4 replies to this topic

[petroabbes]

Gents;

it is stated in API that in case of Fire Vapour Generation Case latent heat is calculated by selecting the lower latent heat value determined by the mass heat of vapourization value from Aspen HYSYS Version 8.6 or performing 5 incremental flashes of 5%.

 

Then for the performing 5 incremental flashes of 5% technically speaking is it a heat input in a series of vessels in cascade mode such a way the liquid mass density is decreased by 5 % ?

[S.AHMAD]

Hi petroabbes,

The reason for using the lowest heat of vaporization is to determine the worst case scenario where the vapor mass rate is the highest possible situation. If I remember correctly, density is not used in the correlation. However vapor density or molecular weight  is used in sizing the PSV orifice area. Lower density should give bigger area.

 

Hope that verify your concerns.

Edited by S.AHMAD, 11 February 2016 - 09:22 AM.

[marchem]

petroabbes,

I am surprised to read that API suggests a specific software and version...

I use, as others here, Prode Properties but as far as I know equivalent results may be obtained with VMG , Chemcad, PROII, Promax  or other software providing you follow the same procedure and adopt equivalent thermo models,

different procedures are possible, you may adopt a sequence of HP flash operations or solve for HV, the volume of vessel being constant etc.

I use Excel and Prode Properties, which allows to solve directly the flash operations and compare the results,  

with a simulator the standard procedure is to vaporize the liquid in several small steps and select worst case as said by Ahmad,

the goal being to evaluate the properties for mixtures as wide boling mixtures, where liquid and vapor compositions may show large variations when vaporizing...

[petroabbes]

what does meam performing 5 incremental flashes of 5% and dividing the required duty for the flash by the mass of vapour liberated including the sensible heat required, is there any exemple that you could provide for me, or steps calculation

[marchem]

if you are not familiar with the methods for calculating these properties I would suggest to ask an expert or use the default values (50 btu/lb) provided by API,

safety valves are critical components and sure you don't wish to specify wrong values.

 

As said, the goal is to estimate an average values for mixtures where properties may show large variations depending from compositions, an example being wide boiling mixtures,

 

with simulators the usual way to calculate latent heat is Hvapor-Hliquid where Hvapor and Hliquid are the enthalpies of vapor and liquid phase, you can estimate an average value by solving a series of flash operations (H-P) which (with a simulator) is about the same to solve a series of heat exchangers,

 

the details will depend from the software, as said  I solve these cases in Excel with Prode Properties,

to calculate latent heat I use the macros =StrSGH(1) - StrSLH(1),  or the difference between (specific) vapor and liquid enthalpy

 

to calculate a series  of flash operations I use the macro =HPF(1,...) which solves directly a H,P flash operation,

 

with a different software you may use a variation of this procedure.