Liquid-level process control system

Fig. 4.27 Block diagram for liquid-level process control system.

Example 4.5 (Liquid-Level Process Control System)... 

The great majority of automatic process control systems involve one or more of only five process variables, namely, pressure, temperature, flow rate, composition, and liquid level. Many of these variables are measured by the same kind of instrument, and indeed, all of them under certain circumstances can be evaluated in terms of pressures. Thus temperature can be measured by the pressure exerted by a confined gas in the gas thermometer the differential pressure across a restriction in a flow line is a measure of flow rate the pressure exerted by a boiling liquid mixture... 

The basic process control system (BPCS) consists of feedback and feedforward control loops that regulate process variables such as temperatures, flow rates, liquid levels, and pressures. Although the BPCS typically provides... 

The traditional approach for process monitoring is to compare measurements against specified limits. This limit checking technique is a standard feature of computer control systems and is widely used to validate measurements of process variables such as flow rate, temperature, pressure, and liquid level. Process variables are measured quite frequently with sampling periods that typically are much smaller than the process setthng time (see Chapter 17). However, for most industrial plants, many important quality variables cannot be measured on-line. Instead, samples of the product are taken on an infrequent basis (e.g., hourly or daily) and sent to the quality control laboratory for analysis. Due to the infrequent measurements, standard feedback control methods fike PID control cannot be applied. Consequently, statistical process control techniques are implemented to ensure that the product quality meets the specifications. 

The above diagram shows a flammable liquid being drawn from a process source into a buffer tank from which it is to be pumped onwards to a treatment stage. A typical level control loop is provided in the process control system to maintain level at say 50% full. A hazard will occur if the level control fails for any reason and the tank becomes full. The tank must have a pressure relief valve by law and if the liquid has to escape via this valve it will form a dangerous vapor cloud. 

A simple instrumented shutdown device would require the features shown in the above diagram i.e. a level switch set to detect extra high level in the tank causes an automatic shutoff valve to close off all liquid feed to the tank. The shutoff valve remains closed until the defect in the process control system has been rectified. 

Three examples of simple multivariable control problems are shown in Fig. 8-40. The in-line blending system blends pure components A and B to produce a product stream with flow rate w and mass fraction of A, x. Adjusting either inlet flow rate or Wg affects both of the controlled variables andi. For the pH neutrahzation process in Figure 8-40(Z ), liquid level h and the pH of the exit stream are to be controlled by adjusting the acid and base flow rates and w>b. Each of the manipulated variables affects both of the controlled variables. Thus, both the blending system and the pH neutralization process are said to exhibit strong process interacHons. In contrast, the process interactions for the gas-liquid separator in Fig. 8-40(c) are not as strong because one manipulated variable, liquid flow rate L, has only a small and indirec t effect on one controlled variable, pressure P. 

Specify special features and materials of construction, such as alloy or nonferrous impingement parts, or entire vessel if affected by process vapor and liquid. Specify special liquid reservoir at base of unit if necessary for system operations. Line units normally have dump traps or liquid outlet of separator, while vessel type often use some type of liquid level control. 

We use a simple liquid level controller to illustrate the concept of a classic feedback control system.1 In this example (Fig. 5.1), we monitor the liquid level in a vessel and use the information to adjust the opening of an effluent valve to keep the liquid level at some user-specified value (the set point or reference). In this case, the liquid level is both the measured variable and the controlled variable—they are the same in a single-input single-output (SISO) system. In this respect, the controlled variable is also the output variable of the SISO system. A system refers to the process which we need to control plus the controller and accompanying accessories such as sensors and actuators.2... 

We can see quickly that the system has unity gain and there should be no offset. The point is that integral action can be introduced by the process and we do not need PI control under such circumstances. We come across processes with integral action in the control of rotating bodies and liquid levels in tanks connected to pumps (Example 3.1, p. 3-4). 

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. 

If the temperature controller is on manual, the level loop cannot work. In this process it probably would be better to reverse the pairing of the loops control temperature with vapor sidestream and control base level with heat input. Notice that if the sidestream were removed as a liquid, the control system would not be nested. Sometimes, of course, nested loops cannot be avoided. Notice that the recommended Scheme B in Fig. 8.10 is just such a nested system. DistOlate has no direct effect on tray temperature. It is only... 

The nature of the interfacial structure and dynamics between inorganic solids and liquids is of particular interest because of the influence it exerts on the stabilisation properties of industrially important mineral based systems. One of the most common minerals to have been exploited by the paper and ceramics industry is the clay structure of kaolinite. The behaviour of water-kaolinite systems is important since interlayer water acts as a solvent for intercalated species. Henceforth, an understanding of the factors at the atomic level that control the orientation, translation and rotation of water molecules at the mineral surface has implications for processes such as the preparation of pigment dispersions used in paper coatings. 

The original GC control system took the form of a central room which monitors the flowllne6, oil, water, and utility sections, plus a smaller satellite control room monitoring the gas compression and gas conditioning section of the plant. Closed loop process control, such as separator liquid level, pressure, flow and temperature control were handled by local pneumatic analog controllers. The key process variables are displayed in the control room via electronic instrumentation. All the key process and equipment trouble alarms are annunciated m the control rooms, plus the on/off status of key machinery and open/close status of key valves are displayed. 

Instruments and Controls. Liquid levels in several of the tanks are automatically controlled with pneumatic instruments. Flows of several of the process streams are controlled and some are recorded. Temperatures of the feed and melt water are automatically controlled temperatures are recorded by two 16-point potentiometer recorders. Pressures are measured at several points in the system but all are manually... 

The use of high or low limits for process variables is another type of selective control, called an override. The feature of antireset windup in feedback controllers is a type of override. Another example is a distillation column with lower and upper limits on the heat input to the column reboiler. The minimum level ensures that liquid will remain on the trays, while the upper limit is determined by the onset of flooding. Overrides are also used in forced-draft combustion control systems to prevent an imbalance between airflow and fuel flow, which could result in unsafe operating conditions. 

In the second control structure (Fig. 2.11b), which does work, the fresh feed makeup of the limiting reactant (.F0B) is flow-controlled. The other fresh feed makeup stream (FCvl) is brought into the system to control the liquid level in the reflux drum of the distillation column. The inventory in this drum reflects the amount of A inside the system. If more A is being consumed by reaction than is being fed into the process, the level in the reflux drum will go down. Thus this control structure employs knowledge about the amount of component A in the system to regulate this fresh reactant feed makeup to balance exactly the amount of B fed into the process. 

R-V Reflux flow controls distillate composition. Heat input controls bottoms composition. By default, the inventory controls use distillate flowrate to hold reflux drum level and bottoms flowrate to control base level. This control structure (in its single-end control version) is probably the most widely used. The liquid and vapor flowrates in the column are what really affect product compositions, so direct manipulation of these variables makes sense. One of the strengths of this system is that it usually handles feed composition changes quite well. It also permits the two products to be sent to downstream processes on proportional-only level control so that plantwide flow smoothing can be achieved. 

A word needs to be said about controller type and controller tuning. Controller algorithm selection and tuning are important to the success of any control system. Two features should be recognized about the Eastman process. First, it is an integrating process with little selfregulation in terms of pressure, liquid levels, and chemical components. Second, there are no tight specifications on any variables. 

The process considered in this chapter is very simple. The reaction involves only one reactant and one product. Two inert components, one light and one heavy, are also present. These inerts must be purged from the system, The major plantwide control consideration is how to adjust the fresh feed of the reactant to balance exactly its rate of consumption by reaction. This is achieved by using the liquid level that is a good indication of the amount of reactant in the system (the base level in the DIB column). 

To maintain the production rate, product quahty, and plant safety requires a data acquisition and control system. This system consists of tenperature, pressure, liquid level, flow rate, and conposition sensors. Computers record data and may control the process. Modem chemical plants use program logic controllers (PLC) extensively. According to Valle-Riestra [20], instrumentation cost is about 15% of pinchased equipment cost for little automatic control, 30% for full automatic control, and 40% for computer control. 

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