Rangeability

It is important to evaluate the region of disturbances for which the control system is able to provide stable effective control and maintains reasonable conversions and product purities. We call this property the rangeability of the control structure. 

The hrst type of disturbance studied is a range of plus or minus step changes in vapor boilup (the production rate handle). Results show that very large positive changes of up to +50% can be handled by the CS7-RR control structure. The purities of both products are maintained within 1% of the desired 95% specihcation. 

However, the robustness of the system for decreases in vapor boilup is more limited. When vapor boilup decreases, tray temperatures throughout the column decrease. Both temperature controllers decrease the fresh feeds. A negative 10% change can be handled, but a 12% decrease shuts off the Fqb stream completely and the system shuts down after a short time. 

The second type of disturbance is a step change in the composition of the fresh feeds. For a change in composition of fresh feed Fqa from pure component A to a mixture of components A and B, the system is dynamically stable, but disturbances larger than 4% of B in this feedstream result in a decrease in bottoms purity below 94%. The distillate purity is above specification. 

Because component B is heavier than component A, when putting some B in the Fqa feedstream, the temperatures throughout the column increase initially. The temperature controllers increase both fresh feeds because they both have direct action. This introduces even more reactant B into the system. The final result is a higher concentration of reactant B in the bottoms (for the same two tray temperatures and the same vapor boilup). 

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Type of trim. Use equal percentage whenever there is a large design uncertainty or wide rangeability is desired. Use linear for small uncertainty cases. 

The "burner" consists of a large number of small burner jets, arranged in a grid pattern inside the stack, near the bottom. To increase the rangeability of the... 

It is clear from q. (7.19) that as the second term in the denominator approaches unity, the required pressure drop goes to infinity So there is a limit to the achievable rangeability of a system. 

Let us define this term as the rangeability index of the system, (R. 

Control valves have orifices that can be adjusted to regulate the flow of fluids through them. Four features important to their use are capacity, characteristic, rangeability and recoveiy. 

Rangeability is the ratio of maximum to minimum flows over which the valve can give good control. This concept is difficult to quantify and is not used much for valve selection. A valve generally can be designed properly for a suitably wide flow range. 

Figure 2.63 illustrates the effect of the distortion coefficient (Dc) on the characteristics of a linear and an equal-percentage valve. As the ratio of the minimum to maximum pressure drop increases, the Dc drops and the equal-percentage characteristics of the valve shift toward linear and the linear characteristics shift toward QO. In addition, as the Dc drops, the controllable minimum flow increases, and therefore, the rangeability (the flow range within which the valve characteristic remains as specified) of the valve also drops. 

The conventional definition of rangeability is the ratio between the maximum and minimum controllable flow through the valve. Minimum controllable flow (Fmin) is not the leakage flow (which occurs when the valve is closed), but the minimum flow that is still controllable and can be changed up or down as the valve stroke is changed. 

This cycling can be eliminated by mounting the control valve in the condensate pipe, but this creates new problems, because when the load decreases, the process is slow steam has to condense before the condensate level is affected, and when the load increases, the process is fast, because blowing out liquid condensate is fast. With such "nonsymmetrical" process dynamics, control is bound to be poor. A better option is to use lifting traps to prevent condensate accumulation. These pumping traps will make temperature control possible even when the heater is under vacuum, but will not improve the problem of low rangeability, and the possible use of two control valves in parallel can still be necessary. 

Pump turndown and rangeability can be increased by operating two or more pumps in parallel. 

Measurement and control of low-flow rates are a requirement in such applications as fuel cells, purging, bioreactors, leak testing, and controlling the reference gas flow in chromatographs or in plasma-emission spectrometers. The most traditional and least expensive low-flow sensor is the variable-area flowmeter. It has a high rangeability (10 1) and requires little pressure drop. Due to its relatively low accuracy, it is limited to purge and leak-detection applications. 

In laminar flow elements, the pressure drop and flow are in a linear relationship. The laminar flow element can be used in combination with either a differential-pressure- or a thermal-type flow detector. These flowmeters provide better rangeability at about the same cost as sonic nozzles. 

Thermal flowmeters also can directly detect low-mass flows without any laminar elements. In that case, they are installed directly into the pipeline as either thermal flowmeters or anemometers. They have a 100 1 rangeability, and can be provided with integral controllers. 

Most laminar flowmeters (also called capillary flowmeters) measure very low flow rates of gases in applications where other types of meters either give marginal performance or cannot be used at all. Commercial units use either capillary elements or matrix shapes (Figure 3.67). The pressure drop generated by these elements is in linear relationship with the process flow. When higher accuracy and rangeability is desired, thermal instead of d/p detectors are used with the laminar flow element. 

Thermal flowmeters are mostly used in gas services, but can also detect liquid flows. They can measure flows from 0 to 10,000 kg/h (22,000 lb/h) with 1 to 2% FS errors. Their rangeability is from 10 1 to 100 1, their operating temperature range is up to 500°C (950°F), and they can operate at pressures from atmospheric to 83 bar (1,200 psig). 

As smart d/p transmitters became available, the orifice rangeability could be increased at a lower cost by using dual-span transmitters. Some smart d/p transmitters are currently available with 0.1% span accuracy, and their spans can be automatically switched based on the value of measurement. Therefore, a 100 1 pressure differential range (10 1 flow range) can theoretically be obtained by automatically switching between a high (10-100%) and a low (1-10%) pressure differential span (in practice, some overlap is recommended). The overall result can be a 1% of AF accuracy over a 10 1 flow range. 

Their disadvantages include their low accuracy, low rangeability, and the fact that they are suitable only for clean liquid, gas, or vapor services unless purged. Their principal limitation is that they measure the flowing velocity only at one point and, therefore, even after calibration, their measurement will be in error every time the velocity profile changes. Therefore, they are used only when low-accuracy volumetric readings are acceptable, such as in HVAC applications. To reduce the effect of velocity profile changes and... 

The flow rate in household units is 0-7 scmh (0-250 scfh), and that of industrial units is up to 4,000 scmh (100,000 scfh). The errors of the standard designs of these sensors are 0.5-1% FS over a rangeability of 200 1, and their high-precision designs are available at 0.5% AF over a 100 1 range. 

Higher precision and rangeability are achieved by eliminating pressure drop and thereby eliminating slip or leakage flows by using a motor drive. This flowmeter is claimed to limit the error to 0.5% over a 100 1 range. 

The chief advantages of gas PD meters are their high accuracy and wide rangeability. Their chief disadvantages are maintenance costs and the fact that wear and tear can degrade their performance. 

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