STEAM TAIL – OPTIMIZATION OF CYCLE BETWEEN HRSG AND STEAM TURBINE IN COMBINED CYCLE POWER PLANTS SVOČ – FST 2017 Bc. Michal Panuška University of West Bohemia in Pilsen, Univerzitni 8, 306 14 Pilsen Czech Republic ABSTRACT Following pages are focused on optimization of a reference combined cycle power plant. Optimization is done from point of view of bottoming (steam) process, which is being actively offered by steam turbine manufacturer as pre- engineered package. Suggested improvements take into account both economic part of project and power output of plant. Power output of the steam turbine was improved by using a double-pressure condenser which replaces a single- pressure one. Cost reduction of the project was achieved by optimizing the layout of machine hall. KEYWORDS steam turbine, combined cycle power plants, condenser, layout, machine hall INTRODUCTION There is a big competition in steam turbine market. Every steam turbine manufacturer has an effort to increase the competitiveness of their product. They try to attain as high efficiency of their product as possible but emphasised is also cost reduction. The goal of this paper is to optimize a reference steam tail which is being actively offered for add-ons to currently operating power plants with gas turbine(s) or newly built plants. As said above, the optimization should be done from technical or/and economic point of view. The actual steam turbine, which will be dealt with, consists of a combined high-pressure and intermediate-pressure part and one double flow low-pressure (LP) part with a downward exhaust to a condenser. The condenser is an equipment where the water steam is condensed. Its electric power output is approx. 200 MW. If there are two or more low pressure parts of a steam turbine, using a multi-pressure condenser is very advantageous. The condensing pressure, which is mainly given by the temperature of cooling water and by its temperature rise, is lower on average even if the initial heat exchange surface is preserved. Lowering the condensing pressure results in a longer steam expansion, which helps to raise the power output of a turbo-generator set. There are two limit options of using this multi-pressure condenser – with preserved heat exchange surface and with retained terminal temperature difference (TTD) between the condensing temperature and outlet temperature of cooling water. As can be seen below, the steam turbine on which this type of condenser will be applied, has one double flow LP part with a common exhaust area to the condenser. This LP part has to be redesigned to provide two different pressure levels in both exhaust areas. The machine halls of steam turbines with double flow LP parts are usually high, because the condensers are under these turbines. It can be lowered by optimizing the layout of machine hall, which brings a huge amount of saved concrete, steel reinforcements and working hours. DOUBLE-PRESSURE CONDENSER As said above, lowering the condensing pressure is the main advantage of using a double-pressure condenser. The cooling water flows through the section where the pressure is lower to the section with higher pressure, unlike a single- pressure condenser with one pressure level (Figure 1). To make the comparison of these options possible, the thermodynamic calculation of the single-pressure condenser has to be done first. Since the heat exchange surface of a single-pressure condenser is known, the calculation of the double-pressure condenser with the same heat exchange surface as in the previous case and with preserved TTD can be done. Single-pressure condenser The heat exchange surface of the single pressure condenser is calculated from: = $ ∙ ℎ ’ −ℎ ) = )* ∙ ,* ∙ *’ − *. = ∙ ∙ , (1) where $ is mass flow of steam, ℎ ’ is enthalpy behind last stage, ℎ ) is enthalpy of saturated condensate, )* is mass flow of cooling water, ,* is specific heat of cooling water, *. , *’ is inlet cooling water temperature, respectively outlet cooling water temperature, is heat exchange coefficient, and
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STEAM TAIL – OPTIMIZATION OF CYCLE BETWEEN HRSG AND STEAM TURBINE
IN COMBINED CYCLE POWER PLANTS SVOČ – FST 2017
Bc. Michal Panuška
University of West Bohemia in Pilsen, Univerzitni 8, 306 14 Pilsen
Czech Republic ABSTRACT Following pages are focused on optimization of a reference combined cycle power plant. Optimization is done from point of view of bottoming (steam) process, which is being actively offered by steam turbine manufacturer as pre-engineered package. Suggested improvements take into account both economic part of project and power output of plant. Power output of the steam turbine was improved by using a double-pressure condenser which replaces a single-pressure one. Cost reduction of the project was achieved by optimizing the layout of machine hall. KEYWORDS steam turbine, combined cycle power plants, condenser, layout, machine hall INTRODUCTION There is a big competition in steam turbine market. Every steam turbine manufacturer has an effort to increase the competitiveness of their product. They try to attain as high efficiency of their product as possible but emphasised is also cost reduction. The goal of this paper is to optimize a reference steam tail which is being actively offered for add-ons to currently operating power plants with gas turbine(s) or newly built plants. As said above, the optimization should be done from technical or/and economic point of view. The actual steam turbine, which will be dealt with, consists of a combined high-pressure and intermediate-pressure part and one double flow low-pressure (LP) part with a downward exhaust to a condenser. The condenser is an equipment where the water steam is condensed. Its electric power output is approx. 200 MW. If there are two or more low pressure parts of a steam turbine, using a multi-pressure condenser is very advantageous. The condensing pressure, which is mainly given by the temperature of cooling water and by its temperature rise, is lower on average even if the initial heat exchange surface is preserved. Lowering the condensing pressure results in a longer steam expansion, which helps to raise the power output of a turbo-generator set. There are two limit options of using this multi-pressure condenser – with preserved heat exchange surface and with retained terminal temperature difference (TTD) between the condensing temperature and outlet temperature of cooling water. As can be seen below, the steam turbine on which this type of condenser will be applied, has one double flow LP part with a common exhaust area to the condenser. This LP part has to be redesigned to provide two different pressure levels in both exhaust areas. The machine halls of steam turbines with double flow LP parts are usually high, because the condensers are under these turbines. It can be lowered by optimizing the layout of machine hall, which brings a huge amount of saved concrete, steel reinforcements and working hours. DOUBLE-PRESSURE CONDENSER As said above, lowering the condensing pressure is the main advantage of using a double-pressure condenser. The cooling water flows through the section where the pressure is lower to the section with higher pressure, unlike a single-pressure condenser with one pressure level (Figure 1). To make the comparison of these options possible, the thermodynamic calculation of the single-pressure condenser has to be done first. Since the heat exchange surface of a single-pressure condenser is known, the calculation of the double-pressure condenser with the same heat exchange surface as in the previous case and with preserved TTD can be done. Single-pressure condenser The heat exchange surface of the single pressure condenser is calculated from:
where 𝑚$ is mass flow of steam, ℎ' is enthalpy behind last stage, ℎ) is enthalpy of saturated condensate, 𝑚)* is mass flow of cooling water, 𝑐,* is specific heat of cooling water, 𝑡*., 𝑡*' is inlet cooling water temperature, respectively outlet cooling water temperature, 𝑘 is heat exchange coefficient, 𝑆 and
𝐿𝑀𝑇𝐷 = 678967:
;< =>?=7:=>?=78
. (2)
The heat exchange coefficient is obtained from [1] and the rest of needed quantities from the heat balance diagram which is calculated by the steam turbine manufacturer.
Figure 1: Single-pressure and double-pressure condenser
Results of this calculation are included in the comparison table below. Double-pressure condenser with preserved heat exchange surface The first option with minimal additional costs is the double-pressure condenser with the same heat exchange surface as in case of the original single-pressure condenser. When this heat exchange surface is known, it is split into two equal sections so are the mass flow of condensing steam and temperature rise of cooling water. The calculation is done as off-design condition. The determined quantity is the condensing temperature, from which the condensing pressure and enthalpy behind the last stage is obtained. Thus all necessary quantities for determining the power gain are known. Double-pressure condenser with preserved TTD The second option of using the double-pressure condenser is with an identical terminal temperature difference as initial condenser. However, it is more expensive. The calculation is the same as in the first case, but is done twice, because both of the pressure levels are solved as two independent condensers. Results of all of these calculations are stated in Tab. 1. As can be seen, the heat exchange surface is increased by 32 %. Noteworthy is the fact, that extending the tube bundle in the condenser results in higher pressure drop of cooling water. This increase in auxiliary consumption is included in the calculation of power gain as well.
Table 1: Comparison of calculated options of condensers
Extension of the steam expansion is evident from Figure 2, where ℎ$ is isoentropic enthalpy drop, ℎ is real enthalpy drop, 𝑝. and 𝑝' is pressure before respectively behind the last stage. It is considered that the change of back-pressure affects only the last stage, 𝑝. is therefore the same in all cases.
Figure 2: Expansion of steam in the last stage in h-s diagram
Design change of LP part The advantages of the double pressure condenser are without any controversy, but there comes a problem with a design solution of the LP part. In the original solution, the exhaust area is common for both flows, which means that pressure level is the same as shown in Figure 3. It is clear that the exhaust space has to be divided into two separate sections. Figure 4 shows how is it done. The outer casing, which is newly formed by two separate exhaust necks, is flanged to the inner casing. The condenser is divided in the middle by a stainless steel plate to provide two different pressure levels possible.
Figure 3: Original design of LP part
Figure 4: New design of LP part
LAYOUT OF MACHINE HALL The height of a machine hall is mainly given by the position of a condenser and by a suction head of condensing pumps. Usually when a steam turbine has a double flow LP part, it uses the downward exhaust, which means that a condenser is under this turbine. This results in a relatively high building of a machine hall. The original layout of the machine hall can be seen in Figure 5. The turbo-generator set level is +12.00 m. One possible way to lower this building is to use a lateral exhaust from the LP part. The condenser is thus next to the LP part. Figure 6 shows that the turbo-generator set level was lowered to +5.5 m. The placement of the condenser is more apparent in Figure 7. Using of this solution saves 14.3 % of concrete, steel reinforcements and working hours during construction.
Figure 5: Longitudinal section through the original machine hall
Figure 6: Longitudinal section through the improved machine hall
Figure 7: Layout of level + 5.50 m of the improved machine hall
CONCLUSION This paper deals with optimizing a reference steam tail package, which is being actively offered by the steam turbine manufacturer to potential clients. At the beginning the approach to the solved problem was described. The second part was focused on replacement of the single-pressure condenser by the double-pressure one. This brings increase in the electrical power output by 546 kW in case of the double-pressure condenser with the same heat exchange surface as the single-pressure one. Using of this solution results in additional investment costs which make approx. 1.9 million CZK. The payback of this investment is 1 year. The profit for whole lifetime of the power plant is 53 million CZK. In the second case, the terminal temperature difference was preserved and the power gain is 1 290 kW, the difference in the costs is 17.1 million CZK and the payback is 3.5 years. If a potential customer agrees with this more expensive solution, the profit for the whole lifetime of the power plant is 104 million CZK. The next suggested improvement is aimed at optimizing the layout of machine hall. The original layout where the condenser is under the steam turbine is replaced by the new one, which has the condenser next to the turbine. The machine hall building was lowered by 6.5 m, and 14.3 % of concrete, steel reinforcements and working hours were saved. Both of these improvements can be combined together and they are not limited to being use only in combined cycle power plants. They can also be applied in fossil, nuclear, biomass power plants, etc. REFERENCES [1] Heat Exchange Institute. Standards for Steam Surface Condensers, 11th edition. Cleveland, 2012. [2] Internal documents of steam turbine manufacturer
