Introduction
feed water tanks. There are different objectives for level control according to the plant context [also see Related Literature /2./]:
Keep level constant (exactly at the set point) – important for levels directly in- fluencing the process; disturbances are passed through to the output (drain).
Keep level as small as possible – if “dead volume” and inventory are undesir- able.
Keep level inside specified limits while using the tank as buffer – changes in level are tolerated to achieve a smooth drain flow.
Figure 2-3 Examples of level control (marked in red) in a typical part of plant (distillation column) [also see Related Literature /1./].
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2.3.2Pressure Control in Tanks
In some cases the pressure control in tanks behaves in a similar way as the level control. The control loop shows an integral behaviour, if the manipulated variable is a gas feed (e.g. admission of inerts) and no pressure loss to the environment ex- ists. Typically a separate purge valve exists in such cases to discharge gases. The pressure controller uses a split range function to access either the feed valve or the purge valve.
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