Levelness instrumentation temperature effects

Also, an adequate emergency feedwater supply is available to allow the plant to remain at hot standby for 8 hours followed by an orderly cooldown to the primary system pressure and temperature at which the Shutdown Cooling System (SCS) can be initiated to continue cooldown to cold shutdown conditions. Level instrumentation and a low level alarm are provided on each EFWST to help the operator align the EFWST from the other train to preclude the tank from being emptied before the changeover to SCS cooling can be effected. 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

This demonstration plant will normally operate at a first-effect boiling point of 250° F., a last-effect boiling point of 120° F., and a discharge sea water concentration factor of 4. The primary control of the process is accomplished by automatic control of steam flow rate, sea water flow rate, and last-effect vacuum. No control is needed for temperature or pressure in the individual effects and heat exchangers, since these achieve their own levels, influenced only by the proportioning of the equipment. The demonstration plant is rather heavily instrumented to permit close surveillance of operating conditions and carrying out of special tests. 

The HAZOP study was instrumental in determining the need for an adequate alarm system on each of the specified controllers. If liquid levels within the column are not well controlled, then either flooding (too much liquid) or plate by-passing bythegas (too little liquid) will occur. Both situations lead to a substantial reduction in absorption efficiency with large increases in emission levels. The other important control parameter was shown to be the temperature. If the temperature in the cooling-coil section rises, then there is an appreciable reduction in absorption. Control of temperature is important in the upper sections of the column because it is here that the greatest effect on emission levels occurs. 

Figure 13 shows a typical pressurizer level system. Pressurizer temperature is held fairly constant during normal operation. The AP detector for level is calibrated with the pressurizer hot, and the effects of density changes do not occur. The pressurizer will not always be hot. It may be cooled down for non-operating maintenance conditions, in which case a second AP detector, calibrated for level measurement at low temperatures, replaces the normal AP detector. The density has not really been compensated for it has actually been aligned out of the instrument by calibration. 

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. 

Ambient temperature variations will affect the accuracy and reliability of level detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in the instrumentation circuitry, and, therefore, affect the calibration of electric/electronic equipment. The effects of temperature variations are reduced by the design of the circuitry and by maintaining the level detection instrumentation in the proper environment. 

Differential pressure-type detectors have already been discussed in connection with both flow (Section 3.9.9) and level (Section 3.11.2) measurements. Therefore, only their ranges and accuracies will be briefly mentioned here. The basic error of d/p transmitters ranges from 0.1 to 0.5% of the actual span. Added to this are the errors caused by the temperature and pressure effects on the span and zero of the instrument. For intelligent transmitters, the pressure and temperature corrections are automatic, and the overall error is 0.1 to 0.2% of span. 

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