Instrumentation cascade control

Greg Shinskey (1988), over the course of a long and productive career at Foxboro, has proposed a number of advanced control" structures that permit improvements in dynamic performance. These schemes are not only effective, but they are simple to implement in basic control instrumentation. Liberal use should be made of ratio control, cascade control, override control, and valve-position (optimizing) control. These strategies are covered in most basic process control textbooks. 

A typical cascade controller-recorder instrument would appear as shown in Figure 28-3. The particular instrument shown is set for remote automatic control. The cascade control set-point temperature is 155 F. The actual temperature being recorded is 145°F. The output signal to the control valve is 132°F. This output signal will eventually increase to match the ISS F set point. 

The cost of process control, as a fraction of the total construction cost of the process, has risen substantially since the early 1960s. Then is it was around 5 %, now it is closer to 25 %. In the 1960s the view was that some instrumentation was necessary but costs should be kept low. As a result plants had the minimum of measurements - just enough for safety and operability. Much of the instrumentation was local to the process, not repeated in the control room, and most of the controllers were single loop with the occasional cascade controller. 

The composition of raw materials, finished products, and samples of the various steps of a reaction is normally measured at the laboratory using the appropriate physical and chemical analytical methods. However, sampling and analysis are timecurrent interest and too late for control decisions to be made. In order to monitor compositions continuously, one needs automatically functioning analytical instruments that can continuously obtain and show the composition of a mixture. Some devices are fast and precise enough to be able to generate signals for control loops. The controllers would then adjust the desired values of other input variables such as flow, temperature, or pressure in a cascade control scheme. 

The figure below shows cascade temperature control of a polymerization reactor, which uses feed heat exchange to adjust the reactor temperature. Using the instrumentation diagram, explain how this cascade control system (both master and slave components) handles the following disturbances. (describe what happens to the reactor temperature.) Assume normal temperatures of coolant (70°F), polymerization feed (200°F), exchanger effluent (100°F), and reactor outlet (800°F). 

Add controls and instrumentation. Depict cascade controls when necessary, add lags and dead time to process measurement if this is expected. 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

The hot section (Fig. 5) is controlled by a cascade loop which is based on a selected pumping rate (150 gpm) and sterilization temperature set in the TIC. Changes in the feed temperature are monitored at TTl which will automatically override the steam supply to keep the temperature at set point. Steam flow rate is monitored (by FE) and flow is automatically compensated should a large drawdown of steam occur elsewhere in the plant. Temperature is recorded at the beginning and end of the hot section. The hot section should be well insulated and special care should be given to the pipe supports for expansion. (Instrumentation symbols used here and in Figs. 3, 5, 6 and 7, conform to the standard symbols of the Instrument Society of America.)... 

Instrumentation response times Sensor problems Time-delays Interactions between process states Interactions between process units Cascade strategies New sensors sensor location. Inferential measurement and control Predictive control. Robust controller designs Selection of control loop pairings. Decoupling control Feedforward strategies... 

FIGURE 28-3 Typical cascade instrument controller-recorder... 

Many interactions and dependencies involve the explicit dependence of one system upon another. For example, many emergency core cooling systems are explicitly dependent upon support systems providing electrical power, instrument air, cooling water, etc. Cascading or propagating failures are also important. For example, a pump may fail to start due to the malfunction of a circuit breaker in the pump control circuit. Categories of explicit dependencies include ... 

---