Process upset control systems

Describe two types of process upset control systems. 

Figure 9-2 shows a generic diagram for the control of a chemical process. The controller will function to minimize or correct for any unexpected disturbances that may upset the process. A control system will measure one of the output variables that must be controlled, Y (e.g., temperature, concentration), and compare it to a desired value Fsp, called the set point. The difference, between the actual value, Y, and the desired value, Tsp, is called the error signal, e. That is,... 

Normal Operation. The designer of a chemical plant must provide an adequate interface between the process and the operating employees. This is usually accompHshed by providing instmments to sense pressures, temperatures, flows, etc, and automatic or remote-operated valves to control the process and utility streams. Alarms and interlock systems provide warnings of process upsets and automatic shutdown for excessive deviations from the desired ranges of control, respectively. Periodic intermption of operations is necessary to ensure that instmments are properly caUbrated and that emergency devices would operate if needed (see Flow measurement Temperaturemeasurement). 

Load sharing or selective load shedding is of interest to many users of hot gas expanders. A particularly successful European FCC application is illustrated in Figure 6-43. The addition of an expander-generator set to the FCC unit at a major refinery presented a challenge because a trip of the expander could upset the process. The company that is the subject of this application case study, GHH Borsig, solved this problem with the installation of a computerized control system and through computer simulation of trips. 

The general material balance of Section 1.1 contains an accumulation term that enables its use for unsteady-state reactors. This term is used to solve steady-state design problems by the method of false transients. We turn now to solving real transients. The great majority of chemical reactors are designed for steady-state operation. However, even steady-state reactors must occasionally start up and shut down. Also, an understanding of process dynamics is necessary to design the control systems needed to handle upsets and to enable operation at steady states that would otherwise be unstable. 

Process control plays an important role in how a plant process upset can be controlled and subsequent emergency actions executed. Without adequate and reliable process controls, an unexpected process occurrence cannot be monitored, controlled and eliminated. Process controls can range from simple manual actions to computer logic controllers, remote from the required action point, with supplemental instrumentation feedback systems. These systems should be designed such as to minimize the need to activate secondary safety devices. The process principles, margins allowed, reliability and the means of process control are mechanisms of inherent safety that will influence the risk level at a facility. 

Because these systems can monitor multiple processes, equipment, and infrastructure and then provide quick notification of, or response to, problems or upsets, SCADA systems typically provide the first line of detection for atypical or abnormal conditions. For example, a SCADA system that is connected to sensors that measure specific water quality parameters shows measurements outside of a specific range. A real-time customized operator interface screen could display and control critical systems monitoring parameters. 

Because the organic chemicals are destroyed in the GPCR reactor by reduction reactions, the main products are gases such as methane, carbon monoxide, and carbon dioxide. These gases, plus the excess feed hydrogen, must be removed at a controlled rate to maintain the set system pressure fluctuations in the system pressure are undesirable and may lead to process upsets. To accommodate the fluctuating reactor loading and gas production, the compressor must be controlled to remove gas from the system at a variable rate. This is accomplished with a variable-speed drive on the compressor. 

The suction pressure of an air compressor is controlled by manipulating an air stream from an off-site process. An override system is to be used in conjunction with the basic loop to prevent overpressuring or underpressuring the compressor suction during upsets. Valve actions are indicated on the sketch below. 

Lightning strikes have resulted in fires in processing facilities. They can also be the cause of electrical and computer control system malfunctions and result in process upsets. 

Process control system response to various upset conditions... 

The plant control system functions to limit temperature rates of change during plant load change and upset events. This is achieved at two different levels. In the time asymptote the control system through control variable set points (with values assigned as a function of steady-state power) takes the plant to a new steady-state condition. The set point values are chosen so that hot side temperatures remain little changed. In the shorter term the control system manages the dynamic response of the plant so that the transition between steady states is stable and with minimal overshoot of process variables. 

An addition to the noted advantages is that the set point of the secondary controller can be limited. In addition, by speeding up the overall cascade loop response, the sensitivity of the primary process variable to process upsets is also reduced, whereas the secondary loop can reduce the effect of control valve sticking or actuator nonlinearity. The primary or outer control loop of a cascade system is usually a PI or PID controller. A properly selected secondary will reduce the proportional band of the primary controller. 

Another technique for mitigating process upsets is to abort the process by means of an emergency process control system. This approach is most commonly employed with reactive chemical systems. Successful methods include ... 

Arkun, Stephanopoulos and Morari (1978) have added a new twist to control system synthesis. They developed the theory and then demonstrated on two example problems how to move from one control point to another for a chemical process. They note that the desirable control point is likely at the intersection of a number of inequality constraints, the particular set being those that give optimal steady-state performance for the plant. Due to process upsets or slow changes with time, the point may move at which one wishes to operate. Also, the inequality constraints themselves may shift relative to each other. Arkun, Stephanopoulos and Morari provide the theory to decide when to move, and then develop alternative paths along which to move to the new... 

A plantwide control design procedure was used to develop a simple but effective regulatory control system for the Eastman process with an on-demand product control objective. With this strategy, control of production rate is essentially instantaneous. Drastic upsets and disturbances are handled by simple proportional-only overrides. 

The waste flow and composition will vary for a number of reasons. Wide variation in the waste stream characteristics may cause process upsets that could take the system out of control and cause equipment failure. The primary method for reducing this variability is to install a tank to collect the waste from the clean room. The tank serves several purposes ... 

Consider a process that consists of a reactor used for the processing of a highly unstable chemical that is sensitive to small increases in temperature. The reactor is equipped with a quench tank to protect the system against a runaway reaction and is monitored by two temperature sensors (see Fig. 17) T, and T2. Sensor T, automatically activates the quench tank outlet valve when it detects a temperature rise above the specified upper limit. Sensor T2 sounds an alarm in the control room to alert the operator to the process upset. When the alarm sounds, the operator closes the reactor inlet valve. The operator also pushes a quench tank valve button in the control room in case the quench valve fails to open. Note that A is the reactant B, the product and C, the quench. 

---