Blowdown of linked liquid filled vessels

[Justice100]

Hi all,

I am looking at emergency blowdown of two liquid filled vessels that are linked via piping at the bottom. The blowdown valve is off the top of one vessel (I have attached a sketch). The liquid is hydrocarbon condensate at its bubble point. Assume for simplicity that both vessels are the same size and same elevation (Infact the upstream vessel is quite a bit bigger).

For the vessel with the blowdown valve, when the valve opens the pressure will drop resulting in flashing and the vapour will be blown down and then the liquid level will continue to drop until 0 barg is reached. Then I run this about 20% of the liquid is flashed. I am struggling with what will happen in the upstream vessel. As the pressure/level in the downstream vessel reduces, similar will happen in the upstream vessel as their levels must be equal. Modelling the entire system as one big vessel with a blowdown valve at the top seems reasonable...

However, I am not sure I really understand what will happen in the upstream vessel. Any thoughts on this?

 

[georgeverghese]

Would agree with your reduction of this to a single vessel from an emergency depressure perspective.

 

[Justice100]

Thanks George.

Out of interest I just ran a simulation for just the upstream vessel where the blowdown valve is on the vessel bottom. The vessel starts at 100 barg but liquid level is completely lost by the time the vessel is at ~20 barg. This can't be reflective of the what happens when the downstream vessel is connected as the liquid level is not lost. I am sure I am missing something simple but I can't understand how this system works as a whole.

 

[shvet]

As the pressure/level in the downstream vessel reduces, similar will happen in the upstream vessel as their levels must be equal.

Incorrect.

Liquid columns in the upstream and downstream vessels are equal (Hup = Hdown)
Pressure in vapor spaces of the upstream and downstream vessels are equal (Pup = Pdown).
The system is balanced SG*g*Hup + Pup = SG*g*Hdown + Pdown
Vapor and liquid phases compositions are equilibrium.

Pressure in vapor space of the downstream vessel drops down (Pup > Pdown).
Systems becomes unbalanced Hup + SG*g*Pup > Hdown + SG*g*Pdown
To become balanced system shall equal left and right parts of this equation. The only variable is the liquid column, which shall decrease. Part of the liquid shall move from the upstream vessel to the downstream one.
A driving force appears, the liquid moves from upstream to downstream.
Liquid level in the downstreama vessel rises up, liquid level in the upstream vessel drops down, the the vapor space in the upstream vessel expands.

The system will seek for a balance until:
- the both parts of the equation becomes equal, or
- the liquid seal gets lost and vapor spaces merge together

Note that the specific graviry (SG) of the liquid and gas density in both vapor spaces are not constant because of the liquid flashing.

I guess you do not understand how complex and unpredictable such case is. A worth exercise for training/studying, a worst one for a sound/responsible engineering.

 

[Justice100]

Thanks Shvet.

I have now run it in a dynamic simulator because it was bugging me so much. The upstream vessel completely empties its liquid via the blowdown valve leaving it as a vapour filled vessel at a pressure of about 40 barg. Then vapour starts to be released via the blowdown valve, with the vapour coming from the upstream vessel gas and flashing from the downstream vessel.

 

[georgeverghese]

Is there not a bidirectional pressure control line on the vapor side that connects both these vessels ?

 

[shvet]

I have now run it in a dynamic simulator because it was bugging me so much. The upstream vessel completely empties its liquid via the blowdown valve leaving it as a vapour filled vessel at a pressure of about 40 barg. Then vapour starts to be released via the blowdown valve, with the vapour coming from the upstream vessel gas and flashing from the downstream vessel.

What do you mean "blowdown"? Blowing down a vapor, a gas, or a liquid? If it is "blow" then why do you "drain" the liquid first?

Anyway your model is inadequate.

Being familiar with how Aspen Dynamics works I am not sure there is a simulator possible to model such without an extensive coding that would script interrelations of these both vessels. I would not treat such model as a sound engineering or an inherently safer design. I guess you do not understand how many assumptions and simplifications are behind it besides of a software limitations.

How are you going to prove that the final model is valid/adequate?
What will both these vessels turn into when the real blowdown goes not as modelled?
Why and for what purpose are you solving such a task? Is it a real project or a study thesis?

 

[Justice100]

By blowdown, I am referring to the standard term used for emergency depressuring of a vessel/system in the oil and gas industry for jet/pool fire protection. The setup is as per my sketch.

The plant is currently under construction, I wasn't involved in the design but I have been asked to review something related to this and noticed this unconventional blowdown / emergency depressuring arrangement as per my sketch, which I've not seen before. Normally, the blowdown valve takeoff is at the top of each vessel.

A vessel survivability analysis has been performed to see if the blowdown rate is sufficient, performed by an outside consultant. I can see two vessels have been considered but details are not clear. A low temperature dynamic simulation has been performed internally modelling the system as 2 vessels but whether this represents what will actually happen is my concern.

 

[shvet]

A low temperature dynamic simulation has been performed ... whether this represents what will actually happen is my concern.

The upstream vessel completely empties its liquid via the blowdown valve

The blowdown valve is off the top of one vessel

The blowdown valve is connected to the top of the vessel but when it opens the liquid is coming out from.
If the model is representative then how is this possible?

I recommend to leave this task as you have not enough expertise to perform such. I am not sure there exists experienced enough eng team to perform such.

Anything can be modelled. Literally anything. I can model how sail spacetrucks haul the ore from the Mars orbit pushing by a solar wind. And the final model can look great, shiny, flawless.
The adequacy/accuracy/limitations of a model is the only issue should be addressed. And you ignore this point.