4 replies to this topic

[Mach21]

Dear experts,

Good day.

 

Currently I am working on study blowdown (de-pressurizing) study of Gas Plant.

HYSYS De-pressurization: A Practical Guide mentioned that, for accurate calculations the detailed Heat Loss model is the one that should be used; otherwise, the contribution of the metal could be completely ignored.

 

 

 

The metal wall thickness must always have a finite value (that is, it cannot be <empty>). To model a vessel without insulation, the insulation value thickness should be zero. Users are also required to enter the specific heat capacity of the material(s), the density of the material(s), and the conductivity of the material(s).

 

 

Here comes a question, what value shall be input for thickness?

I found the internet and Aspen support site have limited discussion on how to obtain the wall thickness.  

 

 

Our team hourly arguments come out with below suggestion but no final decision had been made:

 

1.    Assume mass of the HYSYS depressurizing vessel equal to sum of mass of vessel and piping for the particular blowdown system to be studied.

 

      Wall thickness = sum of metal mass / (surface area x metal density)

 

2.    Thinnest Metal Thickness among piping and vessel for the particular blowdown system to be studied shall be taken for conservative approach.

 

 

Appreciate if somebody could share the knowledge and experience on how to input the wall thickness.

Thank you and have a nice day.

 

 

 

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[Mach21]

any answer/idea from expert is highly appreciated. 

[PaoloPemi]

these difficulties to interpret a "black box" operation available in simulators provide the main reason why I prefer a process library for these applications,
do not misunderstand my point of view, a simulator is useful to obtain quickly a reasonable  estimate but you have to accept the limits of predefined models with little (or no) possibility to customize / trace / debug a iterative procedure as that required to simulate a blowdown.
A rigorous procedure can solve at each step (T1->T2 , time integration) a V-H flash operation where V is the vessel's & piping volume and H is the result of

H(T2) = H(T1) + dH(T)
dH adds the contributes (at each time step) of
heat from an external fire or source
fluid removed from vessel
difference in enthalpy (T1,T2) of fluid, walls etc.

as you see there are several factors related with walls,
weights and temperatures (H2-H1), tickness (heat transfer, conduction, distribution) etc.
from these you can figure out which are the best values to enter in your procedure,

however do not forget the limits of those general purpose operations,
for more accurate estimates there are specific applications or you must create your own model.

[Mach21]

hysys dynamic depressuring unit only account for vessel. The concern is how to include inventories for piping where  piping thickness is different (usually thinner) than vessel thickness. 

[serra]

predefined procedures in simulators have some limits, probably you'll be able to obtain reasonable estimates by introducing correction factors on volumes, areas, weights etc. to consider associated piping.

as alternative you may create a procedure,
see this thread

http://www.cheresour...f-vaporization/

for a blowdown you don't need the initial step (to find initial equilibrium temperature)
while at each time step for heat and mass balance
instead of the H-P flash operation (in a PSV protected vessel pressure is constant)
solve a V-H flash operation (during blowdown pressures & temperatures change)
as discussed above,

good luck