Finite Tank Overflow & Draining

(XTS Module Only)

Finite Tank Overflow

As the transient progresses finite tanks can fill up. If the tank is open, then it is possible for the liquid to reach the top. How does AFT Fathom handle this?

When a finite tank liquid level reaches the top, it is assumed that the liquid spills over the top. Hence the liquid height is maintained at the top of the tank. Note that this causes a loss of mass from the system model.

If the tank is closed, the gas pressure will increase as the tank fills. Thus the tank cannot overflow. If the tank has a maximum pressure (which represents a relief line) then it is possible for the liquid mass to be lost as it would ostensibly flow out the relief line. The liquid level in the tank would then be maintained at the top of the tank.

Finite Tank Drained

When a tank drains to the bottom, it cannot supply any more liquid to the connected pipes. The system behavior after a tank has drained cannot be accurately modeled by AFT Fathom. In a real system, and in the absence of a valve to stop the flow, a drained tank which continues to flow would result in the connected pipes themselves draining and a gas/liquid interface moving down the pipes. AFT Fathom assumes all pipes are liquid full, and cannot model draining pipes.

Since it is not desirable in most applications to have the pipes drain, this limitation is not a significant issue. The user can, for example, use a valve in the pipe which closes when the tank drains. This would use an event transient.

Since AFT Fathom cannot model a tank after it has drained, what does it do? Rather than halt the transient run, which would not be very helpful to the user, AFT Fathom assumes the connecting pipe “turns off” when it no longer has liquid to supply it. To implement this, AFT Fathom sets the pipe’s Special Condition so that it turns the pipe off.

Finite Tank Pipes Uncovered

AFT Fathom allows multiple pipes to connect to a reservoir, all at different elevations. As a tank drains, pipes may be uncovered. AFT Fathom handles this in the same manner as described above for completely drained tank.

If a pipe is uncovered, then there are two possibilities:

• Liquid flows from the pipe and into the tank – AFT Fathom can model this case. The liquid free falls to the liquid surface, and appropriate boundary conditions are used to solve the system.

• Liquid tries to flow from the tank and into the pipe – AFT Fathom cannot model this case. Since the pipe is not covered by liquid, then the tank cannot supply liquid to the pipe and in reality the pipe itself would drain. AFT Fathom does the same thing as described above for a completely drained tank – it sets the pipe flow to zero.

Important: When a tank drains completely or sufficiently to uncover a pipe, AFT Fathom cannot adequately model the system behavior after this point unless a valve is used to stop the flow into the pipe. AFT Fathom will warn the user when this happens.