Technip Using Cfd and Dynamic Simulation Tools for the Design and Optimization of Lng Plants...

7/27/2019 Technip Using Cfd and Dynamic Simulation Tools for the Design and Optimization of Lng Plants (2010.1)

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USING CFD AND DYNAMIC SIMULATION TOOLS FOR THE DESIGN ANDOPTIMIZATION

OF LNG PLANTS

Tania Simonetti1, Dominique Gadelle

1, Rajeev Nanda

2

1. LNG Department, Process Division, Technip France

2. LNG Department, Process Division, Technip Houston

Keywords: 1. CFD; 2. Dynamic simulation; 3. Hot air recirculation ; 4. Air tower design ; 5. Cooldown

procedure; 6. LNG

1 Introduction and objectivesIn recent years, an increase in LNG plant design production capacity and a step out in technology

has been observed; newly designed train capacity has risen to 6.3 MTA for OKLNG project and to 7.8 MTPA

for Qatar individual trains as examples. Along with this development, equipment sizes have grown to exceed

previous common experience while overall plant layouts have evolved towards more spread out or

congested configurations due to the need of installing larger and larger trains.

This context amplifies a need for the best possible design tools, capable not only to investigate and

prove the proper performance of critical pieces of equipment, but also to optimise capital investment in

equipment, piping and layout without compromising the proper performance of the plant.In parallel, enhanced computing capabilities have widened the domain of application of

Computational Fluid Dynamics and Dynamic simulation, allowing these tools to occupy an increasingly

important place in terms of verification and improvement of design. Nowadays, these simulato rs are capable

not only to describe the performance of a single piece of equipment but also to give a complete picture of an

installation and of its response to operation al upsets or procedures.

The purpose of this paper is to illustrate some recent applications of CFD and dynami c simulation,

where these simulators have stepped out of their traditional roles and have been employed to validate

layouts of specific areas or even whole LNG plants, or used as design tools for pieces of equipment and

layout. The use of CFD and dynamic simulation in the applications discussed, in most cases ended up in

significant economi c gains.

2 Computational Fluid Dynamics

CFD represents a powerful simulation tool that allows very accurate mechan ical and thermalmodelling. CFD is based on numerical solution of equations for the conservation of mass, movemen t

quantity, energy (refer to Appendix 1 for equations).

In general, a CFD simulation is built in two parts:

- Geometric model definition via a Computer Aided Design tool

- Mathemati cal solver based on Navier Stokes equation

The simulated domain can be modelled in 2D or 3D: a domain is defined with its boundary conditions. The

equations can be solved in steady or unsteady state.

The results can be presented in graphic form allowing immediate visualisation and interpretation of hydraulic

and thermal profile s.

These principles can be best illustrated through case studies developed during some LNG plant projects,

such Qatargas II (2 x7.8 MTPA LNG production), Qatargas III (2 x7.8 MTPA LNG production), Yemen LNG(2 x 3.45 MTPA LNG production), OKLNG .(2 x 6. 3 MTPA LNG production), Freeport Terminal LNG.

a. Case study: utilizing Air as Heat Source in Air

These air base technologies are very energy efficient, but a careful evaluation need s to be doneto quantify the advantagesbase d on the speci fic site cond itio ns. So me of the main consider atio ns are (i)the lower air temperatureduring the cold months ofthe year may require a suppleme ntary heat source, thatincrease capital and operating costs, (ii) the cold air, dueto negative buoyancy, may tend to recycle back.

Any reci rculat io n wo uld result in reduction in heat transfer area and performance j ustifying rigorousComputation al Fluid Dynamics (CFD) mode llin g, (iii) dealing with fog problem s, asair gets saturated due toits tempe rature reducti on, (iv) handli ng of conden sed moistur e from the air and resulting water disposalissues, (v) demand s on the control system to compensate for variati onsin ambient conditions that requiresdynami c process simul ations foranalysis of the system.

In an air tower or reverse-acting cooling tower, the air exchanges heat with the flowing water bydirect contact. The heat transfer mechanism in an air toweris the reverse ofa cooling t ower. The moisture inthe air condenses as airgets cooler, and there is a net production ofwater in the process. The h eat of


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condensation m akes a signi fican t contribution to the total heat duty. The excesswater is disposed of from theair tower sump.

Figure 1 show s how the air tower can be utilized for LNG vaporization. Although variousschemesare possible to integrate the air tower,one of the typical scheme s is to utilize the shell and tube exchangerfor LNG vaporization withan inte rmediate fluid such as ethy lene orpropylene g lycol flowing in a closed loopcirculation. In such a scheme, the circulating fluid circulates through the loop consisting of LNG vaporizersand intermediate exchangerswhich could be plate and frame type. When the air tower is not operatingduring the winter, theintermediate fluid isheated in a fired heater.In summer, when no heating isrequiredfrom the fired heater, the intermedi ate fluid e xchangesheat with water from the air tower. For flexibility, thesystem would be designed to have partof the heat from the air tower and part from the fi red hea ter. I tisimportant to note that thepower consum ption is significant in circulating the water bypump s forthe system.There is a point of diminishing return to extrac t the heat from the air tower as winter approaches.

Figure 1: U tiliz ing Air Tower for LNG VaporizationThe air towercan be designed conceptually byextending the concept from a normal coolingtower

with the following detailsto be addressed:i. The fill material and type should be tested to confirm that the quantityis adequate.

In the case of the air tower, water condensat ion takes place instead of

evaporation as in the cooling tower.It is important thatappropriate fill material andquantity are used.

ii. The air that comes out of the air tower is at low temperatu re and there is atendency for air to settle down due to negative buoyancy. ComputationalFluidDynamic s (CFD) mode lli ng is required to confirm t he amount of re-circulation andthe impact on design. Due to recirculation of cold air, under some conditions thetower performance candeteriorate significantly.

iii. The wind speed and direction havesignificant impact on the tower performance.Ag ain, the impact can be stud ied from CFDmodelli ng. The location of the air towerbased on the results of CFD mode llin g is key to its successful performance andoptimization ofthe design.

iv. The local ambie nt air tempe rature and f luctuation are also impo rtantcond ition s for understanding t he duration of reduced performance. These

conditions impact the design of the air tower. A backup v aporization system andits design should be also based on the same.

v. There would be a net generation of waterin the air tower due to condensation.Thi s wate r quality is gener ally th e same as rainfall, which should be drained off toa suitable location.

vi. The water that circulates in the air tower and the piping system is moderatelycorro sive. Spe cial metallurgy or int ernal coating for e quipment and pi ping isneeded. Generally, water treatmentby dosing chemicals will be very expensive asthere is a net overflow of water out of the system resulting in a loss of expensivechemi cals. Moreove r thiscould also be a permittingissue.


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Figure 2: Env elope I ndicati ngTemperature Below Ambient

Figure 2 and 3 ill ustrate the en velope of low air te mperature due to cold a ir recirculat ion. Theenve lope show s air r e-ci rcula ting back to the inlet of air tower. I tis important that the amount of recirculationbe computed as it would impact the design of the air tower.The temperature reduction at the inlet of the airtower can significantly reduce the tower performance.

It is important to note that the horizontal fan configuration w ill not pe rform well under low windspeed. This is illustrated by the CFD envelope shownbelow in Figure 3.The low wind speed resultsin thecold air settling nearthe tower intake area.The higher wind speed results in more turbulence, bettermixingand less cold air recircu lation.

Figure 3 : Tempe rature E nvelope fo r Weak Wi nd Spee d

By CFD modell ing the impact of using vertical and horizontal fansin an air tower was studied. In thefinal design for Freeport LNG Terminal the vertical fans were adopted after extensive study of localmeteorolo gical data, plot plan and the site locat ion. The overall contro l system w as extensivel y studied andverified using processdynamic simulations. Also tests were conducted to measureand validate the heatand mass transfer coefficients for the actual fill ma terial used in the air tower.

Figure 4: Comparison ofVertical and Horizontal Air TowerDesign

The maps of velocity vector and surface temperature reveal the impact due to the presence of otherequipment in the plot pl an. The interf erence f rom other equipment o n the air towe r performa nce can not beignored.


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Figure 5: Velocity VectorAround Ai r Tower

Figure 6: Surface Temperature around Air Tower

The analysis of the air tower system as illustrated above required the following aspects to beevaluated in detail:

i. Heat and Mass TransferMechanism: The heat exchange isin reverse directionwhen compared with the standard equipment utilized for similar service. Thecorrelations derived from the cooling tower design required verification throughtesting.

ii. Air Recircul ation: The plot plan and the local meteorological conditions play animpo rtant role in the design ofsuch systems.

iii. Locat ion of Equi pment :The location and orient ation of the air tower on the plotplan were found to be a key factor to its performance. Th e effect of wind speedand direction, prevalent wind direction and interference with otherequipment issignificant.

iv. Temp erature at Site: Average ambient conditions can be misleading for detailedevaluation and design. Detailed evaluation of minimum and maximumtemperatures and changes were found to bevery important for the final designand optimization of thesystem. In some case s, the average temperature at best

may be used for initial snapshot studies at the onset of the project.v. Full Backup Vaporizer during the winter: At Freeport, as in many cases, full

backup vaporizers are required for operation during the colder months. A co stbenefit analysis is required to justify the initial capital investment against fuelsavings and NOxand CO emissions.

vi. Condensation of Water: Excess water would require collection and disposal.Special metallurgicalrequirements wereevaluated, and resultedin i mprovements

such as the internal coating of the wa ter ci rculation pipe .


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b. Case study: CFD application to slug catcher performance assessment

In this case study, CFD simula tion has been used to assess gas distribution and incoming liquid

separation efficiency in a large fingertype slug catcher consisting of 12 x 48fingers.

Gas distribution was successfully simulated using 3Dsegregated implicit solver. This gave velocity

(refer to fig ure 7 here bel ow) and pressure profile throughout the slug catcher.

Figure 7: Slug catcher performance assessment via CFD: velocity profile

Liquid separation efficiency was modelled by injection of liquid droplets. Two droplet injectionmodels were dev elo ped ; the assumption s underlying each one are the following:

i. No shear: in this case all drople ts agglomerate and form liqui d film when theyenter in contact with any wall in the slug catcher

ii. Shear: in this second ca se only the dropletsthat enter in contact with fingerswallsolely, are trapped and agglomerate.

The range of droplet sizes used in the model varies from 1 to 400 m.Based on these CFD simulations, the amount of stopped and escape d dro plets from the slug

catcher could be computed. In either case, the efficiency of the slug catcher in terms of gravitati onalseparation could be assessed by plotting the curves of percentage of trapped droplets against dropletdiameter per each type ofdroplet injectionmodel.

The actual efficiency of the slug catcher in terms of liquid separation versus droplet diame te r issomewhere in between th e no-shear assumption caseplot and shear case a ssumption pl ot (see here bel owfigure 8).


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Figure 8: Slug catcherperformance assessment via CFD:liquid separation efficiency

The geometry, boundary conditions and fluid zones drawn by using the Gambit software. The hydraulic

behaviour simulated with Fluent software.

c. Case study: Hot air recirculation studies in LNG plant

In LNG pla nt, LNG production capacity is directly linked to refrigeration power. This refrigerationpower is dependant uponambient air temperaturebecause of theinflu ence on g as turb ine availab le powe rwhen these are used as mechanical drives, and also b ecause it determines the refrigerant condensingtemperature when air is used as the cooli ng medi a. Consequently , ambient air temperature directly af fectsLNG production.

Hot air reci rculation studies aim toevalua te actual air temperature at the inlet of both gastu rbinesand air coolers. Airtemperature may in fact be higher than suggested by site meteoro logical records due torecirculati on of hot air from sources such as air coolers plum e and exhaust stacks. The results of such astudy are used then to validate the layoutand plot plan of theinstallation.

In the case study described, CFD was used to evaluate hot air recirculation and to validate thelayout of a large, two t rain West African facility . The CFD simula tion model i ncluded two LNG trains, LNG

and LPG tanks.

The model geom etry was built taking into account all large-scale obstacles such as compre ssorhouses, driers, substations, technical rooms,etc and significant detailsin congestedareas, e.g. cable trays,

piperack, zones below main compressors (nozzles, pipes,auxiliaries). On the other hand, downwind unitswere considered to have alesser impact on aircirculation and wereexcluded from the model.

The model took into account atmospheric conditions, e.g. prevailing wind directions, ambient

temperature, wind velocity and turbulenceprofiles.

The sources of hot air for this application were the gas turbines and waste heat recovery unit

exhaust gase s. Refer to figure 9 for viewsof the mode l and hot ai r sources.


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Figure 9: CFD applica tion to hot ai r recirculati on study: twotrains model for CFD simulation.The study le d to the follow ing results:

i. Air temperature rise observed at each air cooler and gas turbine inlet for all theselected wind conditions.

ii. Therma l amplification observation: gas turbines exhaust led to local temperaturerise higher than 50C whilst air coolers caused l ocally tem pe rature rise of 25C.This allowed identifying specific a reas where the air temperature rise with respectto forecastambient temperature may impact the design of equipmen t. Figures 1 0and 11 offer a visual appreciation of the conf igurati on and associa ted a irtemperature rise.

Figure 10: CFD application to hot air recirculation study: thermal amplification.

AIR COOLERS OUTLET310 TO 337K


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Figure 1 1: CFD application to hot air recirculation study: two trains resultin g heat plume.

Validation of the configuration viaair recirculat ion stud y:The air temperature rise wasdetermined for actual site conditions and the performance of affected

equipment was able to be checked.In conclusion, the air recirculation study allowed the efficiency of theLNG plant to be confirmed and to validate t he selec ted plot plan.

d. CFD applied to other engineering studies

CFD has proved to be avaluable tool for a number of other engineering studi es su ch as vapou r/liquid separation, compressor suction line h ydraulics, optimisatio n of compressor suction piping layout,optimisat ion of piping routing upstream of critical separators.

i. Case study: CFD application to vapour/ liquid disengagement in large LNG trains

propane evaporators.

Propane evaporators areat the core of the LNG plant, and good plantperfo rmanc e requires the

pressure drop tobe minimised. A critical issue in these evaporators isthe good separation of liquiddropletsin the ch iller s. The objecti ve of thi sstudy was to evaluate hydraulic behaviour withrespect to pres sure dropand separation efficiency, good separation efficiency tr anslati ng into homogeneous and optimal veloci tyacross the evaporator mesh.

The evaporators studied were HP, MP, LP and LLP chillersof Feed Gas and Mixed Refrigerant in alarge baseload projec t using Air ProductsC3/MR process.

Simulations were built so as to examine only the gaseous phase above High Liquid Level.Thegeome tries of the evaporators including nozzles, headers and wiremesh mist eliminators were fullydescribed in the models.

The results ofthis case study indicate d that the operation ofthese chillers is satisfactory:i. Pressure drops were all within a percentage of operating pressure that is

acceptable for thistype of equipment.ii. Mesh velocities were ina range that isjudged acceptab le for li qu id separation.

Therefore, the study led to the confirmation that the operation of these chillers issatisfactory.In addition a number of recommendations for the design ofthe evaporators were able tobe made,

such as the preferred use of aheader with elbows rather than a T to reduce pressure drop.Such results are especially valuable for large capacity equipment that represent astep out with

respect to experience and referenced equipment sizes.

ii. Case study: CFD application to verific ation of compressors suction line hydraulics

Verification of line hydraulicswith CFD answers a need for design andoptimisation of criticallines,with a tool that is versatile and user friendly. In LNG plantsminimisation of pressure drop in compressorsuction lines brings significant gains in te rms of compressor power, which in turn allows an increase inavailable refrigerationduty and LNG production.

In view ofthis objective, CFDhas been used on a number ofrecent projects to screen compressor

suction lines thoroughly for pressure drop and velocity profi le at t he compre ssor f lange and val idatemodifications.In the case study six compre ssors suction lineswere verified: Low pressure, Medium Pressure, High

Pressure Propane and LowPressure, Medium Pressure, HighPress ure M ixed Refr igerant.


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The CFD geometry took into account the line itself and detailed geometry ofconical fi lters, ventu ris,butte rfly an d non-slam check valves, drums and associated internals (vane type distributors; me sh pad),kettles and outlet manifold.

The CFD output consisted of complete pressure drop and velocity profiles. These results have

allowed the following: i. Verification of compliance with compressor vendor requirem ents for pressure

drop,ii. Analysis of distribution at compressor nozzles and impleme ntation of

modifications to layoutand piping when necessary.The most remarkable modifications that resulted from the study were as fol lows:

i. LP MR suction line: CFD showed that pressure drop exceede d vendorrequirements. In addition poor di stribution was observed at the compressorflange. CFD w as then appl ied to differentline configurationsand sizes leading toa solut ion whe re the dia meter w as incr eased t o 64fro m 60. In fig ure 12, theresults for gas distribution across compre ssor nozzles can be compared for thetwo conf iguratio ns, t he former based on standard (good) enginee ring p racticecriteria applied to line sizing and the latterdesigned using CFD. In addition, as afinal result the resulting pressure drop was reduced by 45% with respect to theini tial con figuration.

ii. MP MR suction reducer geometry was re-specified to improve the velocitydistribution acrossthe inlet nozzle. This allowed a reduction in pressure drop of12%. Figure 13 shows the CFD output from the different tests that lead to thefinally retained arrangemen t. It can be seen tha t CFD output allows a quitestraightforward interpretation ofresult s.

iii. The CFD study first showed pressure drop to be signif icant. From the velocity andpressure drop profile, it was possible to identify the most effective modificationthat consistedin increasing the distribution pipe diameter at the kettle manifold: acomparison of t he two c onfigurations and resulting pressure drops and v elocityprofile can be se en in figure 14.

Figure 1 2 CFD application to compressor suctio n line hydraulics verificati on: LP mixed refrigerant suction

line.


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Figure 1 3: CFD application for compressors suction line hydraulics verification: MP Mixed refrigerant suction

line.

Figure 14: CFD application forcompressors suction line hydraulics verification: modifications in LPPropane suct ion system.

This study showsfirstly how CFD simulation can accurately picturethe pressureand velocity profilesof a given system, allowi ng modelli ng of fi ttings such ascontrol valves, st rainers, check valves and internals.

Pressure drop calculations arefar more accurate than with standard engineering tools.


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Furthermore, CFD allowed severaltests to be performed in order to f ind the optimum solution. Theimprovement in flow pattern resulting from each modification coul d be easily visually appreciatedfrom the

output profil es.

iii. Case study: application of CFD for layout design for refrigerant compressors

suction lines

CFD can be used to optimise compressor layout for minimum investment cost whil e respec tingpressure drop constraints.

On one project aCFD study ofthe Low Pressure Propane Compress or suction li ne wascarried outto find the minimum c ompresso r table height that still met the specifie d maxi mum pres sure d rop thusachieving substantial economic savings. In fact, through CFD it was possible to identify an improvedarrangement for suction line fittings (elbows, strainers) to meet the allowable pressure drop and with anacceptable flow patt ern. Figure 15 shows the effect on velocity profile at the compressor inlet nozzl e for adifference of one metrein compressor table heigh t. Figure 16 shows the nozzle veloc ity profile with the finalgeom etry. In this case study CFD proved to be an effective la youtdesign tool , allowing a clear basis fo r

discarding a costly configuration.

Figure 1 5: Impact of raising compressor suction table height on LP Propane case study: CFD output.


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Figure 1 6: Impact of revised geometry after CFD study for LP Propane case study.

iv. Other case studies: CFD application to verification of critical separators

performances

CFD allows the proper operation of separator internals tobe verified. Such performance verificationsare usually carried out with thepurpose to ensure the correct operation of theseparator, nonetheless CFDcan also be used to relax piping routing criteria upstream critical drums internals with a view to reducingstraight lengths and amore compact insta llati on.

For one LNG project, CFD simulations ofcriti cal separa tors such as the LP MR suction drum and LPC3 suctio n drum l ed to th e relax ing of s traight length requirements between the last elbow and the druminlet, thus simplifying the piping routing. Figure 17 and figure 18 show the results of these CFD simulations:it can be eas ily seen th at the velocity profile at the mesh inlet is rather homogenous, indicating gooddistribution acrossthe section and efficient dropletseparation despite thereduced straight lengthsupstreamthe drums.


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Figure 1 7: LP MR suction drum separator performance: velocity distribution at mesh inlet.

Figure 1 8: LP Propane suction drum separator performance: velocity distribution at mesh inlet.

CFD applied to separatorsmay alternatively lead to separator size reduction, bringing substantialinvestment savings. Asan example, separator volume reduction after CFD verification is summarised in theTable 1: d ata are ta ken from an o ptimi satio n study aimed at reduci ng dru m inve stmen t cost whil emaintaining gas liquid separation performance. In thiscase study all the CFD study drums are fitted withmulti-vane inlet deviceand wiremesh mist eliminators.

Table 1: reductions of critical drums

Drum number Weight of initial Weight after Weight Material


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design (Tons) optimization (Tons) Reduction (Tons)

106 -V - 101 206.7 171.7 35 304L SS

106 - V - 102 148.5 120.2 28.3 304L SS

106 - V - 103 172.6 79.4 93.2 LTCS

106 - V - 104 66.6 60.1 6.5 LTCS

106 - V - 105 63.9 57.6 6.3 LTCS

106 - V - 106 58.5 52.3 6.2 LTCS

106 - V - 107 53.8 48.7 5.1 LTCS

The total weight reduction for the stainless steel drum was 63 tons on each LNG train .

3 Dynamic simulationDynamic simulation allows modelling of transient behaviour in processes bringi ng new information

useful for system design that cannot be represented with static simulations. Dynamic simulation can address

many aspects of process plant design.

In the case studies presented, the problems successfull y studied include cooldown procedure for a

LNG pipe network, and a typical case of compressor dynamic simulation .

a. Case study: Cooldown dynamic study of a LNG pipe network.

The interest of optimisation of cooldown procedure in LNG rundown/loading systems lies in the gain

over LNG and cold gas flows used and in duration shortening for such complex operation.

The case study for this application was the CLP storage rundo wn and loading lines belonging to LNG

production facilities different than those providing cold gas and LNG for the f irst cooldown. It was then of

utmost importance to limit the duration and the flowrate of cold gas and LNG taken from outside production

entity. In this context, dynamic simulation has proven a highly efficient tool to tailor up the cooldown

procedure in the perspective of minimising the use of cold gas and LNG and duration.

In the study for CLP storage cooldown, the network is composed of a rundown system of 4km of 22

pipe, a cross over line 500m of 10 pipe and a loading loop with 16km 36 pipe. The cold gas was brought

through a 3000 m long pipe of 6 size coming from existing external production facilities. The limit of the

system is imposed by the allowable back pressure at cold gas injection point. Pressure at the other end of

the network is set. Figure 1 9 gives a schematic view of the network configuration. It is apparent that the 6

line segment represented the controlling section with respect to allowable pressure drop and allowable

velocity during the transient for the cold gas.

Figure 1 9: Cooldown dynamic study of LNG pipe network: simplified view of the studied network.


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The selected cooldown procedure consists of:

i. Step 1: rundown system cooldown with 20t/h of cold gas during 24 hours.

Maximum allowable back pressure is respected while cooling down the

system to -90C. Loading loop starts to cool down.

ii. Transition: cold gas flowrate reduced to 0t/h between t=24h and t=25h to

allow LNG injection: back pressure is decreased to 1.1 bara and LNG

available at 6.7 bara.

iii. Step 2: once cold gas flowrate is reduced to 0t/h, LNG flowrate is increased

up to 28t/h. The rundown system quickly cools down, and loading li nes

progressively finishes their cooldown in parallel. When LNG arrives liquid at

berth 5 (refer to picture 1 9), parallel circulation must be stopped and loop

circulation is required. At that time, fluid is about -150C and pipes wall

temperature are aroun d -80C. Jetty drum can be partially filled (up to 10-

20%) so that pipes wall temperatures decreases below -100C.

The overall profile of reached temperature versus time obtained via dynamic simulation is shown in

figures 20 and 2 1 for cooldown of rundown lines and cooldown of loading lines respectivel y.

Figure 20: Cooldown dynamic study of LNG pipe network: overall cooldown of rundown lines chart, achievedwall temperature versus elapsed time.


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Figure 2 1: Cooldown dynamic study of LNG pipe network: overall cooldown of loading lines chart, achieved

wall temperature versus elapsed time.

The use of dynamic simulation allows several tests to be run on a built model, in this specific context

this made evident that it is not required to cool down the loading loop with cold gas before letting in LNG

injection. By this method, cooldown duration and involved flowrates of cold gas could be optimised.

Conclusivel y, dynamic simulation applied to network has proven to be an efficient and flexible means

of validating and customising an operating complex procedure.

Figure 22 Cooldown dynamic study of LNG pipe network: overview of CLP cooldown network.


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b. Case study: Dynamic study of flash gas compressor in LNG plant

In this case study, a dynamic simulation applied to this flash gas compressor was performed on the

end flash gas compressor provided downstream the liquefaction facilities in order to confirm the transient

phase operati on such as the start-up sequence of the compressor and the compressor behaviour during

trips. The result s of such simulation permitted to precisely identify the suitability of selected material such as

antisurge valve s size, the need for additional valves, e.g. hot gas bypass, in case of surge during transient,

verification of the proper cooldown rate procedure during start-up in order to protect machine at all operating

conditions. Therefore the outcome of the dynamic simulation has a direct impact over installation andprovided instrumentation.

The studied flash gas compressor was a three stage fixed speed machine, feeding the HP fuel gas

network. Flash gas compressor compress LNG flash gas from 0.24barg up to 28.5barg. First stage is

provided with inlet guide vane valves and suction drum. Each stage is provided with air cooled aftercooler s

to reduce interstage temperature to 45 C.

The dynamic simulation has been carried out with Hysis dynamics; the model has been filled in with

vendor compressor curves depicti ng polytropic height versus suction flow. The pressure drop in pipe s and air

coolers is rated based on design conditions. The valves included in dynamic simul ation are filled in with the

installed Cv, including anti -surge valve data from compressor vendor.

i. Dynamic simulation: start -up sequence development

The flash gas start -up sequence has been defined as follows:

i. Initial conditions correspond to the following configuration: outlet shutdown valve is closed;inlet suction control valve is fully open; inlet guide vanes are closed at 70; antisurge valve

is in manual mode and fully open position; pressure control valve to flare is closed;

compressor loop is pressurised at suction pressure, i.e. 0.24 barg.

ii. Step 1 - motor start -up: in this phase dynamic simulation showed that required load torque

curve providing compressor acceleration stay behind the available torque curve at 70%

and 100% available voltage. Therefore no problem is encountered to start up the

compressor.

iii. Step 2 - compressor cooldown: the compressor has reached nominal speed. A start-up

pressure control val ve is provided to flare gas from discharge, thus allowing to cooldown

the system and place the compressor on line. During this step this valve to flare is opened

in manual mode while cold gas is allo wed in compressor.

iv. Step 3 closure of antisurge valve and opening of antisurge bypass valve: the antisurge

controller is switched to automatic mode, therefore antisurge valve closes down. The

relevant closure time is defined from vendor data and specification. Antisurge bypass

valves open so as to compensate the closure of antisurge valves. The antisurge bypass

valves are used to regulate recirculation so as to slowdown or increase the cooldown rate.

The cooldown rate is set in accordance with machine vendor requirement.

v. Step 4 closure of antisurge bypass valves: after cooldown the antisurge bypass valves

are set back to automatic mode and close down. No impact on compressor operating

parameters is observed. In this phase the cooldown of the compressor is continued

vi. Step 5 At the last phase of cooldown, the antisurge bypass valve s are closed and

compressor suction temperature reaches -63C.

vii. Step 6 Comp ressor on line with process: the IGVs are opened manually from Control

Room in order to increase the discharge pressure of the compressor. The IGVs are

gradually open so as to reach the required pressure to discharge into high pressure fuel

gas network. When this is achieved, the pressure control valve to flare is set to automati c

mode.

The dynamic simulation permitted then to ensure the following:

Compressor motor torque is suffici ent for compressor to reach nominal speed with suction

valve open.

Compressor can be adequately cooled down and put in line via provided pressure controlvalve to flare .

ii. Dynamic simulation: compressor behaviour upon trip

In the dynamic simulator, it is p ossible to carry out a trip scenario once the start-up scenario is

stabilized. The assumption behind a trip dynamic simulation was the blocked outlet case; this assumptionwas selected because for the actual design and configuration represented the worst case scenario.

In picture 22, one can see the evolution of process conditions during transient.


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Figure 2 3: Dynamic simulation: flash gas compressor trip under blocked outlet, exampl e of process

parameters variation.

It is observed that upon blocked out let, as soon as the discharge pressure rises up to 30 bara, the

compressor is shutdown by the process safety logic, then at this point the antisurge valves are fully open ed

(2 seconds time) and inlet suction valve (10 seconds time) is closed. The study showed that each stage

compressor is subject to surge in case of high speeds prior to motor complete stop.

Based on compressor vendor data, the mentioned surge phenomenon under trip conditions may

bring the machine to reverse rotation in case the settling pressure is not reached before total stop of the

motor. If this takes place, the remedy to protect the machine is the installation of a hot gas bypass valve.

However, by analysing this specific dynamic simulatio n data, it was observed that settle out pressure is

reached in 21 seconds after trip, when the motor was still at 22% of nominal speed; therefore the surge

under trip conditions does not pose a threat in view of lifetime of the compressor according to manufacturer.

Conclusivel y, the dynamic simulation not only permitted to observe real behaviour of the machine

upon process upsets and to identify the concerns to machine integrity during transient phases (surge), but

also allowed to take the most suitable countermeasures to ensure the safety of equipment.

4 Conclusions and Way forward

From the above case studies, one can observe that CFD and dynamic simulation offer the following

advantages:

Accurate depiction of flow patterns and response of the systems that are not availablethroughout static simulation

Precise identifications of deviations from process requirements in performance, either in one

single equipment or with respect to overall LNG plant layout

Quantification of such deviations

Several alternates feasible for a given design problem thus allowing to find the optimum

solutions from economic and performance point of view

Easy visual interpretation of flow pattern and consequence of design modifications

Realistic simulation of the dynamic response of a given system, being a unit or the overall

plant, thus leading to tailor procedures (cooldown, but also start up and turn- down are

some of the potential examples) so as to minimise flaring and improve operability of the plant

All the above findings permit to integrate the CFD and dynamic simulat ion as design tools in projects.

This leads to the following:

Confirmation of plant efficiency and LNG guarantee production

Confirmation of layout


19/19

Confirmation of plot plan design

Achievement of CAPEX savings in equipment sizes

Achievement of CAPEX savin gs in piping length

Increase in compactness of installation

Reduction in weight of equipment

Reduction of flaring due to tailored operating procedures

All the above benefits are even more attractive in the near future development of LNG plant ,expected to be on moving supports, where gains in compactness, weight, piping lengths and layout design

are even more crucial issues to designer and investor.

AcknowledgementsMatthieu Chambert, Technip Process Division

Phil Hagyard, Technip LNG product line

Jocelyne Launois, Technip Process Division

Julien Metayer, Technip Process Division

Henri Paradowski, Technip Process Division

Date post:	02-Apr-2018
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Technip Using Cfd and Dynamic Simulation Tools for the Design and Optimization of Lng Plants...

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