55108777 HP Feed Water Performance

7/29/2019 55108777 HP Feed Water Performance

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1. TITLE:H.P. Feed-Water Heater Performance

2.ABSTRACT:

Facing to the paradoxical situation of increasing the efficiency of power plant atlesser 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 correctiveaction 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 ofabnormal 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 discussedbriefly. 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 ofincreasing 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 flowingthrough the economizer section of a boiler. For example, by increasing the feed-watertemperature 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, condensingzone 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 notapplicable. 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 eachheater has a considerable amount of energy and is also used to heat the incoming feed-

water. 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 arrangementfound 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 isused 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 anopen 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. Thedeaerator provides the suction for the main boiler feed-pumps, which sends the feed-

water 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 thehigh pressure and low-pressure heater arrangement.

Fig.2. Arrangement of feed-water heaters in a Power Plant

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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 thetemperature 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 waterheaters, 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 thesame 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 are-1. 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 condensatepumps & 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 isDeaerator.

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 beingused 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, LPHeaters & 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 designcriteria 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 becomputed 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 LPHeaters & 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 maximumdesign 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 waterinlet 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 isprovided 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 condenserdesign temperature. The design temperature of water-box of the heaters shall be the

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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 operatingconditions. However, the use of SS304 tubes permits that tube velocities may bemaintained 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 beincreased 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-h4h2-h5 h2-h4Therefore, the thermal efficiency of the cycle is

= 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 = , where h is the local enthalpy on the expansion line at a given pressure, &

hfis the enthalpy of saturated water at that pressure. Therefore, as seen from the Fig.3.,

h1-h8 = h2-h6= h3- h4 = = constant.

Let the enthalpy rise of feed water in the heater is , which is equal to (h6-h4).Now, h2-h4 = h2-h6+h6-h4 = +

If the total enthalpy rise of feed water is equal to = h8-h4, then,

h1-h6 = h1-h8+h8-h4+h4-h6 = +-Therefore, equation can be written in the form

= 1- 2_____

[(+)(+-)]

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Here, & are fixed & is a variable. So, there is an optimum value of for

which is maximum. On differentiation,

d = 2[(+-)-(+)] =0d

Or, = /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,t = (tboiler saturation tcondenser)

& the corresponding cycle efficiency is

= 1- 2 = 1 - 2 = 2 + 4__

[(+/2)(+-/2)] (+/2)2 (+2)2

For a non-regenerative cycle,

0 = 1 - (h3-h4)

(h1-h4)

Now, h3-h4 = & h1-h4 = h1-h8+h8-h4 = +0 = 1- = __

(+) (+)The efficiency gain due to regeneration,

= - 0 = 2 + 4 _ = 2____

(+2)2 (+) (+)(+2)2

This indicates that is a positive quantity, justifying the fact that due to

regeneration the cycle efficiency increases.

Fig.5. Heater train of a steam power plant ( = /2)

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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 isheated 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. Theenthalpy rise per heater (including the economizer) is thus,

hper 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

htotal = n (hG-h3)

(n+1)Thus, the total temperature rise of feed water, tfw, due to regeneration for the

maximum cycle efficiency is given by

tfw = n tOA

(n+1)where the overall temperature difference is given by

tOA = boiler saturation temperature-condenser temperatureMore is the number of heater, more is the total temperature rise of the feed water,

tfw , 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, tfw0 = 0

If n=1, tfw1 = tOAIf n=2, tfw2 = tOAIf n=3, tfw3 = tOAIf n=4, tfw3 = 4/5 tOA & so on.

By the use of first heater, the gain is tfw1 - tfw0 = tOABy the use of second heater, the gain is tfw2 - tfw1 = tOA - tOA = 1/6 tOABy the use of third heater, the gain is tfw3 - tfw2 = tOA - tOA = 1/12 tOABy the use of fourth heater, the gain is tfw4 - tfw3 = 4/5 tOA - tOA = 1/20 tOA& 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 theincrease 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 reducesthe 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 opencondition, 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 allabnormal 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 ofdesign. 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 & HPHeaters 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 cleaningafter short intervals. Copper alloys are also affecter by ammonium sulphide & oxygen inthe 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 operatingconditions, 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 openedduring 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 corrosionresistance 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 maintaindifferent 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 theeconomizer 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 tubeswith 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 forDrain Cooler & LP Heater. The shape of the water box can be cylindrical or

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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 beheld 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 bewelded 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. Ininstallations 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 willbe 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 de-superheating 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 carefulconsideration 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.

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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 & shellwall 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 thedraining 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-superheatingsection, 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 theymust 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 relativelysimpler. 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

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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 thebottom 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 beimplemented 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 draincooler 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 thesaving 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 theheater 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 theturbine-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 besufficient 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 differentialsbetween 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:

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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 thevalve to position itself without hunting. In a vertical heater installation, however the

surface capacitance of the drain fluid is much reduced, resulting in potentially rapidfluctuation of level during unit load changes. In order to compensate for this rapid flowchanges 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 throughthe 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 principaladvantage associated with the vertical type feed water heaters, is the considerablereduction 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 baymust be utilized for other purpose.

Horizontal high-pressure heaters are usually placed in the operating floor & lowpressure 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 heaterslocated 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 convenientaccess & 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 terminaltemperature 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 specifiedfrequency. This will help in identifying the level of deviations & trending theperformance.

15. H.P. HEATER DESIGN DATA:

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Station: TSTPP Unit: 5

Sl. No. Description Unit Values

1 Unit Capacity MW 500

2 Heater Position (Horizontal/Vertical)

Horizontal

3 HP Heater No. HPH-5 HPH-6

4 No. of Zones (De-

superheating, Condensing,

Drain Cooling

3 3

5 Surface Area m2 1063 1278

6 Extraction steam pressure kg/cm2 16.88 85.5

7 Pressure drop (water side) kg/cm2 0.85 0.85

8 Operating level

(Minimum /Normal/

Maximum)

mm -625/ -540/

-110

-625/ -540/

-110

9 Terminal Temp. difference 0C - 0.3 - 0.3

10 Drain Cooler Approach

temp.

0C 4.8 4.8

11 Temperature rise 0C 37.83 50.13

HP HEATER 5A & 5B HP HEATER 6A & 6B

Description SHELL SIDE TUBE SIDE SHELL SIDE TUBE SIDE

Medium Steam & Drain Feed Water Steam & Drain Feed Water

Design Pr. (Kg/cm2) 24 & Full

vacuum

330 & Full

vacuum

57 & Full

vacuum

330 & Full

vacuum

Design Temp.( C) 224 224/244 273 273/293Test Pr.(Kg/cm2) 36 495 85.5 495

Test Temp. ( C) Ambient Ambient Ambient Ambient

Flow Quantity (T/Hr) 42.591 749.267 78.068 749.267

Inlet Temp.( C) 412.583 165.5 335.25 203.27

Outlet Temp.( C) 170.3 203.33 208.1 191.927

No of passes/Zones 3 2 3 2

No.of Tubes --------- 1347 ----------- 1347

Tube size (O.D. x

Thickness),mm

--------- OD x 13

BWG min

----------- 15.88 x 13

BWG min

MATERIALS

Shell/Channel SA 516 Gr 70 /SA 387 Gr.12CL.1 SA 516 Gr 70

Tubes SA 688 TP 304 SA 688 TP 304

Tube plate(s) SA 350 LF2 SA 350 LF2

Flanges SA 105 SA 105

Nozzles(Tube side) SA 106 GR. B SA 350 LF2

Nozzles(Shell side) SA 182 F11/SA Gr B. SA 350 LF2 & SA 106 Gr B

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16. H.P. HEATER TEST DATA:

Station: TSTPP Unit:5 Test Date: 09/08/05

Sl.

No.

Measurement Unit Run 1 Run 2

1 FW temp. at HPH-5 inlet(5A/ 5B) 0C 168.98 168.86 168.97 168.04

2 Pr. of FW at HPH-5 inlet(5A/ 5B) kg/cm2 200.84 200.77 197.43 198.17

3 FW temp. at HPH-6 inlet(6A/ 6B) 0C 203.63 199.86 203.46 199.28

4 Pr. of FW at HPH-6 inlet(6A/ 6B) kg/cm2 198.03 197.97 197.22 196.98

5 FW temp. at HPH-5 outlet(5A/ 5B) 0C 204.15 203.57 203.85 201.90

6 Pr. of FW at HPH-5 outlet(5A/ 5B) kg/cm2 198.57 198.50 196.35 197.23

7 FW temp. at HPH-6 outlet(6A/ 6B) 0C 253.06 250.06 252.91 251.62

8 Pr. of FW at HPH-6 outlet(6A/ 6B) kg/cm2 196.82 196.18 196.27 195.53

15 HPH-5 level (5A/ 5B) mm -543.6 -550.4 -547.2 -553.2

16 HPH-6 level (6A/ 6B) mm -555.7 -544.6 -549.7 -556.9

5 HPH-5 shell pressure(5A/ 5B) kg/cm2 16.87 16.98 16.76 16.576 HPH-6 shell pressure(6A/ 6B) kg/cm2 42.08 41.94 42.25 40.76

7 HPH-5 extraction temp. (5A/ 5B) 0C 418.97 416.36 416.69 414.13

8 HPH-6 extraction temp. (6A/ 6B) 0C 339.69 332.34 332.20 327.16

9 HPH-5 extraction pressure(5A/ 5B) kg/cm2 16.88 17.00 16.78 16.57

10 HPH-6 extraction pressure(6A/ 6B) kg/cm2 42.10 41.94 42.27 40.87

11 HPH-5 drain temp. (5A/ 5B) 0C 172.22 172.69 173.66 173.37

12 HPH-6 drain temp. (6A/ 6B) 0C 212.43 209.68 208.22 208.83

CONTROL ROOM READINGS:

Station: TSTPP Unit: 5 Test Date: 09/08/05

Sl.No. Description Test Readings

Units Run 1 Run 2

1 Load MW 499.5 485.6

2 Main steam temp. 0C 532.37 539.67

3 MS pressure kg/cm2 165.78 164.86

4 Feed water Flow T/hr 1586 1578

5 SH attemperation flow T/hr 23.21 17.32

6 RH attemperation flow T/hr 11.34 4.63

7 BFP disch. hdr. pr. kg/cm2

201.4 201.98 Condenser vaccum mm of Hg. -0.91 -0.91

9 Barometric pr. kg/cm2 1.01 1.01

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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) isdefined 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 theheater 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 HPH5A HPH5B HPH6A HPH6B

Run 1 Run 2 Run 1 Run 2 Run 1 Run 2 Run 1 Run 2

(i) Pressure of bled

steam(ksc)

16.87 16.76 16.98 16.57 42.08 42.25 41.94 40.76

(ii) Saturated temp

corresponding to bled pr.

(0C)

203.8

9

203.5

6

204.2

1

202.9

9

253.2

2

253.4

6

253.0

4

251.43

(iii) FW temp at heater outlet

(

0

C)

204.1

5

203.8

5

203.5

7

201.9

0

253.1

6

252.9

1

253.0

6

251.62

(iv) FW inlet temp (0C) 168.9

8

168.9

7

168.8

6

168.0

4

203.6

3

203.4

6

199.8

6

199.28

(v) Drip outlet temp (0C) 172.2

2

173.6

6

172.6

9

173.3

7

212.4

3

208.2

2

209.6

8

208.83

TTD (0C) -0.26 -0.29 0.64 1.09 0.06 0.55 -0.02 -0.19

DCA (0C) 3.24 4.69 3.83 5.33 8.88 4.76 9.82 9.55

FW pressure drop (ksc) 2.27 1.08 2.27 0.94 1.21 0.97 1.79 1.45

18. FAULT ANALYSIS:

An increase in either the TTD or DCA, and/or a decrease in the temperature riseindicate the problem with the heater. This deterioration in the performance could be theresult 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 apartial 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 valvetrim. 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 additionalextraction steam will not have to be taken from the turbine.

There are two valves that must be addressed on each heater. These are the normalheater 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 tocontrol 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 steamto 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 thecondenser, which requires additional extraction steam to provide the proper heating. With

the outlet flow going directly to the condenser, flashing will occur during valveoperation.

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 waterheaters:

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 betweenwater 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 tubeplate.

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 bewelded 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 operatingconditions & 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 coolerbypass 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 accumulationof 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 inincreasing 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, theirperformance 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 feed-

water 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, 19752. P.K.Nag, Engineering Thermodynamics, Tata McGraw-Hill, New Delhi, Second

edition, 1995

3. J.H.Horlock, Combined Heat & Power, Pergamon Press, Oxford, 19844. J.K.Salisbury, Steam Turbines and Their Cycles, John Wiley, New Work, 1950

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22.APPENDIX:

55108777 HP Feed Water Performance

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