Siemens PCS 7 manual Examples of Unstable Control Loops, Level Control

Introduction

2.3Examples of Unstable Control Loops

2.3.1Level Control

If the level of a tank with continuous feed is to be controlled via an adjustable drain as actuator (e.g. pump or valve with or without flow control), the control loop shows integrating behaviour.

An equilibrium condition of the level only exists if the drain is exactly equal to the feed. The level permanently decreases until the tank is empty, if the drain is in- creased stepwise starting at this equilibrium condition. In contrast the level perma- nently increases until the tank overflows, if the drain is decreased stepwise starting at the equilibrium condition.

Figure 2-2 Types of level control, taken from Related Literature /1./.

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Exception natural drain: If only a valve with constant pressure behind exists in the drain of the tank, the drain flow depends according to the drain formula of Toricelli not only on the valve position (manipulated variable of the controller) but also

nonlinearly on the level h itself (proportional to 2gh with acceleration of gravity

g). Compensation can be reached after small steps in the valve position in such control loops. The level is decreasing if the valve at the drain is opened a little bit starting at the equilibrium condition. Thus the hydrostatic pressure at the bottom of the tank is decreasing, and accordingly the drain flow is decreasing until a new equilibrium condition is reached. Anyway, the application of the techniques de- scribed in the following is helpful for stabilization of a natural drain too, as the mechanism of compensation only works for small steps and cannot be modelled linearly.

In process plants there are many tanks where level control is necessary, e.g. surge drums, separation tanks, stirred tank reactors, column sumps, feed water tanks,

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