Homogeneous Equilibrium Isentropic Flashing for Two Phase PSV Sizing

[Sawsan311]

Dear All,

As we all know, when performing the HEM method for sizing the two phase PSV we select the point giving maximum G flux or the point where the inlet pressure during flashing is reaching the backpressure whichever occurs first. However, I would like to enquire on the principle behind targeting the choked flow condition in this case, is it because of the need to consider the compressible fluid effect in making the throat pressure at the PSV nozzle helping the fluid to reach the sonic velocity where the flow will no longer increase as long as the inlet stagnation pressure remains same and exceeds almost two times the downstream backpressure.

In other words, what is the rational behind targeting the choked flow in HEM,

Appreciate your views

Thanks

 

[don1980]

With the exception of PSVs with very low set pressure, choking occurs across the orifice of almost all PSVs in vapor or 2-phase service. Thus it's appropriate to size the orifice based on this choke-flow condition, which of course affects the mass flux.

 

[pierreick]

Hi,
You may consider the document attached to support your work.
Good luck
Pierre

 

[KevinNZ]

Have a look at ISO 4126-10