carlo.stenali
Active Member

In a vessel protected by a PSV the temperature inside the vessel may reach values higher than those allowable for vessel's materials, for these cases a depressuring system is required.
Prode Properties allows to calculate the temperature when the PSV opens, these values of temperature and pressure can be compared with the MAWP and MAWT and decide if a depressuring system is required.
The basis of the procedure is the flash operation at specified volume and pressure.
The procedure calculates the temperature and phase equilibria when PSV opens.
To define the composition from Excel open Prode Editor, select stream 5 (or whichever you prefer) and define the composition.
In cells B1 and B2 enter operating pressure and temperature,
the procedure will calculate specific volume in cell B3.
In cell B4 enter the discharging pressure for PSV,
the procedure will calculate the fluid temperature at specified discharging pressure.
To view compositions at specified discharging pressure open Prode Editor and select the stream
psvsimul.xls

In a vessel protected by a PSV the temperature inside the vessel may reach values higher than those allowable for vessel's materials, for these cases a depressuring system is required.
Prode Properties allows to calculate the temperature when the PSV opens, these values of temperature and pressure can be compared with the MAWP and MAWT and decide if a depressuring system is required.
The basis of the procedure is the flash operation at specified volume and pressure.

The procedure calculates the temperature and phase equilibria when PSV opens.
To define the composition from Excel open Prode Editor, select stream 5 (or whichever you prefer) and define the composition.
In cells B1 and B2 enter operating pressure and temperature,
the procedure will calculate specific volume in cell B3.
In cell B4 enter the discharging pressure for PSV,
the procedure will calculate the fluid temperature at specified discharging pressure.
To view compositions at specified discharging pressure open Prode Editor and select the stream

for additional information see this thread
'http://www.egpet.net/vb/threads/27092-Excel-library-for-process-calc-s-including-distillation'

Prode Properties allows to size / rate a PSV for critical or two-phase flows with different models including HEM (Homogeneous Equilibrium Model)
The HEM model for the nozzle applied in Prode Properties solves

hin+1/2*vin^2 = ho+1/2*vo^2

(in = inlet, o = orifice)
where vo (for a critical flow) is the speed of sound
ho, vo calculated at vena contracta conditions

this model works well for critical and two-phase flows,
it is also applicable for subcritical flows within certain limits.
For subcritical flows Prode calculates the area solving a constant energy operation but since usually for a nozzle we do not specify the internal area of pipe (which is required for calculating vin in above formula the procedure estimates the value, this may generate errors when the rate vo/vin is low.
For such cases it is suggested to use as alternative to ISPF() the method EPF

double t = EPF(integer stream, double p, double E, double aout, double et)

this method allows to specify both inlet and outlet condition thus modeling a adiabatic, irreversible expansions when the contribute of kinetic energy cannot be neglected.

HEM MODEL
HEM (Homogeneous Equilibrium Model) is based on two assumptions (Homogeneous flow)
1) velocity of gas an liquid phases are equal
2) vapor and liquid phase are in thermodynamic equilibrium
this is a general purpose model which gives good results for a broad range of problems

HNE MODEL
in HNE model (Homogeneous Non-Equilibrium Model) the vapor and liquid phases are not in thermodynamic equilibrium,
HNE has found (Fauske, Schmidt ...) to describe well the behaviour of subcooled fluids which reach two-phase equilibria in a nozzle, pipe

etc. for example the fluid residence time could be too short for a significant vaporisation.

SOME APPLICATIONS WITH PRODE PROPERTIES

Isentropic Nozzle model
this method alows to design/rate safety/relief devices as PSV etc. for critical and two-phase flow.
The Isentropic Nozzle model allows to define
1) HEM Homogeneous Equilibrium (Solution of Mass Flux integral)
2) HNE Homogeneous Non-equilibrium (HEM with Boling Delay and Gas-Liquid Slip Contributes)
3) HNE-DS , Homogeneous Non-equilibrium
4) NHNE Non-homogeneous Non-equilibrium

PIPE
this method allows to simulate single phase, two-phases, multiphase flow on circular pipes,
for multiphase flow different models are available including HEM (Homogeneous Equilibrium)

Speed of Sound
the methods StrMSS(), EStrMSS() in Prode Properties allow to calculate the speed of sound for gas, liquid and gas+liquid (mixed phases) with

HEM model

A TEST CASE
Prode Properties allows to compare HEM (Homogeneous Equilibrium Model) against different models as HNE (Homogeneous Non-Equilibrium Model)

and NHNE (Non-Homogeneous Non-Equilibrium Model),
this allows to select the most suitable pressure relief valve for a specific application

Read More

cbadia wrote
for a two phase discharge I am sizing a relief valve with the isentropic Nozzle unit available in Prode Properties,
the procedure allows to specificy the HEM (Homogeneous Equilibrium Model) and NHNE (Non-Homogeneous Non-Equilibrium Model) for isentropic flow.
While the HEM model gives results consistent with those calculated by Excel page nozzle.xls distributed with the program the NHNE model requires specific settings and may give results quite different.
Which is the best option (HEM or NHNE) for modeling a PSV discahrge ?

I think it depends from several factors as for example the size of the valve and the physical properties of the fluid,
my version of Prode Properties has three models
HEM (Homogeneous Equilibrium Model)
HNE-DS (Homogeneous Non Equilibrium, simplified model)
NHNE Non Homogeneous Non Equilibrium)
the last two require a specific parameter which for HNE-DS is available in several papers while for NHNE you have to estimate the value, generally I select HEM and I do a test with HNE-DS