Computers & Fluids 94 (2014) 30–36

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Computers & Fluids

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Modelling of valve induced water hammer phenomena in a districtheating system

http://dx.doi.org/10.1016/j.compfluid.2014.01.0350045-7930/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +370 37 401 922.E-mail addresses: algis@mail.lei.lt (A. Kaliatka), minde@mail.lei.lt (M. Vaišnoras),

valinc@mail.lei.lt (M. Valincius).

Algirdas Kaliatka, Mindaugas Vaišnoras, Mindaugas Valincius ⇑Laboratory of Nuclear Installation Safety, Lithuanian Energy Institute, Breslaujos str. 3, LT-44403 Kaunas, Lithuania

a r t i c l e i n f o

Article history:Received 5 July 2013Received in revised form 5 January 2014Accepted 29 January 2014Available online 10 February 2014

Keywords:Accident analysisDistrict heatingModellingRELAP5Water hammer

a b s t r a c t

Water hammer is one of the most dangerous phenomena in liquid or liquid/gas systems, because it cancause failure of the system integrity. A water hammer in a district heating system is investigated in thisarticle. The reasons of the water hammer phenomenon are investigated quite well in the scientificliterature. However, the conditions when the water hammer may occur depend on the specific systemand the thermal–hydraulic specifics of the system. In this paper an accident scenario of blackout in apump station is investigated and the analysis of fast check valve closure due to a pump station blackoutis presented in the paper. A computer code RELAP5 was employed to perform accident analysis. The anal-ysis showed that under some hypothetical conditions, pressure peak could exceed the value used duringthe hydraulic tests of the pipelines.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Water hammer is a pressure or momentum transient in a closedsystem caused by a rapid change in fluid velocity. It is classifiedaccording to the cause of the velocity change. Generally waterhammer can occur in any thermal–hydraulic systems and it isextremely dangerous for the thermal–hydraulic system since, ifthe pressure induced exceeds the pressure range of a pipe givenby the manufacturer, it can lead to the failure of the pipeline integ-rity. Water hammer features are described in excellent textbooks,such as Wylie and Streeter (1985), Moody [7] and Chaudhry andHanif [1].

There are three basic types of severe water hammer occurringat piping systems that can result in significant system damage[12]:

� valve-induced water hammer;� void-induced water hammer;� condensation-induced water hammer.

The cases, when the water hammers occurred in the pipelinesystems as the consequences of standard actions such as start-upor shut-down of systems and components, opening or closure ofvalves, switch-over from one component (e.g. pump, heat

exchanger) to another, belong to the first type of water hammer.The most severe water hammers may be caused due to rapid isola-tion valve closure.

The interaction of sub-cooled water with condensing steam in apipeline may be the cause of a void-induced (second type) waterhammer. In this case, the pressure difference between the part ofpipeline filled by water and part where the condensation of steamappears accelerates water in the pipe, and the water hammer ap-pears when the water column is abruptly stopped by the closedend of the pipe.

The third type of water hammer usually is related to steam sys-tems. Pressure pulses in the presence of liquid and vapour (forexample due to accumulation of condensate (water) trapped in aportion of horizontal steam piping) can lead to rapid condensationof the vapour, leading to the so-called condensation-induced waterhammer.

In this paper, a water hammer phenomenon in a district heatingnetwork was analysed. Generally, the sub-cooled single phasewater is used as a coolant in district heating systems (DHS), there-fore, we concentrating only on the first type – water hammer dueto a fast valve operation.

2. Modelling of valve-induced water hammer phenomena inpipeline systems

To understand the processes in pipeline system in the case offirst type of water hammer (valve-induced water hammer), theexperimental investigations performed at Fraunhofer Institute for

Nomenclature

CHP combined heat and power (plant)DHS district heating systemFSI Fluid–Structure InteractionDx length of computational volume (m)dt time step (s)

dtcrnt courant time step (s)t time (s)

A. Kaliatka et al. / Computers & Fluids 94 (2014) 30–36 31

Environmental, Safety and Energy Technology, UMSICHT (Fig. 1)should be mentioned [2]. Water hammer tests conducted in thisfacility – depressurization inducing cavitation water hammer initi-ated by a fast valve closure. After valve closure at time t = 0 s, thepressure decreases to saturation pressure because the liquid moveson. Thus big vapour bubbles are created. Since the pressure at thereservoir is constant, the liquid flows backwards; the bubble con-denses downstream at the (still closed) valve and causes a pressurepeak (cavitational hammer).

To understand and be able to model this phenomenon, theexperimental data [8] obtained in the UMSICHT experimental facil-ity was employed. In Lithuanian Energy Institute, the water ham-mer has been numerically investigated since the year of 2000 [9].For the numerical modelling the system thermalhydraulic codeRELAP5 was used taking into account that there is wide experiencein use of this program tool in the institute.

The computer code RELAP5 [4] was developed by Idaho (U.S.)National Laboratory for the analysis of nuclear reactors cooling cir-cuit failures. Fluid flow is described by one-dimensional, non-homogeneous and non-equilibrium two-fluid model, using sixequations: energy, mass and momentum conservation equationsfor both liquid and vapour phases. It uses flow regime dependentapproach, semi-empirical closure equations of interfacial energy,mass and momentum transfer are used in the conservation equa-tions. Also, specific modules to model critical flow, heat release,pumps, valves, branches, etc., are implemented into the code. TheRELAP5 code, being highly generic, has found use in a variety offluid transient problems, including water hammer analysis in pip-ing systems [10]. Unfortunately, the RELAP5 code has a significantlimitation in prediction of complex water hammer phenomena.The standard water hammer and the Fluid–Structure Interaction(FSI) theory consider the correction of the speed of sound in a fluidas a consequence of the pipe elasticity. Obviously the elasticity oftubes also influences the speed of sound and one of the featuresnot covered by RELAP5 code is elasticity of the pipe wall, whichaffects the propagation of the pressure waves in the pipe. Pressurewaves are weaker if pipe elasticity is taken into account. Unfortu-

Fig. 1. Perspective view of pilot plant pipework [2].

nately, but FSI models are not integrated into RELAP5 code. Despitethe fact that RELAP5 is not the most appropriate or user-friendlycode for solving water hammer problems, but it is well docu-mented, easy to access and able to produce reliable results, andtherefore is commonly used for such purposes in many countries.For instance, the RELAP5 code has been applied to study the waterhammer event due to clearing of a water slug downstream of asafety relief valve [3], to investigate water cannon phenomenon[10] and the dynamic of a liquid slug driven by a non-condensablegas [11]. In all these studies, it has been demonstrated that thecode reasonably predicts the physical behaviour of the transientevent.

The benchmarking analysis of experimental measurements andcalculation results was performed by LEI specialists [5]. Sufficientlyclose agreement between the calculation results using RELAP5code and the measured values of peak pressure in test case per-formed at UMSICHT test facility was found (Fig. 2).

During simulation of UMSICHT experiment it has been noticedthat initial values of input parameters (both from initial andboundary conditions, and RELAP5 code models, assumptions andcorrelations) have a significant impact on the water hammer mod-elling results. The investigations of benchmarking have shown twogroups of parameters [5]:

(1) parameters of thermal–hydraulic system conditions

� initial fluid velocity;� pressure;� water temperature;� flow energy loss coefficient in the piping;� valve closure time;

(2) parameters of modelling� calculation time step;� scheme nodalization.

It has been determined from the benchmarking analysis thatthe most contributing factors with respect to the maximum pres-sure values are [5]:

� the ratio of current time step (dt) and current Couranttime step (dtcrnt) should not exceed 0.1 at modelling ofwater hammer transients using RELAP5;

Fig. 2. Pressure history downstream the closed valve [5].

32 A. Kaliatka et al. / Computers & Fluids 94 (2014) 30–36

� the length of one computational volume Dx should notexceed 0.5 m in the modelled pipelines where water ham-mer is expected to occur;

� at modelling it is necessary to properly select initial con-ditions (pressure and temperature) also to check up val-ues of flow energy loss coefficients in modelledpipelines to obtain correct initial fluid velocity in system;

� it is very important to correctly simulate the valve, toobtain proper time of it closing.

Gained experience from benchmarking water hammer calcula-tions, which was used for the nuclear reactors licensing purposes,can be used also for the analysis of processes in long distance com-plicated water pipeline systems – i.e., district heating system.

3. Development of Kaunas city district heating pipeline networkmodel

As it was described in the above, previous studies have shownthat RELAP5/MOD3.3 computer code is able to simulate waterhammer phenomena in pipeline systems. This computer code is in-tended to be used for the accident analysis in nuclear reactors.However, there are other thermal hydraulic systems, which canexperience the same phenomena, and where this tool (RELAP5) isapplicable, e.g., DHS. A DHS is a hydraulic system, which useswater as a heating agent. RELAP5, being a complex system hydrau-lics computer code, is able to calculate all the phenomena observedin a district heating network. This computer code has previouslybeen used to perform analysis of pipe break accident in a DHS[6] and showed that it can perform decently.

The thermal–hydraulic model of the system is very useful forassessing a variety of network modifications and steady-state con-ditions (for example: construction of a new line, closure of a sepa-rate branch of the pipeline, etc.). The model becomes very handy ifit is able to simulate transients (the trip of circulation pumps indistrict heating supply network, etc.) and more importantly, theemergency processes (processes in the case of pipeline break, lossof gas, oil or water, the pressure changes in the system, etc.).

The Kaunas city (Lithuania) heat supply network supplies theheat to the 105 thousand flats and more as 3 thousand companiesand organizations. The total installed thermal power, used for dis-trict heating in Kaunas city is 679 MW. The main Combined Heatand Power plant of Kaunas (Kaunas CHP plant) is located in theeastern part of the city. There is one more CHP plant with a smallercapacity (265.8 MW thermal) – ‘‘Petraši�unuz’’ CHP plant, and a few

Fig. 3. Layout of Kaunas h

pump stations (‘‘Jonavos’’, ‘‘Šilko’’ and ‘‘Pergales’’) are used to en-sure necessary water circulation in the heat supply network. Thetotal length of the main pipelines is about 100 km, while the lengthof pipelines, connecting main network with the consumers’ heatsupply units is an additional 300 km. The layout of Kaunas districtheating pipeline network is shown in Fig. 3.

Kaunas district heating network model, developed for RELAP5code, was based on the information about the length, diameter,roughness and elevation change of pipe sections in the network,as well as the information about heat consumption. The heat con-sumption data was used to determine the required water flowrates through the consumers’ heating systems. Heat removal as aprocess was not modelled, since the model was developed toinvestigate system hydraulics, rather than temperature profilesor other thermodynamic parameters of the network. The modelof the entire district heating system is too complicated to illustrate.Every single line in Fig. 3 means two pipelines – supply and return,which are modelled as in principle demonstrated in Fig. 4 duringthe development of the model.

Both supply and return lines were modelled using RELAP5‘‘pipe’’ elements. Since RELAP5 is not a dedicated district heatingsystem modelling software, there is no possibility to define ‘‘twopipe’’ or ‘‘three pipe’’ pipelines. Therefore, the supply and returnpipelines must be modelled separately. The consumers heatingsystems are modelled using valve elements, connecting the supplyand return lines in the specified places of the heat supply network.The required water flow rates through the consumers’ heating sys-tems were set by tuning the flow energy loss coefficient for thevalve, which models the specified consumer heating system. Thisis exactly the same way as in a real consumer heating system,which also uses a valve on the heating water inlet, to adjust the re-quired heating water flow rate through the heat exchanger. In suchway all connected consumer heating systems were modelled. Themain hot water source (Kaunas CHP plant) is modelled by two vol-umes (one for hot water supply and one for water return line). Thepressures in these volumes are set constant, depending on theinvestigated operation regime – 0.78 MPa for water supply source,and 0.23 MPa for water return sink (gauge pressure). The pumps in‘‘Pergales’’, ‘‘Šilko’’, ‘‘Jonavos’’ pump stations and ‘‘Petraši�unuz’’ CHPplant are modelled by special RELAP5 pump models, with specifiedcapacities and pump head data.

When the model has been developed, it was calibrated by usingmeasured pressure values at several locations. The model has beenused for various normal operation calculations in order to definepressure drop over the pipelines, to evaluate possibility of

eat supply network.

Fig. 4. Principle of model developed for RELAP5 code. Apostrophe (‘) marks a return line, ‘‘C’’ is a consumer, connected to the system.

Fig. 5. Simplified layout of Kaunas central part (‘‘Pergales’’) pump station. 1 – pumpbypass line valve (opening time 35 s); 2 – a throttle valve (closing time 35 s); 3 –pump; 4 – check valve; 5 – safety relief valve (opens permanently if pressureexceeds 0.72 MPa).

A. Kaliatka et al. / Computers & Fluids 94 (2014) 30–36 33

connecting new consumers to the network, etc. The results havebeen compared between two different kinds of software andshowed quite good agreement. However, the most important andinteresting task was to model accident scenarios, where transientprocesses are important and static pressure drop models, whichare implemented in various dedicated DH network modellingsoftware do not work. This model has also been used to performanalysis of transients during pipe break accident in a DHS [6].

4. Transients in the DH network in case of a pump trip in apump station

Kaunas city centre is located below the heat source (KaunasCHP plant) and elevation change is about 40 m. At 0.5 MPa pres-sure in the supply line at the higher elevation (0 m), the pressureis about 0.9 MPa at the lower elevation (�40 m). To ensure watersupply in Kaunas city centre, the pump station is equipped witha throttle valve and a pump (in Fig. 3 it is marked as ‘‘Pergales’’pump station). A simplified layout of the pump station is presentedin Fig. 5. As it can be seen from the scheme, the supplied water flowis throttled by a throttle valve (2) from 0.9 MPa down to 0.55 MPaand supplied to the city centre. The pressure of returning water isincreased from 0.33 MPa up to 0.8 MPa by pumps (3). In case ofpump trip in ‘‘Pergales’’ pump station, water is not pumped fromthe city centre, and the pressure in water supply lines startsincreasing. To prevent the pressure increase to dangerous limitand to avoid accidents (e.g. damaging consumers’ heating sys-tems), the pump station is equipped with a safety system(Fig. 5): valve (1) automatically opens and reduces the pressurein the supply line; the valve (2) automatically closes and termi-nates the water supply to the city centre; check valve (4) preventsthe backflow; the safety relief valve (5) prevents overpressure incircuit (in case of accident the heating water is discharged throughthis valve to the environment). The diameter of the safety reliefvalve is 200 mm.

The pressure change in the pump station during the pump tripin ‘‘Pergales‘‘ pump station, when all safety systems work as de-signed is presented in Fig. 6. After the pump trip (Fig. 6a), thepressure in the city centre starts increasing until it reaches0.72 MPa downstream of the throttle valve (Fig. 6b). Then thethrottle valve starts closing while the bypass valve starts opening

at the same time. In this particular case these valves open/closequite slowly, therefore, the pressure in the return line from thecity centre upstream of the pump station reaches 0.72 MPa andthe relief valve opens (Fig. 6c). The throttle valve is closed in42 s after beginning of the accident, thus the discharge of waterfrom the safety relief valve is terminated. As we can see, opera-tion of the safety systems ensured that the pressure will not ex-ceed the dangerous limits.

5. Analysis of the valve-induced water hammer phenomenon incase of blackout in the pump station

In case of the blackout, the activation of bypass and throttlevalves, which are electro-motor driven, would also fail. Thus, theadditional analysis was performed, assuming failure of the bypassand the throttle valves. In this case the opening of the safety reliefvalve prevents the overpressure in the district heating pipeline cir-cuit and consumers’ heat supply units (Fig. 7). However, becausethe throttle valve remains open, the water from the district heatingpipeline network is discharged through the relief valve to the envi-ronment, leading to high losses of heating water. In this case onlythe check valve, which is in the passive safety system in the pumpstation (see Fig. 5) is closing after pump trip, preventing the backflow in water return line in the section between CHP plant andpump station.

Fig. 6. Pressure in ‘‘Pergales’’ pump station in the case of pump trip, when all safetysystems operate as designed: a – pump trip (t = 0 s), b – pressure in water supplyline downstream the throttle valve reaches 0.72 MPa (t = 8 s), c – pressure in thereturn line upstream the pump reaches 0.72 MPa (t = 10 s) – opening of the reliefsafety valve.

Fig. 7. Pressure in ‘‘Pergales’’ pump station in the station blackout case.

Fig. 8. Water flow velocity through the check valve (assuming check valve nevercloses).

Fig. 9. Pressure in the pipeline connected to the check valve.

34 A. Kaliatka et al. / Computers & Fluids 94 (2014) 30–36

The numerical studies were carried out using the best estimatecomputer code RELAP5. The result is one best estimate value perparameter, valid only for a given case. A recommendation fromKaliatka et al. [5] was used, that the current time step to thecurrent Courant time step ratio is dt/dtcrnt 6 0.1. The ratio of dt/dtcrnt 6 0.04 was used for the analysis of this accident. The nodelength of 0.5 m near the valve was used as the recommendationfrom Kaliatka et al. [5].

As it was mentioned in the introduction of this article, the mostcommon event which causes the water hammer in a single phase,all-liquid piping system is a fast closure of a valve. In case of black-out in ‘‘Pergales’’ pump station, the check valve is one of the safetymeasures to prevent backflow in the return line, so that the waterdoes not leak from the pipelines through the safety relief valve. Thecheck valve installed in the pump station is wafer type swing checkvalve. Such valve automatically closes due to flapper disk weightwhen there is no water flow or there is back flow. As backflow can-not accelerate before closing the valve, there is no water hammerobserved under normal operation of the valve. However, this valveis always open under normal conditions and it can become stuckand fail to close during the accident. Thus, for the detailed analysisthe worst scenario – i.e. the valve closure when there’s the highestwater back flow in the return line, was selected. Also, since thewater velocities are quite small in these large 600 mm diameterpipelines, an additional hypothetical scenario was analysed withsafety relief valve diameter set to a diameter of a full 600 mm

pipeline, instead of actual valve diameter of 200 mm. Larger waterdischarge area increases water velocity and should lead to biggerpressure peaks when the check valve closes.

The first task was to determine the time when the backflowreaches its maximum velocity. Therefore, the pump station black-out accident scenario was modelled with the check valve alwaysopened. As we can see from Fig. 8, the maximum backflow velocityof 0.7 m/s is reached at 40 s after the blackout in the case with200 mm diameter safety relief valve, and 0.9 m/s at 49 s in the casewith 600 mm diameter safety relief valve.

The next task was to model transients in the pipelines when thecheck valve closes at 40 s and 49 s in each case respectively. To re-ceive the conservative results, a very fast check valve closure wasassumed – the valve closes in 10 ms. Calculated pressures of bothcases in the pipeline connected to the check valve are presented inFig. 9. The results show that the maximum pressure peak reachesabout 1.7 MPa in the case with the 200 mm diameter safety reliefvalve and about 2.0 MPa in the case with the 600 mm diametersafety relief valve. These pressure peaks are quite big, but annualhydraulic tests of the pipelines are performed under pressure of2 MPa. Therefore, these pressure peaks do not exceed hydraulictest pressure limit. The main reason why the pressure peak is notas big as in the UMSICHT experimental facility is that the initialflow velocity is significantly lower. In the UMSICHT case it was

A. Kaliatka et al. / Computers & Fluids 94 (2014) 30–36 35

4.5 m/s while in our analysed cases it is only 0.7 m/s and 0.9 m/s.Although, as it was expected, higher initial flow velocity resultedin higher pressure peak in the second case.

Let’s analyse the processes in the pipeline (Fig. 10), where thehighest pressure peak is reached. Before closing the check valve(while the water is discharging through a safety relief valve) thepressure at higher elevation decreases to saturation pressure. Attime interval t > 14 s in this pipeline at higher elevation (location‘‘2’’ in Fig. 10) the steam appears due to cavitation (see Fig. 11).After closing the check valve, the flow momentum is transformedinto pressure and pressure peak appears (see Fig. 12 at t = 40 sand t = 49 s) in the pipe connected to the check valve (location‘‘1’’ in Fig. 10). The pressure wave then travels through the pipelineto the highest elevation (Fig. 10 from location ‘‘1’’ to location ‘‘2’’).When the pressure wave reaches the pipeline at highest elevation,the pressure becomes higher than saturation pressure, and steamcollapses (see Fig. 11 at t = 42 s and t = 51 s). At the same time

Fig. 10. A fragment of the pipeline from the pump station up to the highestelevation part of the pipeline with 2 marked reference locations. Location ‘‘1’’ – 0 mfrom the check valve, elevation 0 m; ‘‘2’’ – 2115 m from the check valve, elevation+38 m.

Fig. 11. Void fraction at reference location ‘‘2’’.

Fig. 12. Pressure at reference locations ‘‘1’’ and ‘‘2’’.

the single pressure peak in the part of pipeline at higher elevationappears (see Fig. 12 at t = 41.3–44 s and t = 50.3–53). After thesteam collapses, the pressure wave travels back to the check valvecausing the second pressure peak (see Fig. 12 at t = 43–45 s andt = 52–54.5 s), which is not a typical shape of a water hammer.As we can see from Fig. 11, the steam fraction increases after thesafety relief valve opens and then starts decreasing even beforeclosing the check valve – this happens due to flow changing itsdirection, accelerating, and then stabilizing in this pipeline. Thecheck valve closes and steam collapses when the steam fractionis already significantly decreased.

It was shown that the second pressure wave is caused by steamcollapse. Collapsing larger quantity of steam could cause higherpressure peaks, therefore, further investigations were performed.The case with the 600 mm diameter safety relief valve was mod-elled and the check valve was closed in the model at different timemoments. The pressure peaks in the pipeline connected to thecheck valve (Fig. 10, location ‘‘1’’) were compared and the resultsare shown in Fig. 13. Void fraction at highest elevation (Fig. 10,location ‘‘2’’) is shown in Fig. 14. After performing a set of numer-ical experiments (Fig. 13 and Fig. 14 only shows a few of them), thehighest pressure wave of 2.1 MPa was observed when the checkvalve closes at time t = 43 s.

Comparing these test cases we can conclude, that the moresteam collapses, the bigger pressure oscillations occurs betweenthe first and second pressure peaks. However, this study showedthat larger quantities of steam collapsing do not always lead to

Fig. 13. Pressure in the pipeline connected to the check valve (location ‘‘1’’), whenthe check valve closes at different time moments, marked in the legend.

Fig. 14. Steam fraction in the pipeline at highest elevation (location ‘‘2’’), when thecheck valve closes at different time moments, which are marked in the legend.

36 A. Kaliatka et al. / Computers & Fluids 94 (2014) 30–36

the bigger pressure peak. The highest pressure peak of 2.1 MPa isabove the hydraulic test pressure limit and could be potentiallydangerous for pipelines.

The analysis of ‘‘Pergales’’ pump station blackout accident sce-nario showed that even in the worst case scenario, the pressuredue to water hammer would not exceed hydraulic test pressurelimit. The pressure limit was reached only in a hypothetical sce-nario, assuming different physical system configuration. Suchnumerical analysis of water hammer phenomenon is very impor-tant, because there is no possibility to perform real life tests in areal integral network. Therefore, transients during accidents canbe estimated only numerically.

6. Conclusions

The water hammer can occur in any water filled pipeline and itis extremely dangerous since, if the pressure induced exceeds thepressure range of a pipe given by the manufacturer, it can lead tothe failure of the pipeline integrity. In this paper the water hammerphenomena, induced fast closure of valve in the pipeline of districtheating system was analysed.

For the modelling of accident scenarios in a district heating net-work the nuclear reactor accident analysis computer code RELAP5was applied. The pressure transients during pump trip in a pumpstation were modelled. The model was demonstrated to be ableto take into account safety system operation logics, realistic pumpsand valves operation.

In case of blackout accident in a pump station, the pressurepeak due to water hammer was predicted to be lower than thehydraulic test pressure limit. Even when the conditions were setto the most conservative scenario (check valve closes at highestwater velocity through the valve), it took to assume different phys-ical system configuration (600 mm diameter safety relief valve) toreach the hydraulic test pressure limit.

As it was demonstrated in this paper, the pressure peaksdepend on: (1) initial water velocity before closing the check valve;(2) steam appearing due to cavitation, and steam collapsing. A de-tailed study has shown that the pressure peaks due to steam col-lapse depends on time moment when it collapses. Under some

conditions a steam collapse process could cause a pressure peak,which is potentially dangerous for the system.

The performed analysis allows to evaluate the consequences ofthe possible accidents in the district heating system and to assessthe reliability of this system.

Acknowledgement

This research was funded by a grant (No. ATE-04/2012) fromthe Research Council of Lithuania. This research was performedin cooperation with the SC –Kaunas Energy.

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