P.K.Vasudevan., B.Sc, M.C.A.,D.C.P., Junior Chemist / MTPS.What is Generator chemistry Hydrogen gas is used as a means of removing heat from turbine generators ( rotors) DM water ( Stator water) is used to remove heat from stator coils STATOR WATER COOLING SYSTEM Why Generators are cooled by Stator water ? Since , electron flow in a conductor in Zig Zag path, conductor heats up If temperature increases, resistance value also increases, which in turn limits the voltage In order to increase the current carrying capacity of the conductors To reduce the degrading of stator coils and insulating materials Makeup water Expansion Tank . Makeup water Expansion Tank To Ejector Pump . Makeup water Expansion Tank To Ejector Pump Cooler . Makeup water Expansion Tank To Ejector Pump Cooler Filter / Screen . Makeup water Expansion Tank To Ejector Magnetic Filter Pump Cooler Filter / Screen . Makeup water 5 to 10 % water Expansion Tank POLISHING UNIT To Ejector Magnetic Filter Pump Cooler Filter / Screen . Makeup water Expansion Tank POLISHING UNIT To Ejector GENERATOR Magnetic Filter Pump Cooler Filter / Screen . Makeup water Gas Trap Expansion Tank POLISHING UNIT To Ejector GENERATOR Magnetic Filter Pump Cooler Filter / Screen . Makeup water Gas Trap Expansion Tank POLISHING UNIT To Ejector GENERATOR Magnetic Filter Pump Cooler Filter / Screen . This water is cooled by in an external heat exchangers 600 MW .Where the stator water is cooled? And how it is cooled? The hollow Conductors allow the Stator water to carry away the heat generated by direct contact. What are all the limitations for stator water? Conductivity PH Dissolved oxygen Dissolved Carbon dioxide Copper Pick up Total Iron as Fe Velocity / rate of flow Temperature Pressure . Conductivity What is conductivity? It will reveal the purity of water indirectly I.6 = Total dissolved solids It is the basic criteria of quality for the water It should be maintained in low level to eliminate the possibility of flash over Continuous monitoring of conductivity using online meters .e Conductivity X 0. What are all the factors which will increase the conductivity? circuit Dissolved carbon dioxide Copper and Iron particles due to corrosion Other impurities from makeup water During Exhaustion of polishing unit . 0 µs / cm With polishing unit not > 2.0 µs / cm Unit trip = 21.0 µs / cm High alaram = 13.0 µs / cm Purging and make up with fresh water Using a by pass ion exchange polishing plant.What is the limit & How to maintain the limit? Less than 2 micro siemens / cm circuit Without polishing unit not > 5. . What is Polishing unit? circuit Polishing is generally designed to treat polish 2 – 5 % of total stator water flow The polishing unit contains a bed of mixed resins consisting strong Cation resin in H – form and strong Anion in OH form Cation will remove the positive particles and the Anion will remove the negative particles present in water Resin Resin Structure . Exchange reaction During Operation At Cation R – H + Cu2O R – Cu + H2O At Anion R – OH + H2CO3 R – CO3 + H2O . Where these resins are separated and regenerated then finally mixed up together .After some period of service the polishing unit should be regenerated with suitable Acid and Alkali. The resins in the polishing units are transferred in to another regenerate vessel. Exchange reaction During Regeneration At Cation [ with 5 % acid] R – Cu + HCl R – H + CuCl2 At Anion [ with 5 % alkali] R – CO3 + NaOH R – OH + H2 CO3 . Sodium hydroxide etc.pH The measurement of PH will tell the stator water is weather Acidic or alkaline Acidic PH is due to entry of Chloride or CO2 which are not permitted Alkali PH is due to entry of ammonia.5 . The PH should be around 7 ± 0. O (8.0 to 10 ppm i.e 8 to 10 mg / liter) D. is removed by using ejector and Vacuum tank which will liberate D. to atmosphere .DISSOLVED OXYGEN circuit D.O is the main reason for corrosion D. Of D.O.O normally enters through the makeup water which is high Conc.O. The vacuum in the Tank is maintained around 300 mm Hg Dissolved Oxygen content in the cooling water should be kept at the lowest below 50 ppb to avoid corrosion circuit Copper Corrosion CORR vs pH CORR vs O2 Due to the presence of D.O the copper and Iron will under corrosion Cu + H2O Cu2O / Cuo Fe + H2O Fe2O3 These salts dislodge from the surface and form blockage, electrical shorting and leading further problems Copper Corrosion Cupric Oxide – tenorite – CuO – black compound , Which will be produced in high Oxygen environment Cuprous Oxide – cuprite – Cu2o – reddish black compound , will be found along with cupric oxide in low oxygen environment Total copper should less than 150 ppb When copper Oxide forms Stay where it forms , protecting the metal from further oxidation (Passivation) Break off and transport to and become trapped by inline filters or at a hot area flow restriction Loss of cooling water flow will result in localised hot spots and possible catastrophic Figure Dissolve and redeposit in a hot area. O? Regulate the makeup water flow in order maintain the DO. so that it does not raise abruptly The makeup water can be passed through special de – oxidizing resin column to remove oxygen ( 500 MW) Continuous monitoring of DO level in stator water By Using Oxygen removal membrane .How to control D. The use of vacuum or nitrogen sweep gas will promote the oxygen removal by ensuring low partial pressure of oxygen in the sweep stream. . The oxygen removal membrane system allows dissolved oxygen from demineralised makeup water to escape into the nitrogen sweep gas due to the difference in partial pressure of oxygen between the two phases. DISSOLVED CARBON DIOXIDE The presence of CO2 will decrease the PH and increase the conductivity of the water. The CO2 comes from the atmosphere forms carbonic acid which aids the acid corrosion CO2 + H2O H2CO3 H2CO3 + Cu Cu ( OH) 2 It can release carbonates from the anion resin allowing the formation of insoluble copper carbonate in the stator strands . Maintaining required Vacuum in the expansion Tank Monitoring of CO2 level in the laboratory .Prevention of CO2 As it is coming from the atmosphere avoid contact with the same ( atmospheric air) This can be done by providing sealing pots with lime at the vents of DM water Storage Tanks or by providing plastic balls floating on the surface in the storage tanks. 005 ppm .Copper & Iron Pickup The corrosion products of copper and Iron are formed due to the presence of DO & CO2 Usually it will increase slightly in the start up and maintain steadily below 0. Prevention of Cu & Fe Corrosion Correct quality in DM water & proper Blow down Maintaining the level of DO & CO2 Drawing water directly from the DM plant without adding any chemicals Bi–Monthly measurement of copper & Iron sea water cooled heat exchanger may use a primary cooler with DM water as coolant . & rate of flow Temperature raise will cause the decrease in PH Higher the temperature rise will decrease the solubility of gases Lower the temperature below the limit will cause dew formation in the winding surface. Which will affect the insulation resistance & corrosion .Temperature. Pressure Generator is cooled by Hydrogen gas and water . water pressure will be lower than hydrogen gas pressure to avoid water leak On the other hand gas trap will check any gas leak . The differential pressure must be maintained scrupulously If water pressure is higher than gas pressure there is a possibility of water into casing Starvation in the winding may increase in the conductor Hence . Rate of flow / Velocity The velocity of water must be low enough to prevent erosion It is an important parameter to be monitored. . to be safely within the limits as prescribed by the manufacturers. . Advantages over air The density is lowest of all gases and is ¼ of that of air Lesser windage and friction less due to less density Requires less fan power Increase in efficiency The heat transfer capacity of hydrogen is about twice that of air Increase in pressure increase in out put For every 1.0 ksc pressure increase 0.5% increase out put But more pressure will increase the density of gas in turn will increase the windage loss . Advantages over air High thermal conductivity and specific heat ensures effective heat removal from heated surfaces ( approx. 10 times that of air) Reduction of machine size Reduction of size of hydrogen coolers The degradation of insulation process cannot occur in hydrogen atmosphere Dirt and moisture free atmoshphere Increase in machine life . Dis .Advantages Purity Vs windage loss Complicated control system Gas tight seal oil is necessary Hydrogen is explosive nature when it mixes with air ratio 75:25 or 95 : 5 For evacuation or admission of Hydrogen another medium (Carbon dioxide ) is required . 69 : 510 °C .0156 : 0.0 KSC Thermal conductivity ( air = 1) at 3.Characteristics of Hydrogen Molecular weight Relative density ( air = 1) Boiling point Heat Transfer Co-efficient at 3.4 : 6.06952 : 252 C : 4.0 KSC Ignition Temperature when it Mixes with air : 2. Thermal conductivity of gases related to Air at 100 C Hydrogen sulphide Argon Chlorine Sulphur dioxide Methane Carbon monoxide Carbondioxide Ammonia Water Vapour Oxygen Nitorgen Helium Hydrogen Air 56 70 35 44 141 97 71 106 78 101 98 554 672 100 . while parallel flow passes through the rotor. Four hydrogen coolers are located horizontally inside the casing for cooling the hydrogen gas Regenerative heaters are provided for removing Hyd. coolers moisture from hydrogen gas.Hydrogen gas Circulation The cooler stator core and the rotor conductors are cooled by High purity of hydrogen gas Hydrogen enters the generator casing through an axially – oriented distribution pipe at the top The rotor fans circulate hydrogen over the end windings and through the stator core. . Hydrogen purity Moisture content in hydrogen . 1.Parameters to be maintained The following Two parameters are prime importance. 2. The Hydrogen gas purity can be measured using Orsat apparatus Burrell gas analyser .Hydrogen purity The hydrogen purity should be maintained around 98% (min) in the generator. 99 = 99.ORSAT APPRATUS A known volume of hydrogen is passed through alkaline pyrogallol solution which absorbs Oxygen and the reduction in volumes gives the amount of impurity Example: Oxygen consumption is 0.75(N2) = 0.99 ml H2 Gas purity = 100 – 0.1 % .2 X 79)/ 21 = 0.2(O2) + 0.75 Total = 0.2ml Then Nitrogen volume is(0. 10 97.05 98.33 3.3 Nitrogen ml 0.29 4.24 0.01 3.76 Total ml 0.6 0.9 1 .13 1.67 96.71 95.76 Hydrogen Purity % 99.19 95.95 1.38 2.2 0.63 3.62 97.8 0.43 1.4 0.81 4.86 3.39 3.14 96.88 2.26 2.57 98.50 1.7 0.5 0.75 1.90 2.Oxygen consumption ml 0. Burrell Gas Analyser This is direct method to find Hydrogen purity A known volume of the gas is taken and oxygen present as impurity is allowed to react with catalyst as copper balls The reduction in volume corresponds to the amount of hydrogen present in it . which is carried to the generator.Moisture in Hydrogen gas Because of its low molecular mass it can retain vast amount of moisture. . By gravimetric method a known volume of hydrogen taken and moisture is condensed by some means of absorbents ( P2O5) and weight of the moisture is thus determined. the temperature at which air must be cooled at constant pressure in order for it to become saturated with respect to a plane surface of water" The temperature at which the gas is fully saturated with water vapour If the temperature falls below this. water will deposit on any available surface If dew point if gas is known then the moisture content can be calculated from smithosonian tables (Vap/press) H circulation 2 ...Dew point of gas What is dew point? table ". What are the limits? Limit for moisture =15 g/m3 at 44 oC 3.5 Kg/cm2 gauge pressure of H2 The dew point of hydrogen in generator should be maintained below 0 °C. If dew point reaches 10 °C it should be purged with fresh hydrogen If the dew point reaches 18 °C. serious consideration should be given (trip) . Generally dew point should be less than 32 °F Hyd.CEGB RECOMMENDATION High humidity can induce stress corrosion cracking on retaining rings and cause stator winding shorts The dew point of hydrogen ( at casing pressure) must be 20 °C lower than the temperature of the cooled hydrogen emerging from the coolers. coolers . pressure. no diff.P. and there is no means to circulate hydrogen through drier .What are the care should be taken? Maintain the D. changed when required and regenerated After each cycle or regeneration moisture should be properly purged When unit under shut down .of water & H2 Gas H2 at each cooler out let is 5 o C cooler than coldest part of the stator water flow Hydrogen drier should be in service continuously. 05 % W/W ( 500 ppm) . Continuous monitoring using Dew point meter is recommended Drier performance should be checked The commonest cause of moisture ingress to the hydrogen is Seal oil Hence. Hence . all the vapour ejectors and seal oil vacuum pumps should be in service to minimize moisture transfer from seal oil. moisture in seal oil should be below 0. . . Burrell Hydrogen gas Analyser Hydrogen gas fill line Electric furnace Water Seal Copper balls ( Catalyst) . Vent Filled with lime DM Water Storage Tank Lime + Carbon di oxide Calcium Carbonate . Valve Volume 100 ml Zero mark Hydrogen gas inlet Alkaline Pyrogallol solution Water Seal Orsat apparatus . Ph Neutral . Stator water circuit Makeup water Gas Trap Expansion Tank POLISHING UNIT To Ejector GENERATOR Magnetic Filter Pump Cooler Filter / Screen . . . Return end ( Exciter end) . Stator water in let and out let line ( turbine end) . . . . Common Salts present water . . . CATION RESIN . . . Gravity .(During Regeneration) Sp. Sample Table Temperature Dew Point Relative Humidity 80 80 =100 % 90 100 80 80 =67% = 47% 110 80 =33% . T . System Storage Tank Pressure filter Cation Exchanger Pump DM water Tank Anion Exchanger Mixed Bed Pump C.O.De Gasser R.S. Water from cation out let Co2 WILL BE REMOVED AIR COMPOSITION = 21 % OXYGEN + 79 % NITROGEN Air from blower DE – GASSER TOWER DE CAROBONISED WATER SATURATED WITH OXYGEN . Mg++. Na+ Polishing Unit Pure water with out any salts . Ca++. Fe++.Cu++. Cu++. Na+ R – Na + Cu2O R – Cu + NaOH Polishing Unit All the salts in the form of Sodium (Na+) . Fe++. Mg++. Ca++. Specific Gravity • Specific Gravity of any liquid = Density of the liquid/ Density of water ( in Weight / Volume) Example: Specific gravity of oil = (950 grams/liter) / ( 1000 grams/liter) = 0.950 ( no units) .