Flash Tank

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Navigation: Main Page -> Models -> Energy Transfer Models

Related Links: Flash Tank 2, Flash Train

General Description

The newer Flash Tank 2 can be used. The Flash Tank is used to 'flash' a feed stream to produce a liquid and vapour product at a set pressure. The liquids and vapours leave the unit separately.

There are two operational modes for the flash tank, a) as a stand-alone unit or b) as part of a Flash Train. The operational mode is decided by the overall configuration of the flowsheet in which the Flash Tank is located. For example, if the Flash Tank is connected to other flash tanks, Barometric Condensers and / or heat exchangers, the model will become part of the entire Flash Train. The user does not have to specify that the unit is part of the Flash Train, SysCAD will do this automatically. If the Flash Tank is connected to units other than the previous three, i.e. Feeders, tanks, etc; then the Flash Tank is a stand-alone unit and can be configured as such.

If the Flash Tank is inserted as part of a Flash Train, see Flash Train for a description of the theory and variables. This documentation will only discuss the variables for a 'stand alone' Flash Tank.

The Vapour Liquid Equilibrium section allows the user to configure which species to flash and / or the flashing method, fully described in Vapour Liquid Equilibrium (VLE).

The Reaction Block (RB) allows the user to configure any number of reactions within the flash tank, fully described in Reaction Block (RB).



The diagram shows the default drawing of the flash tank, with the required connecting streams. When the user inserts a flash tank into a flowsheet, he may choose a different drawing from a pull down menu.

Inputs and Outputs

Label Required
Number of Connections Description
Min Max.


1 Required




Flash tank feed






Liquor outlet






Vapour outlet

Behaviour when Model is OFF

If the user disables the unit, by un-ticking the On tick box, then the following actions occur:

  • All liquids and solids in streams connected to the 'Feed' inlet will flow straight out of the 'Liquor' outlet;
  • All vapours in streams connected to the 'Feed' inlet will flow out of the 'Vapour' outlet;
  • The pressure of the outlet streams will be the pressure of the input stream, or the minimum pressure of all input streams if more than one inlet;
  • No Flashing or reactions will occur.

So basically, the unit will be 'bypassed' without the user having to change any connections.

Model Theory

The unit is configured to achieve vapour : liquid equilibrium at a required pressure (or temperature) using the user defined VLE method. Any solids that may be in the stream entering the unit are ignored in the flash calculations. However the enthalpy balance does include the solids.

For further information on the theory for the VLE calculations see Vapour Liquid Equilibrium (VLE).


The following shows the sequence of events if sub model options are switched on. See next heading for more information.


Data Sections

The default access window consists of several sections:

  1. FlashTank1 tab - This first tab contains general information relating to the unit.
  2. RB - Optional tab, only visible if the Reactions are enabled.
  3. VLE - Only visible if the VLEquilibrium is enabled (If it is NOT enabled, then the Flash Tank will operate as a pipe)
  4. QFeed - Optional tab, only visible if ShowQFeed is enabled. This page shows the properties of the mixed stream as the feed to the tank.
    • This is before any Evaluation Block models are evaluated.
  5. QProd - This page shows the properties of the Flash Tank discharge as a single stream.
    • This is AFTER the flash calculations, but before the split to liquid and vapour streams.
  6. Info tab - Contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
  7. Links tab, contains a summary table for all the input and output streams.
  8. Audit tab - Contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.

Flash Tank Page

Unit Type: FlashTank1 - The first tab page in the access window will have this name.

Tag (Long/Short) Input / Calc Description/Calculated Variables / Options
Tag Display This name tag may be modified with the change tag option.
Condition Display OK if no errors/warnings, otherwise lists errors/warnings.
ConditionCount Display The current number of errors/warnings. If condition is OK, returns 0.
GeneralDescription / GenDesc Display This is an automatically generated description for the unit. If the user has entered text in the 'EqpDesc' field on the Info tab (see below), this will be displayed here. If this field is blank, then:
  • For pipes, SysCAD will use the text in the 'ConnectDesc' field - this displays the Source and Destination tags of the pipe;
  • For Feeders/Cross Page Connectors, SysCAD will display the state of the unit, i.e. if it is a Feeder or a Sink, etc.;
  • For other units SysCAD will display the unit class ID.


On Tickbox This variable in used to enable flashing in the unit. If this not selected, the material will flow out of the liquor outlet with no change in state, i.e. the unit acts as a pipe.


Method Atmospheric All flash calculations will be done at Atmospheric Pressure. The atmospheric pressure is calculated by SysCAD based on the user defined elevation (default elevation is at sea level = 101.325 kPa). The elevation can be changed on the Environment tab page of the Plant Model.
RequiredP All flash calculations will be done at the user specified pressure.
Saturated All flash calculations will be done at the saturated pressure. (This option is only valid if the flash tank is part of a Flash Train configuration.)
RequiredT All flash calculations will be done at the user specified temperature.
PressureReqd / P_Reqd Input The Required Flash Pressure - This is only visible if the RequiredP Method is selected.
TemperatureReqd / T_Reqd Input The Required Flash Temperature - This is only visible if the RequiredT Method is selected.
Result Calc This field displays the operating pressure of the flash Tank.
VapSpModel Input Standard - The default species model. This sets the species model to be used in the vapour exit stream. This selection has no effect on the model operation.
More options may be available, depending on the user configurations.
Reactions List Box Reaction Block (RB) is enabled here. If this is 'On' then the associated page, RB becomes visible and may be configured.
Note: The user does not have to configure a reaction file, even if this block is checked.
VLEquilibrium List Box Vapour Liquid Equilibrium (VLE) is enabled here. This MUST be enabled for the unit to operate as a Flash Tank. When it is enabled the associated page, VLE becomes visible and may be configured.
If this field is disabled then the flash tank will operate as a pipe and all feed material will report to the Liquor outlet.
ShowQFeed Tick Box QFeed and associated tab pages (eg Qm) will become visible if this is enabled. These tabs will show the properties of the combined feed stream to the Unit. These values are BEFORE any flashing calculations or reactions occur.
TrackVapFd Tick Box Selecting this will add a warning message and error status colour to the flash tank if vapour is present in the feed stream.
Vapour Entrainment
FlashVapToLiquid Input This field allows the user to set a proportion of the flash vapour bypassing to the liquid stream. The default is 0%.
OtherGasToLiquid Input This field allows the user to set a proportion of any non-condensable gas bypassing to the liquid stream. The default is 0%.


VapourCalcs Calc This displays the compound(s) used to calculate the vapour pressure.
TemperatureIn / Ti Calc The flash tank feed temperature.
BPE Calc The Boiling Point Elevation of the Flash Tank discharge. This is the BPE value that is used for the Flash calculations.
WaterSatT Calc The water saturation temperature at the flash tank pressure. This is used to calculate the BPE. (BPE = Saturation T - WaterSatT)
FlashT Calc The calculated temperature at the operating pressure of the Flash Tank.
FlashP Calc The calculated operating pressure of the Flash Tank.
VapourMassFlow / VQm Calc The mass flow of vapour leaving the Flash Tank (this includes any vapour entrained in the liquor stream).

Flash Train Macro Model

These extra fields are only visible if the unit is part of a Flash Train.

VapourMassFlow / VQm Calc The quantity of flashed vapour.
PressureReqd / P_Reqd Calc The required flash pressure calculated by the Flash Train.
FlashTrain Calc A unique tag assigned to the Flash Train by SysCAD. Each unit in the Flash Train will have the same tag in this block.
FlashTearBlock Display Displays the name of the tear block that is part of the Flash Train.
FlashTrainEqp List This contains a list of all of the equipment tags in this Flash Train. For example, the list might be as follows:



PAdvBase Input Additional damping or acceleration for calculated pressure change (DP) for the iteration. DP = DP * PAdvBase when PAdvExtra=0.
PAdvExtra Input Additional damping or acceleration for calculated pressure change (DP) for the iteration. DP = DP * (PAdvBase + PAdvExtra*(VQmReqd-VQm)/Max(VQmReqd,VQm)).

Adding this Model to a Project

Insert into Configuration file

Sort either by DLL or Group.





Heat Transfer: Flash Tank(1)



Energy Transfer


Heat Transfer: Flash Tank(1)

See Project Configuration for more information on adding models to the configuration file.

Insert into Project


Insert Unit

Heat Transfer

Flash Tank(1)

See Insert Unit for general information on inserting units.

Hints and Comments

  1. There may be mass entering or leaving the unit via the Reaction Block source or sink, so if the outgoing mass is not the same as the incoming mass, this may be a place to check first.
  2. If the VLE check box is not ticked the Flash Tank operates as a pipe only. All the material entering the unit leaves in the same state via the Liquor outlet.
  3. The information in the VLE Section must be correct for the Flash Tank to operate as required. If the VLE configuration requires information that is not available, then the flashing will not occur.