{"id":17864,"date":"2018-05-18T18:12:10","date_gmt":"2018-05-18T18:12:10","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=17864"},"modified":"2022-11-16T16:32:54","modified_gmt":"2022-11-16T16:32:54","slug":"principle-of-operation-of-turbine-generator","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-power-plant\/turbine-generator-power-conversion-system\/principle-of-operation-of-turbine-generator\/","title":{"rendered":"Principle of Operation of Turbine Generator"},"content":{"rendered":"
<\/div>\n

Most nuclear power plants<\/strong> operate a single-shaft turbine-generator<\/strong> that consists of one multi-stage HP turbine<\/strong> and three parallel multi-stage LP turbines<\/strong>, the main generator and an exciter. HP Turbine<\/strong> is usually a double-flow impulse turbine (or reaction type) with about 10 stages with shrouded blades and produces about 30-40% of the gross power output of the power plant unit. LP turbines<\/strong> are usually double-flow reaction turbines with about 5-8 stages (with shrouded blades and free-standing blades of the last 3 stages). LP turbines produce approximately 60-70% of the gross power output of the power plant unit. Each turbine rotor is mounted on two bearings, i.e., there are double bearings between each turbine module.<\/p><\/div><\/div>

<\/iframe><\/div><\/div><\/div>
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\"Steam<\/a>
Schema of a steam turbine of a typical 3000MWth PWR.<\/figcaption><\/figure>\n<\/div><\/div>
<\/div>\n
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From Steam Generator to Main Steam Lines – Evaporation<\/h2>\n
\"Steam<\/a>
Steam Generator – vertical<\/figcaption><\/figure>\n

The power conversion system of typical\u00a0<\/span>PWR<\/span><\/a>\u00a0begins in the\u00a0<\/span>steam generators<\/span><\/a>\u00a0in their shell sides. Steam generators are\u00a0<\/span>heat exchangers<\/span><\/strong>\u00a0that convert\u00a0<\/span>feedwater into steam<\/span><\/strong>\u00a0from heat produced in a\u00a0<\/span>nuclear reactor core<\/span><\/a>. The feedwater (secondary circuit) is heated from\u00a0<\/span>~230\u00b0C 500\u00b0F\u00a0<\/span><\/strong>(preheated fluid by regenerators) to the boiling point of that fluid\u00a0<\/span>(280\u00b0C; 536\u00b0F; 6,5MPa)<\/span><\/strong>. Heat is transferred through the walls of these tubes to the lower pressure secondary coolant located on the secondary side of the exchanger where the coolant evaporates to pressurized steam\u00a0<\/span>(<\/span><\/strong>saturated steam<\/span><\/strong><\/a>\u00a0280\u00b0C; 536\u00b0F; 6,5 MPa)<\/span><\/strong>. The saturated steam leaves the steam generator through a steam outlet and continues to the\u00a0<\/span>main steam lines<\/span><\/strong>\u00a0and further to the\u00a0<\/span>steam turbine<\/span><\/strong>.<\/span><\/p>\n

\"Steam<\/a>These main steam lines are cross-tied (e.g.,, via steam collector pipe) near the turbine to ensure that the pressure difference between any steam generators does not exceed a specific value, thus maintaining system balance and ensuring uniform heat removal from the Reactor Coolant System (RCS). The steam flows through the\u00a0<\/span>main steam line isolation valves<\/span><\/strong>\u00a0(MSIVs), which are very important to the high-pressure turbine from the safety point of view. Directly at the inlet of the steam turbine, there are<\/span>\u00a0throttle-stop valves<\/span><\/strong>\u00a0and\u00a0<\/span>control valves<\/span><\/strong>. Turbine control is achieved by varying these turbine valves openings. In the event of a\u00a0<\/span>turbine trip<\/span><\/strong>, the steam supply must be isolated very quickly, usually in a fraction of a second, so the stop valves must operate quickly and reliably.<\/span><\/p>\n

<\/span>Evaporation of water at high pressure - Energy balance in a steam generator<\/div>
\n
\"Steam<\/a>
Steam Generator – vertical<\/figcaption><\/figure>\n

Calculate the amount of primary coolant, which is required to evaporate 1 kg of feedwater<\/strong> in a typical steam generator<\/a>. Assume that there are no energy losses. This is only an i<\/span>dealized example.<\/p>\n

Balance of the primary circuit<\/strong><\/p>\n

The hot primary coolant (water 330\u00b0C; 626\u00b0F; 16MPa<\/strong>) is pumped into the steam generator<\/strong> through the primary inlet. The primary coolant leaves (water 295\u00b0C; 563\u00b0F; 16MPa)<\/strong> the steam generator through the primary outlet.<\/p>\n

hI, inlet<\/sub> = 1516 kJ\/kg<\/p>\n

=> \u0394hI<\/sub> = -206 kJ\/kg<\/p>\n

hI, outlet<\/sub> = 1310 kJ\/kg<\/p>\n

Balance of the feedwater<\/strong><\/p>\n

\"Steam<\/a>
Temperature gradients in a typical PWR steam generator.<\/figcaption><\/figure>\n

The feedwater (<\/span>water 230\u00b0C; 446\u00b0F; 6,5MPa<\/span><\/strong>) is pumped into\u00a0<\/span>the steam generator<\/span><\/strong>\u00a0through the feedwater inlet. The feedwater (secondary circuit) is heated from\u00a0<\/span>~230\u00b0C 446\u00b0F<\/span><\/strong>\u00a0to the boiling point of that fluid\u00a0<\/span>(280\u00b0C; 536\u00b0F; 6,5MPa)<\/span><\/strong>. Feedwater is then evaporated, and the pressurized steam\u00a0<\/span>(<\/span><\/strong>saturated steam<\/span><\/strong><\/a>\u00a0280\u00b0C; 536\u00b0F; 6,5 MPa)<\/span><\/strong>\u00a0leaves the steam generator through the steam outlet and continues to the steam turbine.<\/span><\/p>\n

hII, inlet<\/sub> = 991 kJ\/kg<\/p>\n

=> \u0394hII<\/sub> = 1789 kJ\/kg<\/p>\n

hII, outlet<\/sub> = 2780 kJ\/kg<\/p>\n

Balance of the steam generator<\/strong><\/p>\n

Since the difference in specific enthalpies is less for primary coolant than for feedwater, it is obvious that the amount of primary coolant will be higher than 1kg. To produce 1 kg of saturated steam from feedwater, about 1789\/206 x 1 kg = \u00a08.68 kg<\/strong> of primary coolant is required.<\/p><\/div><\/div>

<\/span>Isobaric Heat Addition<\/div>
\n
\"Rankine<\/a>
Rankine Cycle – Ts Diagram<\/figcaption><\/figure>\n

Isobaric heat addition <\/strong><\/a>(in a heat exchanger \u2013 boiler) \u2013 In this phase (between state 2 and state 3), there is a constant-pressure heat transfer to the liquid condensate from an external source since the chamber is open to flow in and out. \u00a0The feedwater (secondary circuit) is heated to the boiling point (2 \u2192 3a) of that fluid and then evaporated in the boiler (3a \u2192 3). The net heat added is given by Q<\/strong>add<\/sub><\/strong> = H<\/strong>3 <\/sub><\/strong>– H<\/strong>2<\/sub><\/strong><\/p><\/div><\/div><\/div>\n

From Turbine Valves to Condenser – Expansion<\/h2>\n
\"Rankine<\/a>
Rankine cycle – Ts diagram<\/figcaption><\/figure>\n

Typically most nuclear power plants<\/strong> operate multi-stage condensing steam turbines<\/strong>. In these turbines, the high-pressure stage receives steam<\/a> (this steam is nearly saturated steam \u2013 x = 0.995 \u2013 point C at the figure; 6 MPa<\/strong>; 275.6\u00b0C) from a steam generator and exhausts it to moisture separator-reheater (MSR – point D). The steam must be reheated to avoid damages caused to the steam turbine blades by low-quality steam<\/a>. High water droplets can cause rapid impingement and erosion of the blades, which occurs when condensed water is blasted onto the blades. To prevent this, condensate drains are installed in the steam piping leading to the turbine. The moisture-free steam is superheated by extraction steam from the high-pressure stage of the turbine and by steam directly from the main steam lines.<\/p>\n

\"Source:<\/a>
Source: TVO \u2013 Olkiluoto 3 NPP www.tvo.fi\/uploads\/julkaisut\/tiedostot\/ydinvoimalayks_OL3_ENG.pdf<\/figcaption><\/figure>\n

The heating steam is condensed in the tubes and is drained to the feedwater system. The reheater heats the steam (point D), and then the steam is directed to the low-pressure stage of the steam turbine, where it expands (point E to F). The exhausted steam then condenses in the condenser. It is at a pressure well below atmospheric (absolute pressure of 0.008 MPa<\/strong>) and is in a partially condensed state (point F), typically of a quality near 90%. The turbine\u2019s high and low-pressure stages are usually on the same shaft to drive a common generator, but they have separate cases. The main generator produces electrical power, which is supplied to the electrical grid.<\/p>\n

\"Wet<\/a><\/p>\n

<\/span>Expansion in the high-pressure stage of the steam turbine<\/div>
A high-pressure stage of steam turbine operates at steady state with inlet conditions of \u00a06 MPa, t = 275.6\u00b0C, x = 1 (point C). Steam leaves this turbine stage at a pressure of 1.15 MPa, 186\u00b0C, and x = 0.87 (point D). Calculate the enthalpy difference<\/strong><\/a>\u00a0(the work done by HP Turbine) between these two states.\n

The enthalpy for the state C can be picked directly from steam tables<\/a>, whereas the enthalpy for the state D must be calculated using vapor quality:<\/p>\n

h<\/em><\/strong>1, wet<\/sub><\/em><\/strong> = <\/em><\/strong>2785 kJ\/kg<\/strong><\/p>\n

h<\/em><\/strong>2, wet<\/sub><\/em><\/strong> = h<\/em><\/strong>2,s<\/sub><\/em><\/strong> x + (1 \u2013 x ) h<\/em><\/strong>2,l<\/sub><\/em><\/strong> \u00a0= 2782 . 0.87 + (1 \u2013 0.87) . 790 = 2420 + 103 = 2523 kJ\/kg<\/strong><\/p>\n

\u0394h = 262 kJ\/kg = W<\/strong>HP<\/sub><\/em><\/strong><\/p><\/div><\/div>

<\/span>Isentropic Expansion<\/div>
\n
\"Rankine<\/a>
Rankine Cycle – Ts Diagram<\/figcaption><\/figure>\n

Isentropic expansion <\/strong><\/a>(expansion in a steam turbine) \u2013 Steam from the boiler expands adiabatically from state 3 to state 4 in a steam turbine to produce work and then is discharged to the condenser (partially condensed). The steam works on the surroundings (blades of the turbine) and loses an amount of enthalpy equal to the work that leaves the system. The work done by the turbine is given by W<\/strong>T<\/sub><\/strong> = H<\/strong>4<\/sub><\/strong> \u2013 H<\/strong>3<\/sub><\/strong>. <\/strong>Again the entropy remains unchanged.<\/p><\/div><\/div><\/div>

<\/div>\n

From Condenser to Condensate Pumps – Condensation<\/h2>\n

\"Condenser<\/a>The main condenser condenses the exhaust steam from the main turbine\u2019s low-pressure stages and the steam dump system. The exhausted steam is condensed by passing over tubes containing water from the cooling system.<\/p>\n

The pressure inside the condenser<\/strong> is given by the ambient air temperature (i.e., water temperature in the cooling system) and by steam ejectors<\/strong> or vacuum pumps<\/strong>, which pull the gases (non-condensable) from the surface condenser eject them to the atmosphere.<\/p>\n

The lowest feasible condenser pressure is the saturation pressure corresponding to the ambient temperature (e.g.,, the absolute pressure of 0.008 MPa, <\/strong>which corresponds to 41.5\u00b0C<\/strong>). Note that there is always a temperature difference between (around \u0394T = 14\u00b0C<\/strong>) the condenser temperature and the ambient temperature, which originates from condensers\u2019 finite size and efficiency. Since neither the condenser is a 100% efficient heat exchanger, there is always a temperature difference between the saturation temperature (secondary side) and the temperature of the coolant in the cooling system. Moreover, there is design inefficiency, which decreases the overall efficiency of the turbine. Ideally, the steam exhausted into the condenser would have no subcooling<\/strong>. But real condensers are designed to subcool the liquid by a few degrees Celsius to avoid the suction cavitation<\/strong><\/a> in the condensate pumps. But, this subcooling increases the inefficiency of the cycle because more energy is needed to reheat the water.<\/p>\n

\"Rankine<\/a>
Decreasing the turbine exhaust pressure increases the network per cycle and decreases the vapor quality of outlet steam.<\/figcaption><\/figure>\n

The goal of maintaining the lowest practical turbine exhaust pressure is a primary reason for including the condenser in a thermal power plant. The condenser provides a vacuum that maximizes the energy extracted from the steam, resulting in a significant increase in network and thermal efficiency. But also this parameter (condenser pressure) has its engineering limits:<\/p>\n