Siemens PCS 7 Manual De Usuario
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Introduction
10
MPC Level
V 1.0, Beitrags-ID: 42200753
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feed water tanks. There are different objectives for level control according to the
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./].
2.3.2
Pressure 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.