Discussion on the lag in instrument control.
Thread Content
During the operation of the gasification furnace, a common problem at present is the lag in coal powder control caused by the instruments. Now, let those with insight speak! 1. There is a mechanism underlying it – why does this phenomenon occur. Wasn’t this taken into account when designing the shell? 2. In actual operation, what form does it take? Let’s talk about the specific phenomena. 3. How to solve problems in this area.
Instrument lag exists not only in Shell and Texaco processes; it can be said that lag is a problem in any control system that relies on instruments. Why? Because this is related to the working principle of the instruments and the composition of the control system. 1. Instrument detection: Instrument detection involves converting physical quantities that are not electrical into electrical quantities, which are then fed into the control system. When measuring density, velocity, pressure, and temperature, some parameters can be measured immediately after adjustment, such as velocity. However, there is always a phenomenon of pure lag, which refers to the distance between the control mechanism and the sensing mechanism, as well as the time it takes for the material to travel through this distance; this is what is known as pure lag. Some are small, some are large. The pure lag of temperature is the most obvious. Speed can be detected very quickly, but this also leads to changes that happen too fast, affecting control. Therefore, the tachometer (and other instruments as well) requires a damping time, which objectively increases the phenomenon of pure lag. 2. Control system: Apart from the time required for signal transmission, the control system also desires that the parameters being regulated not change too rapidly. If changes occur too quickly, the system needs to respond swiftly, which leads to frequent actions by the regulating mechanism (the control valve), further causing fluctuations in the parameters being regulated – thus creating a vicious cycle (unlike positive feedback). To achieve timely regulation, it is necessary to tune the PID parameters, as we desire a rapid response. However, the system has poor stability. If a disturbance occurs, the system output will fluctuate excessively. This has happened with Shell units. 3. The response of the control valve is also one of the factors. Some control valves respond slowly, resulting in delayed regulation. Personal opinion
In fact, it’s very difficult to control the lag in instrument control; what matters is finding ways in practical operations to minimize the impact of this lag. For example, when increasing the load, it’s better to do so slowly rather than quickly, so that oxygen can keep up properly, thereby reducing the risk of water contamination or blockages