arrangement of the feed water heater will operate over entire load range with little or no component maintenance or attention from plant operator. The performance of HP Heaters can be analysed by monitoring the terminal temperature difference (TTD), drain cooler approach (DCA), the pressure drop on the feed-water side & the temperature rise across the heater. To monitor these, it is desirable to carry out simplified routine performance test on feed water heaters at a specified frequency. This will help in identifying the level of deviations & trending the performance.
15. H.P. HEATER DESIGN DATA: 15
Station: TSTPP Sl. No. 1 2 3 4 5 6 7 8 9 10 11
Description Unit Capacity Heater Position (Horizontal/ Vertical) HP Heater No. No. of Zones (Desuperheating, Condensing, Drain Cooling Surface Area Extraction steam pressure Pressure drop (water side) Operating level (Minimum /Normal/ Maximum) Terminal Temp. difference Drain Cooler Approach temp. Temperature rise
Unit: 5 Unit MW
Values 500 Horizontal HPH-5 3
HPH-6 3
m2 kg/cm2 kg/cm2 mm
1063 16.88 0.85 -625/ -540/ -110
1278 85.5 0.85 -625/ -540/ -110
0
- 0.3 4.8
- 0.3 4.8
37.83
50.13
C C
0
0
C
HP HEATER 5A & 5B Description SHELL SIDE TUBE SIDE Medium Steam & Drain Feed Water 2 Design Pr. (Kg/cm ) 24 & Full 330 & Full vacuum vacuum Design Temp.(º C) 224 224/244 Test Pr.(Kg/cm2) 36 495 Test Temp. (º C) Ambient Ambient Flow Quantity (T/Hr) 42.591 749.267 Inlet Temp.(º C) 412.583 165.5 Outlet Temp.(º C) 170.3 203.33 No of passes/Zones 3 2 No.of Tubes --------1347 Tube size (O.D. x --------OD ¾” x 13 Thickness),mm BWG min MATERIALS Shell/Channel SA 516 Gr 70 /SA 387 Gr.12CL.1 Tubes SA 688 TP 304 Tube plate(s) SA 350 LF2 Flanges SA 105 Nozzles(Tube side) SA 106 GR. B Nozzles(Shell side) SA 182 F11/SA Gr B.
16
HP HEATER 6A & 6B SHELL SIDE TUBE SIDE Steam & Drain Feed Water 57 & Full 330 & Full vacuum vacuum 273 273/293 85.5 495 Ambient Ambient 78.068 749.267 335.25 203.27 208.1 191.927 3 2 ----------1347 ----------15.88 x 13 BWG min SA 516 Gr 70 SA 688 TP 304 SA 350 LF2 SA 105 SA 350 LF2 SA 350 LF2 & SA 106 Gr B
16. H.P. HEATER TEST DATA: Station: TSTPP Sl. No. 1 2 3 4 5 6 7 8 15 16 5 6 7 8 9 10 11 12
Unit:5
Test Date: 09/08/05
Measurement
Unit
Run 1
FW temp. at HPH-5 inlet(5A/ 5B) Pr. of FW at HPH-5 inlet(5A/ 5B) FW temp. at HPH-6 inlet(6A/ 6B) Pr. of FW at HPH-6 inlet(6A/ 6B) FW temp. at HPH-5 outlet(5A/ 5B) Pr. of FW at HPH-5 outlet(5A/ 5B) FW temp. at HPH-6 outlet(6A/ 6B) Pr. of FW at HPH-6 outlet(6A/ 6B) HPH-5 level (5A/ 5B) HPH-6 level (6A/ 6B) HPH-5 shell pressure(5A/ 5B) HPH-6 shell pressure(6A/ 6B) HPH-5 extraction temp. (5A/ 5B) HPH-6 extraction temp. (6A/ 6B) HPH-5 extraction pressure(5A/ 5B) HPH-6 extraction pressure(6A/ 6B) HPH-5 drain temp. (5A/ 5B) HPH-6 drain temp. (6A/ 6B)
0
168.98 200.84 203.63 198.03 204.15 198.57 253.06 196.82 -543.6 -555.7 16.87 42.08 418.97 339.69 16.88 42.10 172.22 212.43
C kg/cm2 0 C kg/cm2 0 C kg/cm2 0 C kg/cm2 mm mm kg/cm2 kg/cm2 0 C 0 C kg/cm2 kg/cm2 0 C 0 C
Run 2 168.86 200.77 199.86 197.97 203.57 198.50 250.06 196.18 -550.4 -544.6 16.98 41.94 416.36 332.34 17.00 41.94 172.69 209.68
168.97 197.43 203.46 197.22 203.85 196.35 252.91 196.27 -547.2 -549.7 16.76 42.25 416.69 332.20 16.78 42.27 173.66 208.22
168.04 198.17 199.28 196.98 201.90 197.23 251.62 195.53 -553.2 -556.9 16.57 40.76 414.13 327.16 16.57 40.87 173.37 208.83
CONTROL ROOM READINGS: Station: TSTPP Sl.No. Description 1 2 3 4 5 6 7 8 9
Load Main steam temp. MS pressure Feed water Flow SH attemperation flow RH attemperation flow BFP disch. hdr. pr. Condenser vaccum Barometric pr.
Unit: 5 Test Readings Units MW 0 C kg/cm2 T/hr T/hr T/hr kg/cm2 mm of Hg. kg/cm2
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Test Date: 09/08/05 Run 1 499.5 532.37 165.78 1586 23.21 11.34 201.4 -0.91 1.01
Run 2 485.6 539.67 164.86 1578 17.32 4.63 201.9 -0.91 1.01
OTHER TECHNICAL SPECIFICATIONS: WEIGHTS HPH 5A & 5B HPH 6A & 6B Dry (Kgs) 45250 58050 During Operation (Kgs) 49300 62550 Floaded (Kgs) 56250 70100
17. CALCULATION: For a closed feed water heater, the Terminal Temperature Difference (TTD) is defined as: TTD = saturation temperature of bled steam – feed water temperature at heater outlet. If the value of TTD for a heater is too small, it is good for plant efficiency, but the heater size increases. If the value is too small, the cycle efficiency will be reduced. The extracted steam on condensation gets sub-cooled in the drain cooler & is removed as drip. The Drain Cooler Approach Temperature (DCA) is defined as: DCA = Drip outlet temperature – Feed water inlet temperature The results obtained are shown in tabular form as below: Sl.No
Heater No
(i)
(iv)
Pressure of bled steam(ksc) Saturated temp corresponding to bled pr. (0C) FW temp at heater outlet (0C) FW inlet temp (0C)
(v)
Drip outlet temp (0C)
(ii) (iii)
TTD (0C) DCA (0C) FW pressure drop (ksc)
HPH5A Run 1 Run 2 16.87 16.76
HPH5B Run 1 Run 2 16.98 16.57
HPH6A Run 1 Run 2 42.08 42.25
HPH6B Run 1 Run 2 41.94 40.76
203.8 9
203.5 6
204.2 1
202.9 9
253.2 2
253.4 6
253.0 4
251.43
204.1 5 168.9 8 172.2 2 -0.26 3.24 2.27
203.8 5 168.9 7 173.6 6 -0.29 4.69 1.08
203.5 7 168.8 6 172.6 9 0.64 3.83 2.27
201.9 0 168.0 4 173.3 7 1.09 5.33 0.94
253.1 6 203.6 3 212.4 3 0.06 8.88 1.21
252.9 1 203.4 6 208.2 2 0.55 4.76 0.97
253.0 6 199.8 6 209.6 8 -0.02 9.82 1.79
251.62 199.28 208.83 -0.19 9.55 1.45
18. FAULT ANALYSIS: An increase in either the TTD or DCA, and/or a decrease in the temperature rise indicate the problem with the heater. This deterioration in the performance could be the result of any or all of the following causes: a. Fouled heater tubes (either steam or water side or both). b. Internal leakage (leakage through the water box partition plate resulting in a partial internal bypassing of the heater, or, tube-to-tube sheet leakage resulting in feed water leaking to the steam side). c. External leakage (through the bypass valve).
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d. Plugged tubes (reducing the heat transfer area, while increasing tube velocity). The detailed Fault tree is shown in the adjoining Fish-Bone diagram drawn.
Fig.7. Cause-Effect Diagram for Feed-Water Heater Performance
TTD is an indication of the ability of the surface to transmit heat under a given set of conditions. This ability is determined by the overall heat transfer coefficient. The principal factors affecting the heat transfer coefficient are: • Tube material, diameter, length & arrangement • Feed water velocity • Tube cleanliness
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•
Non-condensable gases in the steam or water spaces of the heater
Steam usually contains non-condensable gases, which if allowed to accumulate, will affect the performance of the heater. To make certain that such condition does not occur, the vent valves should be adjusted so that sufficient quantity of escapes to cause the temperature rise of the water passing through the heater to remain both maximum & constant. This usually provides adequate assurance that no heating surface is blanketed with non-condensable gases. Tight shutoff is a necessity for these applications to protect the integrity of the valve trim. The normal heater drain should be supplied with tight shutoff for situations where the heater is bypassed and leakage between heaters is possible. Tight shutoff will ensure that damage such as wire draw will be prevented. Heater efficiency will also be maintained as the warm condensate will not be lost to the condenser and additional extraction steam will not have to be taken from the turbine. There are two valves that must be addressed on each heater. These are the normal heater drain valves and the emergency or high-level dump drain valves. Both of these valves can dramatically affect unit efficiency. The normal heater drain valve is used to control the condensate level in the heater and the flow of condensate to the next successive heater. As stated above, the incoming condensate is under saturated conditions. As the condensate passes through the valve, the condensate flashes to steam to aid in the heat transfer process. Flashing of steam can lead to severe erosion effects due to the high velocity steam carrying entrained water droplets. The emergency heater drain can have a similar impact on unit performance. This valve is used only on high level alarms; therefore, tight shutoff is necessary to ensure proper performance. Any condensate that leaks through the valve will go directly to the condenser, which requires additional extraction steam to provide the proper heating. With the outlet flow going directly to the condenser, flashing will occur during valve operation. In order to combat the flashing damage while providing proper control, Fisher recommends the use of an angle valve with a downstream liner. Since flashing is directly related to the downstream pressure, there is no way to eliminate it from occurring. Therefore, it is necessary to protect the valve and associated equipment from any of the damaging effects. The angle valve design directs the flow into the center of the valve away from the valve body and downstream pipe walls.
19. RECOMMENDATIONS: Based on the above discussions, following are the recommendations for feed water heaters: 1. All the HP Heater tubes shall be welded to tube sheets & then roller expanded. LP Heater tubes shall be roller expanded to tube sheet.
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2. The water box or channel section of all heaters shell should be of carbon steel, fabricated or forged construction. Water box should be of hemispherical shape. Sufficient space area should be provided in between water box & tube plate for efficient & smooth entry of feed water & ease maintenance of the tubes. 3. Water box of all the HP Heaters & LP Heaters shall be welded to the tube plate. 4. The shells of all the heaters shall be welded to the tube plate. 5. All connections for drains, feed water & steam at the heaters shall be welded to leak-tight. 6. In the HP Heaters the steam leaving the de-superheater section at the full duty shall be above the saturation temperature by a sufficient margin to ensure that no condensation will occur in the tubes under normal operating conditions & steam leaving the de-superheating section will not cause droplet impingement in the condensing section. 7. To avoid excessive velocity in the drain cooling section during emergency draining operation to condenser & lower heater, a separate drain cooler bypass connection shall be provided on the shell of the heater. 8. All openings on the HP Heater channel shall be self-sealing type. 9. The fouling factors on the tube side & the shell side in different zone shall be as per HEI standards. 10. Periodical checking should be done to avoid any leakage or accumulation of non-condensable gases. For this proper venting has to be ensured during the operating condition of the heaters. 11. Heater water level & operating condition should be maintained as close as possible to the designed values.
20. CONCLUSION: The importance of feed-water heater is judged not only from the role they play in increasing the cycle efficiency, by making the cycle consistently approach towards a regenerative one, but also, from the fact that they reduces thermal stresses in the pipelines & water-wall tubes by increasing the inlet temperature of the drum. Moreover, their performance is a direct indication in saving of equivalent amount of fuel cost, there by adding towards the economic running of the plant. Hence, proper monitoring of the feedwater heater parameters is a must for efficient & economic running of the power plants
21. BIBILOGRAPHY: 1. R.W.Haywood, Analysis of Engineering Cycles, Pergamon Press, Oxford, 1975 2. P.K.Nag, Engineering Thermodynamics, Tata McGraw-Hill, New Delhi, Second edition, 1995 3. J.H.Horlock, Combined Heat & Power, Pergamon Press, Oxford, 1984 4. J.K.Salisbury, Steam Turbines and Their Cycles, John Wiley, New Work, 1950
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22. APPENDIX:
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