8 replies to this topic

[SafetyUser]

Hi

I want to calculate with HYSYS the velocity for a gas stream expanding to atmosphere through a PSV orifice.
The idea was to use the change in enthalpy across the PSV.

Unfortunately what I see from HYSYS is that enthalpy down and upstream remains unchanged i.e. the expansion is iso-enthalpic.

Can anybody advise me how to calculate the exit velocity with HYSYS?

thanks

[ARAZA]

Hi There,

If I were you, I wouldn't use Hysys for a simple velocity calculation. For the velocity calculation from the PSV outlet, you need rated capacity of the PSV, backpresure in the header, moleculer weight of the gas. Calculate density of the gas and then use the equation of continuity to determine the velocity.

Hope this helps.

ARAZA

Hi

I want to calculate with HYSYS the velocity for a gas stream expanding to atmosphere through a PSV orifice.
The idea was to use the change in enthalpy across the PSV.

Unfortunately what I see from HYSYS is that enthalpy down and upstream remains unchanged i.e. the expansion is iso-enthalpic.

Can anybody advise me how to calculate the exit velocity with HYSYS?

thanks

[fallah]

Hi There,

If I were you, I wouldn't use Hysys for a simple velocity calculation. For the velocity calculation from the PSV outlet, you need rated capacity of the PSV, backpresure in the header, moleculer weight of the gas. Calculate density of the gas and then use the equation of continuity to determine the velocity.

Hope this helps.

ARAZA

In my opinion,we also need discharge relief temperature.

[SafetyUser]

Hi There,

If I were you, I wouldn't use Hysys for a simple velocity calculation. For the velocity calculation from the PSV outlet, you need rated capacity of the PSV, backpresure in the header, moleculer weight of the gas. Calculate density of the gas and then use the equation of continuity to determine the velocity.

Hope this helps.

ARAZA

In my opinion,we also need discharge relief temperature.

thanks you all,
I can't leave HYSYS out of consideration since the velocity calculation is one of the major outputs I have to calculate within a more complex dynamic problem.

But of course I can have from HYSYS all the parameters upstream and downstream the orifice: all but the enthalpy.
Other options could be use the volume flow or the change in pressure and density but I feel the results I obtained are unlikely.

[ARAZA]

Fallah is absolutely right in quoting that we also need the relief temperature, with temp, Pressure and Moleculer weight calculate density, apply equation of continuity and calculate velocity. For this simple calculations, you do not need enthalpy or iso-thermal calculations.

Is that satisfies your query or am I missing something here??

ARAZA

Hi There,

If I were you, I wouldn't use Hysys for a simple velocity calculation. For the velocity calculation from the PSV outlet, you need rated capacity of the PSV, backpresure in the header, moleculer weight of the gas. Calculate density of the gas and then use the equation of continuity to determine the velocity.

Hope this helps.

ARAZA

In my opinion,we also need discharge relief temperature.

thanks you all,
I can't leave HYSYS out of consideration since the velocity calculation is one of the major outputs I have to calculate within a more complex dynamic problem.

But of course I can have from HYSYS all the parameters upstream and downstream the orifice: all but the enthalpy.
Other options could be use the volume flow or the change in pressure and density but I feel the results I obtained are unlikely.

[JoeWong]

Hi

I want to calculate with HYSYS the velocity for a gas stream expanding to atmosphere through a PSV orifice.
The idea was to use the change in enthalpy across the PSV.

Unfortunately what I see from HYSYS is that enthalpy down and upstream remains unchanged i.e. the expansion is iso-enthalpic.

The statement "The idea was to use the change in enthalpy across the PSV" is confusing. Do you mind to advise how you want to relate change in enthalpy to velocity ?

In PSV, it is a nozzle instead of an orifice. If the PSV nozzle has good "curved" inlet, the process be isentropic in the nozzle follow by isenthalpic process after the nozzle.

You may consider process across PSV is isenthalpic by looking at PSV as an "orifice". I believe this approximation is sufficient for your exercise.

Can anybody advise me how to calculate the exit velocity with HYSYS?

Are you interested in velocity at PSV nozzle, PSV flange or tail pipe (PSV end or tail pipe exit end ?) ?

In HYSYS, there is a spreadsheet like excel. You can program you simple velocity calculation in this spreadsheet. Beside there is pipe module in HYSYS...

[SafetyUser]

The statement "The idea was to use the change in enthalpy across the PSV" is confusing. Do you mind to advise how you want to relate change in enthalpy to velocity ?

I considered that in flow through a converging nozzle, enthalpy is converted to kinetic energy at constant entropy, i.e. h2 - h1 = (v^2)/2. This enthalpy drop increases as the pressure falls.
This conversion of energy reaches a limit at a velocity defined in the same way as sonic velocity for a gas and this velocity defines the flowrate through the nozzle.

In PSV, it is a nozzle instead of an orifice. If the PSV nozzle has good "curved" inlet, the process be isentropic in the nozzle follow by isenthalpic process after the nozzle.

You may consider process across PSV is isenthalpic by looking at PSV as an "orifice". I believe this approximation is sufficient for your exercise.

I do want to look at PSV as an orifice but if enthalpy across the valve remains unchanged thus what I have with the spreadsheet formulas is zero velocity.

Are you interested in velocity at PSV nozzle, PSV flange or tail pipe (PSV end or tail pipe exit end ?) ?

In HYSYS, there is a spreadsheet like excel. You can program you simple velocity calculation in this spreadsheet. Beside there is pipe module in HYSYS...

I'm interested at the velocity after complete expansion to atmospheric conditions. I don't have modeled a piece of pipe after the psv.
The fact is that using different simple formulas (volume flow in the spreadsheet or change in pressure & density) results in different velocity outcomes.

Moreover, I think the HYSYS version I'm using doesn't support dynamics calculations for pipe module.

[SafetyUser]

The statement "The idea was to use the change in enthalpy across the PSV" is confusing. Do you mind to advise how you want to relate change in enthalpy to velocity ?

I considered that in flow through a converging nozzle, enthalpy is converted to kinetic energy at constant entropy, i.e. h2 - h1 = (v^2)/2. This enthalpy drop increases as the pressure falls.
This conversion of energy reaches a limit at a velocity defined in the same way as sonic velocity for a gas and this velocity defines the flowrate through the nozzle.

In PSV, it is a nozzle instead of an orifice. If the PSV nozzle has good "curved" inlet, the process be isentropic in the nozzle follow by isenthalpic process after the nozzle.

You may consider process across PSV is isenthalpic by looking at PSV as an "orifice". I believe this approximation is sufficient for your exercise.

I do want to look at PSV as an orifice but if enthalpy across the valve remains unchanged thus what I have with the spreadsheet formulas is zero velocity.

Good Heavens,
I suspect I cannot use h2 - h1 = (v^2)/2 unless h1 is the final enthalpy after the isentropic flash.
Now I must change my problem in having the final isentropic flash...

[Muhammad Adnan]

Hi

I want to calculate with HYSYS the velocity for a gas stream expanding to atmosphere through a PSV orifice.
The idea was to use the change in enthalpy across the PSV.

Unfortunately what I see from HYSYS is that enthalpy down and upstream remains unchanged i.e. the expansion is iso-enthalpic.

Can anybody advise me how to calculate the exit velocity with HYSYS?

thanks

Dear Safety User;

In my opinion, You should use the simple basic engineering relation; Make your model in hysys with two streams and one PSV. Get PSV to Pope at its set pressure pressure simply slightly increasing upstream pressure. After popping get actual volumetric flow of downstream stream from its property window. Now, you know Actual volumetric flow rate and the Area of the orifice so, use V= q / A relation to resolve to get velocity i.e. V.

Note: Remember to convert q into m3/sec from m3/hr.

I hope this may have resolved your problem.

Take care and have a nice life.
Feel free to ask any question.

Best regards,

Muhammad Adnan Hanif

Process Engineer

adnan_haninf@ffc.com.pk