10 replies to this topic

[J_Leo]

Dear Friends,

 

I have light liquid hydrocarbon (C3,C4) to relieve through the PSVs, for block outlet case. Some flashing is expected for some PSVs. To what extent, do we need to use the two phase methods? If only a very small fraction of flashing, can we use liquid equations? I have done this before, now, I want to get your opinions. 

 

Thank you,

-Leo

[fallah]

Dear Friends,

 

I have light liquid hydrocarbon (C3,C4) to relieve through the PSVs, for block outlet case. Some flashing is expected for some PSVs. To what extent, do we need to use the two phase methods? If only a very small fraction of flashing, can we use liquid equations? I have done this before, now, I want to get your opinions. 

 

Thank you,

-Leo

 

Leo,

 

IMO, for very small fraction of flashing there would be no problem using liquid equation...

[J_Leo]

Fallah,

 

Thank you. Do you have a rule of thumb about the "small amount"? 

 

Regards,

Leo

[fallah]

 

Do you have a rule of thumb about the "small amount"? 

 

 

Leo,

 

No specific rule of thumb, but a few percent flashing fraction, say below 5% ...

Edited by fallah, 30 November 2017 - 04:18 PM.

[latexman]

I think it depends on how long the outlet line is.  If the outlet line is short, more.  If the outlet line is long, less.

[Bobby Strain]

There is no reason NOT to size as a flashing liquid. It's a simple enough calculation. HEM method is readily available. And you can use your process simulator for flashing. VMG simulator has the capability built into the simulator.

 

Bobby

[J_Leo]

Fallah, Laxeman, and Bobby,

 

Thank you very much for your advice. The only problem is there is limited access to the simulation software with my current employer. The big oil and gas downturn has made my work environment worse than before. I really hope everything will go back to normal soon.

 

Regards,

-Leo

[Bobby Strain]

So, post the composition along with relieving rate and temperature and set pressure and I'll reply with a required size using the HEM procedure. Then you can compare it to an alternate sizing method of your choice. Maybe someone else will also respond, too. Many have good software at no cost. But remember, it's your responsibility.

 

Bobby

[serra]

Leo,

about your question (PSV design for flashing liquid) there are several equilibrium and non-equilibrium models available,

see for example this thread

 

https://www.cheresou...-design-rating/

 

HEM model should revert to standard model for the cases of all liquid or all gas

but for gas it results more accurate than standard model (integration with real properties)

[J_Leo]

Hi All,

 

I just want to give an update. I have used the two-phase flow equation from API 520 Part I to calculate the orifice size. Compared to using single liquid phase, the orifice size using two-phase equation is even smaller though there is significant flashing through the orifice. I think the reason is: Kd of 0.65 is used for liquid sizing and Kd of 0.85 is used for two-phase. Smaller Kd gives a bigger orifice size. Only when the fraction of flashing exceeds a certain value, the orifice size will be bigger using two-phase equation.

 

-Leo

[serra]

Leo,

if I remember correctly Kd should be the effective coefficient of discharge, the values 0.85 and 0.65 are given for preliminary sizing,

anyway I agree that Kd has a large influence on required discharge area,

you may find useful also this thread

https://www.cheresou...r-2-phase-flow/