PSV 2-phase Sizing Direct Integration 1

[PSVSizer]

Dear all,

I have recently been asked to size 2-phase relief PSV.
Based on recent, API 520 two methods were available depending on the degree of rigorous-ness.
For quick and dirty method, Leung's 2-point Omega and for rigorous, direct integration.
Comparisons of two methods agree very well when it is critical flow, i.e. Kb close to 1.0. Discrepancies arise when Kb departs from 1.0.
I have found the reason behind this is that Leung's method takes into account critical and sub-critical flow in the Mass flux calculations. Following the direct integration method, the mass flux is always the same no matter whether you're flowing against a huge back pressure or not.

Could someone clarify whether this observation is correct?

Thank you.

 

[Latexman]

PSVSizer,

Sorry, I cannot see your point. Leung's Omega method used some physical property and thermodynamic assumptions to derive a linear function for the two phase density in terms of pressure and the fluid properties. Yes, this function is used in the mass flux calculations, but it's results has nothing to do with whether the flow is critical or sub-critical. It's purely a physical property estimation technique whose results depend on pressure and the fluid properties. Therefore, I don't see how the Leung method "takes into account critical and sub-critical flow". Maybe, you can explain it further.

As for "Following the direct integration method, the mass flux is always the same no matter whether you're flowing against a huge back pressure or not." Two phase choking can occur at only 10-20% below the upstream pressure. When choked, reducing the downstream pressure (i.e. less back-pressure) has no effect. Thus, this gives the impression that a "huge back pressure" has no effect, however if the back-pressure should exceed the critical pressure, fundamentally the flow should decrease below critical flow.


Good luck,
Latexman

 

[PSVSizer]

Hi Latexman,

Thank you for the reply.

I probably haven't made the point clear.
It's perhaps misleading of me to say that Leung's omega has anything to do with critical or non-critical flow.

API method of Leung's omega details calculation of Mass Flux, G which depends on whether the flow is critical. If critical, the G is not a function of backpressure ratio and if it's subcritical, the formula for G is a function of the ratio.

On the other hand, the direct integration detailed in API (Dec 2008) does not details any correction or different equations to be used for non-critical flow.

Regards,

PSVsizer

 

[PSVSizer]

Hi Latexman,

I have finally found the error in my calculations.

For Direct Integration, if you have subcritical flow, I just need to step back the pressure up to the back pressure of the PSV and quote the Mass flux and calculate the area based on that mass flux rather than find the maximum mass flux as detailed in API.

Thanks.

Regards,

PSVsizer

 

[Latexman]

PSVSizer,

You are spot on now! Glad I could help.

Good luck,
Latexman