14 replies to this topic

[furio]

Hi all,

I'm working on a crude oil test separation train (capacity 20000 bpd) constituted by a first stage 3-phases separator operating at 500 psig, a second stage 3-phases separator operating at 70 psig and an atmospheric tank that constitutes the final stage of separation.

I’m trying to size the PSV for the second stage separator considering the scenario in which the inlet control valve (that controls the level in HP separator) is in a fully opened position. In the event of loss of liquid level, the rate of the vapour that flows into the LP separator (at initial condition, that is in critical flow) is very very high, about 40 times higher than the normal rate of vapour inflow to the HP system and the resulting PSV discharge area is several times higher than orifice T. How can I proceed?

Moreover, when I consider the scenario in which the water or oil outlet control valve is in fully closed position, the required relieving rate is the difference between the maximum inlet and the maximum outlet flows: but which phase I have to consider, liquid or gas?

Thank you very much for your time.

[djack77494]

furio,
You have not adequately described your situation to get a good quality recommendation. I'll make the assumptions that the HP Separator removes much of the incoming water and vapors and that the liquid oil stream then goes to the LP Separator. (A common alternative is to use the HP Separator to separate all of the liquids [oil + water] from the vapor phase.) Proceeding with my assumption, you must develop the scenario of failure of the liquid level control system in the HP Separator. Such a failure would cause a high oil flowrate into the LP Separator. If left uncorrected, you could easily deplete the oil phase and have vapor blowby from the HP to the LP Separator. In between you would have the wide open oil level control valve. You would have to design for its capacity to pass vapors under this scenario. Depending on the owner's philosophy and the specifics of your design, you may or may not be able to take credit for the normal vapor outlet flow from the LP Separator. Whatever excess there is must be handled by the PSV.

[JoeWong]

It is common to have large vapor flow when you have a level control valve failure.
40 time seem excessive...

One of the typical example is the Amine absorption column (normally at very high pressure i.e. 60 barg) with liquid level maintained by a level control valve. Downstream of the control valve will have very lower design pressure i.e. 16 barg. Whenever this LCV failed, the load is extremely high...

Typically 2 ways to limit the flow :
i) Mechanical stop on the level control valve
ii) Restriction orifice on the liquid line

[furio]

Well met dear colleagues,

Thank you for your responses: the djack interpration is correct, the system is composed by a HP 3-phase separator followed by a LP separator fed by the separated oil stream. The calculation I made was wrong but the gas flow rate that could pass from HP to LP separator in case of LCV failed open remains rather high. In any case, in order to limit the flow and then limit the size of LP separator PSV, JoeWong suggests a mechanical stop on the level control valve or a restriction orifice on the liquid line. In your experience, the use of the restriction orifice on a line crossed by crude oil containing possible dissolved and suspended solids is adequate?

Thank you for your time.

[fallah]

In your first post,you mentioned:

"In the event of loss of liquid level, the rate of the vapour that flows into the LP separator (at initial condition, that is in critical flow) is very very high, about 40 times higher than the normal rate of vapour inflow to the HP system"

As per above,by providing "Internal Mechanical Stop" at e.g. 110% of normal flowrate on LCV you can eliminate Gas Blow-by case from failure scenarios,otherwise you have to consider design pressure of LP separator equal to HP one and size the PSV on LP based on gas blow-by case.

Utilizing RO with the hole size equal to LCV nozzle size at e.g. 110% of normal flowrate would be possible with poor controlability and as you mentioned possibility of dissolved and suspended solid accumulation.

[furio]

Hi fallah thank you for you response. You say that "providing "Internal Mechanical Stop" at e.g. 110% of normal flowrate on LCV you can eliminate Gas Blow-by case from failure scenarios" mumble mumble...but if the control system fails and valve remains partailly open (@ maximum flowrate, as you say) is it not possible a loss of liquid level and have vapor blowby from the HP to the LP Separators?

[fallah]

mumble mumble...but if the control system fails and valve remains partailly open (@ maximum flowrate, as you say) is it not possible a loss of liquid level and have vapor blowby from the HP to the LP Separators?

Good point!..Yes,it is possible,but normally there is a safeguard as below:

A separate LT that has an interlock with a SDV installed upstream of LCV closes mentioned SDV on LL alarm condition and therefore prevents loss of liquid level in upstream vessel.

[jprocess]

otherwise you have to consider design pressure of LP separator equal to HP one and size the PSV on LP based on gas blow-by case.

Dear Mr.Fallah,

I disagree with this hint. There is no need to consider the LP separator design pressure equal to HP separator when adequate relief device(s) are already considered and engineered for blow-by case.
The similar example can be the one that JoeWong mentioned.("One of the typical example is the Amine absorption column (normally at very high pressure i.e. 60 barg) with liquid level maintained by a level control valve. Downstream of the control valve will have very lower design pressure i.e. 16 barg."). I should complete this case by considering the solvent regenerator with operating pressure of about 1 barg.
No one set the design pressure of absorption column downstream equal to this column.

[fallah]

Dear Mr.Fallah,

I disagree with this hint. There is no need to consider the LP separator design pressure equal to HP separator when adequate relief device(s) are already considered and engineered for blow-by case.
The similar example can be the one that JoeWong mentioned.("One of the typical example is the Amine absorption column (normally at very high pressure i.e. 60 barg) with liquid level maintained by a level control valve. Downstream of the control valve will have very lower design pressure i.e. 16 barg."). I should complete this case by considering the solvent regenerator with operating pressure of about 1 barg.
No one set the design pressure of absorption column downstream equal to this column.

Dear My Friend,

If you consider SDV at the inlet of LCV that closes by an interlock with LL alarm come from dedicated LT on the absorber you are completely right.

In the case under discussion high flowrate of the gas in gas blowby case may lead to high capacity PSVs on downstream vessel that may not be practicable.

[jprocess]

If you consider SDV at the inlet of LCV that closes by an interlock with LL alarm come from dedicated LT on the absorber you are completely right.

In the case under discussion high flowrate of the gas in gas blowby case may lead to high capacity PSVs on downstream vessel that may not be practicable.

This is a matter of how to cope against gas blow by and there are some corrective actions for this BUT the design pressure of absorber downstream equipment will NOT be the same with absorber. This is my main idea. I did not discuss about gas blow by case.

[fallah]

the design pressure of absorber downstream equipment will NOT be the same with absorber.

Let me complete the above as below:

The design pressure of downstream equipment (e.g. separator) will NOT be the same as upstream equipment (e.g. absorber) if adequate facilities would be provided to prevent downstream one to be subjected to upstream pressure.

[jprocess]

the design pressure of absorber downstream equipment will NOT be the same with absorber.

Let me complete the above as below:

The design pressure of downstream equipment (e.g. separator) will NOT be the same as upstream equipment (e.g. absorber) if adequate facilities would be provided to prevent downstream one to be subjected to upstream pressure.

But this is in contrast with your previous post:
"As per above,by providing "Internal Mechanical Stop" at e.g. 110% of normal flowrate on LCV you can eliminate Gas Blow-by case from failure scenarios,otherwise you have to consider design pressure of LP separator equal to HP one and size the PSV on LP based on gas blow-by case."

In the previous post you proposed to set the LP separator design pressure the same with HP and also to provide pressure relief device engineered for blow by case.

[fallah]

But this is in contrast with your previous post:
"As per above,by providing "Internal Mechanical Stop" at e.g. 110% of normal flowrate on LCV you can eliminate Gas Blow-by case from failure scenarios,otherwise you have to consider design pressure of LP separator equal to HP one and size the PSV on LP based on gas blow-by case."

In the previous post you proposed to set the LP separator design pressure the same with HP and also to provide pressure relief device engineered for blow by case.

Per my above statement you are right,but i meant "or" wrongly mentioned "and" . Below is the corrected statement:

"......you have to consider design pressure of LP separator equal to HP one or size the PSV on LP based on gas blow-by case."

One more correction on my statement:Even with providing adequate safeguards,Gas Blowby case should be considered as possible scenario in the cases under discussion (e.g. absorber and downstream separator).

[JoeWong]

Good point!..Yes,it is possible,but normally there is a safeguard as below:

A separate LT that has an interlock with a SDV installed upstream of LCV closes mentioned SDV on LL alarm condition and therefore prevents loss of liquid level in upstream vessel.

Such safeguard SIL shall sufficiently high...otherwise, gas blowby is remain valid...

...Internal Mechanical Stop...

What exactly is "internal mechanical stop" ? Any image or web link to understand ?

[fallah]

Such safeguard SIL shall sufficiently high...otherwise, gas blowby is remain valid...
What exactly is "internal mechanical stop" ? Any image or web link to understand ?

Agree with your first statement.

About your request for "internal mechanical stop" i am trying to provide.