EP3473822B1

EP3473822B1 - Steam-recycling system for a low pressure steam turbine

Info

Publication number: EP3473822B1
Application number: EP17197248.2A
Authority: EP
Inventors: Jiri Kucera
Original assignee: Doosan Skoda Power sro
Current assignee: Doosan Skoda Power sro
Priority date: 2017-10-19
Filing date: 2017-10-19
Publication date: 2023-06-07
Anticipated expiration: 2037-10-19
Also published as: ES2949859T3; EP3473822C0; PL3473822T3; EP3473822A1; KR20190044018A; KR102085622B1

Description

Technical Field

The present invention is related to a bypass system for a power plant with a low pressure steam turbine and to a method of processing of a steam generated in a power plant during a low flow mode of a low pressure steam turbine.

Background Art

In a typical steam turbine plant, a steam is produced in a steam generator (boiler) and passes through the steam turbine. The steam that has passed through the steam turbine is directed through an exhaust system to a condenser. In a typical steam turbine plant, there is a main surface condenser disposed after the exhaust of the steam turbine to cool and condense the steam.
The main surface condenser is connected to a cooling tower via a cooling line. A cooling medium, e.g. water, circulates in the cooling line between the cooling tower and the main condenser and has a certain inlet temperature when entering the main condenser and a certain higher outlet temperature when leaving the condenser and flowing back to the cooling tower.
In the main condenser, the cooling medium circulating in the cooling line cools and condenses the hot steam coming from the exhaust of the steam turbine and a produced liquid condensate is collected and returned to steam generator to continue the cycle.
The steam turbine may work in different modes. Typically it works in an operation mode, in which the full amount of steam produced in the steam generator passes the steam turbine. However, the steam turbine often needs to be run in a maintenance/non-production/low flow mode, in which only a small amount of steam (low mass flow) is allowed through the turbine. In this case, because of steam production constraints on the steam generator side, the rest of the steam produced in the steam generator must be diverged through a bypass line, bypassing the steam turbine.
Thus, the mass flow of the steam passing through the bypass line - the so called bypass steam - is much higher than the mass flow of the steam passing through the steam turbine and increases because of attemperation (spraying by cold condensate).
In a typical steam turbine plant, the exhaust line (containing steam coming from the exhaust system of the steam turbine) and the bypass line (containing the bypass steam) are merged before entering the main condenser, such that the low mass flow exhaust steam from the exhaust of the steam turbine and the high mass flow bypass steam are merged before entering the condenser and then condensed together in the main condenser, which is shown in Fig. 4. However, this configuration has several drawbacks, possibly leading to a failure of the steam turbine.
The performance of the main condenser is considerably affected by the steam load entering the condenser. The more the condenser is loaded, the worse vacuum, i.e. higher back pressure, is created. The back pressure acts in the exhaust line between the exhaust of the steam turbine and the main condenser. Depending on the steam load, the back pressure can be relatively high and affects the steam volumetric flow from the exhaust of the steam turbine. Reduction of the steam load would lead to lower temperatures in the main condenser and so to lower backpressure.
In the normal operation mode of the steam turbine, the steam mass flow from the exhaust of the steam turbine is relatively high. The exhaust volumetric flow is high enough even if the back pressure is relatively high too, so in principle, the negative effects are negligible in the normal operation mode. However, in the maintenance/non-production/low flow mode of the steam turbine, the combination of high back pressure together with the low steam mass flow creates low exhaust volumetric flow. In that case a so called "backward flow" on the blade root may occur. The steam is "sucked" from the exhaust on the blade root and transported toward the blade tip. These phenomena together with the high back pressure may lead to overheating and vibrations of last stage blades (LSBs) and to an overall deterioration of the material of the blades. Moreover, the steam that is forced back towards the LSBs may contain water drops, which may cause erosion of the material of the last stage blades of the steam turbine, again causing deterioration of the material. The deterioration of the blade material considerably contributes to the shortening of the lifetime of the steam turbine.
A common solution to the above-described drawback is a limitation applied in the maintenance/non-production/low flow mode of operation. Either a lower limit on a mass flow through the steam turbine, or an upper limit on the operation time of the steam turbine is provided. Although such limitations might have been sufficient so far, they are not desirable nowadays. The main reason is that systems of varying energy output (for example of renewable energy sources) became much more common as input energy sources for systems such as turbine power plants than in the past. The varying input energy increases the need of low load operation modes. Therefore, the application of such limitations is undesirable and non-application leads to deterioration of material of the last stage blades and consequently to shortening of the lifetime of a turbine.
Overall, the major advantage of the presented invention is a widening of the operating range of the steam turbine in the maintenance/non-production/low flow mode, such that current limitations on the minimum mass flow or maximum operation time in the maintenance/non-production/low flow mode are exceeded.
Documents EP2980474 A1 and US2014/165565 A1 disclose the technical features of the preamble of independent claim 1.

Disclosure of the Invention

The bypass system according to the invention may be implemented to any low pressure steam turbine and/or steam turbine plant.
The presented solution applies primarily to operation in the maintenance/non-production/low flow mode. Although generally it is not necessary to consider these aspects in the operation mode, in principle, the presented solution may be used in any mode, where at least part of the steam is bypassed through the bypass line.
The aim of the present invention is to overcome the above-described drawbacks by providing a bypass system with additional steam-condensing means, thus allowing for reduction of the mass flow of the steam entering the main surface condenser, reduction of the pressure in the main condenser, reduction of the back pressure in the exhaust system and reduction of the deterioration of the material of the last stage blades, thus widening the operation range of the maintenance/non-production/low flow mode.
When the low mass flow exhaust steam from the exhaust of the steam turbine and the high mass flow bypass steam are merged before entering the condenser and then condensed together in the main condenser, the pressure in the condenser rises depending on the amount of the bypassed steam up to the nominal pressure, i.e. the pressure in a typical operation mode, or even higher, and the aforementioned drawback occur. However, by providing a bypass system with additional steam-condensing means, the pressure in the main condenser is reduced to 40% to 60% of the nominal pressure, which enables the unwanted effects to be eliminated, or, at least, significantly reduced, because the bypass system with at least one additional steam-condensing means allows the high mass flow of bypass steam, or at least a major part of it, to be cooled and condensed separately from the low mass flow exhaust steam.
The exhaust line from the exhaust of the steam turbine ends directly in the main condenser. The bypass line for transporting the high mass flow steam from the steam generator is separated from the exhaust line, i.e. it is not connected to the exhaust line before entering the main condenser. Instead, the bypass line comprises at least one additional steam-condensing means, such that the high mass flow bypass steam, or at least a major part of it, is cooled and condensed separately from the low mass flow exhaust steam. Thus, the steam load entering the main surface condenser corresponds to the low mass flow exhaust steam from the exhaust of the steam turbine only and the high mass flow bypass steam is cooled and condensed separately. As a result, the main surface condenser is not overloaded and the created backpressure is lower.
Thus, the steam generated in the power plant during the low flow mode of the low pressure steam turbine is in the low flow mode processed in two parallel processes: the exhaust steam coming from the exhaust of the low pressure steam turbine through the exhaust line is condensed in a main condenser and the bypass steam coming from the steam generator through the bypass line is condensed in additional steam-condensing means, separately from the exhaust steam.
While pressure in the main condenser shall remain in the range of 40% to 60% as given above, a pressure in the additional condensing means may be higher, for example 0.5 - 3 bar, but may be even higher. In general, the pressure in the additional condensing means depends on a type of the condensing means and on the operating parameters recommended by a producer. The higher is the pressure, the lower are the requirements as to the size of the condensing means. In addition, with the lower pressure of e.g. 0.5 - 1 bar, the requirements as to the strength of the walls of the additional condensing means are also lower.
The additional steam-condensing means have to be connected to a cooling system in order to cool the bypass steam entering the additional steam-condensing means from the bypass line. The cooling system comprises a cooling means for cooling of a cooling medium and a cooling line for transporting a cooling medium in a cooling system between the cooling means and the additional steam-condensing means.
The cooling means may comprise a typical cooling tower, where a small part of the cooling medium evaporated and the medium is cooled down, or it comprises a heat exchanger, wherein the cooling medium is cooled down by air. Alternatively, the condensate may be used directly as a cooling medium, in which case, the main condenser is a direct contact condenser. In yet another alternative, the cooling means may comprise a direct natural water supply or any other suitable water supply. In this case there is no cooling tower in the cooling system, because the water is supplied directly from a natural source.
Although the additional condensing means may be provided with their own cooling system, in a preferred embodiment, they are connected to the cooling system of the main condenser, wherein the cooling system of the main condenser comprises the cooling line connecting the main condenser to the cooling means, such that the cooling medium, e.g. water, enters the main condenser to cool the steam in the main condenser.
In the cooling system of the main condenser, the cooling medium has a certain inlet temperature at an intake of the main condenser when entering the main condenser and a certain higher outlet temperature at an outtake of the main condenser when leaving the main condenser and returning back to the cooling means.
The bypass line comprises the additional steam-condensing means in a form of an additional condenser, which is positioned separately from the exhaust line and from the main condenser.
The additional condenser is connected to the cooling system, preferably, to the cooling system of the main surface condenser.
When connected to the cooling system of the main condenser, the additional condenser may be connected to the main condenser either in series or in parallel.
In the case of serial connection, the additional condenser is positioned in the outtake of the cooling medium from the main condenser, such that the inlet temperature of the cooling medium entering the additional condenser corresponds to the outlet temperature of the cooling medium leaving the main condenser. First, a low mass flow exhaust steam is cooled in the main condenser (the cooling medium comes directly from the cooling means, e.g. a cooling tower, and has the lowest possible temperature), then the bypass steam is cooled in the additional condenser (the temperature of the cooling medium is higher than when coming directly from the cooling means, but it is still sufficient for the purpose of cooling the bypass steam), and only then the cooling medium returns to the cooling means to cool down.
Alternatively, the parallel connection allows both the exhaust steam in the main condenser and the bypass steam in the additional condenser to be condensed by a cooling medium coming directly from the cooling means. In this way, both the main and the additional condenser receive the coolest possible medium, thus improving condensation of the low mass flow exhaust steam in the main condenser, as well as condensation of the bypass steam in the additional condenser. The outtake of the main condenser and the outtake of the additional condenser lead back to the cooling means; they may lead back to the cooling means separately, or they may merge before entering the cooling means.
In any case, the condensate produced in the additional condenser is returned to continue the main cycle, preferably after having been merged with the condensate from the main condenser.
The high mass flow bypass steam is cooled and condensed separately, such that the steam load entering the main surface condenser corresponds to the low mass flow exhaust steam from the exhaust of the steam turbine only. As a result, the main surface condenser is not overloaded and the created back pressure is considerably lower. In turn, the operation range of the low pressure steam turbine in the maintenance/non-production/low flow mode widens.
In another embodiment, the additional steam-condensing means may, in addition to the additional condenser, further comprise a heat pump. In this embodiment, the bypass line may be split into a first part and a second part, the first and the second part being two separate lines, the first part comprising the additional condenser and the second part comprising a heat pump. Alternatively, the additional condenser may be integrated as a part of the heat pump. In any case, part of the bypass steam is directed to the additional condenser and part of the bypass steam is directed to the heat pump, the heat pump being driven by the bypass steam.
The amount and the properties of the bypass steam redirected through the heat pump may be controlled by a valve positioned before the entering the heat pump.
The heat pump is positioned separately from the exhaust line and separately from the main condenser. The heat pump is connected to a cooling system, preferably to the cooling system of the main surface condenser. In the heat pump, the heat is absorbed from the cooling medium, such that it is cooled down. The absorbed heat is transferred by the heat pump into the return part of the cooling line, where the cooling medium returns to the cooling means.
The cooling medium cooled down by the cooling means passes through the heat pump before entering the main condenser. Thus, the temperature of the cooling medium entering the main condenser after passing the heat pump is lower than the temperature of the cooling medium entering the main condenser directly from the cooling means, i.e. even lower than in the other embodiments. Temperature difference between the cooling medium entering the heat pump and leaving the heat pump may be up to 20 degrees Celsius, preferably between 5 and 15 degrees, likely about 10 degrees.
The additional condenser may be connected in the system in the same manner as described above, i.e. connected preferably to the cooling system of the main surface condenser either in parallel or in series. When connected in series, it should be connected in the outtake of the cooling medium from the main condenser in order not to warm up the cooling medium before entering the main condenser.
The cooling line containing the cooling medium directly from the cooling means is preferably split into two parallel lines - the "heat pump cooling line" and the "condenser cooling line".
The cooling medium of the "heat pump cooling line" passes through the heat pump only, such that the heat pump is cooled down, and returns directly to the cooling means.
When the main condenser and the additional condenser are connected in series, the cooling medium of the "condenser cooling line" passes first through the heat pump, then through the main surface condenser and finally through the additional condenser before it returns to the cooling means. Thus, the cooling medium is first cooled down in the heat pump. Then, having a temperature, which is lower than the temperature of the cooling medium entering the main condenser directly from the cooling means, the cooling medium enters the main condenser and cools down the steam passing through the main condenser. Subsequently, it enters the additional condenser and cools down the part of the bypass steam directed through the first part of the bypass line to the additional condenser. Finally, after having passed through the additional condenser, it is preferably coupled with the cooling medium of the "heat pump cooling line" and it returns to be cooled down by the cooling means and returned to the cycle again.
When the main condenser and the additional condenser are connected in parallel, the cooling medium of the "condenser cooling line" passes first through the heat pump. Then it simultaneously passes through the main surface condenser and through the additional condenser before it returns to be cooled down by the cooling means. Thus, the cooling medium is first cooled down in the heat pump. Then, having a temperature, which is lower than the temperature of the cooling medium entering the main condenser directly from the cooling means, the cooling medium enters the main condenser and the additional condenser simultaneously, such that the low mass steam passing through the main condenser and part of the bypass steam directed through the additional condenser are cooled down by the cooling medium of the same, lowest possible, temperature. Finally, the "condenser cooling line" is preferably coupled with the cooling medium of the "heat pump cooling line" and it returns to be cooled down and returned to the cycle again.
As the energy of the bypass steam is applied to drive the heat pump, the bypass steam cools down and condenses, and it is brought back to the system. The remaining part of the bypass steam is cooled and condensed by the additional condenser. The condensate from the additional condenser is preferably merged with the condensate from the main condenser. The condensate is then returned to the main cycle.
This embodiment is especially advantageous, because the large amount of energy comprised in the bypass steam can be used instead of being lost, which is usually the case in the maintenance/non-production/low flow mode. Moreover, the steam load entering the main surface condenser corresponds to the low mass flow exhaust steam from the exhaust of the steam turbine only and is cooled by a cooling medium of the temperature, which is lower than the temperature of the cooling medium entering the main condenser directly from the cooling means. As a result, the main surface condenser is not overloaded and the created back pressure is considerably lower. In turn, the operation range of the low pressure steam turbine in the maintenance/non-production/low flow mode becomes wider.
Furthermore, the arrangement enabling the heat pump may be especially beneficial for system with expected fluctuations of the temperature of the cooling medium. As will be described in Example 4, these fluctuations may cause the pressure in the main condenser to rise up to relatively high values, e.g. even up to the nominal pressure, i.e. pressure of the normal operation mode. The engagement of the heat pump in the system allows the temperature of the cooling medium to decrease to a lower level. Moreover, it also allows the temperature of the cooling medium to be relatively constant, such that at least major fluctuations are eliminated.
In an example, the bypass line may comprise the heat pump only. The heat pump may be connected in the system substantially in the same manner as described above, i.e. to the cooling system of the main condenser.
In this example, the bypass steam is directed through the heat pump, the heat pump being driven by the bypass steam, such that the bypass steam cools down and condenses as the energy of the bypass steam is applied to drive the heat pump.
The cooling medium from the cooling means passes through the heat pump before entering the main condenser, such that the temperature of the cooling medium entering the main condenser is lower than the temperature of the cooling medium entering the main condenser directly from the cooling means.
In any case, the condensate produced by the additional steam-condensing means is returned to continue the main cycle. Preferably, the condensate is merged with the condensate produced by the main condenser. In one embodiment, the additional steam-condensing means may be connected to a condensate tank of the main condenser by a condensate line, such that the condensate produced by the additional steam-condensing means is merged with the condensate of the main condenser before being returned to the system.
Preferably, the additional steam-condensing means and/or the condensate line may further comprise means for controlling the properties of the condensate drained off the additional steam-condensing means. In particular, a pressure lowering means may be enabled in order to balance the pressure in the additional steam-condensing means with the pressure of the main condenser, which is lower than the pressure in the additional steam-condensing means. The pressure lowering means may comprise a valve or any other suitable means. Similarly, a temperature control means may be enabled in the system

Brief description of figures

Fig. 1 shows a part of the steam turbine plant with a steam-recycling system according to embodiment, where the steam-condensing means comprises a condenser connected in series with the main surface condenser.
Fig. 2 shows a part of the steam turbine plant with a steam-recycling system according to embodiment, where the steam-condensing means comprises a condenser connected in parallel with the main surface condenser.
Fig. 3 shows a part of the steam turbine plant with a steam-recycling system according to embodiment, where the steam-condensing means comprises a combination of the condenser and the heat pump.
Fig. 4 shows a part of the steam turbine plant with a known steam-recycling system with the main surface condenser only.

Examples of preferred embodiments

In the following examples, cooling means comprise a cooling tower and water is used as a cooling medium, but in principle any suitable cooling means and cooling medium could be used.

Example 1

In the first example, the embodiment with the additional condenser connected to the main surface condenser in series is described and can be seen in Fig. 1.
The additional condenser 6 is positioned separately from both the exhaust line 21 and the main condenser 3. It is connected to the cooling system of the main surface condenser 3 and positioned in the outtake 32 of the cooling medium from the main condenser 3, such that the inlet temperature of the cooling medium entering the additional condenser 6 corresponds to the outlet temperature of the cooling medium leaving the main condenser 3.
First, a low mass flow exhaust steam is cooled in the main condenser 3 (the cooling medium comes directly from the cooling tower 4 and has thus the lowest temperature possible in this embodiment), then the bypass steam is cooled in the additional condenser 6 (the temperature of the cooling medium is higher than when coming directly from the cooling tower 4, but it is still sufficient for the purpose of cooling the bypass steam), and only then the cooling medium returns to the cooling tower 4 to cool down.
The condensate produced in the additional condenser 6 is delivered, through a line 8, to the condensate tank 10 of the main condenser 3 to be merged with the condensate produced by the main condenser 3 and returned to the cycle.
The high mass flow bypass steam is cooled and condensed separately, such that the steam load entering the main surface condenser 3 corresponds to the low mass flow exhaust steam from the exhaust 2 of the steam turbine 1 only. As a result, the main surface condenser 3 is not overloaded and the created back pressure is considerably lower. In turn, the operation range of the low pressure steam turbine in the maintenance/non-production/low flow mode widens.

Example 2

In the second example, the embodiment with the additional condenser 6 connected to the main surface condenser 3 in parallel is described and can be seen in Fig. 2.
The principle of the bypass system in the Example 2 is the same as in the Example 1, i.e. the high mass flow bypass steam is cooled and condensed separately, and the steam load entering the main surface condenser 3 corresponds to the low mass flow exhaust steam from the exhaust 2 of the steam turbine 1 only. The main surface condenser 3 is not overloaded and the created back pressure is considerably lower.
The additional condenser 6 is again positioned separately from both the exhaust line 21 and the main condenser 3. There is, however, difference in the cooling of the main condenser 3 and the additional condenser 6. The parallel connection allows both the exhaust steam in the main condenser 3 and the bypass steam in the additional condenser 6 to be condensed by a cooling medium coming directly from the cooling tower 4, thus providing lower temperature of the cooling medium for condensation of both the low mass flow exhaust steam in the main condenser 3, as well as condensation of the bypass steam in the additional condenser 6.
The advantage of this embodiment, in comparison to the embodiment of Example 1, is that there is lower pressure loss in the cooling system, so the final energy consumption of cooling water circulation pumps (energy consumption required to run the cooling system) is lower.

Example 3

In the third example, the embodiment with the additional condenser 6 in combination with the heat pump 7 is described and can be seen in Fig.3.
In this case, the bypass line 5 is split into a first part 5a and a second part 5b, the first and the second parts 5a, 5b being two separate lines. In the example, the first part 5a comprises the additional condenser 6 and the second part 5b comprises a heat pump 7, such that part of the bypass steam is directed to the additional condenser 6 and part of the bypass steam is directed to the heat pump 7.
The additional condenser 6 may be connected in the system in the same manner as described above in Example 1. The additional condenser 6 is connected to the cooling system of the main surface condenser 3, in series to the main condenser 3, in this example, such that it is connected in the outtake 32 of the cooling medium from the main condenser 3 in order not to warm up the cooling medium before entering the main condenser 3.
However, the additional condenser 6 may as well be connected to the main condenser 3 in parallel, as described in Example 2.
Alternatively, the additional condenser 6 may be integrated as a part of the heat pump 7.
The heat pump 7 is positioned separately from the exhaust line 21 and separately from the main condenser 3. The amount and the properties of the bypass steam redirected through the heat pump may be controlled by a valve (not shown) positioned preferably at an entry of the heat pump 7.
The heat pump 7 is connected to the cooling system of the main surface condenser 3.
The cooling line containing the cooling medium directly from the cooling tower 4 is split into two parallel lines - the "condenser cooling line" 41a, and the "heat pump cooling line" 41b.
The cooling medium of the "condenser cooling line" 41a passes first through the heat pump 7, then through the main surface condenser 3 and finally through the additional condenser 6 before it returns to the cooling tower 4. Thus, the cooling medium is first cooled down in the heat pump 7. Then, having a temperature, which is lower than the temperature of the cooling medium entering the main condenser 3 directly from the cooling tower 4, i.e. lower than in previous embodiments, the cooling medium enters the main condenser 3 and cools down the steam passing through the main condenser 3. Temperature difference between the cooling medium entering the heat pump 7 and leaving the heat pump 7 may be up to 20 degrees Celsius, preferably between 5 and 15 degrees, likely about 10 degrees.
Subsequently, the cooling medium enters the additional condenser 6 and cools down the part of the bypass steam directed through the first part 5a of the bypass line 5 to the additional condenser 6.
Finally, after having passed through the additional condenser 6, the cooling medium is preferably coupled with the cooling medium of the "heat pump cooling line" 41b (described below) and it returns to the cooling tower 4 to be cooled down and returned to the cycle again.
The cooling medium of the "heat pump cooling line" 41b passes through the heat pump 7 only, in order to cool down the heat pump 7 itself (i.e. to remove the collected thermal energy). After having passed the heat pump 7, the cooling medium of the "heat pump cooling line" 41b returns directly to the cooling tower 4.
In this example, the condensate produced in the heat pump 7 is delivered, through line 9, into the condensate tank 11 of the additional condenser 6, and is then, together with the condensate produced in the additional condenser 6, delivered, through a line 8, to the condensate tank 10 of the main condenser 3 to be merged with the condensate produced by the main condenser 3 and returned to the cycle. The condensate lines 8 and 9 may also be placed separately or in any configuration suitable for returning the condensate to the cycle.

Example 4

In the following section, the calculated parameters of the steam flow are provided, which support the effect of implementing the additional condensing means in the bypass system of the steam turbine plant.
The results are provided for a 250 MW steam turbine comprising a first HP/IP (high pressure, intermediate pressure) part and a symmetric second LP (low pressure) part. Nevertheless, the presented solution is not limited to this particular type of turbine.

The parameters of the steam at the exhaust of the steam turbine are summarized in Table 1 below. The presented parameters comprise mass flow, pressure, density, volumetric flow and the volumetric flow relative to the volumetric flow of the reference nominal load.

Table 1

Row nr.	Load	Mass flow of the exhaust steam	Pressure of the exhaust steam	Density of the exhaust steam	Volumetric flow of the exhaust steam	Volumetric exhaust flow relative to volumetric exhaust flow of the reference nominal load
		m [kg/s]	p [bar]	P [m³/kg]	V [m³/s]	V/V₀ [%]
1	Reference nominal load	123	0,0409	0,0328	3750	100,0%
2	Reference minimal load	17,11	0,0569	0,034	503	13,4%
3	Example 1	17,11	0,0222	0,0132	1296	34,6%
4	Example 2	17,11	0,0246	0,0147	1164	31,0%
5	Example 3	17,11	0,0159	0,009501	1801	48,0%

Row nr.1 describes parameters of a reference nominal load, i.e. condensing load, which considers a typical operation mode with disabled bypass system and corresponds to a nominal mass flow, nominal pressure p₀, nominal density and nominal volumetric flow V₀. The mass flow is the same for the reference minimal load and for the three given examples and corresponds to a 14% of the nominal flow, which is typical minimal mass flow entering the main condenser.
In the row nr.2, a reference minimal load is provided, which corresponds to a typical steam turbine plant, where the exhaust line and the bypass line are merged before entering the main condenser, such that the low mass flow exhaust steam and the high mass flow bypass steam are merged before entering the condenser and then condensed together in the main condenser as per Fig. 4. It can be seen that the pressure in the main condenser may reach values even higher than the nominal pressure. Generally, the pressure in the main condenser rises depending on the amount of the bypassed steam.
Further rows refer to the parameters calculated for individual examples: row nr.3 refers to the parameters of Example 1, corresponding to the embodiment with additional condenser in serial connection. Parameters of Example 2, corresponding to the embodiment with additional condenser in parallel connection are provided in row nr.4, and parameters of Example 3, corresponding to the embodiment comprising a combination of the additional condenser and the heat pump, are provided in row nr.5.
Comparing the parameters of individual examples with the parameters of the reference minimal load, it can be seen that the pressure of the exhaust steam is considerably lower in the examples than in the case of the reference minimal load, it decreases to 40% to 60% of a nominal pressure p₀.

Further calculations and several scenarios can be seen in Table 2. The calculations are based on the assumption that the typical minimal steam mass flow eps into the condenser is 14% of the nominal flow, that the temperature rise of the cooling water ΔTcw and terminal temperature difference TTD of the main condenser are linearly dependent on the steam mass flow into the condenser, and that the temperature of the cooling water tcw at the inlet of the main condenser remains constant. The last scenario of Table 2 - "Scenario based on change of cooling water temperature", shows how the situation worsens if the last assumption is not fulfilled. For example, in a scenario that the condenser, which was assumed to use 20°C inlet cooling water temperature (tcw), is operated in the "summer" minimal load with a cooling water of 30°C, the ratio of minimal pressure vs. nominal pressure pk'/pk may rise up to around 90%. Even though, the arrangement suggested by the present invention is beneficial, especially the arrangement enabling the heat pump, because it allows the temperature of the cooling water tcw to decrease to a lower level. Moreover, it also allows this temperature to be relatively constant, such that at least major fluctuations are eliminated.

Table 2

Sensitivity analysis of minimal loads of the condenser
tcw °C	Δ Tcw °C	TTD °C	eps %	Δ Tcw' °C	TTD' °C	Tsat °C	Tsat' °C	pk bar	pk' bar	eps' %	pk'/pk %	pk" bar	pk"/pk %
Sensitivity to coolinc water temperature
15	10	3,5	14,0%	1,4	0,49	28,5	16,89	0,039	0,019	28,3%	49,4%	0,017	43,8%
20	10	3,5	14,0%	1,4	0,49	33,5	21,89	0,052	0,026	27,6%	50,7%	0,023	45,2%
25	10	3,5	14,0%	1,4	U.49	38,5	26,89	0,068	0,035	26,9%	52,0%	0,032	46,5%
30	10	3,5	14,0%	1,4	0,49	43,5	31,89	0,089	0,047	26,3%	53,3%	0,042	47,8%
35	10	3,5	14,0%	1,4	0,49	48,5	36,89	0,115	0,062	25,7%	54,5%	0,056	49,1%
40	10	3,5	14,0%	1,4	0,49	53.5	41,89	0,147	0,082	25,2%	55,7%	0,074	50,4%

Sensitivity to cooling water temperature rise
20	8	3,5	14,0%	1,12	0,49	31,5	21,61	0,046	0,026	25,1%	55,8%	0,023	50,6%
20	10	3,5	14,0%	1,4	0,49	33,5	21,89	0,052	0,026	27,6%	50,7%	0,023	45,2%
20	12	3,5	14,0%	1,68	0,49	35,5	22,17	0,058	0,027	30,3%	46,2%	0,023	40,4%

Sensitivity to TTD (terminal temperature difference) of the condenser
20	10	2	14,0%	1,4	0,28	32	21,68	0,048	0,026	25,7%	54,5%	0,023	49,2%
20	10	3,5	14,0%	1,4	0,49	33,5	21,89	0,052	0,026	27,6%	50,7%	0,023	45,2%
20	10	5	14,0%	1,4	0,7	35	22,1	0,056	0,027	29,6%	47,3%	0,023	41,6%

Scenario based on change of cooling water temperature
tcw	ΔTcw	TTD	eps	ΔTcw'	TTD'	Tsat	Tsat'	pk	pk'	eos'	pk'/pk
°C	°C	°C	%	°C	°C	°C	°C	bar	bar	%	%
20	10	3,5	14,0%	1,4	0,49	33,5	21,89	0,052	0,026
25	10	3,5	14,0%	1,4	0,49	38,5	26,89	0,068	0,035	15,3%	91,3%
30	10	3,5	14,0%	1,4	0,49	43,5	31,89	0,089	0,047

Legend of Table 2

tcw	°C	Cooling water temperature at the inlet of the condenser
ΔTcw	°C	Nominal cooling water temperature rise across the condenser
TTD	°C	Nominal terminal temperature difference of the condenser
eps	%	Ratio of steam mass flows at the condenser inlet (typical minimum flow vs. nominal flow)
ΔTcw'	°C	Cooling water temperature rise across the condenser in the minimal load
TTD'	°C	Terminal temperature difference of the condenser in the minimal load
Tsat	°C	Saturation temperature in the condenser in nominal load
Tsat'	°C	Saturation temperature in the condenser in minimal load
pk	bar	Pressure in the condenser in nominal load (derived from Tsat)
pk'	bar	Pressure in the condenser in minimal load (derived from Tsat')
eps'	%	"Ratio of ratios" of mass flow vs condenser pressure. It well correlates to the ration of the exhaust volumetric flows
pk'/pk	%	Ratio of condenser pressures in minimal vs. nominal loads
pk"	bar	Hypothetical minimal pressure in condenser derived from the inlet cooling water temperature only
pk"/pk	%	Ratio of condenser pressures : hypothetical minimum vs. nominal pressure

Furthermore, referring back to Table 1, the volumetric flow of the exhaust steam is much higher in the individual examples than in the case of reference minimal load, corresponding to relative volumetric flow V/V₀ greater than 30% for the examples and much lower value of about 13% for the reference minimal load.
The 30% value substantially determines whether or not the backward flow occurs in the exhaust of the steam turbine. Below the threshold, the effects of the back steam flow are considerable in the exhaust of the steam turbine and at the root of the last stage blade, as described in the literature: according to M. Gloger et al. (Designing of LP bladings for steam turbines, VGB Kraftwersktechnik 69, No. 8, August 1989), a dynamic stress of a last stage blade (determined by alternating stress amplitude of a tip of a blade in this case) increases considerably when the volumetric flow decreases below 30%. This was confirmed also later, by Sigg et al. (Numerical and experimental investigation of a low-pressure steam turbine during windage, Proc.IMechE Vol. 223 Part A: J. Power and Energy), where it was observed that beneath 34% of the relative mass flow, a backflow starts on the hub, and worsens with decreasing relative mass flow.
These effects are strongly reduced, when the relative volumetric flow V/V₀ is above the threshold of 30%.
The ratio of the pressure in the main condenser to the mass flow entering the condenser is in the range of 25% to 30%, which is well correlated with the relative volumetric flow. When considering individual examples, and based on the abovementioned assumptions (see Table 2), the results are around the boundary where the problematic ventilation regime starts.
In the last scenario, when the last assumption is not fulfilled, i.e. when the cooling water temperature is not constant, it may happen that this ratio may decrease to 15%, i.e. to the problematic ventilation regime. Even though, the arrangement suggested by the present invention is beneficial, because it decreases the pressure in the main condenser. As already mentioned above, the arrangement enabling the heat pump may be especially advantageous in this case, because it allows the temperature of the cooling medium to decrease to a lower level and even to be relatively constant, such that at least major fluctuations are eliminated.
In conclusion, in all three presented embodiments (Example 1, 2 and 3) the pressure in the main condenser is considerably lower in comparison to the known solution. On the other hand, the relative volumetric flow is above 30% in all three cases, i.e. above the problematic ventilation regimes. Thus, keeping the main condenser pressure between 40% - 60% by enabling the additional steam-condensing means allows for lowering, or even eliminating, the above-described unwanted effects, thus reducing the deterioration of the material of the last stage blades, and thus widening of the operation range of the maintenance/non-production/low flow mode.

Claims

A steam-recycling system for a power plant comprising a steam generator and a low pressure steam turbine (1), the steam-recycling system comprising:
a main condenser (3),

a cooling system,

an exhaust line (21) for transporting a low mass flow exhaust steam from an exhaust (2) of the low pressure steam turbine (1) to the main condenser (3), and

a bypass line (5) for transporting a high mass flow steam from the steam generator for condensation without passing through the low pressure steam turbine (1),

wherein the bypass line (5) is separated from the exhaust line (21),

and wherein the bypass line (5) comprises at least one additional steam-condensing means (6,7),

characterized in that the additional steam-condensing means comprises an additional condenser positioned separately from the exhaust line and from the main condenser, and the additional steam-condensing means (6,7) being connected to the cooling system, and the cooling system being adapted for cooling of the steam entering the main condenser (3) and the steam entering the additional steam-condensing means (6,7).
The steam-recycling system according to claim 1, wherein the steam-recycling system is configured to have a minimal pressure in the main condenser (3) of 40% to 60% of the nominal pressure in the main condenser (3), wherein the nominal pressure is defined as the pressure in the main condenser (3) at the nominal load.
The steam-recycling system according to any one of the preceding claims, wherein the cooling system comprises a cooling means (4) adapted for cooling of a cooling medium, and a cooling line (41) for transporting the cooling medium in the cooling system between the cooling means (4), the main condenser (3), and the additional steam-condensing means (6,7).
The steam-recycling system according to claim 1, wherein the additional condenser (6) is configured to have a pressure of 0.5 -3 bar.
The steam-recycling system according to claim 1, wherein the additional condenser is positioned in series to the main condenser (3) in an outtake (32) of the cooling line (41) of the main condenser (3).
The steam-recycling system according to claim 1, wherein the additional condenser (6) is positioned in parallel to the main condenser (3).
The steam-recycling system according to any one of the preceding claims, wherein the additional steam-condensing means comprises a heat pump (7).
The steam-recycling system according to claim 7, wherein the bypass line (5) comprises a first part (5a) and a second part (5b), the first part comprising the additional condenser (6) and the second part comprising the heat pump (7).
The steam-recycling system according to claim 7 or 8, wherein the bypass line (5) comprises a valve for controlling the amount of the bypass steam flowing into the heat pump (7).
A power plant comprising:
a steam generator,

a low-pressure steam turbine,

an exhaust system of the low pressure steam turbine, and

a steam-recycling system according to any one of the preceding claims.
A method of processing of a steam generated in a power plant during a low flow mode of a low pressure steam turbine (1), using the steam-recycling system according to any one of claims 1 to 9, comprising the step of
- condensing an exhaust steam in a main condenser (3),
characterized in that it further comprises the step of

- condensing a bypass steam in at least one additional steam-condensing means (6, 7) separately from the exhaust steam, wherein the at least one additional steam-condensing means (6, 7) comprises an additional condenser (6).
The method of processing of a steam according to claim 11, wherein the additional steam-condensing means (6, 7) comprises a combination of the additional condenser (6) and a heat pump (7).