Depressurisation: A Practical GuideThis guide has been prepared based upon questions frequently asked regarding the Dynamic Depressuring utility introduced in HYSYS 3.0. It should pro ide users !ith an e"planation ho! to use the utility and correctly interpret the results. It is di ided into three sections# $.0 % er ie! &.0 'dding and (onfiguring the )tility 3.0 *"ample +roblem 1.0 Overview Why are there two Depressuring utility options? The original Depressuring utility in HYSYS !as a pseudo,dynamic calculation based on a series of steady state calculations. The Dynamic Depressuring utility !as introduced in HYSYS 3.0 to allo! users to perform proper time,dependant calculations. ' HYSYS Dynamics licence is NOT required to use this ne! utility. What can this utility be used for? The Depressuring utility can be used to simulate the depressurisation of gas- gas,liquid filled esselspipelines and systems !ith se eral connected essels or piping olumes depressuring through a single al e. .eferences to / essel0 in this guide can also refer to piping or combinations of the t!o. What types of depressuring calculations can be performed? There are t!o ma1or types of depressuring calculations a ailable# • • Fire Mode is used to model a essel or pipe under fire conditions. This mode has three sub,types# 2ire- 2ire 3etted and 'lternati e 2ire. Adiabatic Mode is used to model the blo!do!n of pressure essels or piping !ith no e"ternal heat supplied. ' more in depth discussion of the different methods follo!s in Section &.0. 2.0 Addin and !on"i urin t#e $tilit% How to add the utility ' Depressuring utility can be added to the case by selecting 4Tools4 4)tilities4- highlighting 4Depressuring , Dynamics4 and pressing the 4'dd )tility4 5utton. You may note that the original Hyprotech Technical Support Knowledge Base Article $ Depressuring model is still sho!n on the 4' ailable )tilities4 menu- this option !ill be discontinued after ersion 3.0.$ and all e"isting models !ill be con erted to the ne! Dynamic utility. !onnections How to connect the utility to a stream %n the 4Design4 tab- 4(onnections4 page- choose the stream that represents the fluid you !ant to use as the source for the depressuring. If you ha e a single essel- for e"ample- the stream !ould be the feed stream into the essel. 'ttaching the stream to the utility is accomplished as sho!n in the ie! belo!. +ress the arro! and select the inlet stream from the drop,do!n list. Entering Vessel Parameters Ideally- the essel si6e !ill be kno!n and this data can be entered into the appropriate fields on the form sho!n abo e. If the essel si6e is unkno!n- then the essel si6ing utility in HYSYS can be used to estimate the required parameters. The initial liquid olume is normally calculated at the normal liquid le el 7899:. The heads of the essel are not taken into account so the olume !ill be the liquid in the cylindrical portion only. If the feed stream is t!o,phase- the equilibrium composition of the liquid !ill be calculated. If an initial liquid olume is not specified- HYSYS !ill take a olume equal to the olumetric flo! of the feed liquid o er one hour. This may be disproportionate to the total essel olume. HYSYS does not take account of the heads in a essel so olumes and areas are calculated as for a cylinder. The total essel olume is calculated from the diameter and height 7or length for a hori6ontal essel:. To account for piping or head olume contributions- a small amount can be added to the height or length of the essel. If the condition of the system at settle out are such that the apour is superheated- HYSYS !ill not allo! a liquid in entory. The settle out conditions for mi"ed sources and olumes are calculated on a constant enthalpy- olume and mass basis. Hyprotech Technical Support Knowledge Base Article & (orrection 2actors allo! for ad1ustments to the amount of metal in contact !ith the top or bottom of the essel. This can also be used to account for additional no66les- piping- strapping or support steel!ork in close contact !ith the essel. HYSYS !ill use the heat content of this metal !hen performing the calculations. This is analogous to adding- for e"ample- ten percent to the essel mass to account for fittings. !on"i ure &trip !#arts 3hen the Depressuring utility is run- all data is stored using strip charts. Three default strip charts are added !hen the utility is added. It is possible to remo e ariables by deselecting the appropriate ariable in the 4'cti e4 column. ' ariable can be added by pressing the 4'dd ;ariable4 button and selecting it from the list of simulation ariables. 'ny configuration to the strip charts should be done before the utility is run- other!ise any ne! ariables !ill not be stored. To ie! data in tabular form- press the 4;ie! Historical Data<4 button. To ie! data in graphical form- press the 4;ie! Strip (hart<4 button. 'eat Flu( Para)eters %n this page- the type of depressuring to be performed is specified. The different modes and their respecti e equations are described here. • Fire Mode can be used to simulate plant emergency conditions that !ould occur during a plant fire. +ressure- temperature and flo! profiles are calculated for the application of an e"ternal heat source to a essel- piping or combination of items. Heat flu" into the fluid is user defined using the follo!ing equation# Q = C1 + C2 × time + C3 ( C4 − TVESSEL ) + C5 × LiquidVolumetime=t LiquidVolumetime=0 Hyprotech Technical Support Knowledge Base Article 3 The 2ire equation can also be used to simulate the depressuring of sub,sea pipelines !here heat transfer occurs bet!een sea!ater and the pipeline. If (3 !as equal to )'- (= !as equal to T$ and ($(& and (> !ere equal to 6ero- the abo e equation !ould reduce to# Q =UA( ∆T ) • Fire *etted Mode uses similar heat flu" parameters to those used in 2ire mode. Three coefficients# ($- (& and (3 must be specified. The equation used by HYSYS is an e"tension to the standard AP+ equation for heat flu" to a liquid containing essel. ' !etted area is required and used to calculate the heat transfer into the essel. The follo!ing notes are based on e"tracts from ?uide for +ressure,.elie ing and Depressuring System- '+I .ecommended +ractice >&$- 2orth *dition- @arch $AAB. The amount of heat absorbed by a essel e"posed to an open fire is affected by# a: The type of fuel feeding the fire b: The degree to !hich the essel is en eloped by the flames 7a function of si6e and shape: c: 'ny fireproofing on the essel The follo!ing equations are based on conditions !here there is prompt fire fighting and adequate drainage of flammable materials a!ay from the essel. AP+ ,-uation 7field units: AP+ ,-uation 7metric units: Q = 21000 × F × A 0.82 C D total absorption to !etted surface 75T)Eh: 2 D en ironmental factor ' D total !etted surface 7ft &: C D total absorption to !etted surface 7kFEs 2 D en ironmental factor ' D total !etted surface 7m &: Q = 43.116 × F × A0.82 Environmental Factor Table > on +age $B of '+I >&$ lists 2 factors for arious types of essels and insulation. 2or a bare essel- 2 D $. 2or earth,co ered storage- 2 D 0.03. 2or belo!,grade storage- 2 D 0. 2or insulated essels- users should consult the reference and select an 2 alue based on the insulation conductance for fire e"posure conditions. Wetted Area The surface area !etted by the internal liquid content of the essel is effecti e in generating apour !hen the e"terior of the essel is e"posed to fire. To determine apour generation it is only necessary to take into account that portion of the essel that is !etted by liquid up to B.Gm 7&>ft: Hyprotech Technical Support Knowledge Base Article = abo e the source of the flame. This usually refers to ground le el but it can be any le el capable of sustaining a pool fire. The follo!ing table indicates recommended olumes for partially filled essels. ;olumes abo e B.Gm are normally e"cluded as are essel heads protected by support skirts. T%pe o" .essel 9iquid full 7e.g.# treaters: Surge drums- knockout drums and process essels 2ractionating columns 3orking storage Spheres and spheroids 1 Portion o" /i-uid +nventor% 'll 7up to B.Gm: 8ormal operating liquid le el 7up to B.Gm: 8ormal le el in the bottom plus liquid hold up from all the trays dumped to the normal le el in the column bottom. Total !etted surface only calculated up to B.Gm 1 @a"imum in entory le el 7up to B.Gm: *ither the ma"imum hori6ontal diameter or B.Gm- !hiche er is greater .eboiler le el is to be included if the reboiler is an integral part of the column. The HYSYS equation is an e"tension of the standard '+I equation. Therefore- in field units- ($ !ill be &$000 multiplied by the en ironmental factor- 2 and (& !ill 0.H&. 7In most cases- ($ !ill be equal to &$000:. Q = C1 × (WettedAreatime=t ) C2 3etted area at time t is defined by the follo!ing equation# LiquidVolumetime =t WettedAreatime =t = WettedAreatime =0 × C3 × 1 − LiquidVolume time =0 Hyprotech Technical Support Knowledge Base Article > The follo!ing table is an e"ample sho!ing ho! the ( 3 term affects the !etted area calculation. 'n initial liquid olume of Gm 3 and a !etted area of >00 m & !ere gi en. (3 $ 0.B> 0.> 0.&> 0 3etted 'rea 3etted 'rea 3etted 'rea 3etted 'rea 3etted 'rea 7m&: 7m &: 7m &: 7m&: 7m &: >00.0 >00.0 >00.0 >00.0 >00.0 333.3 3B>.0 =$G.B =>H.3 >00.0 &>0.0 3$&.> 3B>.0 =3B.> >00.0 $GG.B &>0.0 333.3 =$G.B >00.0 Ti)e 1)inutes2 /i-uid .olu)e 1) 32 .olu)e 0atio 0 > $0 $> G = 3 & $.0 0.B 0.> 0.3 Therefore if a (3 alue of 0 is used- the initial !etted area is used throughout the calculations. This could represent a !orst case scenario. 'lternati ely- if a (3 alue of $ !as used- the olume !ould ary proportionally !ith the liquid olume. This !ould represent a ertical essel. H ! ! "#$#%& 'uild ()$* +,-W, .!!/E Depressuring Heat 2lu" *quation is incorrect if 2ield units are selected. If the fire !etted equation is used !hile field units are selected 7i.e.# 5T)Eh:- the heat flu" equation used by the Depressuring utility !ill be incorrect. There is a problem !ith the con ersion bet!een SI and 2ield units. Instead of using the normal '+I coefficient of &$000- the alue of ($ should be multiplied by B 7i.e.# $=B000:. This !ill correct for the unit con ersion problem. 5ecause of this defect- the follo!ing equations should be# AP+ ,-uation ,-uation $nits Area $nits C D $=B000 I 2 ' 0.H& 5T)Eh ft& C D $>>&0$ I 2 ' 0.H& JFEh m& C D =3.$$G I 2 ' 0.H& JFEs m& • Alternative Fire Mode uses the 5olt6man constant to take into account radiation- forced con ection- flame temperature and ambient temperature. The method may be considered as an alternati e method to the '+I standard. Q = Atotal × ε × ε ! × % (T + 2$3.1# ) − ( TV + 2$3.1# ) + out"ideU × ( Tamb − TV ) 4 4 ( ( ) ) !here# ' total Kf K k Tf T outside ) Tamb D total !etted surface area D flame emissi ity D essel emissi ity D 5olt6man constant D flame temperature D essel temperature D con ecti e heat transfer bet!een essel and air D ambient air temp generally ranges from 0.& to 0.> 7for burning hea y H(s: generally ranges from 0.> to $ 7for polished metal: equals >.GBI$0 , H 3Em & J= $>00 J and up!ards Hyprotech Technical Support Knowledge Base Article G • Adiabatic Mode can be used to model the gas blo!do!n of pressure essels or piping. 8o e"ternal heat is applied so no parameters need to be entered in this section. Heat flu" bet!een the essel !all and the fluid is modelled as the fluid temperature drops due to the depressurisation. Typical use of this mode is the depressuring of compressor loops on emergency shutdo!n. • $se &preads#eet is an option that allo!s the user access to the spreadsheet used by the depressuring utility. ;alues can be altered in this spreadsheet and additional equations substituted for calculation of the heat flu". It is recommended that this option only be used by ad anced users. 'eat /oss Para)eters There are three types of Heat 9oss models a ailable# $. None# does not account for any heat loss &. &i)ple# allo!s the user to either specify the heat loss directly or ha e it calculated from specified alues 3. Detailed# allo!s the user to specify a more detailed set of heat loss parameters &i)ple Model Hyprotech Technical Support Knowledge Base Article B • • • 'n o erall ) alue can be specified in this section. Heat Transfer 'rea is the cylindrical area of the essel !ith no allo!ance for head area. This alue is calculated using the essel dimensions specified on the 4(onnections4 page. )sing the Simple Heat 9oss @odel- heat loss from the essel is calculated using the follo!ing formula# Q = UA(T luid − Tambie&t ) Detailed Model The duty can be applied to the essel !all or directly to the fluid. The former !ould be used to model a fire and the latter to model a heater. There are four portions of the model to be set up. They are ?eneral(onduction- (on ection and (orrelation (onstants. 0eneral The ?eneral section allo!s the user to manipulate .ecycle *fficiencies and the ambient temperature. The default alue for all three .ecycle *fficiencies is $00L. This means that all material in the essel has been flashed together and is in thermodynamic equilibrium. If the .ecycle *fficiencies !ere to be reduced a portion of the material !ould by,pass the flash calculation and the apour and liquid !ould no longer instantaneously reach equilibrium. In this case- the phases may ha e different temperatures. Hyprotech Technical Support Knowledge Base Article H )nfortunately- there is no single typical number suggested for these parameters. The best option !ould be to try arious scenarios and obser e the results. 1onduction The (onduction parameters allo! the user to manipulate the conducti e properties of the !all and insulation. The metal !all thickness must al!ays ha e a finite alue 7i.e.# it cannot be MemptyN:. To model a essel !ithout insulation- the insulation alue thickness should be 6ero. )sers are also required to enter the specific heat capacity of the material7s:- the density of the material7s: and the conducti ity of the material7s:. Some typical alues for metals are# Metal @ild steel Stainless steel 'luminium Titanium (opper 5rass 1onvection The (on ection ie! allo!s users to manipulate the heat transfer coefficient for inside and outside the essel as !ell as bet!een apour and liquid material inside the essel. Densit% kgEm BHG0 BA30 &B$0 =>=0 HA30 H>00 3 &peci"ic 'eat kFEkg J 0.=&0 0.>$0 0.A$3 0.>&3 0.3H> 0.3B0 T#er)al !onductivit% 3Em J G3 $>0 &0$ &3 3H> $$0 Hyprotech Technical Support Knowledge Base Article A To use a set of fi"ed ) alues- the 4)se 2i"ed )4 option should be selected. If the ) alues are unkno!n- the user can press the 4*stimate (oefficients 8o!4 button and ha e HYSYS determine the ) alues. In order to ha e HYSYS ary the ) alues throughout the depressuring scenario- select the 4(ontinually )pdate )4 alue. 1orrelation 1oefficients This feature gi es users the opportunity to manipulate the coefficients used in the heat transfer correlation. 5y selecting 4)se Specified (onstants4- the user may manually enter the constants used in the heat transfer correlations. The equation !hich determines the outside heat transfer coefficient for air is# ∆T ( = C × le&'t( m The equation used for the other three correlations is# +u = C × ( *r × )r ) 3here# 8u D 8usselt 8umber ?r D ?rashof 8umber m Hyprotech Technical Support Knowledge Base Article $0 +r D +randtl 8umber .alve Para)eters The ;al e +arameters page allo!s users to select the type of al es to be used for both apour and liquid ser ice. In most cases- either the 2isher or the .elief al e should be used for al e si6ing. Their equations are more ad anced than some of the others and can automatically handle choked conditions. 2urthermore- these t!o al e types support other options that can be accessed through the al e property ie! accessible through the Depressuring sub,flo!sheet. The se en a ailable al e types are described in the sections that follo!. Fisher The 2isher option uses the standard al e option in HYSYS. It allo!s the user to specify both al e ( and percent opening. 5y pressing the 4Si6e ;al e4- the al e can be si6ed for a gi en flo! rate. Hyprotech Technical Support Knowledge Base Article $$ %nce the appropriate Si6ing (onditions ha e been enteredpress the 4Si6e ;al e4 button to ha e HYSYS determine the al e ( . 2elief Valve The relief al e option uses the standard HYSYS relief al e. The user can specify orifice area 7or diameter:- relief pressure and full open pressure. The user is required also to specify an orifice discharge coefficient. To ha e the relief al e open at all times- enter a full open pressure that is lo!er than the final e"pected essel pressure and a set pressure that is only slightly lo!er than the full open pressure. !upersonic The supersonic al e equation can be used for modelling systems !hen no detailed information on the al e is a ailable. The discharge coefficient 7( d: should be a alue bet!een 0.B and $. + $ refers to the upstream pressure and ρ$ the density. F = C d × A × ( )1 × ρ1 ) 0.# !ubsonic The subsonic al e equation can also be used for modelling systems !hen no detailed information on the al e is a ailable but the flo! is sub,critical. This can occur !hen the upstream pressure is less than Hyprotech Technical Support Knowledge Base Article $& t!ice the backpressure. The discharge coefficient 7(d: should be a alue bet!een 0 and $. The area 7': should be a alue bet!een 0.B and $. +$ refers to the upstream pressure and ρ$ the density. ( )1 + )ba,% ) × ( )1 − )ba,% ) F = Cd × A × ρ1 )1 0.# +back refers 5ack +ressure It is possible to ha e the depressuring scenario cycle bet!een pressure build,up and relief. To perform this analysis- ensure a reasonable pressure differential and increase the number of pressure steps. 3asoneilan This equation !as taken from the @asoneilan catalogue. It can be used for general depressuring al es to flare. 3hen this option is selected- the user must specify ( and (f. The remaining parameters in the equation are set by the Depressuring utility. F = C1 ×C! ×C ×- ×( ) 1 × ρ1 ) !here# ($ ( (f Yf y +$ ρ$ 0eneral D D D D D D D D $.GGG3 7SI )nits: 3H.HG 72ield )nits: al e coefficient 7often kno!n from endor data: critical flo! factor y , 0.$=Hy3 e"pansion factor upstream pressure upstream density 0.# The ?eneral al e equation is based on the equation used to calculate critical flo! through a no66le as sho!n in Perry's Chemical Engineers' Handbook . It should be used !hen the al e throat area is kno!n. 8ote that this equation makes certain limiting assumptions concerning the characteristics of the orifice. F = Cd × A! × . term × ( ' , × )1 × ρ1 × % ) !here# (d ' Jterm k +$ D D D D D discharge coefficient throat cross sectional area 2 20 % +1 / % +1 ratio of specific heats 7(pE( : upstream pressure %+ 1 0.# Hyprotech Technical Support Knowledge Base Article $3 ρ$ $ D upstream density +age >,$=- *quation >.&0 7Gth *dition: O +age $0,$>- *quation $0.&G 7B th *dition: ,o Flow This option indicates that there is no flo! through the al e. /se !preadsheet .ecommended only for ad anced users- this option allo!s the user to customise a al e equation by editing the al e spreadsheet found inside the Depressuring sub,flo!sheet. +ressing the 4;ie! Spreadsheet<4 button !ill open the spreadsheet. Discharge 1oefficient 3hen the relief- supersonic- subsonic or general al e is selected- the user is required to specify a discharge coefficient. This correction factor accounts for the ena contracta effect. ;alues ranging from 0.G to 0.B are typically used. In order to disregard this effect- set the discharge coefficient equal to $. Options Hyprotech Technical Support Knowledge Base Article $= 4+; 3ork Term (ontribution4 refers to the isentropic efficiency of the process. ' re ersible process should ha e a alue of $00L and an isenthalpic process should ha e a alue of 0L. 2or gas,filled systems- alues range from HBL to AHL. 2or liquid filled systems the number ranges from =0L to B0L. ' higher isentropic efficiency results in a lo!er final temperature. Operatin !onditions -perating Parameters %perating pressure refers to the initial essel pressure. 5y default- this alue is the pressure of the inlet stream. The time step si6e refers to the integration step si6e. It may be a good idea to reduce the step si6e if the flo! rate is significantly larger than the olume or if the essel depressurises in a relati ely short amount of time 7P3s:. Vapour -utlet !olving -ption *ither the Dynamic Depressuring utility can sol e for the final pressure or the ( E'rea required to achie e a specified final pressure. The 4(alculate +ressure4 option uses the specified areaE( to determine the final pressure. The final pressure is gi en !hen the Depressuring Time has elapsed. 4(alculate 'rea4 is a ailable for .elief- Supersonic- Subsonic and ?eneral al es. 4(alculate ( 4 is a ailable for 2isher and @asoneilan al es. The t!o options differ only in the type of alue calculated. 5ased on '+I- it is normal to depressure to >0L of the staring pressure or to $00 psig. 5efore the calculations start- the user must specify an initial ( or area. If the depressuring time is reached before the final pressure is achie ed- then the calculations stop and a ne! ( or area is calculated using the final pressure. The calculations are repeated until the final pressure is reached in the gi en amount of Hyprotech Technical Support Knowledge Base Article $> depressuring time. The user may specify a ma"imum number of iterations and a pressure tolerance to impro e con ergence. If the user !ishes to stop the calculations at any time- the M(T.9N M5.*'JN keys can be used. 3hen the utility has stopped running- the final calculated alue is displayed here. This is the desired final pressure. Per"or)ance %nce all the required information has been submitted- a yello! bar that reads 4.eady To (alculate4 !ill appear at the button of the Depressuring ie!. +ress the 4.un4 button to start the calculations. %nce the utility has run- users can go to the 4+erformance4 4Summary4 page to ie! the results. Hyprotech Technical Support Knowledge Base Article $G 1.0 ,(a)ple Proble) !imple Fire Depressuring In the e"ercise- the required al e si6e for depressuring a ertical essel to >0L of its operating pressure in a 2ire 3etted case !ill be calculated. Select the +eng,.obinson equation of state- add the required components and then add a stream !ith the follo!ing properties and molar flo!s# &trea) Na)e Temperature +ressure !o)ponent @ethane *thane +ropane i,5utane n,5utane i,+entane n,+entane n,He"ane Feed $0H ( $000 k+a Molar Flow 30.0 kmolEh 30.0 kmolEh 30.0 kmolEh 30.0 kmolEh 30.0 kmolEh 30.0 kmolEh 3&>.0 kmolEh 30.0 kmolEh 7&&G.= 2: 7$=>.0= psia: 7GG.$3H lbmolEh: 7GG.$3H lbmolEh: 7GG.$3H lbmolEh: 7GG.$3H lbmolEh: 7GG.$3H lbmolEh: 7GG.$3H lbmolEh: 7B$G.=A> lbmolEh: 7GG.$3H lbmolEh: Hyprotech Technical Support Knowledge Base Article $B To attach the Dynamic Depressuring utility to the stream- open the stream property ie!- go to 4'ttachments4 4)tilities4 and press 4(reate<4. Select 4Dynamic Depressuring4 from the list of a ailable utilities. +ress the 4'dd )tility4 button. 3: Select 4Dynamic Depressuring4 =: +ress 4'dd )tility4 &: +ress 4(reate<4 $: ?o to 4'ttachments4 4)tilities4 *nter the follo!ing essel information on the 4Design4 4(onnections4 page# .ariable Na)e Height Diameter Initial 9iquid ;olume &+ $nits =.>0 m $.&> m $.=> m 3 Field $nits $=.BG ft =.$0$ ft >$.&$ ft 3 *nter the follo!ing information on the 4Heat 2lu" +arameters4 section of the 4Heat 2lu"4 page# Hyprotech Technical Support Knowledge Base Article $H .ariable Na)e %perating @ode *quation )nits ($ (& (3 Initial 3etted 'rea .alue 2ire 3etted kFEh 0.$3A= 0.H&00 0.0000 =.> m & 7=H.== ft &: *nter the follo!ing information on the 4;al e +arameters4 page# .ariable Na)e ;apour 2lo! *quation ( L %pening .alue 2isher $0 )S?+@ B0L %n the 4%ptions4 page- enter a +; 3ork Term of 405. %n the 4%perating (onditions4 page- select 4!alculate !v4 and enter a final pressure of 600 7Pa 7B&.>& psia:. %nce you ha e submitted the required information- press the 4.un4 button to e"ecute the calculations. *"plore the strip charts- analyse the results and ans!er the follo!ing questions# 3hat si6e al e !as required to achie e the depressurisationQ 3hat is the peak flo! through the al eQ )sing the default alues pro ided- try the 4Simple4 heat loss model. 3hat ( is calculatedQ 3hat is the peak flo!Q )sing the default alues pro ided- try the 4Detailed4 heat loss model. 3hat ( is calculatedQ 3hat is peak flo!Q kgEh kgEh kgEh Hyprotech Technical Support Knowledge Base Article $A
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