Siemens PCS 7 2.3.2 Pressure Control in Tanks

Introduction

10 MPC Level

V 1.0, Beitrags-ID: 42200753

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./].

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.