Three-element level control

Three-element level control is most commonly applied to the control of water level in steam drums on boilers. However it is applicable to many other situations where tight level control is required and is made difficult by unusual dynamics. 

The first most commonly encountered problem is swell. The water in the steam drum contains vapour bubbles which expand if the pressure in the drum is reduced, thus increasing the liquid level. So, if there is an increase in steam demand which causes a transient drop in drum pressure, the level controller will reduce the flow of water in order to correct for the apparent increase in level. Of course, on increasing steam demand we need an increased water flow. The pressure in the drum will ultimately be restored, for example by a pressure controller on the steam header increasing the boiler duty, and the level controller will ultimately increase the water flow. However, for the controller to be stable, its initial behaviour means that it will have to act far more slowly than the tight controller defined in Equation (4.14). 

This problem may be solved by using a dp type level instrument that effectively measures the mass of liquid in the drum rather than its volume. Since the effect is caused by a reduction in the fluid density, rather than an increase in its inventory, an instrument measuring the head of liquid will respond correctly. However some of the increase in level may be due to bubbles expanding in the tubes supplying the drum which causes additional water to enter the drum. Further local legislation may dictate, for safety reasons, that actual liquid level must be measured and used for control. Under these circumstances the problem can be alleviated by applying a correction term to the level measurement. 

The term K is determined empirically from process data and has the effect of increasing the level measurement transmitted to the controller when the measured pressure increases above the normal operating pressure. 

The level controller may also have to cope with inverse response. The boiler feed water ideally is heated in the economiser to the boiling point of water at drum pressure. However, this is often not achieved so that when the cooler water enters the drum it will cause a drop in temperature, thus causing bubbles to collapse and the water level to drop. While controllers can generally be tuned to handle inverse response, they have to be tuned to act more slowly to avoid instability. 

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In this example it would be incorrect to apply a ratio algorithm. We do not wish to keep the two fuel flows in proportion. However there are situations were either algorithm may be used. For example, in Chapter 4, we described how the three-element level control scheme for a steam drum may be adapted to either approach. 

Feedforward control was not widely used in the process industries until the 1960s (Shinskey, 1996). Since then, it has been applied to a wide variety of processes that include boilers, evaporators, solids dryers, direct-fired heaters, and waste neutralization plants (Shinskey et al., 1995). However, the basic concept is much older and was applied as early as 1925 in the three-element level control system for boiler drums. We will use this control application to illustrate the use of feedforward control. 

By definition, the FW regulator provides BW level control through the use of various sensing elements at the normal operating water level (NOWL). Three types of FW regulator are described in the next sections. 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

The three hierarchical levels are interconnected by information flowing from the strategic level via the tactical level to the operational level, and the other way around. From upper to the lower level, the information flow is related to the environment on the strategic level, which is the organizational values and norms. However, as Thompson (Thompson, 1967) identified, the tactical control level can allow the operational level to operate as a relatively closed system. The tactical level provides a buffer between the uncertain environment and stability of resources required for uninterrupted production on the operational level. In this way the influences from the external environment on the operational level will be reduced to a minimum. The information flow going from lower to upper level is related to the operational process or transformations. The top down flow provides the restrictions and conditions for the transformation, while the bottom up flow provides information about the status of inputs, outputs, and resources of the transformations. The horizontal information flows are between different control elements on one hierarchical control level. 

In addition to erythrocytes, blood contains white blood cells, called leukocytes, of several types, and platelets, also called thrombocytes, which control blood clotting. Hematopoiesis (from the Greek, haimo, for blood, and poiein for to make ) is the process by which the elements of the blood are formed. The marrow of bone contains so-called stem cells which are immature predecessors of these three types of blood cells. Chemicals that are toxic to bone marrow can lead to anemia (decreased levels of erythrocytes), leukopenia (decreased numbers of leukocytes), or thrombocytopenia. Pancytopenia, a severe form of poisoning, refers to the reduction in circulatory levels of all three elements of the blood. One or more of these conditions can result from sufficiently intense exposure to chemicals such as benzene, arsenic, the explosive trinitrotoluene (TNT), gold, certain drugs, and ionizing radiation. Health consequences can range... 

Boiler controls have already been described in Section 2.2, so their discussion here will be limited and oriented toward power generation. The level control of the steam drum of an HRSG is very similar to that of fired boilers, except that up to 30% of nominal steam flow, a single-element controller is usually used. Above 30%, the loop is bumplessly transferred to a three-element control (Figure 2.116). 

We have developed a semi-synthetic animal diet in which the levels of trace elements can be carefully controlled (31). We selected three dietary conditions control (C) 66 ppm Mn and 5ppm Cu low-Mn low-Cu (L) 2.5 ppm Mn and 0.5 ppm Cu and Mn-deplete (D) no added Mn and 5 ppm Cu. Female Sprague Dawley rats were randomly divided at weaning into the three dietary groups. Food and distilled water were provided jacj. 14h4tTim for the entire experimental period of twelve months. The weight gain of all animals was approximately the same. 

The European standard EC8-3 (CEN 2005) defines three Umit states, i.e. Umit states of damage limitation (DL), of significant damage (SD), and of near collapse (NC). The limit states are defined at the element level by the rotations in the moment - rotation relationship of the plastic hinge. In test examples presented in this paper only the probability of the exceedance of the NC limit state was evaluated. The NC limit state at the element level is reached when the rotation in plastic hinge exceeds the ultimate rotation, which corresponds to a 20% drop in strength. At the structure level, there is a lack of definitions of the limit states in codes. In this study, it has been conservatively assumed that the most critical column controls the state of the structure. Consequently, the structure is assumed to attain the NC limit state when the first column attains the NC limit state. Furthermore, it has been conservatively assumed that the NC limit state corresponds to the failure of the structure. The collapse of masonry infills does not influence the NC limit state. 

Process-variable feedback for the controller is achieved by one of two methods. The process variable can (I) be measured and transmitted to the controller by using a separate measurement transmitter with a 0.2-I.0-bar (3-15-psi pneumatic output, or (2) be sensed directly by the controller, which contains the measurement sensor within its enclosure. Controllers with integral sensing elements are available that sense pressure, differential pressure, temperature, and level. Some controller designs have the set point adjustment knob in the controller, making set point adjustment a local and manual operation. Other types receive a set point from a remotely located pneumatic source, such as a manual air set regulator or another controller, to achieve set point adjustment. There are versions of the pneumatic controller that support the useful one-, two-, and three-mode combinations of proportional, integral, and derivative actions. Other options include auto/manual transfer stations, antireset windup circuitry, on/off control, and process-variable and set point indicators. 

Hierarchical Control, in which a three-level command hierarchy is established each lower-level echelon element keys on those in the next higher echelon on each time step of the evolution. 

Blood was studied in a group of virtually healthy adolescents aged 14-17 from two localities in the Ukraine, where pesticide exposure differed by a factor of three, though the pesticide content in food products, drinking water, air and soil in the experimental zone was not higher than public health standards permitted. In Azerbaijan there was a difference of 100 times in the amounts of pesticides used in the experimental and control localities, while the pesticide contamination of elements of the environment and food products in the experimental zone was 2-50 times higher than acceptable levels [A97]. Table 3.6 shows the results. 

Because, for each of the three cases, 20 precursors are analysed, a comparison between the results of all three cases is possible. From the results of the analysis it appears that in all three cases, most precursors were enabled by the ineffective judgement element on the tactical control level. In companies A and B, the judgement element on the operational control level is the ineffective control element enabling the second most frequent precursors, while for company C the judgement element contains the most precursors on the strategic control level and appears to be a critical element at each control level for all three companies. Moreover, relatively more ineffective control elements are present on the higher (tactical and strategic) control levels in company C (18), than in company A (12) and B (11). 

So, summarising the situation, the company s management on the strategic control level, had to decrease their costs under pressure from stockholders. Both, the current information and historical information from the transformation and its deviations were not available. These three types of latent conditions led to the ineffective observation on the strategic control level, i.e. the failure to realise the necessity that engineers with the necessary expertise should be present on site. So, the latent conditions that caused the ineffective observation element on the strategic control level, are transformation, history, and external environment. ... 

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