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PSV Sizing for sub cooled liquid which flahses while in the PSV itself

PSV Sizing for sub cooled liquid which flahses while in the PSV itself

PSV Sizing for sub cooled liquid which flahses while in the PSV itself

Hello all
2 questions if I may regard subcooled liquid sizing which goes through flashing inside the PSV.
1) PSV Discharge nozzle sizing :
Is there a sizing criteria for the discharge nozzle of a PSV ?
I didn’t see any relation to that nozzle . All the sizing equations and scenarios always relate to the inlet nozzle and the orifice size. Why isn’t the discharge being treated?
Should a criteria of 0.7 x sonic velocity ? ( or sonic velocity ) be used ?
This is especially important in cases where the inlet is subcooled and PSV it goes partial/full flashing inside the PSV.
In that case, to the best of my judgment, a substantial difference should be between inlet nozzle and outlet nozzle

2) Subcooled liquid flashing - sizing :
I have a specific case that I am debating for a while on how to size it.
This is a subcooled liquid that goes through flashing through the PSV.

To the best of my judgment, section C 2.3 of API 520,part I should be used.
Do you agree?
In that case, when I check omega and all criteria, its seems I am in the “high sub cooling region” which require using equation: C.42 ( or D.11 in older versions of the code):

Do you agree?
What will be the correct sizing for that valve( especially outlet nozzle ? ) and why isn’t the vapor criteria any factor in this kind of sizing ?

Below are the exact data for my case, I will appreciate your help for the correct sizing:
- Water ( with small amount of soluble CO2,~ 0.2% wt ).
- Vapor pressure of composition: 70.3 psia
- PSV set pressure : 200psig @ 300 F
- Block flow scenario (110%), Ambient pressure : 11.5 psia, therefore :
- Reliving pressure : 231.5 psia
- Operating temperature: 283 F
- Volumetric Flow : 4500 GPM
- Mass Flow : 2,051,325 lb/Hr
- Liquid density : 56.833 lb/ft^3
- Density at 90% of saturation pressure ( P=63.27 psia ) :
Vapor density =0.18 lb/ft^3,liquid density = 56.8 lb/Ft^3

Thanks you all in Advance

RE: PSV Sizing for sub cooled liquid which flahses while in the PSV itself

You may find many threads discussing these topics,
You can specify orifice and inlet rating, for many fluids, critical flow condition for pin/pout > 2 , given t, p you can calculate speed of sound (critical flow) for two phase flow with HEM or similar methods (again, several threads discussing the topic), for PSV sizing (flashing mixtures) you can consider HEM or HNE-DS methods, normally HEM is more conservative but it depends from specific case....

RE: PSV Sizing for sub cooled liquid which flahses while in the PSV itself

Thanks PaoloPemi.

2 question:

1) Can you send me some links to these threads ?
2) I saw many threads debating which is the correct equation to use ( HEM or HNE ), but my question was relating to :

- What should be the criteria for the discharge nozzle size ( sonic velocity ? )
- Is it indeed correct that, when I take in this case the API equation ( I believe this is the HEM ,the more conservative case,if I remember correctly ), does it indeed correct to use only the liquid properties for the orifice and inlet sizing ,and to use :

G= 96.3 (row_l_0(P_0-P))^0.5


RE: PSV Sizing for sub cooled liquid which flahses while in the PSV itself

1) try search Posts button
2) Given discharging flow and conditions one can calculate orifice area (with HEM or HNE-DS methods), then a manufacturer can propose a specific model
3) omega method adopts a simplified HEM formulation, for mixtures one may wish to use a software (there are several free applications available, see my posts on previous threads)

RE: PSV Sizing for sub cooled liquid which flahses while in the PSV itself

One comment to add. Using one the "omega" methods above is probably satisfactory, but if the stagnant fluid is close to or above its critical point, you should use the direct integration method. "Flashing inside the relief" per se is not an issue - this is what happens when the fluid is at saturation.

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