In the modern industrial production process, the designed process control system can not only overcome the external disturbance under normal working conditions, but also achieve smooth operation, and must also consider safe production under accident conditions.

In order to effectively prevent the occurrence of production accidents and reduce the number of driving and parking, a control scheme that can adapt to short-term production anomalies and improve control quality is developed, that is, selective control.

Selective control is a combined control method that superimposes the logical relationship formed by the constraints in the production process onto a normal automatic control system. That is, in a process control system, two controllers (regulators) (or more than two transmitters) are provided, and control signals that can adapt to production safety conditions are selected through high and low value selectors to achieve production. Automatic control of the process.

When the production process approaches the danger limit zone, but has not yet entered the danger zone, a control scheme for controlling unsafe conditions will replace the control scheme under normal production conditions through the high and low selectors (the normal regulator is at Open loop state) until the production process is back to normal.

Then, the original control scheme is restored to work by the selector. This selective control system is therefore also referred to as an automatic protection system, or soft protection system.

The selective control system features a selector. The selector can be connected to the output of two or more regulators to select the control signal; it can also be connected to the output of several transmitters to select the measurement signal to suit the needs of different production processes; The position of the selector in the system structure is different, and the selective control system can be divided into two types of selectors located at the output of the regulator.

Principles and design principles of selective control systems

The main feature is that the two regulators share a single regulating valve. Under normal production conditions, the output signals of the two regulators are sent to the selector at the same time, and the control signal output from the normal regulator is sent to the regulating valve to realize automatic control of the production process. When the production is not normal, the normal regulator is replaced by a selector instead of the regulator until the production situation is restored.

The feature of this selective system is that several transmitters use a single regulator. There are usually two purposes for selecting one, one of which is to select the highest or lowest measured value; the other is to select a reliable measurement. In a fixed-bed reactor, in order to prevent the catalyst from being burnt out at a high temperature, several temperature detecting points are installed at different positions in the fixed catalyst bed of the reactor, and the temperature detecting signals of the respective points pass through the high value selector. The highest temperature detection signal is selected as the measured value, and the temperature is automatically controlled to ensure the safety of the catalyst layer of the reactor.