BEST OPERATING & MAINTENANCE PRACTICES RELATED TO 21 MW & 16 MW STEAM TURBO-GENERATORS Dr T. G. Sundara Raman Seshasayee Paper & Boards Limited IPPTA Zonal Seminar Best Maintenance Practices in Pulp & Paper Mill to Improve ProfitabilityPune 18 th July

Steam turbo-generators Generator Winding Section Power generation enhancement is directly related to increase in current produced [I**2] In turn, it heats up the core winding [stator and rotor]. It results in winding temperature increase. Heat is dissipated to air in closed circulation on the exterior enclosed air duct. Hot air in turn is cooled by cooling water flowing inside the finned tubes of the heat exchanger. The cycle continues with windings being kept below a set temperature with alarm set at 85 deg C and trip at 95 deg c.

Steam turbo-generators in SPB –Erode Unit CPP- 21 MW Double Extraction Condensing Steam Turbo-generator of BHEL Chemical Recovery Complex- 16 MW Extraction Back Pressure Steam Turbo- Generator of BHEL

Power input rating of the Generators Rated Power 21 MW16 MW Power Factor 0.80 Power input [MVA]

CASE MW STEAM TURBO-GENERATOR

. 21 MW STG Air Water Circuit -Existing

21 MW Steam Turbo-GENERATOR Temperature record of Generator Winding and other related temperatures PF : High [ ] Before Cleaning of HX After Cleaning of HX Temperature reduction,°C Date/Time05/06/14: 14 h19/06/14 : 20 h P.F Power GenerationMW15.3 Winding Temperature # 1 CC # 2 CC --- # 3 CC # 4 CC # 5 CC # 6 CC # 7 CC # 8 CC Cold air temperature CC 54/56496 Hot air temperature CC 65/6658/597 CW in temperature CC CW out temperature CC 42393

CASE MW STEAM TURBO-GENERATOR16 MW STEAM TURBO-GENERATOR

16 MW STG Air Water Circuit -Existing

Power input rating of the Generators Rated Power 21 MW16 MW Power Factor Power input [MVA]

16 MW Steam Turbo-GENERATOR Temperature record of Generator Winding and other related temperatures IMPACT OF HIGH & LOW PF ON WINDING & AIR TEMPERATURES Power Factor Temperature increase,°C Date/Time13/06/1411/06/14 Cleaning of HXAfter Power GenerationMW Winding Temperature # 1 CC # 2 CC # 3 CC # 4 CC # 5 CC # 6 CC Cold air temperature CC 42/4144.6/44.43 Hot air temperature CC 59.6/ /58.83 CW in temperature CC 2728 CW out temperature CC 3435 ΔT : 7°C

21 MW Generator Issue : High Winding Temperature High Cooling water inlet temperature of ~ °C due to poor cooling tower performance. Cooling water chemical treatment need to be improved & should be commensurate with the operating COC. All the 8 coolers are on line.

16 MW Generator Issue : High Winding Temperature CW temperature differential across generator had gone up from 4°C to 7°C. More than heat dissipation, probably cooling water flow through generator cooler would have dropped. CW flow restriction needs to be looked into & cleared.

Winding temperature increase Due to Stator & Rotor Core heat losses directly proportional to square of increasing current Copper & Iron Core heat loss with increase in Winding temperature associated with the above Frictional Heat & Field Losses Reduced Power factor results in Reactive Power loss Harmonic current heat losses

Heat loss reduction achieved through Increase in Power Factor Reduced Generation load Harmonic loss reduction

Winding temperature reduction Achieved through - Lowered hot air temperature Lowered cold air temperature Lowered inlet cooling water temperature High cleanliness of air ducts & HX exterior Minimal dirt/deposits over the winding coils Ensuring Quality cooling water devoid of suspended impurities & foulant ( scales).

MEASURES ADVOCATED Quality cooling water to be ensured Periodic cleaning of heat exchanger tubes ( water- side) Occasional cleaning of air side/duct off dust/dirt Occasional cleaning of winding coil exterior ( during annual shut of the STG), if there is approach. Cooling tower & chemical conditioning of CW to be in order.

CONCLUSIONS & RECOMMENDATIONS Quality cooling water to be ensured Periodic cleaning of heat exchanger tubes ( water-side) Occasional cleaning of air side/duct off dust/dirt Occasional cleaning of winding coil exterior ( during annual shut of the STG), if there is approach.

CONCLUSIONS & RECOMMENDATIONS Ensure desired rate of cooling water flow ( say to maintain 2.5 to 3.5 Deg C temperature differential) Cooling tower & chemical conditioning of CW to be in order. High Power factor leads to lowered heat losses from the Generator resulting in reduction in Winding temperatures. All of the above if practiced on a sustained basis would ensure longevity of the Generator unit, thereby resulting in 100% availability of the same.

GALVANIC CORROSION OF HX & MITIGATIONAL MEASURES

Cathodic Protection Tube sheet and finned tubes are of different materials.Tube sheet and finned tubes are of different materials. Hence there will be Galvanic Corrosion due to high Electrochemical Potential difference between the materials resulting in corrosion of tube sheet.Hence there will be Galvanic Corrosion due to high Electrochemical Potential difference between the materials resulting in corrosion of tube sheet. Hence Zinc anode ( sacrificial anode) pieces are installed over the tube sheet cover to ensure longevity life of tube sheet/tubes.Hence Zinc anode ( sacrificial anode) pieces are installed over the tube sheet cover to ensure longevity life of tube sheet/tubes.

WAY FORWARD For ensuring Winding temperatures to be well within the recommended limits, be it at full load or low power factor operation of the Generator, both water and air sides are to be cleaned off deposit/silt/foulant on sustained basis.For ensuring Winding temperatures to be well within the recommended limits, be it at full load or low power factor operation of the Generator, both water and air sides are to be cleaned off deposit/silt/foulant on sustained basis. As for cooling water, the quality should be good devoid of foulant; the cooling water temperature should be preferably <34°C & the flow should be maintained to have low temperature difference (say 3 to 4 °C).As for cooling water, the quality should be good devoid of foulant; the cooling water temperature should be preferably <34°C & the flow should be maintained to have low temperature difference (say 3 to 4 °C). Zinc bars attached to the tube sheet should be checked at any available opportunity to be healthyZinc bars attached to the tube sheet should be checked at any available opportunity to be healthy The successful operating concept shall be extended to all Generator Air water Heat exchangers for increased ProductivityThe successful operating concept shall be extended to all Generator Air water Heat exchangers for increased Productivity

. 27. Good Maintenance Practice Ensures Productivity