Control loops pressure

Fig. 7. Instmment components of a control loop, where A = process measurement devices, in this case, pressure measurement B = transducer ...

Many misconceptions exist about cascade control loops and their purpose. For example, many engineers specify a level-flow cascade for every level control situation. However, if the level controller is tightly tuned, the out-flow bounces around as does the level, regardless of whether the level controller output goes direcdy to a valve or to the setpoint of a flow controller. The secondary controller does not, in itself, smooth the outflow. In fact, the flow controller may actually cause control difficulties because it adds another time constant to the primary control loop, makes the proper functioning of the primary control loop dependent on two process variables rather than one, and requites two properly tuned controllers rather than one to function properly. However, as pointed out previously, the flow controller compensates for the effect of the upstream and downstream pressure variations and, in that respect, improves the performance of the primary control loop. Therefore, such a level-flow cascade may often be justified, but not for the smoothing of out-flow. 

Regulatory Control For most batch processes, the discrete logic reqmrements overshadow the continuous control requirements. For many batch processes, the continuous control can be provided by simple loops for flow, pressure, level, and temperature. However, very sophisticated advanced control techniques are occasionally apphed. As temperature control is especially critical in reactors, the simple feedback approach is replaced by model-based strategies that rival if not exceed the sophistication of advanced control loops in continuous plants. 

G. D. Anderson s article recommends initial controller settings for those control loops set on automatic rather than manual for a plant startup. For liquid level, the settings depend upon whether the sensor is a displacer type or differential pressure type, or a surge tank (or other surge) is installed in the process ... 

Process flow diagrams are more complex and show all main flow streams including valves to enhance the understanding of the process as well as pressures and temperatures on all feed and product lines within all major vessels and in and out of headers and heat exchangers, and points of pressure and temperature control. Also, information on construction materials, pump capacities and pressure heads, compressor horsepower, and vessel design pressures and temperatures are shown when necessary for clarity. In addition, process flow diagrams usually show major components of control loops along with key utilities. 

Establish the rado of the naximurn anticipated flow rate for system, Qjj, to the design basis rate, Q,d or Q,m/Q.d-When Qv] is not known, nor can it be anticipated, use Qm/Qd of 1.1 for flow control and 1.25 for level pressure and temperature control valves to anticipate the flow rate transients as the control loop recovers from a disturbance [9]. 

The ARC is controlled by its own hardwired control module. The temperature is monitored by a set of seven thermocouples connected in series which measure the difference between the temperature of the sample and that of its surroundings. The temperature is maintained by heaters which receive their inputs from the control module. A pressure transducer is attached to the sample container, giving both an analog readout on a pressure gauge and a digital readout on the control module panel. It should be noted that pressure is monitored but it is not part of the control loop. 

By deliberately changing the pressure (in a loop), the temperature response followed immediately [1]. This proved that control of pressure is cmcial for obtaining stable temperature baselines. 

Disturbance, such as upstream pressure, which specifically leads to changes in the fuel gas flow rate is now drawn to be part of the secondary flow control loop. (A disturbance such as change in the process stream inlet temperature, which is not part of the secondary loop, would still be drawn in its usual location as in Section 5.2 on page 5-7.)... 

Controllers are the controlling element of a control loop. Their function is to maintain a process variable (pressure, temperature, level, etc.) at some desired value. This value may or may not be constant. 

Area 300 is controlled using a distributed control system (DCS). The DCS monitors and controls all aspects of the SCWO process, including the ignition system, the reactor pressure, the pressure drop across the transpiring wall, the reactor axial temperature profile, the effluent system, and the evaporation/crystallization system. Each of these control functions is accomplished using a network of pressure, flow, temperature, and analytical sensors linked to control valves through DCS control loops. The measurements of reactor pressure and the pressure differential across the reactor liner are especially important since they determine when shutdowns are needed. Reactor pressure and temperature measurements are important because they can indicate unstable operation that causes incomplete reaction. 

The interface with the process at the other end of the control loop is made by the final control element. In a vast majority of chemical engineering processes the final control element is an automatic control valve which throttles the flow of a manipulated variable. Most control valves consist of a plug on the end of a stem that opens or closes an orifice opening as the stem is raised or lowered. As sketched in Fig. 7.5, the stem is attached to a diaphragm that is driven by changing air pressure above the diaphragm. The force of the air pressure is opposed by a spring. 

The basic reason for using different control-valve trims is to keep the stability of the control loop fairly constant over a wide range of flows. Linear-trim valves are used, for example, when the pressure drop over the control valve is fairly constant and a linear relationship exists between the controlled variable and the flow rate of the manipulated variable. Consider the flow of steam from a constant-pressure supply header. The steam flows into the shell side of a heat exchanger. A process liquid stream flows through the tube side and is heated by the steam. There is a linear relationship between the process outlet temperature and steam flow (with constant process flow rate and inlet temperature) since every pound of steam provides a certain amount of heat. 

In addition to the basic control loops, all processes have instrumentation that (1) sounds alarms to alert the operator to any abnormal or unsafe condition, and (2) shuts down the process if unsafe conditions are detected or equipment fails. For example, if a compressor motor overloads and the electrical control system on the motor shuts down the motor, the rest of the process will usually have to be shut down immediately. This type of instrumentation is called an interlock. It either shuts a control valve completely or drives the control valve wide open. Other examples of conditions that can interlock a process down include failure of a feed or reflux pump, detection of high pressure or temperature in a vessel, and indication of high or low liquid level in a tank or column base. Interlocks are usually achieved by pressure, mechanical, or electrical switches. They can be included in the computer software in a computer control system, but they are usually hard-wired for reliability and redundancy. 

The common types of control loops are level, flow, temperature, and pressure. The type of controller and the settings used for any one type are sometimes pretty much the same from one application to another. For example, most flow control loops use PI controllers with wide proportional band and fast integral action. 

C. PRESSURE LOOPS. Pressure loops vary from very tight, fast loops (almost like flow control) to slow averaging loops (almost like level control). An example of a fast pressure loop is the case of a valve throttling the flow of vapor from a vessel, as shown in Fig. 7.15o. The valve has a direct handle on pressure, and... 

Figure shows the choke control loop in detail. The position choke controller, ZIC, is located in the GC control room at Skid 7 and can be operated in the remote mode, i.e. under the control of the HOC computer, or in the local mode in which case the GC operator enters a setpoint value and the controller drives the choke to the proper position. Estimates based on both the pressure and choke relationship are available to the MOC operator on CRT displays. 

Hence, a theoretical reject pressure can be calculated from pressures measured in the field, together with a constant input from a ratio-cootrol unit. The theoretical pressure proportional to the reject flow rate is then used as the set point in the control loop. The measured variable pn, in the field can then be adjusted automatically by trimming a pressure-control valve in the reject line until the calculated pn) equals the measured pnj. 

Fig. 7.95. Block diagram of sampled data pressure control loop shown in Fig. 7.94...

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