Water level sensing

Feedback is information in a closed-loop control system about the condition of a process variable. This variable is compared with a desired condition to produce the proper control action on the process. Information is continually "fed back" to the control circuit in response to control action. In the previous example, the actual storage tank water level, sensed by the level transmitter, is feedback to the level controller. This feedback is compared with a desired level to produce the required control action that will position the level control as needed to maintain the desired level. Figure 3 shows this relationship. 

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. 

A closed-loop control system is one in which control action is dependent on the output. Figure 2 shows an example of a closed-loop control system. The control system maintains water level in a storage tank. The system performs this task by continuously sensing the level in the tank and adjusting a supply valve to add more or less water to the tank. The desired level is preset by an operator, who is not part of the system. 

The upper hydrocarbon pump is switched on and off by a product probe, which measures the resistivity of fluids adjacent to the intake of the hydrocarbon pump. When the product probe senses the higher-resistivity hydrocarbon, it switches on the product pump, which removes the accumulated LNAPL. The water level in the well rises in response to the removal of overlying product. As soon as the water reaches the product probe, the probe shuts down the hydrocarbon pump until the probe again senses that a sufficient quantity of free hydrocarbon has accumulated within the well. The well continues to cycle in this manner as long as free hydrocarbon is entering the well. 

Density of the fluid whose level is to be measured can have a large effect on level detection instrumentation. It primarily affects level sensing instruments which utilize a wet reference leg. In these instruments, it is possible for the reference leg temperature to be different from the temperature of the fluid whose level is to be measured. An example of this is the level detection instrumentation for a boiler steam drum. The water in the reference leg is at a lower temperature than the water in the steam drum. Therefore, it is more dense, and must be compensated for to ensure the indicated steam drum level is accurately indicated. 

Like conventional RP devices, these employ a reduced-pressure zone between the checks and, by continuously sensing line pressure, operate a relief valve to keep the pressure in the zone below the supply pressure. How ever, they offer one very important advantage—the ability to protect against backsiphonage. If supply pressure ever drops to atmospheric or below, the water level in the reduced-pressure zone is automatically lowered to the point w here an air gap greater than the diameter of the supply pipe is created within the unit. 

The tidal analysis of this study was based on archived data of prediction runs on the BSH s operational model system (Dick et al., 2001). As has been pointed out above, these studies only made sense because the model had been trained for independent tides (Muller-Navarra 2002), and it has been shown that by incorporating this improvement it has been possible to compute much more realistic water levels, at least with respect to strictly periodic water level flucmations (Miiller-Navarra and Lange, 2004). Otherwise important tidal constituents would be missing in the Baltic water-level time series, especially those in the Baltic Proper, Bay of Bothnia, and Gulfs of Finland and Bothnia. It is possible to extract time series for all grid points from the BSH s model archive and carry out a harmonic analysis in exactly the same way as with measured time series. 

Satellite remote sensing also allowed the estimating of several physical and hydrological parameters of the Aral Sea and its watershed basin [19]. For example, the volume variations of the Aral Sea have been calculated by combination of optical satellite imagery and in situ water level [32]. More recently [16, 23] have estimated the volume variation of the Large and Small Aral by combination of a precise digital bathymetry map (DBM) of the basin, with level variation deduced from satellite radar altimetry, for the period 1993-2004. 

In addition to desirable emulsions, a recurring feature in many process industries is the rag layer, a gel-like emulsion that forms and accumulates at the oil-water interface in the separation vessels of many industrial processes. Rag layers tend to concentrate a range of emulsion stabilizing components. Once formed, it can also trap additional components that would otherwise have creamed or settled out of the way. Rag layers can interfere with level-sensing monitors, short-out electrostatic grids, promote channeling flows, and, of course, prevent oil or water separation [87,88],... 

Ordinary level control does not work in steam drums or kettle boilers. The problem is the lower density of the boiling water as compared to the greater density of the non-boiling hot water in the external level sensing chamber. I ve addressed this problem in my book, Troubleshooting Process Plant Control (Wiley, 2009). 

The boiling water will be at a 10 to 50 percent higher level inside the steam drum than the water in the level-sensing chamber or gauge glass. There are two ways of dealing with this problem ... 

As I ve described in my book. Troubleshooting Process Plant Control, any time the steam inlet control valve becomes fully open, the deaerator temperature, pressure, and water level will become unstable and the deaerator stripping section will flood. Flood in the sense that boiling water may erupt from the atmospheric vent. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

In a sense this subdivision of the composition of the atmosphere is arbitrary since some of the so-called contaminants are derived partly or wholly from natural sources. However, in that their concentrations vary appreciably within very narrow geographical limits, they may be distinguished from the contents of Table 2.8 (with the possible exception of water vapour). Table 2.6 lists those contaminants which are important from a corrosion standpoint. Excluded are contaminants found only in very specific locations, e.g. in the vicinity of a chemical works. The concentrations given are intended only to indicate general levels in the usual classification of environments and not to define a particular environment. 

A sense of scale is important for understanding how chemistry at the macroscopic level is related to the behavior of atoms at the microscopic level. Atoms are extraordinarily small, and there are vast numbers in even very tiny objects. The diameter of a carbon atom is only about 150 trillionths of a meter, and we would have to put 10 million atoms side by side to span the length of this dash -. Even a small cup of coffee contains more water molecules than there are stars in the visible universe. 

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