A Two Phase System
Shown above is a simple flash vessel. The aim of this is to separate a stream into vapour and liquid by altering the pressure (and hence temperature). There are three control loops.
- Flow control on feed line On a flash vessel it is necessary to have the flow of one stream controlled. This is to ensure that the throughput is known.
This flow control operates the same as that shown above i.e if the flow decreased then the valve is opened and vice versa.
- Pressure control on vapour line The composition of the resulting streams depends on the pressure in the vessel. If the pressure increases then the valve is opened to allow more vapour through. If the pressure decreases then the valve is closed to allow the pressure to build back up again.
Notice, however, that both these actions will have an effect on the level of liquid in the tank. If the pressure decreases then more liquid will vapourise and the level will decrease. If the pressure increases then the level will rise.
- Inventory control on liquid line The shaded valve denotes an inventory loop. This is to ensure that the mass balance around the vessel balances i.e. what goes in must come out.
In this case it is simply a feedback liquid level control loop. If the level increases then the valve is opened and if the level decreases then the valve is closed. Thus the vessel should not overflow or run dry.