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# AFT Arrow Model - Vacuum Relief Valve / Tank Vapor Strems

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## AFT Arrow Model - Vacuum Relief Valve / Tank Vapor Strems

(OP)
Good afternoon all,

Has anyone been able to model a vacuum relief valve in the software AFT Arrow? I have several tanks that will be connected to a vent header line and wanted to show a relief valve opening up if the vent header line vacuum level went below the relief valve set pressure.

The mass flow rates of specific vapors from the tanks are also wanted for a material/heat balance. If I have a tank with a known mixture of liquids and known liquid/gas properties of the individual components, is there a way to determine how much of each component will be in the gas/vapor stream going into the header?

Thanks!

### RE: AFT Arrow Model - Vacuum Relief Valve / Tank Vapor Strems

I don't use Arrow, however
If you use absolute pressures, then definition of a vacuum is irrelevant and a vacuum vent becomes just another valve with a set pressure.
The program I use allows me to specify
Inlet Node connection
Outlet Node connection
Set_Pressure and
%_Open vs %_travel

The "inlet" and "outlet" is just a sign convention to define the direction of positive flow.
Positive flow direction is from Inlet to Outlet. It does not mean flow is necessarily in that direction. If the program then tells you the flow is negative, it just means the flow is going from outlet to the inlet Node.

Direction of flow is determined by the pressure at the Inlet Node and the pressure at the Outlet Node and is always from higher to lower pressure. If you use absolute pressures, then definition of a vacuum is irrelevant. -0.5BarG (vacuum) is really 0.5 BarA. If the absolute tank pressure is higher than the header pressure, then flow is from tank to header. If the absolute header pressure is higher than the tank pressure, then flow is from header to tank. The program always knows which direction flow is going.

Pressure in a tank above a liquid level is always determined by the total of the partial pressure of each component making up the liquid mixture inside the tank. If the partial pressure of benzene is 1 BarA and the partial pressure of heptane is 0.6 BarA then Total tank pressure is 1+0.6 = 1.6 BarA.
The molar amount of each component in the vapor is proportional to the partial pressure of each vapor to the total tank pressure. The flow rate of each vapor is in the proportion of its partial pressure to total tank pressure. Here the flow of benzene vapor will be 1/1.6 x total flow rate. Here the flow of heptane vapor will be 0.6/1.6 x total flow rate. The flow direction of the vapor will be determined by pressure in the tank vs pressure in the header. Partial pressures are determined by finding the vapor pressure of each component in the mixture at the given tank temperature. Usually I look that up in NIST data files, or some other reference.

If flow is a vapor from header to tank, you must somehow know the composition of the vapor in the header. Note that the composition of the liquid in the tank will change, if header vapor composition is different from tank vapor composition. That gets complicated fast.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

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