Feed Heaters Performance

Feed Heaters & Condenser Performance

By:

M.V. Pande Director NPTI, Badarpur

Steam St eam & Feed Feedwa watter Cy Cycle cle

Effect of no. of feed-water heaters on thermal efficiency of the cycle

Vital Measures of an Operating Heater •

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Terminal temperature difference (TTD) = inlet steam saturation temperature -Feedwater outlet temperature Drains cooler approach (DCA) = shell drains outlet outlet temper temperatur ature e - feedwat eedwater er inlet inlet temper temperatur ature e Feedwater Feedwater temperature temperature rise (TR) = feedwa feedwater ter outlet outlet temper temperatur ature e - feedwat eedwater er inlet inlet temper temperatur ature e

Zones of Feed Water Heaters

Zones of Feed Water Heaters

Horizontal Feed Water Heater

Vertical Feed Water Heater

HP Heater

HP Heater

HP Heater Installation

Key Performance Indicator •

TTD - Terminal Tempera Temperature ture Difference Difference

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TTD = TS TS - FW OUT OUTLE LET T TEMP TEMP TS saturation temperature corresponding to shell pressure

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DCA

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TR

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PRESSURE DROP

Thermal profile in different zones of H P HEATER

High TTD Causes

TTD and Feed Water Temp.

High Hig h Drain Drain Cool Cooling ing Appr Approac oach h - DCA

(Level control valve )

Temperature Profile of a closed closed Feed Water Heater

Effect of HP Heaters TTD on NTHR (Net Turbine Heat Rate)

Effect of LP Heaters TTD on NTHR (Net Turbine Heat Rate)

Effect of DCA on NTHR (Net Turbine Heat Rate)

Low Temperature Rise TR = FW outle outlett temp temp - FW inlet inlet temp temp

Feed Heaters Survey •

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Feed heaters survey evaluate the performance of heaters & predicts the deterioration deterioration causes Following parameters are noted down - Steam Steam press pressur ure e at heat heater er - Steam Steam temper temperatur ature e at at heater heater - Feed Feed water water inlet inlet & outlet temper temperatur ature e - Heater Heater drai drain n temper temperatur ature e

Feed Heaters Survey  Evaluate the steam flow to each heater by heat balance  Compare the flow values with optimum Calc ulate T.T.D. for each heater  Calculate  The results indicate following problems - The elevated elevated TTDs on the heater heater train suggests water water side contamination (oil) - The high steam steam flow to to particular heater heater may may be due to lower feed water inlet temperature,suggesting the problem in previous heater

Sample Sampl e Data Data for for Feedw Feedwat ater er Hea Heate terr

Calculations

Deterioration • Air accumulation • Steam side fouling • Water side fouling • Drainage defects • Parting plane leakage

Air accumulation • Increased TTD • Possible elevation of steam-to-heater

temperature • Reduced temperature rise of feed water or

condensate. • 0.5 % steam is venting inevitable for good

venting

Steam side fouling • Progressive increase of TTD • Drain temperature unaffected • Reduced feed water temperature rise

• Eventual tube failure due to mechanical

weakening • Accumulation of debris in the heater shell.

Water side fouling •

Gradual increase of TTD. T TD.

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Oil

 – LPT bearing oil through seals

heaters, worst hit at  – Deposition occurs in HP heaters, highest pressure heater

Drainage def defects ects •

Damage Damaged d flsahbo flsahbox intern internals als

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Reduced orifice opening

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Enlarged orifice opening

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Heater drain CV/ bypass valve malfunction.

Parting plane leakage • Short circuiting of FW • TTD high • DCA high

• TR less

HP Heat Heater er Perf Performan ormance ce Repor Reportt - 210 MW MW

Partition Plane Damage

Heat Exchanger Tubes Scaling

SURFACE CONDENSER

EFFECT OF VARYING THE BACK PRESSURE •

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A large amount of the extra work is done by the steam, when the back pressure is reduced. However, the trouble is that as the back pressure improves, certain losses also increase1) CW Pumping Power. 2) Leaving losses. 3) Reduced condensate Temperature. 4) Wetness of the steam.

Increased CW Pumping Power •

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Assuming that the CW inlet temperature is low enough, the back pressure can be reduced by putting more and more CW through condenser tubes. However, this will require more CW pumping power and the gain from improved back pressure must be offset against extra power absorbed by the pumps. So, the CW pumps should be run only when the cost of running the pump is equal to, or less than the gain in output from the machine.

Increasing leaving loss •

The steam leaves the last row at a velocity which depends upon the conditions prevailing at the point. As this velocity is not utilized usefully, usefully, it represents a loss of possible work known as the leaving loss. So velocity steam through fixed annulus must also double. But leaving losses varies as a s square of the velocity. velocity. So it will increase four times.

Reduced condensate temperature / increased bled steam •

The condensate in the condenser is at saturation temperature temperature corresponding to the back pressure. It back pressure is reduced, saturation temperature temperature will drop. When it enters enters first LP heaters it will be cooler than before before consequently more steam will automatically be bled to the heater. The extra steam is no longer available to do work in the turbine will be deprived of some work.

Increase wetness of the steam •

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The lower the back pressure, the greater the wetness of steam. The extra moisture could result in damage to the moving blade. Also with increased wetness, volume of steam is reduced water droplets being heavier than steam moves slowly. So the front edge of moving blades have to push the droplets out of the way. This can cause damage to blades. Therefore, Therefore, it is usual to fit satellite erosion shields to the leading edge to reduce this damage. As a rough guide, it can be assumed that every 1% wetness will reduce efficiency of associated stage by 1%. The reduction in back pressure will result in net improvement in heat consumption until a point is reached beyond which benefit due to improve back pressure is outweighed by the losses and heat consumption increase.

Blades erosion damage due to wet steam

Condenser Low Vacuum Causes

Vacuum Efficiency & Condenser Condenser Efficiency Vacuum Efficiency  It is the ratio of the actual vacuum at the steam inlet to the maximum obtainable vacuum in a perfect condensing plant, i.e., it is the ratio of actual vacuum to ideal vacuum. •

Condenser Ef Efficiency ficiency  In thermal power plants, the purpose of a surface condenser is to condense the exhaust steam from a steam turbine to obtain maximum efficiency, and also to convert the turbine exhaust steam into pure water (referred to as steam condensate) so that it may be reused in the steam generator generator or boiler as boiler feed water. •

Condenser Condition Graph

Deviation due to CW inlet temperature •

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Plot a line vertically from the actual CW inlet temperature to intersection with the optimum CW rise. Then plot horizontally to the intersection intersection with optimum terminal difference difference (TTD) line, and then vertically downwards to cut the saturated steam temperature line to obtain the corresponding back pressure. (Refer fig in previous slide) Hence the loss due to the high CW inlet temperature can be calculated by subtracting the optimum value from the actual back pressure pressure

Deviation due to C.W. flow •

Plot a line from the actual CW inlet temperature temperature vertically to the intersection intersection with actual CW rise Then plot horizontally to the optimum TTD T TD,, then vertically downward to the saturation steam temperature to obtain the actual back pressure. the difference difference between the actual back pressure and the optimum back gives the loss due to the incorrect CW flow.

Deviation due to air/dirty tubes •

The effect of the air and dirty tubes on the heat transfer is to increase the TTD above optimum. As they both give the same effect , they are clubbed together in this exercise. Plot line from the actual CW inlet temperature to the actual CW rise and then across the actual TTD line plotting vertically downwards to adja djacent steam tempe mperatur ture bac back pressu ssure. So the deviation due to air/dirty tubes can be found out.

CASE STUDY Khaparkheda Khaparkheda Thermal Thermal Power Power Station Station (MSPGCL) (MSPGCL)

Design Data. Condenser tubes specification specification 1. Tube diameter (outside diameter) 25.4mm 2. Tube thickness 1.0mm 3. Tube diameter ( inside diameter) 23.4mm 4. Tube length 7.5m 5. Number of tubes 19208 6. Cross sectional area per tube t ube 430.05m3 7. Surface area per tube 0.60m2 8. Total surface area of tube 11495 m2 9. Specific heat of CW 1.00kcal/kg/0C

10. Density of CW 1000kg/ m3 11. Condenser vacuum 650.06mm of Hg 12. Condenser back pressure 0.09kg/cm2 13. Sat. temperature at condenser back pressure 43.6 oC 14. Average temperature of CW inlet 30.500C 30.50 oC 15. Average temperature of CW at outlet 39.20 oC 16. CW temperature rise across condenser 8.79 oC 17. Terminal temperature difference 4.40 oC

Calculation for the TEST-1 1) If all values of C.W. Inlet (C.W.), C.W. Rise (C.W.R) ,T.T.D. are as per design then C.W.I+C.W.R.+T.T.D. = 30.5 + 8.79 +4.4 = 43.7 = 89.11 mbar 2) If only inlet water temperatur temperature e is actual ac tual then (C.W.I)a.+C.W.R.+T.T.D.=29 +8.79 +4.4 = 42.2= 82.46 mbar 3) If both the inlet temperature temperature and temperature temperature rise is actual then (C.W.I)a.+C.W.R)a.+T.T.D.=29 +10.2 +4.4 = 43.6 = 89.11 mbar 4) If all parameters parameters are actual then (C.W..I)a.+(C.W.R)a.+(T.T.D)a=29+10.2 +7.1 =46.3 = 102.4 mbar Now effect of various parameters is as a follows #1) Cooler Circulating Water improves vacuum by (82.46 (82.46 -89.11) -89.11) = -6.65 -6.65 mbar = - 5 mm of Hg. #2) Dirtiness of tubes deteriorates deteriorates vacuum by (89.11 (89.11 - 82.46) 82.46) = 6.65 mbar mbar = 5 mm of Hg. #3) Air ingress deteriorates deteriorates vacuum by

Conclusion: #1) Inlet temperature is slightly lower than design value and hence little improvement in vacuum by 5 mm of Hg. #2) Tubes are dirty which deteriorates vacuum by 5mm of Hg. #3) There is a little air ingress which causes vacuum deterioration deterioration by 10mm of Hg.

Calcula Calc ulation tion for for the Test Test - 2 1) If all values of C.W. Inlet C.W. Inlet (C.W.), C.W. Rise (C.W.R) , T.T.D. are as per design thenC.W.I.+C.W.R.+T.T.D. =30.5 +8.79+4.4 =43.7 = 89.11 mbar 2) If only inlet water temperatur temperature e is actual a ctual then (C.W.I)a. +C.W.R.+ T.T.D.= 22 + 8.79 + 4.4 =35.19 = 56.6 mbar 3) If both the inlet temperature temperature and temperature temperature rise is actual then (C.W.I)a.+(C.W.R)a.+ T.T.D.=22 + 10 +4.4 =36.4 =61.18 mbar 4) If all parameters parameters are actual then (C.W.I)a.+(C.W.R)a.+(T.T.D)a= 22+10+7.9=39.9=73.15 mbar Now effect of various parameters is as a follows #1) Cooler Circulating Water improves vacuum by (56.6 - 89.11) 89.11) =-32.5 =-32.5 mbar =-24.4 =-24.4 mm of Hg. #2) Dirtiness of tubes deteriorates deteriorates vacuum by (61.18 (61.18 - 56.6) = 4.58 4.58 mbar =3 mm mm of Hg. #3) Air ingress deteriorates deteriorates vacuum by (73.15 (73.15 - 61.18) 61.18) = 12 mbar mbar = 9 mm of Hg. Hg.

Conclusion #1) Inlet temperature is slightly lower than design value and hence little improvement in vacuum by 24.4 mm of Hg. #2) Tubes are dirty which deteriorates vacuum by 3mm of Hg. #3) There is a little air ingress which causes vacuum deterioration deterioration by 9mm of Hg.

Calcul Cal cula atio tion n for for the TES TEST T-3 1) If all values of C.W. Inlet (C.W.), C.W. Rise (C.W.R) , T.T.D. are as per design then C.W.I.+C.W.R.+T.T.D.=30.5 + 8.79 +4.4 = 43.7 89.11 mbar 2) If only inlet water temperature is actual then (C.W.I)a.+C.W.R.+T.T.D. =27.82 + 8.79 +4.4 =41 = 73.5 mbar 3) If both the inlet temperature and temperature rise is actual then (C.W.I)a.+(C.W.R)a.+T.T.D.=27.82 + 11 +4.4 = 43.2 = 87.11 mbar 4) If all parameters are actual then (C.W.I)a.+(C.W.R)a.+(T.T.D)a=27.82+11+15.4 = 54.22 = 151.62 mbar Now effect of various parameters is as follows #1) Cooler Circulating Water improves vacuum by (73.5 - 89.11) 89.11) = -15.61 -15.61 mbar = -11.74 -11.74 mm of of Hg. #2) Dirtiness of tubes deteriorates vacuum by (87.11 (87.11 - 73.5) = 13.61 13.61 mbar =10.23 =10.23 mm of Hg. #3) Air ingress deteriorates vacuum by (151.62 (151.62 - 87.11) 87.11) = 64.51 mbar mbar = 48.5 mm of Hg.

Conclusion #1) Inlet temperature is slightly lower than design value and hence little improvement in vacuum by about 13 mm of Hg. #2) Tubes are dirty which deteriorates vacuum by 10.23 mm of Hg. #3) There is a little air ingress which causes vacuum deterioration by around 50 mm of Hg