any base load onshore LNG plants use Air-Fin-Coolers (AFC)o remove heat for refrigeration cycle in liquefaction process.ince the required duty for cooling for refrigeration cycle isuge in recent large capacity base load onshore LNG plants,
arge number of AFCs are applied for the required large dutynd normally mounted on the center pipe rack of the LNGrocess train. For example, 4–5 MMTPA (Million Metric Ton Pernnual) capacity LNG process train has over 250 AFC fans and
hose AFCs are normally mounted on the center piperack ofhe LNG process train.
Several ongoing LNG plant projects are planning to applyodularization concept in order to mitigate environmental
mpact and difficulty of remote site construction by usingabrication yard for plant construction as a substitute of site
onstruction. The modularized onshore LNG plant equipment
∗ Corresponding author. Tel.: +81 45 682 8505; fax: +81 45 682 8850.E-mail addresses: [email protected] (M. Tanabe), atsumi@yReceived 23 September 2011; Received in revised form 11 July 2012; Ac
has to be mounted on the module structure for supporting theLNG process equipment and for allowing sea and land trans-portation. The first deck level is normally approximate 4 min height and large voids are left under the deck surroundedby 2 m deep module structure girders (Tanabe and Miyake,2010). To minimize gas accumulation in these spaces is theone of important safety aspects in the modularized onshoreLNG plant. Thus, the modularized LNG plant has higher explo-sion risk than normal stick-built LNG plant, and then the ACHmay become important indicator for safety.
High wind velocity has been observed in actual LNG pro-cess train site using AFC air cooling process (measured in2009–2010). The high wind velocity increases especially in gapsfor safety separation and maintenance access, compared withthe process area, which is congested. The forced ventilationair flow by AFC in LNG process facilities layout is shown inFigs. 1 and 2.
nu.ac.jp (A. Miyake).cepted 4 September 2012
There are several ways to reduce risk by explosion,e.g., reducing possibility of flammable gas accumulation,
neers. Published by Elsevier B.V. All rights reserved.
352 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366
LNG
Fig. 1 – Air flow by AFC in
mitigating explosion consequence (separation distance andequipment layout) and making structures withstand theexpected blast load (blast resistant design). This basic prin-ciple is applied for both onshore stick-built and modularizedLNG plants.
Although the equipment layout and the blast resistantdesign are commonly applied as countermeasure for explo-sion risk in actual plant design, such as the gap between
congested regions in view of minimization of explosion
Fig. 2 – Air flow by AFC in LNG
process train (plan view).
consequence for both stick-built and modularized plants(van den Berg and Versloot, 2003; Mores et al., 1996; Huseret al., 2009; Paterson et al., 2000; Pitblado et al., 2006), thereducing the possibility of flammable gas accumulation is notcommonly taken as a reliable safeguard since quantificationof the ventilation effect in open area is complex.
The AFC fans are normally stopped in emergency condi-tions, as the AFC is not considered safeguard, but only as
process equipment (i.e., heat exchangers). Process design of
process train (side view).
Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366 353
Require detailed vendordata (later phase)Difficulties in definingdetailed model for largescale geometrySignificantcomputational time andcost dictates that fewsimulations are able tobe run in a practicaltimescale
Not used in thisstudy
Simplified CFD model To determinetrends of in/outflows (targetvolumea)consideringhigh/low packingdensity
Actual geometryonly for largeequipmentPorosity used forcongested area(wherecalculatedporosity is lessthan 0.9)Constant AFC airflow
Early results due tothe use ofpreliminary dataCanprovide/quantifyarea ventilationtrends, and hence abasis for comparisonof design optionsSave computationaltime and cost
Sensitivity not based ondetailed geometryFlow patterns arisingfrom small scalegeometry might not beaccurately capturedThe effect of variationsof incoming air flow onfan performance is notcaptured
Used in thisstudy
a Target volumes in this study are above deck area, below deck area and gap.
Aatwvtiru
t(tvAltDfm
1
2
cg
1
2
FC specifies the required duty for the process fluid coolingnd AFC is designed to provide the required air flow rate forhe duty accordingly. However, the Air Change per Hour (ACH),hich is an indicator of ventilation and the function of area
olume and air flow rate, is not normally calculated. Thus, inhe current standard design practice, the AFC forced air flows not effectively used for enhancing the ventilation (i.e., toeduce possibility of flammable gas accumulation), in partic-lar with modular design, during emergencies.
The study is planned in two steps (1) to quantify ventila-ion by ACH as general indication for ventilation effect and2) to check gas dispersion trend (e.g., buoyancy, release direc-ion). This paper covers the first step and estimates the forcedentilation effect of AFC (i.e., the increase of ACH due to theFC induced air flow over natural ventilation) inside modu-
arized LNG process trains which have higher congestion thanraditional onshore stick built LNG plant. Computational Fluidynamics (CFD) analysis has been used for the estimates and
or evaluating the design measures for increasing ACH withoutodifying AFC process design, such as
. The increase of ventilation in the modules and gaps due towind conditions and orientation of the trains
. The impact on AFC induced air ventilation of– modules separation distances– deck material
Based on the findings from this paper, the second step willover gas dispersion study using CFD for the following modeleometries and leak parameters
. Geometries– wind conditions and orientation of the trains– modules separation distances– deck material
. Leak parameters
– leak release direction (downward and horizontal)– buoyancy (methane gas, LNG, propane).
2. Methodology
2.1. Strategy
The simplified geometry is structured, rather than accurategeometry, for the CFD model in this study. The advantage anddisadvantage of the both models are summarized in Table 1.Although detailed air flow behavior around small equipmentand piping can be identified by the detailed CFD model, itrequires significant computational cost. The simplified modelcan provide area (i.e., above module, below module and gap)ventilation, which is sufficient indicator for ventilation, andcomparison among design options by quantifying the areaventilation with reasonable computational cost.
2.2. Study basis
The basic design data of an LNG plant of 4 MMTPA capacity(recent typical base load LNG single train capacity) is used forthe study as follows:
• Plant capacity: 4 MMTPA• AFC mounted height on the center piperack: 23.4 m• Total induced air flow rate by AFCs: 22,620 m3/s• Size of LNG process train: 400 m (L) × 250 m (W)• Size of module: 40 m (W) × 40 m (L) × 17 m (Height to top
deck) including module deck height of 4 m (below deck)• Size of AFC mounted piperack: 336 m (L) × 32 m (W) × 23.4 m
(H)• Number of fan of AFC per train: 282.
2.3. Air change per hour
The increase in ventilation due to the AFC forced air flow is
evaluated based on the increase of Air Changes per Hour (ACH)compared to that for natural ventilation, used as a datum.
354 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366
Fig. 4 – Image of narrow separation distance.
The Air Change per Hour (ACH) is calculated based on thefollowing formula.
Qa = VmodR
3600(1)
where
Qa : Air Flow Rate (m3/s)
Vmod : Free Module Volume (m3)
R : Air Change Rate per Hour
Since the ACH calculation is simply related to free volumein the area and air flow rate passing through the area, it isimportant to correctly identify the detailed air flow inside thearea (Horan and Finn, 2008; Matsuura et al., 2010; Deru andBurns, 2003). Therefore, CFD analysis has been used.
2.4. Expected design sensitivity
The detailed air flow inside the process train, which is a com-ponent of the ACH calculation, is affected by such factors as
• pressure loss in the module• wind direction relative to process train orientation• wind speed• separation distance between modules• module deck material.
Taking into account above factors, the following two majorengineering options (module separation distance and deckfloor material) are identified and their sensitivity of increaseACH is evaluated by CFD.
In case the wind direction is perpendicular to the processtrain axis, AFC induced air flows passes through the modulesand the gaps between them as shown in Fig. 3. Since a modulehas certain “density” and a resulting resistance to air flow, thedifference between the module and the gap “densities” resultsin an unequally distributed air flow, i.e., greater flow velocitythrough the gap than the modules. Thus, there is a potential
advantage using narrow gaps in order to increase the air flowrate (better ventilation) in the gap as shown in Fig. 4.
Fig. 3 – Image of wide separation distance.
Selection of module deck material is one of the key designelements for modularized plant. There are two materials thatcan be used for deck floor: solid plate or grating. The solid platedeck has the advantage of reducing site work for grade pavingunder module, since spillage is collected on the deck (Fig. 5).This concept is used in offshore plants, i.e., FPSO to avoid spillon the ship hull deck. Grating deck instead has the advantageof enhancing natural ventilation, thus reducing the amountof gas accumulation under the deck (Fig. 5). This concept issimilar to traditional onshore plant, i.e., enhancing the naturalventilation.
3. CFD analysis model
3.1. General
The CFD analysis was conducted by MMI Engineering, UK. CFX(ANSYS) is used for this study due to large number of sourceterms and required mesh size.
The AFC ventilation is evaluated in terms of ACH by For-mula (1). The air flow passing through the “target” volume (e.g.,under the 1st floor deck, above 1st floor deck and gap betweenmodules) is measured for each face of the volume under con-sideration. The ACH is calculated based on the net inflow andoutflow to/from the target volume.
3.2. Assumed atmospheric conditions
The following are the major model and cases assumptionsused for this study.
• atmospheric temperature: 300.15 K• atmospheric stability class: D (neutral condition)• Vertical wind profile: Wind measurement height is 10 m
with logarithmic velocity profiles as shown in Fig. 6. Thewind speed at ground level is lower than that far aboveground due to the existence of an assumed atmosphericboundary layer, i.e., friction with the ground.
Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366 355
Fig. 5 – Side view of deck concept (solid plate vs. grating decks).
d sp
3
TTw
Fig. 6 – Win
.3. Simulated scenarios
he CFD analysis has been done for the cases shown in
able 2. The 2 wind speed cases are selected for the study,hich are 5 m/s and 10 m/s. Since AFC forced ventilation
a AFC-on: Induced flow by AFC fans.b AFC-off: Natural flow without AFC fans.
eed profile.
air flow effect is evaluated comparing to natural ventilation,lower atmospheric wind speed cases than 5 m/s are expectedto have better results for AFC ventilation effect (i.e., larger
increase of ACH in AFC-on case comparing to ACH in AFC-offcase).
Run cases
AFC-onAFC-off5 m/s10 m/s
d Perpendicular wind direction toprocess train in the longitudinaldirection (from North)Parallel wind direction to processtrain in the longitudinal direction(from West)
8 m15 m25 mGratingPlating
356 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366
RefrigerationCompressor
Package
RefrigerationCompressor
Package
Local ElectricalRoom
LocalInstrument
Room
Geometry in this box is
included in CFD geometry
Symmetry planes
ind
Fig. 7 – N-W
3.4. Simulation model
The CFD model is developed as below.
• N-Wind model and W-Wind modelIn order to keep the computational grid to a practical size,only the central part of the LNG train is included in the com-putational model as illustrated in Fig. 7 (N Wind case) and8 (W Wind case).For simulations in which the imposed wind direction is fromthe north, it is assumed that there is geometric and flowsymmetry in the planes shown in the Fig. 7. The module lay-out for the fixed module separation simulations with windfrom the north is based on this layout, with two typicalmodules included to both the north and south of the AFCs.For simulations in which the imposed wind direction is fromthe west, it is assumed that there is geometric and flow sym-metry in a plane between the two banks of AFCs, shown in
the Fig. 8. The module layout for the fixed module separa-tion simulations with wind from the west is based on this
Fig. 8 – W-Wind
model area.
layout, with four typical modules included to the south ofthe AFCs.
• Model geometry and boundaryCFD geometry and boundary is as Figs. 9 and 10.For the N wind simulations (Fig. 9), the east and westdomain boundaries are symmetry planes. All other bound-aries are defined to be far from the geometry to ensurea stable solution. The North boundary is located approx-imately 260 m upstream of the North modules; the Southboundary is located approximately 260 m downstream ofthe South modules. The domain length in the N–S direc-tion is 650 m. The top boundary of the domain is locatedapproximately 140 m above the ground level.For the W wind simulations (Fig. 10), the north boundaryis a symmetry plane. All other boundaries are defined tobe far from the geometry to ensure a stable solution. Thisis located half way between the two banks of AFCs. TheWest boundary is located 250 m upstream of the first AFCs.
The East boundary is located 300 m downstream of thefinal AFCs, so the width of the domain varies with module
model area.
Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366 357
Fig. 9 – CFD model boundary for N-Wind case(
•
perpendicular to the process train axis).
separation. As for the N wind simulations, the top bound-ary of the domain is located approximately 140 m above theground level.
AFC ModelAFC fans are modeled using the AFC air flow rate takenfrom actual LNG plant data. The number of AFC is 18. EachAFC has several bays and each bay has 3 fans. Total num-ber of fans is 282 and total volumetric flow through AFC is22,620 m3/s. In the CFD model, only AFC fans included inthe CFD geometry is modeled. The flow rate per fan is cal-culated as the total for that designated type of fan divided
by the number of fans of that type. The raised temperaturesare set at AFC outlet. The inlets to the AFCs are defined as
Fig. 10 – CFD model boundary for W-Wind c
Fig. 11 – Mesh cut plane for N Wind (Perpendicu
domain outlets with no constraint on temperature. The out-lets from the AFCs are defined as domain inlets, with the airentering the domain constrained to have the temperaturedefined by the actual design information. The temperatureis applied uniformly across the boundary.
• MeshCFD model mesh is established as shown in Figs. 11 and 12.Finer mesh size is applied to the modules and AFC and themesh size is gradually enlarged toward the model bound-ary. The minimum and maximum mesh sizes are 0.5 m and5 m. The total number of mesh elements is summarized inTable 3.
• EquationsThe equations used in the CFD are summarized in Table 4.The large equipment is modeled explicitly in the CFD model.The loss term due to small pieces of equipment is estab-lished as a friction factor only (Figs. 13–15).Module congestion is represented by applying porosityless than unity and a flow resistance source term for themomentum equation based on the Modified Porosity Dis-tributed Resistance (MPDR) model (Vianna, 2009).
In CFX there is a significant computational cost associatedwith using porosity not equal to unity. Hence the full porousmodel is only applied to congested regions with a volumeporosity � below 0.90. It is judged that regions with poros-ity greater than this would not affect the flow significantlyenough to justify the computational expense of using porosityless than unity, since the terms in equations for porosity lessthan unitiy would be almost identical to those in equationsfor porosity equal to unity in Table 4.
It would be impractical to calculate the volume porosity
� for each computational cell in the domain, so each iden-tified congested region is assumed to have uniform volume
ase (parallel to the process train axis).
lar to the Process Train Axis) simulations.
358 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366
ralle
Fig. 12 – Mesh cut plane for W Wind (pa
porosity throughout the region. The volume porosity for eachcongested region is calculated independently from the actual
model data, with the region boundaries identified throughmanual inspection of the model. The locations of these regions
Table 3 – Number of mesh elements.
Model Module se
8 m
Total number of mesh elements for N-Wind model 3,794,883
Total number of mesh elements for W-Wind model 5,169,127
Table 4 – Equations in CFD.
Terms Equations
Turbulence k−ε
Energy ∂(�h)∂t + ∇ · (� U h) = ∇ · (� ∇ T) + � :
∇ U + SE
Steady/Transient Steady state simulations
Continuity Porosity equal to unity:∂�∂t + ∇ · (� U) = 0Porosity less than unity:∂∂t � � + ∇ · (� K · U) = 0
CFX thermal energy equationSimplified equation suitable for low Mach numberflows of compressible gasesConvergence criteria of 1e−5 was used� represents the volume porosity and K representsthe area porosity tensor
� represents the volume porosity and K representsthe area porosity tensorThe term SM in the momentum equationsrepresents a momentum source term. This includesthe effects of buoyancy and any user defined flowresistance.
where Aw is the wetted area of obstacles per unitvolume (calculated based on porosity) and fi is thefriction factor.Since Aw is different for each congested region, andfi is different for each congested region andcoordinate direction, a different value of Kloss,i isspecified for each congested region and coordinatedirection.
Rei = �|ui |Dn�
Rei = �|ui |D�
Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366 359
Fig. 13 – Module 3D model assessed in CFD analysis (bird’seye view).
Fig. 14 – Module 3D model assessed in CFD analysis (norths
dm
-
Fig. 15 – Module 3D model assessed in CFD analysis (eastside).
ide).
irection, and is a function of the following which were deter-ined manually by inspection of the 3D model:
orientation of the obstacles relative to the flow direction
Fig. 16 – Location of congested reg
- typical dimension of the obstacles- pitch between the obstacles- hydraulic diameter of the spaces between the obstacles.
The parameters used to calculate friction factors are shownin Table 6.
4. CFD analysis results and discussion
The results of the CFD analysis are shown in Tables 7–9. Theresults are summarized based on the effect of AFC-on, separa-tion distance, and deck floor material in Sections 4.1, 4.2 and4.3, respectively.
4.1. Increase of ACH due to AFC-on
The effect of AFC air flow through modules is shown in Table 7.The major observation on the ACH increases due to AFC-on isthat the ACH increases especially below deck area (Deck EL.+4 m) in both N-Wind and W-Wind cases.
The increase in percentage of ACH due to the AFC-on with
5 m/s wind speed over ACH due to AFC-off is
ions in modules in piperack.
360 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366
Fig. 17 – Location of congested region in module.
Table 5 – Volume porosities for congested regions.
• N-Wind (perpendicular to the process train axis) case:- 22% whole module (as minimum)- 82% below deck (as minimum)- 179% in N-GAP (upwind side GAP) below deck level
• W-Wind (parallel to the process train axis) case:- 7% whole module (as minimum)- 36% below deck (as minimum)- 36% in GAP12 below deck level (as minimum).
Note: The increase in percentage of the ACH at low windspeed (5 m/s) is much higher than that at high wind speed
Table 6 – Parameters used to calculate friction factors.
Region Hydraulic diameter, Dh (m)
Piperack-1 N/A
Piperack-2 N/A
Piperack-3 4.05
Piperack-4 4.05
Piperack-5 0.67
Piperack-6 0.42
Piperack-7 N/A
Equipment Module-1 5.07
Equipment Module-2 4.48
(10 m/s) because the AFC-on air flow speed (approx. 2 m/s,constant) and the atmospheric wind speed (5 m/s) are close.
In the upwind gap in the case of perpendicular wind direc-tion to the process train axis (N-Wind case), the increase inpercentage of ACH is significant, as higher wind speed due tothe AFC induced air flow enhances the ACH in the gap. Theperpendicular train orientation to wind direction has betterdispersion effect in the gap for flammable gas accumulation,and it contributes to reduce the possibility to form larger gascloud covering several modules and gaps, which would resultin larger explosion impact when ignited.
With natural ventilation (AFC-off), the below deck ACHis smaller than the above deck due to vertical wind speedprofile, i.e., at ground level the horizontal speed of atmo-spheric wind is slower than mid-air due to friction withthe ground. Consequently with natural ventilation, the pos-sibility of gas accumulation below deck is higher thanabove deck area. The effect enhancing the ACH belowdeck by AFC-on is an effective measure to reduce theamount of gas accumulation below deck in the case ofleak.
Figs. 18 and 19 show the difference of air flow streamlines
between AFC-on and AFC-off. In the case of AFC-on (induced
Note: The results are based on 8 m Separation Distance.
Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366 363
air fl
oc
4
Tavitad
rate, i.e., the “channel effect” with basically no crosswind in
Fig. 18 – AFC-on
air flow), a vertical component in the flow (streamlines) isbserved. Refer to Section 4.3 for discussion for the verticalomponent.
.2. Effect of separation distance
he effect of the separation distance between modules on their flow with AFC-on is shown in Table 8. The major obser-ation on the effect of the separation distance on the ACHs that the ACH increases with shorter distance. Specifically,he increase in percentage of ACH in N-GAP (below deck) with
8 m separation distance over ACH with a 25 m separation
istance is
Fig. 19 – AFC-off air fl
ow streamlines.
• 25% (5 m/s N-Wind case – perpendicular to the process trainaxis)
• 250% (5 m/s W-Wind case – parallel to the process train axis).
In the N-wind case, this increase is simply the result ofwind speed increase by narrowing the separation distancebetween modules (gap). The ACH for W-Wind in the gaps isrelatively larger than N-Wind. However, in this case, the ACHincrease is not the results of wind speed change (the windspeed is almost constant throughout modules and gaps) butof reduced gap volume which results in a higher air change
the gaps (refer to Fig. 20).
ow streamlines.
364 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366
Fig. 20 – Wind flow by difference of separation distances for W-Wind.
between decks (by the increase of vertical component of
4.3. Effect of deck floor material
The effect of deck material on ACH is shown in Table 9. Thereare no major changes in the ACH observed for both, the AFC-on and the AFC-off, due to the effect of the deck material.However, with AFC-on below deck there is a small increase ofACH in the case of grating.
The small increase in percentage of ACH by grating belowdeck over ACH by plating is
• 5 m/s N-wind (perpendicular to the process train axis) case:- 20–30% for upwind modules- 10–20% for downwind modules
• 5 m/s W-wind (parallel to the process train axis) case:- 10–20%.
This increase is the result of better dispersion in thebelow deck area due to grating with the AFC-on, becauseAFC-on increases the vertical component of the air flow insidemodules (refer to Figs. 21 and 22). This effect also reducesgas accumulation in the below deck area and the explosionoverpressure by reducing the confinement.
Based on the above results (Sections 4.1–4.3) in the Step-1ventilation study, the following design approaches are identi-fied as possible optimization for the use of AFC-on ventilationfor reducing possibility of flammable gas accumulation inonshore modularized LNG plant.
• The AFC fans should be kept running even in emer-
gency conditions to reduce the amount of flammable gas
accumulation. (Normally AFC fan motors are stopped uponemergency shutdown condition.) Since the boil off gas ofLNG is denser than air due to its cryogenic temperature,methane gas cloud will be formed at ground level andtrapped for a sufficient length of time which may causeexplosion in congested areas, e.g., equipment and piping.Therefore, although normally AFC fan motors are stoppedupon emergency conditions, to ensure a higher degreeof safety, i.e., better ventilation, the AFC should be keptrunning even after a leak has been detected. This recom-mendation is based on the fact that the fan motors arenormally explosion proof type suitable for the hazardousarea classification Zone-2 operation to minimize the igni-tion probability. In order to further reduce the ignitionprobability, it is also worthwhile to consider applicationof Zone-1 operation certified motor. It is to be noted thatrecently in one LNG project, the use of Zone-1 motor (forvariable speed motor) has been specified.
• From ventilation aspect, the LNG train axis should be per-pendicular to the prevailing wind direction to increase ACHin the gaps and below deck. If the probability of the prevail-ing wind direction is very high, it is worthwhile to considerusing shorter separation distance between modules to fur-ther increase the ACH. However, the hazards of shorterseparation distance should be considered in the design.
• When, considering other aspects such as hot air circulation,the train axis is set parallel to the prevailing wind direction,the separation distance should be as great as possible toavoid the “channel effect” (refer to Section 4.2).
• The grating deck floor has no major effect on the ACH.However, it is worthwhile to consider using grating deckfloor in order to reduce the amount of gas accumulation
the air flow in modules). This is also better for reducing the
Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366 365
Fig. 21 – Effect of AFC and deck material on ventilation (solid plate).
k ma
arapoptitb
sl
Fig. 22 – Effect of AFC and dec
explosion overpressure by reducing the confinement (referto Figs. 21 and 22).
The above design measures shall be carefully evalu-ted also from other aspects, such as adverse effect byeducing the separation distance, hot air circulation, oper-tion/maintenance aspects. Especially, hot air circulationhenomenon and mitigation measures shall be evaluated inrder to minimize the reduction of the AFC performance androduction rate when the train orientation is perpendicularo the wind direction. However, since modularized approachncreases congestion of the plant and creates large voids underhe module deck, which are confined by very large girders, weelieve that the proposed approach shall be considered.
The future paper (Step-2) will further evaluate gas disper-
ion effect considering other parameters, such as buoyancy ofeaked gas and release direction.
terial on ventilation (grating).
5. Conclusion
The AFC induced air flow in modularized LNG plants wasevaluated using CFD analysis to optimize the use of AFC-onventilation during emergencies.
The results showed that the increases in ACH in modulesdue to AFC-on and train orientation were more than 80% in thecase of perpendicular wind direction to the process train axis(N-Wind) and more than 30% in the case of parallel wind direc-tion to the process train axis (W-Wind). Especially, the increasein ACH in the upwind gap was significant (more than 170%) inthe case of perpendicular wind direction to the process trainaxis (N-Wind).
The increases in ACH due to the separation distancesbetween modules vary significantly. The increase in ACH inthe gaps with shorter separation distance between modules
were observed to be 25% for N-Wind and 250% for W-Wind(ACH with 8 m separation/ACH with 25 m separation at 5 m/s
366 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 351–366
wind). However, the increased ACH in the W-Wind case wasmainly due to the “channel effect”. Crosswind flow, which pre-vents flammable gas accumulation in the gaps, was increasedwhen separation distance between modules is increased.
No major increases of ACH were observed with grating deckmaterial for both, the AFC-on and the AFC-off. However, thegrating deck enhanced vertical component of the AFC-on airflow inside modules.
Overall, the increase of the ACH is mainly due to the AFC-on and train orientation. The separation distance and the deckmaterials have moderate impact (increase) on the ACH in spe-cific conditions or areas. Based on the results, four designapproaches (Section 4.4) were proposed to optimize the useof AFC-on ventilation for onshore modularized LNG plant.
The effect of AFC ventilation should be further evaluatedby gas dispersion study using CFD (Step-2) in order to identifythe detailed behavior of leaked gas in the modules and thegaps.
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