13 replies to this topic

[pymorty]

Hi,

 

I'm working on emissions control project with Vapor Recovery Units. We are calculating the flash gas generated within the storage tank when the crude oil enters. I'm using Vasquez-Beggs equation to get estimation on vapor flow rate, here's where I have some doubts:

 

These kind of equations use upstream crude oil separation pressure, the higher the separation pressure the higher the flash gas flow rate that occurs in the tank, but, since the tanks operate at atmospheric pressure, the maximum flash would occur despite the upstream pressure, wouldn't it?

 

I ask this since a supplier quoted VRU for a given flow considering 300 psig for the crude oil separation pressure, but a coworker noted the actual upstream pressure is 170 psig and stated that the VRU should be sized then for lower capacity. Would that be the case? If I ran a HYSYS simulation with those parameters the flow rate won't change with increasing the differential pressure by means of changing the separation pressure (as long as Storage tank is fixed at atm pressure), so it doesn't matter if upstream separation pressure is 170, 300 or 500 psig, the atm pressure in the tank does the job, doesn't it?

 

 

 

[horatorres]

The first question is yes, you will find the maximum flash at atmospheric pressure..

 

As a general rule, when you design a GOSP, it has to recovery as much C5+ as possible.  If your client wants reduce from 300 psi to 14,7   they will loss BB of liquid... Any way they should know it...  If they don’t have a liquid recovery and send the gas to flare  it does matter to reduce it in that way,,, normaly there are 2 o 3 flash to recovery as much liquid as possible and it also reduce the power requirement of the compressor to feed the gas pipeline.

 

Do the simulation with both cases 300 psi and 170 psi, it does change a lot.. Check than your client gave to you the right gas composition.

 

Horacio

[fallah]

Hi,

 

I'm working on emissions control project with Vapor Recovery Units. We are calculating the flash gas generated within the storage tank when the crude oil enters. I'm using Vasquez-Beggs equation to get estimation on vapor flow rate, here's where I have some doubts:

 

These kind of equations use upstream crude oil separation pressure, the higher the separation pressure the higher the flash gas flow rate that occurs in the tank, but, since the tanks operate at atmospheric pressure, the maximum flash would occur despite the upstream pressure, wouldn't it?

 

I ask this since a supplier quoted VRU for a given flow considering 300 psig for the crude oil separation pressure, but a coworker noted the actual upstream pressure is 170 psig and stated that the VRU should be sized then for lower capacity. Would that be the case? If I ran a HYSYS simulation with those parameters the flow rate won't change with increasing the differential pressure by means of changing the separation pressure (as long as Storage tank is fixed at atm pressure), so it doesn't matter if upstream separation pressure is 170, 300 or 500 psig, the atm pressure in the tank does the job, doesn't it?

 

Hi,

 

Is the upstream pressure changed from 300 psig to 170 psig with the same crude composition?

[pymorty]

 

Hi,

 

Is the upstream pressure changed from 300 psig to 170 psig with the same crude composition?

 

  Hi Fallah,

 

Yes, it is with the same crude composition. Just changing the upstream pressure with all the other parameters kept the same.

[pymorty]

The first question is yes, you will find the maximum flash at atmospheric pressure..

 

As a general rule, when you design a GOSP, it has to recovery as much C5+ as possible.  If your client wants reduce from 300 psi to 14,7   they will loss BB of liquid... Any way they should know it...  If they don’t have a liquid recovery and send the gas to flare  it does matter to reduce it in that way,,, normaly there are 2 o 3 flash to recovery as much liquid as possible and it also reduce the power requirement of the compressor to feed the gas pipeline.

 

Do the simulation with both cases 300 psi and 170 psi, it does change a lot.. Check than your client gave to you the right gas composition.

 

Horacio

 Thank you for your elaboration, horatorres,

 

The doubt with the upstream pressure change is that given the case the crude composition is exaclty the same, it shouldn´t make any difference if I change the upstream separation pressure if anyway I´m sending it to an atmospheric tank. 

 

I know If the crude oil composition was taken from a 300 psig separation condition it would make a difference, but the composition I got was taken at 170 psig, so if I put that composition in the simulator, that´s why I wouldn´t see more vapors flashing at atm pressure even if I set a higher separation pressure upstream, would it?

 

Basically if I want to see any change in the vapor recovery in the simulator, by increasing the upstream pressure, it should be accompanied by its corresponding composition at that pressure, so the vapor flashing potential can be calculated, otherwise, if I use the same composition I won´t see any change just by changing the upstream pressure. Am I right?

Edited by pymorty, 10 May 2021 - 08:18 AM.

[fallah]

 

 

Hi,

 

Is the upstream pressure changed from 300 psig to 170 psig with the same crude composition?

 

  Hi Fallah,

 

Yes, it is with the same crude composition. Just changing the upstream pressure with all the other parameters kept the same.

 

 

 

Then your coworker is right, i.e. with reduction in upstream pressure the VRU should be sized for lower capacity...

Edited by fallah, 10 May 2021 - 11:59 AM.

[pymorty]

Then your coworker is right, i.e. with reduction in upstream pressure the VRU should be sized for lower capacity...

 

 

But the composition analysis was made taking sample at the lower separation pressure (170 psig). At first the supplier used Vasquez-Beggs empirial equation considering 300 psig, so Vapors would be higher, but afterwards the compositional analysis was made taking the 170 psig separation pressure. I agree the VRU should be resized for lower capacity, but no for the reason of just using a lower upstream pressure.

 

 

My question is, if I enter in HYSYS certain crude composition analysis made at certain separation pressure I will obtain the most vapor recovery just by sending it to an atmospheric tank, but changing the upstream pressure keeping the rest of the parameters won´t show any change in the vapor recovery amount.

 

If I take a sample at higher separation pressure and then I input those parameters in the simulator it will certainly increase the vapors generated, but not because I just increased separation pressure, but the composition that is differente in the higher pressure condition, therefore more solution gas in the crude oil stream with potential to flash when droping the pressure to atmosphere.

[fallah]

 

 

 

 

But the composition analysis was made taking sample at the lower separation pressure (170 psig). At first the supplier used Vasquez-Beggs empirial equation considering 300 psig, so Vapors would be higher, but afterwards the compositional analysis was made taking the 170 psig separation pressure. I agree the VRU should be resized for lower capacity, but no for the reason of just using a lower upstream pressure.

 

 

My question is, if I enter in HYSYS certain crude composition analysis made at certain separation pressure I will obtain the most vapor recovery just by sending it to an atmospheric tank, but changing the upstream pressure keeping the rest of the parameters won´t show any change in the vapor recovery amount.

 

If I take a sample at higher separation pressure and then I input those parameters in the simulator it will certainly increase the vapors generated, but not because I just increased separation pressure, but the composition that is differente in the higher pressure condition, therefore more solution gas in the crude oil stream with potential to flash when droping the pressure to atmosphere.

 

 

Hi,

 

The red color statement of yours in above is in contradiction with your previous statement as:

 

"Hi Fallah,

 

Yes, it is with the same crude composition. Just changing the upstream pressure with all the other parameters kept the same."

 

Please clarify the point...

Edited by fallah, 10 May 2021 - 01:04 PM.

[pymorty]

 

 

 

Hi,

 

The red color statement of yours in above is in contradiction with your previous statement as:

 

"Hi Fallah,

 

Yes, it is with the same crude composition. Just changing the upstream pressure with all the other parameters kept the same."

 

Please clarify the point...

 

 

The actual situation is that we just have one composional analysis, which was made with the sample at 170 psig separation pressure. The text in red is the condition I assume we'd need in order to see a difference in the vapors flashing, not just increasing upstream pressure in the simulator, but updating the composional analysis with the higer separation conditions, wich we don´t have. So assuming that having the information we have, considering an escenario where the flash flow rate increases is not possible, we'd need the new analisys at 300 psig separation pressure.

 

So the increase in flash gas is not given by the higher differencial pressure, but for the different composition of the crude oil.

[Bobby Strain]

So, you must estimate the composition at the higher pressure. Should be a simple task having the crude composition at 170 psig and a known temperature. But you should design for the uncertainty using this approach.

 

Bobby

[pymorty]

So, you must estimate the composition at the higher pressure. Should be a simple task having the crude composition at 170 psig and a known temperature. But you should design for the uncertainty using this approach.

 

Bobby

 

Thank you Bobby,

 

So, for me to be clear, as long as I just use the crude composition obtained at 170 psig, in Hysys I won´t get more flash gas in the atm Tank just by increasing the upstream pressure.

[Bobby Strain]

That is not what I said. Read my response more carefully.

 

Bobby

[breizh]

Hi,

Did you try the search engine in this forum?

https://www.cheresou...z-beggs-method/

 

Good luck

Breizh

[pymorty]

Hi,

Did you try the search engine in this forum?

https://www.cheresou...z-beggs-method/

 

Good luck

Breizh

Yes I did. That´s  where part of my doubt comes from.

 

Thanks.