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.