85279029 Depressuring a Practical Guide

8/13/2019 85279029 Depressuring a Practical Guide

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Depressurization: A PracticalGuide

This guide has been prepared based upon frequently asked questions regarding theDynamic Depressuring utility introduced in Aspen HYSYS 3.0. . !t e"plains ho# touse the utility and correctly interpret the results. !t is di$ided into four sections%

.0 &$er$ie#

'.0 Adding and (onfiguring the )tility

'. (onnections * +essel (onfiguration'.' (onfiguring the Strip (harts'.3 Heat ,lu" -arameters'. Heat /oss -arameters'. +al$e -arameters'.1 &perating &ptions

3.0 2ain -oints to emember

.0 Appendices

1.0Overview

Why has the old depressuring utility gone?The original Depressuring utility in Aspen HYSYS #as a pseudo4dynamiccalculation based on a series of steady state calculations. The Dynamic Depressuringutility #as introduced in Aspen HYSYS 3.0. to allo# users to perform proper time4dependant calculations. An Aspen HYSYS Dynamics licence is NOT required to usethis ne# utility.

In version 3.2 onwards, you now only have the option to run the new DynamicUtility. The dyndepressuring.tpl file in the templates sub-directory of the

spen !"#"# 3.2 installation should be dated $%&'(&2''( or later. "ou can

download the latest version from the website. )#ee *nowledgebase #olution+$$322 at http &&support.aspentech.com


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What can this utility e used !or?

The Depressuring utility can be used to simulate the depressuri5ation of gas6 gas4liquid filled $essels6 pipelines6 and systems #ith se$eral connected $essels or piping$olumes depressuring through a single $al$e. eferences to 7$essel8 in this guidecan also refer to piping or combinations of the t#o.

What types o! depressuring calculationscan e per!or"ed?

There are t#o ma9or types of depressuring calculations a$ailable%

Fire Mode is used to model a $essel or pipe under fire conditions. This mode has three sub4types%

o ,ire

o ,ire Stefan :olt5mann

o ,ire A-! '

Adiabatic Mode is used to model the blo#do#n of pressure $essels or piping #ith no e"ternal heat supplied.

A more in depth discussion of the different methods follo#s in Section '.0.

2


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#.0Adding and $on!iguring the%tility

&ow to add the utilityA Depressuring utility can be added to the case by selecting Tools | Utilities on themain menu bar6 highlighting Depressuring Dynamics and clicking the Add Utility

button. After you ha$e selected the )tility6 al#ays rename the )tility to somethingthat is recogni5able the ne"t time you open the case ;for e"ample6 DP-V !"#-Fire


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/igure $

(ntering 'essel Para"eters!deally6 the $essel si5e #ill be kno#n and this data can be entered into theappropriate fields on the form sho#n in ,igure .

The initial liquid $olume is normally calculated at the normal liquid le$el ;=//


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Aspen HYSYS does not take account of the heads in a $essel6 so $olumes and areasare calculated as for a simple cylinder. The total $essel $olume is calculated from thediameter and height ;or length for a hori5ontal $essel


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#.* &eat +lu, Para"eters&n the Design tab6 'eat ,lu" page6 the type of depressuring to be performed isspecified. The different modes and their respecti$e equations are described here.

There are fi$e types of Heat ,lu" models a$ailable%

Adiabatic Mode * no e"ternal heat isapplied

Fire Mode * models heat from a fireusing a general equation

Fire AP+ ,! * models heat from a fireusing an equation based on A-! '

Fire - )te an .olt/mann * models heatfrom a fire using a radiation equation Use )preads*eet * allo#s the user tocustomi5e the equation used

Adia atic -odeThis can be used to model the gas blo#do#n of pressure $essels or piping. =oe"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 dueto the depressuri5ation. Typical use of this mode is the depressuring of compressorloops on emergency shutdo#n.

/igure 3

&


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+ire -ode

,ire 2ode can be used to simulate plant emergency conditions that #ould occurduring a plant fire. -ressure6 temperature6 and flo# profiles are calculated for theapplication of an e"ternal heat source to a $essel6 piping6 or combination of items.Heat flu" into the fluid is user defined using the follo#ing equation%

( )0

3'

=

=+++=time

t timeVESSEL me LiquidVolu

me LiquidVoluC T C C timeC C Q

The ,ire @quation can also be used to simulate the depressuring of sub4sea pipelines#here heat transfer occurs bet#een sea#ater and the pipeline. !f the follo#ing holdtrue%

( 3 )A ( T and ( ( ' and ( 0

then the pre$ious equation #ould reduce to%

( )T UAQ =

/igure (

'


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+ire AP /#1

,ire A-! ' uses similar heat flu" parameters to those used in ,ire mode. Threecoefficients% ( 6 ( '6 and ( 3 must be specified. The equation used by Aspen HYSYSis an e"tension to the standard A-! equation for heat flu" to a liquid containing$essel. A #etted area is required and used to calculate the heat transfer into the$essel.

The follo#ing notes are based on e"tracts from Guide for Pressure-Relieving and De ressuring S!stem" AP# Re$ommended Pra$ti$e %&' 6 ,ourth @dition6 2arch BBC.

The amount of heat absorbed by a $essel e"posed to an open fire is affected by%

The type of fuel feeding the fire The degree to #hich the $essel isen$eloped by the flames ;a function of si5e and shapeetted Area $ariable #ill only be completed if cases from earlier$ersions of Aspen HYSYS ;pre 3.'< are loaded.

The Aspen HYSYS equation is an e"tension of the standard A-! equation.Therefore6 in field units6 ( #ill be ' 000 multiplied by the en$ironmental factor6 ,and (' #ill be 0. ' by default. ;!n most cases6 ( #ill be equal to ' 000


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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 generation6 A-! recommends that you only take into account that portion ofthe $essel that is #etted by liquid up to C.1m ;' ft< abo$e the source of the flame.!ndi$idual companies may de$iate from this figure6 so be sure to check. This usuallyrefers to ground le$el6 but it can be any le$el capable of sustaining a pool fire. Thefollo#ing table indicates recommended $olumes for partially filled $essels. +olumeshigher than C.1m are normally e"cluded as are $essel heads protected by supportskirts.

Type of essel 4ortion of 5i6uid Inventory 7ef 4I 2'

5i6uid full )for e8ample, treaters All up to % -

#urge drums, 1noc1out drums, and

process vessels

.o! al ope!ating li"uid level up to % -

/ractionating columns .o! al level in the botto plus li"uid hold up f!o all the t!ays du ped tothe no! al level in the colu n botto # /otal wetted su!face only calculatedup to %01eboile! level is to be included if the !eboile! is an integ!al pa!t of thecolu n#

9or1ing storage a i u invento!y level up to % -

#pheres and spheroids 5ithe! the a i u ho!izontal dia ete! o! % , whicheve! is g!eate!

!f a ( 3 $alue of 0 is used6 the initial #etted area is used throughout the calculations.This could represent a #orst4case scenario. Alternati$ely6 if a ( 3 $alue of #ereused6 the $olume #ould $ary proportionally #ith the liquid $olume. This #ould

represent a $ertical $essel.

))


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+ire )te!an oltz"ann

This mode uses the :olt5mann constant to take into account radiation6 forcedcon$ection6 flame temperature6 and ambient temperature. The method may beconsidered as an alternati$e method to the A-! standard.

( )( ) ( )( )V am0 f v f total T T outsideU T 1 AQ ++= .'C3 >here%

A total Total #etted surface area

f ,lame emissi$ity Ienerally ranges from 0.' to 0. ;for burning hea$y H(s

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85279029 Depressuring a Practical Guide