Analysis of Low-Power Steam Turbine With One Extraction for Marine Applications

The paper presents thermodynamic (energy and exergy) analysis of low-power steam turbine with one extraction for marine applications. Analyzed steam turbine is divided in two parts High Pressure (HP) part before steam extraction and Low Pressure (LP) part after steam extraction. Analysis shows that HP turbine part produces the majority of cumulative turbine power and consequentially has higher mechanical, energy and exergy losses when compared to LP turbine part. Regardless of heavier operating conditions, LP turbine part has higher effi ciencies and lower specifi c losses (in both energy and exergy analysis) when compared to HP turbine part. Whole analyzed turbine has energy and exergy effi ciencies equal to 62.84% and 65.58%, while energy and exergy turbine losses are 696.74 kW and 618.50 kW. Cumulative produced power at the turbine shaft outlet is equal to 1178.40 kW. Steam extraction which divides analyzed turbine on HP and LP part can deliver a notable amount of heat to any marine heat consumer, what represents a signifi cant advantage of observed turbine in comparison with similar low-power marine steam turbines which usually does not have steam extractions.


INTRODUCTION / Uvod
Steam turbines are widely used components in power plants for electricity production all over the world [1,2]. Such steam turbines are high-power, complex components which consist of several cylinders and a number of turbine stages in each cylinder [3,4]. Small, low-power steam turbines are often used in such power plants for the drive of several power consumers, e.g. water pumps [5].
In marine applications, steam turbines can be used in various combinations and in various systems. The most common usage of steam turbines in marine applications is in the engine rooms of the LNG (Liquefi ed Natural Gas) carriers, which still has the steam propulsion system as a dominant one [6,7]. In such marine steam propulsion system all of the steam turbines (main turbine and auxiliary) are low-power turbines (main turbine maximum power usually did not exceed 30 MW, while auxiliary turbines maximum power usually did not exceed 5-6 MW) [8,9]. Auxiliary low-power steam turbines in such marine systems (turbo-generators for the electricity generator drive or steam turbines for water pumps drive) are simple steam turbines without steam extractions [10]. and ideal (isentropic) steam expansion process whose description requires assumption of constant steam specifi c entropy during the expansion process [14]. Figure 1 General scheme of the analyzed low-power steam turbine with marked operating points Slika 1. Opća shema analizirane parne turbine male snage s označenim radnim točkama As analysis in this paper is performed not only for the entire steam turbine, but also for each turbine part, energy analysis requires comparison of real (polytropic) and ideal (isentropic) steam expansion processes for each turbine part. Expansion processes (ideal and real) of each analyzed steam turbine part begin at the same steam pressure and fi nished at the same steam pressure. Therefore, for the HP part of the analyzed turbine the ideal steam expansion process (points 1-2is, Fig. 2) is compared with real steam expansion process (points 1-2, Fig. 2), while for the LP part of the analyzed turbine the ideal steam expansion process (points 2-3is, Fig. 2) is compared with real steam expansion process (points 2-3, Fig. 2) in order to be able to perform complete energy analysis of the entire turbine and both of its parts.
Exergy analysis of presented steam turbine (or any other steam turbine [15]) is based on real (polytropic) steam expansion process (points 1-2-3 in Fig. 2), which is also valid in exergy analysis of each turbine part.
From Fig. 2 can be observed that the majority of real steam expansion process through the analyzed turbine is in the superheated steam area, while only the last section of LP turbine part falls under the steam saturation line. Figure 2 Ideal (isentropic) and real (polytropic) steam expansion through the analyzed turbine presented in h-s diagram -plotted from [16] Slika 2. Idealna (izentropna) i stvarna (politropska) ekspanzija pare kroz analizanu turbinu prikazanu u h-s dijagramu -nacrtanom iz [16] Several steam turbine producers in their catalogues off er lowpower steam turbines for marine applications which have at least one steam extraction in order to ensure simultaneous power and heat production [11]. One of such low-power steam turbine, which has one steam extraction and is made for the usage in the marine applications, is analyzed in this paper. Thermodynamic (energy and exergy) analysis of steam turbine is performed, based on operating parameters provided by the producer.
Analyzed low-power steam turbine is divided in two parts -High Pressure (HP) part before steam extraction and Low Pressure (LP) part after steam extraction. Thermodynamic analysis is performed for each turbine part independently as well as for the whole turbine. Mechanical losses of each turbine part as well as of the whole turbine are included in the analysis. It is concluded that LP part produces less power in comparison with HP part, but has notable higher effi ciencies and lower losses (from both the energy and exergy point of view). Energy and exergy fl ow streams throughout each turbine part and the whole turbine are also presented and discussed.

DESCRIPTION AND OPERATING CHARACTERISTICS OF THE ANALYZED LOW-POWER STEAM TURBINE WITH ONE EXTRACTION / Opis i radne značajke analizirane parne turbine male snage s jednim oduzimanjem pare
Analyzed low-power steam turbine consists of two parts -the fi rst is High Pressure (HP) part between steam entrance into the turbine and steam extraction, while Low Pressure (LP) part can be seen between steam extraction and steam outlet from the turbine (condenser inlet), Fig. 1. Steam produced in steam generator [12] expanded fi rstly in HP part of the turbine, after which follows steam extraction. Extracted steam can be led to any marine heat consumer. Remaining steam mass fl ow rate expanded through LP part of the turbine and after expansion is led to the main marine steam condenser, which is usually shell and tube type heat exchanger [13].
Steam turbine analyzed in this paper and presented in Fig. 1 drives electricity generator with the necessary usage of gearbox which reduces turbine revolutions and ensures proper drive of any power consumer (not necessary electricity generator only). Thermodynamic analysis of this steam turbine is based on steam expansion processes -real (polytropic) expansion process whose description requires turbine operating points presented in Fig. 1

General equations for the thermodynamic analysis of any control volume or system / Opće jednadžbe za termodinamičku analizu bilo kojeg kontrolnog volumena ili sustava
Thermodynamic analysis (energy and exergy analysis) of any control volume or system is based on the fi rst and the second law of thermodynamics. The fi rst law of thermodynamics defi nes energy analysis [17], which is independent of the environment in which analyzed control volume or a system operates [18]. On the other side, exergy analysis of any control volume or system is based on the second law of thermodynamics [19] and it takes into account the parameters of the ambient in which control volume or a system operates [20].
Mass fl ow rate balance for any control volume or system in steady state, when the mass fl ow rate leakage did not occur, is [21]: (1) Disregarding potential and kinetic energy, the energy balance of a control volume or system can be defi ned, according to [22,23], by using the following equation: (2) Any fl uid fl ow has a certain amount of energy which can be defi ned as [24,25]: (3) The overall defi nition of any control volume or system energy effi ciency, according to [26,27] is: Disregarding potential and kinetic energy, the exergy balance of a control volume or system can be defi ned, according to [28,29], by using the following equation: (5) In the Eq. 5, ε is specifi c exergy, which is defi ned as [30,31]: (6) while heat X  is the net exergy transfer by heat (at temperature T), which is defi ned, according to [32,33], as: Amount of exergy for any fl uid fl ow can be defi ned according to [34,35] as follows : The overall defi nition of any control volume or system exergy effi ciency, according to [36,37] is:

Analysis of low-power steam turbine with one extraction for marine applications / Analiza parne turbine male snage s jednim oduzimanjem za primjenu u pomorstvu
Thermodynamic (energy and exergy) analysis of low-power steam turbine with one extraction for marine applications (along with the analysis of each turbine part) is performed by using turbine operating points presented in Fig. 1 and Fig. 2. The mechanical effi ciency of the whole turbine and both of its parts is assumed to be 95% (according to data from [38,39]).

Low-power steam turbine with one extraction energy analysis / Energijska analiza parne turbine male snage s jednim oduzimanjem
Real (polytropic) developed power of each turbine part and the whole turbine is: -HP part: (10) -LP part: (11) -Whole turbine:

OPERATING PARAMETERS OF LOW-POWER STEAM TURBINE WITH ONE EXTRACTION / Radni parametri parne turbine male snage s jednim oduzimanjem
Thermodynamic (energy and exergy) analysis requires knowledge of steam operating parameters (temperature, pressure and mass fl ow rate) at each turbine operating point presented in Fig. 1. Those operating parameters are found in the analyzed steam turbine producer catalogue and presented in Table 1. Steam specifi c enthalpy, specifi c entropy and specifi c exergy at the same turbine operating points were calculated by using NIST-REFPROP 9.0 software [16].
It should be noted that steam specifi c enthalpy after ideal (isentropic) expansion in HP turbine part (operating point 2is, Fig. 2) is calculated also with NIST-REFPROP 9.0 software [16] by using steam pressure at the HP turbine part outlet and the same steam specifi c entropy as at the HP turbine part inlet (operating point 1, Fig. 2). The identical procedure is used for calculation of steam specifi c enthalpy after ideal (isentropic) expansion in LP turbine part. The same calculation procedure can be found in the literature, applied during the energy analyses of other steam turbines [40,41].
Specifi c exergies in each turbine operating point (presented in Table 1) are calculated for the base ambient state (the base ambient conditions) which is in this paper taken as proposed in [42]: -pressure: p 0 = 100 kPa = 1 bar, -temperature: T 0 = 298 K = 25 °C.
Steam turbine producer for defi ned operating parameters in Table 1 specifi es an electricity generator driving power of 1100 kW. This power also includes losses in the gearbox and in electricity generator, Fig. 1. Therefore, produced power at the turbine outlet shaft should be higher than the specifi ed value for mentioned losses.

The results of energy analysis / Rezultati energijske analize
The results of low-power steam turbine energy analysis showed that the majority of real (polytropic) turbine power is produced in HP turbine part (66.10%) while LP turbine part produces  Fig. 3 (a). For the steam operating parameters specifi ed by turbine producer and presented in Table 1, cumulative real (polytropic) developed power at the turbine shaft outlet is equal to 1178.40 kW. This turbine power is calculated by using Eq. 12, which takes into account mechanical losses in both turbine parts. The accuracy of calculated real developed power at the turbine shaft outlet (1178.40 kW), along with used assumption for mechanical losses inside both turbine parts, can be easily validated. If this power is reduced for losses in the gearbox (according to [38] gearbox effi ciency can be taken between 95% and 96%) and for losses in the electricity generator (according to [39] electricity generator effi ciency is around 98%), Fig. 1, electricity generator driving power will be around 1100 kW, as specifi ed by turbine producer.
Identical to turbine real developed power, the majority of turbine mechanical losses appears in HP turbine part ( Fig. 3 (b)), while the mechanical losses of the whole turbine are equal to 62.02 kW.
Ideal (isentropic) power of the whole analyzed steam turbine is calculated by using Eq. 18 and is equal to 1875.14 kW. The majority of ideal power of the whole turbine (71.75%) refers to the HP turbine part, while only 28.25% refers to the LP turbine part, Fig. 4 (a).
Energy losses of each turbine part and whole turbine are calculated by using Eq. 19 -Eq. 21 and presented in Fig. 4 (b). Energy loss of the whole turbine is equal to 696.74 kW and the highest amount of that loss (81.33%) refers to the HP turbine part. It is interesting to observe this fact, because LP turbine part operates in much heavier conditions when compared to HP part (last stages of LP turbine part operates with wet steam which, due to water droplets, increases inner losses). However, HP turbine part has much higher energy loss (in comparison to LP part) due to signifi cantly higher steam mass fl ow rate.
Energy losses of steam turbines or its parts are reverse proportional to turbine (or its parts) energy effi ciencies [10]. The same conclusion can be obtained when compared Fig. 4 (b) and Fig. 5. LP part of the analyzed steam turbine has an energy effi ciency of 75.43% -much higher in comparison to the HP turbine part which energy effi ciency equals 57.89%. The energy effi ciency of the whole turbine is 62.84%, what is expected for low-power steam turbines [43,44].  Specifi c energy loss of the steam turbine or any of its parts is defi ned by using the same methodology as for specifi c fuel consumption, which is commonly used parameter for tracking the internal combustion engine process [45]. Specifi c energy loss of steam turbine or each turbine part is obtained by dividing of energy loss with real developed power (for the whole turbine as well as for each turbine part, Eq. 25 -Eq. 27). Specifi c energy loss presents which part in the real developed power takes an energy loss (for the whole turbine or each of it parts). From Fig.  5 can be seen that specifi c energy loss of HP turbine part has a share of 72.75% in real developed power of this turbine part. The same parameter for the LP turbine part is equal to 32.57% and for the whole turbine is equal to 59.13%.
Regardless of worse operating conditions (wet steam, high steam volume and speed), from Fig. 5 it can be seen that LP part of the analyzed turbine has much higher energy effi ciency and much lower specifi c energy loss in comparison with the HP turbine part.
Energy fl ow streams of the analyzed steam turbine (turbine real developed power and steam fl ow streams), presented in Fig. 6 are calculated by using general equations (Eq. 1 -Eq. 4) and Eq. 10 -Eq. 27 as well as steam operating parameters from Table 1. It should be noted that analyzed turbine parts (HP part and LP part) are separated in Fig. 6 in order to present energy fl ow streams of each turbine part and of the whole turbine.
Real (polytropic) produced power of the whole turbine (of the both turbine parts) is 1240.42 kW and it is reduced for mechanical losses of both turbine parts (62.02 kW), therefore power which can be used for power consumer drive (power at the turbine shaft outlet) is equal to 1178.40 kW, Fig. 6.
While presenting energy fl ow streams of the analyzed steam turbine (or any other steam turbine), energy balance equation (Eq. 2) considered only mechanical losses of each turbine part (and cumulative mechanical losses of the whole turbine) as the real steam turbine energy dissipation parameters.
In comparison with energy analysis, exergy analysis of the same steam turbine will take into account additional losses of steam (along with mechanical losses). Such steam losses (steam exergy destruction) are caused by taking into account parameters of the ambient (pressure and temperature) in which analyzed turbine operates. The energy analysis did not take into account the parameters of the ambient; therefore additional steam energy losses did not occur [46].
It should be noted that, due to the lack of data, steam mass fl ow rates lost on the front and rear gland seals of the analyzed turbine are not taken into account in this analysis. It can be assumed that the cumulative mass fl ow rate lost on both turbine gland seals (front and rear) is approximately equal to 1% of the steam mass fl ow rate which enters in the analyzed turbine [47,48].

The results of exergy analysis / Rezultati eksergijske analize
Exergy losses (exergy destructions) of the whole turbine and each turbine part are calculated by using Eq. 28 -Eq. 30. Similar to energy losses, exergy loss of HP turbine part takes a share of 76.15% in cumulative exergy loss of the whole turbine. Therefore, LP turbine part share in the cumulative exergy loss of the whole turbine is only 23.85%, Fig. 7. As can be seen from Fig. 7, cumulative exergy loss (cumulative exergy destruction) of the whole turbine is equal to 618.50 kW, and it consists of steam exergy destruction and mechanical losses of both turbine parts (HP and LP part). The exergy effi ciency of the analyzed steam turbine and both of its parts have the same trend as energy effi ciency - Fig. 5 and Fig. 8. LP turbine part has signifi cantly higher exergy effi ciency (73.03%) in comparison with the HP turbine part (62.32%), while the whole turbine exergy effi ciency is equal to 65.58%, Fig. 8. LP turbine part has lower exergy than energy effi ciency, while the exergy effi ciencies of HP turbine part and the whole turbine are higher in comparison with energy effi ciencies, Fig. 5 and Fig. 8.
Specifi c exergy destruction (specifi c exergy loss) of the whole turbine and each turbine part is calculated by using Eq. 34 -Eq. 36 and is obtained by dividing of exergy loss (exergy destruction) with real developed power. Specifi c exergy destruction of HP turbine part is much higher in comparison with the LP turbine part (60.47% vs. 36.93%), while the whole turbine has specifi c exergy destruction equal to 52.49%, Fig. 8. LP turbine part has higher exergy than energy specifi c loss, while HP turbine part and the whole turbine have much lower specifi c exergy losses when compared to specifi c energy losses, Fig. 5 and Fig. 8.
Exergy fl ow streams of steam and power throughout the analyzed turbine, according to operating parameters from Table  1 as well as by using general equations (Eq. 5 -Eq. 9) and Eq. 28 -Eq. 36 are presented in Fig. 9. Taking into account the parameters of the ambient (pressure and temperature) generates additional steam exergy loss (when compared to energy fl ow streams) which for the whole turbine is 556.48 kW. In the cumulative exergy destruction of the whole turbine are included mentioned steam exergy loss (steam exergy destruction) and mechanical losses.
The infl uence of the ambient temperature change on steam turbine exergy effi ciencies and exergy losses is usually low [49,50]. Still, it can be concluded that an increase in the ambient temperature reduces turbine exergy effi ciency and increases turbine exergy destruction [8]. For the same mechanical effi ciency of each turbine part and the whole turbine, mechanical losses will remain the same as presented in Fig. 3 (b) and in Fig. 9, regardless of the ambient temperature.

CONCLUSIONS / Zaključci
This paper focuses on the thermodynamic (energy and exergy) analysis of low-power steam turbine with one extraction for marine applications . According to the producer data, losses and effi ciencies of the whole turbine and both turbine parts (HP and LP part) are calculated. Energy and exergy fl ow streams are also presented throughout the analyzed turbine along with mechanical losses as an integral part of the analysis. The major conclusions obtained from the presented analysis are: -HP turbine part, in comparison with the LP turbine part, produces higher power (ideal and real), but also has higher mechanical, energy and exergy losses. -Regardless of the fact that LP turbine part operates in much heavier conditions in comparison with the HP turbine part (with wet steam, which has a high volume fl ow), LP turbine part has signifi cantly higher energy and exergy effi ciencies and lower specifi c energy and exergy losses (destructions). -In the turbine energy analysis, the only losses are mechanical losses (if the steam which passes through front and rear turbine gland seal is neglected). In the turbine exergy analysis additional loss occurs (steam exergy destruction). -The whole observed turbine has energy and exergy effi ciencies equal to 62.84% and 65.58%, while energy and exergy turbine losses are 696.74 kW and 618.50 kW, what are expected effi ciencies and losses for low-power steam turbine. -Analyzed turbine at the shaft outlet produces real power equal to 1178.40 kW, which can be used for any power consumer drive. -Presented steam turbine, along with produced power, delivered a notable amount of heat (by steam extraction) to any heat consumer or more of them.