The Feedback Control Loop: Controller Characteristics (2)

Another, perhaps the most important, controller parameter is the control action, which is set as either ‘‘direct’’ or ‘‘reverse’’. If not set correctly, positive feedback in the control loop would result in unstable operation with the valve reaching a wide open or closed limit. By convention, if the valve position is to increase as the measurement increases, then the controller is considered ‘‘direct’’ acting.

By first determining the process action, then specifying the opposite controller action, the desired negative feedback loop is achieved. A typical flow loop is a good example as follows: the process action is ‘‘direct’’ because the flow increases as the valve position is increased, therefore the controller action should be specified as ‘‘reverse’’.

The actual output signal from the controller will further depend upon the specified failure mode of the valve. For example, a fail-closed valve will require an increase-to-open signal, whereas a fail-open valve will require an increase-to-close signal. Most industrial controllers will have a separate parameter to specify the required signal for the failure mode of the valve. In order to minimize confusion, rather than displaying actual output, most controllers display an ‘‘implied valve position’’, which indicates the desired position of the valve.

The response characteristics of a direct acting PID controller are shown in Figure 3.2. For illustrative purpose, a step change to the measurement is made and held constant without feedback. In response to this disturbance, the independent contributions of each controller mode are provided in Figures 3.2(A, B and C), and the combined PID response is presented in Figure 3.2(D). Note that the Proportional mode has an immediate effect on the output, as defined by its algebraic relationship. The Integral mode keeps changing the output at a constant rate as long as the constant error persists. The Derivative mode provides an initial exaggerated response, which decays rapidly since the measurement stops changing after the initial step disturbance.

Although there are many ways to implement PID modes into a controller, the ISA standard algorithm is an ideal, non-interacting combination of the modes. This algorithm is a relatively new standard, made feasible by digital implementation. Note that many previously published tuning guidelines have been developed based upon various analog implementations of an interacting, series combination of these modes.

The Feedback Control Loop: Controller Characteristics (1)

The design of the valve, process, and measurement should be made such as to minimize deadtime in the loop while providing a reliable, more linear response; then the controller can be tuned to provide the best performance, with an acceptable operating margin for robustness. The PID controller is the most widespread and applicable control algorithm, which can be tuned to provide near optimal responses to load disturbances. PID is an acronym for Proportional, Integral and Derivative modes of control.

Proportional mode establishes an algebraic relationship between input and output. The proportionality is set by a tunable gain parameter. This unitless parameter, controller gain (Kc), specifies percent change in output divided by percent change in input. On earlier versions of PID controllers, an alternate parameter, Proportional Band (PB), was defined as the percent change in input required to cause a 100 percent change in output. Thus by combining definitions, these two terms are related as follows: Kc = 100/PB.

The Integral mode is sometimes referred to as ‘‘reset’’ because it continues to take action over time until the error between measurement and set point is eliminated. The parameter to specify this action is Integral time, which can be thought of as the length of time for the controller to repeat the initial proportional response if the error remained constant. Note that as this parameter is made smaller, the reset increases as the control action is repeated in a shorter period of time. Some controllers use an alternate parameter, Reset, that is the reciprocal of Integral time and is referred to as repeats/unit time. This latter approach is perhaps more intuitive in that as the Reset parameter is increased, there is more reset action being applied.

The Derivative mode is sometimes referred to as ‘‘rate’’ because it applies control action proportional to the rate of change of its input. Most controllers use the process measurement, rather than the error, for this input in order to not have an exaggerated response to step changes in the set point. Also, noise in the process measurement is attenuated by an inherent filter on the Derivative term, which has a time constant 1/8 to 1/10 of the Derivative time. Even with these considerations, process noise is a major deterrent to the use of Derivative mode.

The Feedback Control Loop: Measurement Characteristics

Sensor type and location as well transmitter characteristics, noise, and sampled data issues also can affect loop performance. Most continuous measurement sensors and transmitters have relatively fast dynamics and a noise filter, which can be approximated by a first-order lag with a one or two second time constant. Temperature sensors are somewhat slower as the sensor is in a thermowell, and these measurements have a larger, 15–30 second time constant.

Noise is often a problem in flow, pressure, and level measurements. Because flow is a very fast loop, controller tuning can be set to ignore noise by using low gain and rely on a large amount of reset to take significant action only on sustained deviations. On slower, non self-regulating loops like level, noise in the measurement can degrade potential control performance by preventing the use of higher gains and/or derivative action in the controller.

Excessive filtering of a signal to reduce noise would add effective deadtime to the loop, thus degrading the loop performance. One technique for reducing high amplitude, high frequency noise, without introducing an excessive lag, is to rate limit the signal to a rate comparable to the largest physically realizable upset. This approach chops off peak noise and allows a smaller time constant filter to effectively reduce the remaining lower amplitude, high frequency noise.

Non-continuous measurements, such as produced by the sample and hold circuitry of a chromatograph, can introduce significant deadtime into a loop. Also, the nature of the periodic step change in value prevents the use of derivative action in the controller.

Distributed Control Systems often sample the transmitted signal at a one second interval, sometimes faster or slower depending upon the characteristics of the process response. One concern related to sample data measurement is aliasing of the signal, which can shift the observed frequency. However at a one second sample interval, this has seldom been a problem for all but the fastest process responses. A general rule for good performance is to make the period between scans less than one-tenth of the deadtime, or one-twentieth of the lag in the process response.

The Feedback Control Loop: Process Characteristics

An agitated tank is often used as an example of a first-order lag process. However, mixing in real tanks falls far short of the ideal well-mixed tank. Real tanks have composition responses that are a combination of a first-order lag and deadtime. If the pumping rate of the agitator (Fa) is known, the deadtime (Td) of the real tank may be estimated by the following equation: Td = V/(F+Fa), where V is the volume of the tank and F is the flow through it.

Process responses often consist of multiple lags in series. When these lags are non-interacting, the resulting response is predominantly deadtime, varying linearly with the number of lags in series. However when these lags are interacting, such as the trays on a distillation column, the resulting response remains predominantly a first-order lag with a time constant proportional to the number of lags squared.

Other process characteristics that affect control performance are both steady-state and dynamic non-linear behavior. Steady-state non-linear behavior refers to the steady-state gain varying, dependent upon operating point or time. For example, the pH of a process stream is highly non-linear, dependent upon the operating point on the titration curve. Further, depending upon the stream component composition, the titration curve itself may vary over time.

Non-linear dynamic behavior can occur due to operating point, direction, or magnitude of process changes. For example, the time constant of the composition response for a tank will depend upon the operating point of liquid level in the tank. Some processes will respond in one direction faster than in the other direction, particularly
as the control valve closes. For example, liquid in a tank may drain quite rapidly, but once the drain valve closes the level can only rise as fast as the inlet stream flow allows. The magnitude of a change may cause different dynamic response whenever inherent response limits are reached. Process examples may include a transition
to critical flow, or a transition from a heat transfer to a mass transfer limiting mechanism in a drying processes.

These non-linearities are the main reason an operating margin must be considered when tuning the controller. If the loop is to be robust and operate in a stable manner over a wide range of conditions, conservative values of the tuning parameters must be chosen. Unfortunately, this results in poorer performance under most conditions. One technique to handle known non-linearities is to provide tuning parameters that vary based upon measured process conditions.

Rules Of Thumb : Filtration

1. Processes are classified by their rate of cake buildup in a laboratory vacuum leaf filter: rapid, 0.1–10.0 cm/sec; medium, 0.1–10.0 cm/min; slow, 0.1–10.0 cm/hr.
2. The selection of a filtration method depends partly on which phase is the valuable one. For liquid phase being the valuable one, filter presses, sand filters, and pressure filters are suitable. If the solid phase is desired, vacuum rotary vacuum filters are
   desirable.
3. Continuous filtration should not be attempted if 1/8 in. cake thickness cannot be formed in less than 5 min.
4. Rapid filtering is accomplished with belts, top feed drums, or pusher-type centrifuges.
5. Medium rate filtering is accomplished with vacuum drums or disks or peeler-type centrifuges.
6. Slow filtering slurries are handled in pressure filters or sedimenting centrifuges.
7. Clarification with negligible cake buildup is accomplished with cartridges, precoat drums, or sand filters.
8. Laboratory tests are advisable when the filtering surface is expected to be more than a few square meters, when cake washing is critical, when cake drying may be a problem, or
   when precoating may be needed.
9. For finely ground ores and minerals, rotary drum filtration rates may be 1500 lb/(day)(sqft), at 20 rev/hr and 18–25 in. Hg vacuum.
10. Coarse solids and crystals may be filtered by rotary drum filters at rates of 6000 lb/(day)(sqft) at 20 rev/hr, 2–6 in. Hg vacuum.
11. Cartridge filters are used as final units to clarify a low solid concentration stream. For slurries where excellent cake washing is required, horizontal filters are used. Rotary disk filters are for separations where efficient cake washing is not essential. Rotary drum filters are used in many liquid- solid separations and precoat units capable of producing
    clear effluent streams. In applications where flexibility of design and operation are required, plate-and-frame filters are used.

The Feedback Control Loop: Valve Characteristics

Control valves have unique characteristics of their own which can significantly affect the performance of a loop. The steady-state gain of the valve relates controller output to a process flow. How this flow affects the controlled variable of the process defines the range of control. For servo control, the range of control would be defined as the range of setpoints achievable at a given load. For regulator control, it would be defined as the range of loads for which the given setpoint could be maintained. Attempting to operate outside the range of control will always result in the valve being either fully open or closed and the controlled variable offset from setpoint.

The steady-state gain of a control valve is determined at its operating point, since its gain may vary somewhat throughout its stroke. Valves have internal trim that provide a specified gain as a function of position, such as Linear, Equal Percentage, or Quick Opening inherent characteristics. Typically, the trim is chosen such that the installed characteristics provide an approximately linear flow response. Thus for a valve operating with critical gas flow, Linear trim would provide an approximately linear flow response. An Equal Percentage trim may be used to provide a more linear response for gas or liquid flow where line pressure drop is equal or greater than the valve pressure drop. The Quick Opening trim is usually not chosen for linear response in continuous control applications, however, it provides a high gain near the closed position, which is useful for fast responding pressure relief applications.

One common non-linear characteristic of control valves is hysteresis, which results in two possible flows at a given valve position, depending upon whether the valve is opening or closing. In the steady-state, hysteresis limits resolution in achieving a specific flow with its desired effect on the process. Dynamically, hysteresis also creates pre-stoke deadtime, which contributes to total loop deadtime, thus degrading the performance of the loop. Prestroke deadtime is the time that elapses as the controller output slowly traverses across the dead band before achieving any change in actual valve position or flow.

The use of a valve positioner can significantly reduce both hysteresis and thus pre-stroke deadtime. A valve positioner is recommended for all control loops requiring good performance. Typical hysteresis may be 2–5% for a valve without a positioner, 0.5–2% for a valve with an analog positioner, and 0.2–0.5% for a valve with a
digital positioner.

On some control loops, a variable-speed drive on a pump, fan or blower may be used as the final element connecting the controller output to the process. Variable-speed drives provide fast and linear response with little or no hysteresis and therefore are an excellent choice with respect to control performance. As the initial cost of variable-speed drives continues to decrease, their use should become a more widespread practice.

Rules Of Thumb : Extraction, Liquid–Liquid

1. The dispersed phase should be the one that has the higher volumetric rate except in equipment subject to backmixing where it should be the one with the smaller volumetric rate. It should be the phase that wets the material of construction less well. Since the holdup of continuous phase usually is greater, that phase should be made up of the less expensive or less hazardous material.
2. Although theory is favorable for the application of reflux to extraction columns, there are very few commercial applications.
3. Mixer–settler arrangements are limited to at most five stages. Mixing is accomplished with rotating impellers or circulating pumps. Settlers are designed on the assumption that droplet sizes are about 150 mm dia. In open vessels, residence times of 30–60 min or superficial velocities of 0.5–1.5 ft/min are provided in settlers. Extraction stage efficiencies commonly are taken as 80%.
4. Spray towers even 20–40 ft high cannot be depended on to function as more than a single stage.
5. Packed towers are employed when 5–10 stages suffice. Pall rings of 1–1.5 in. size are best. Dispersed phase loadings should not exceed 25 gal/(min) (sqft). HETS of 5–10 ft may be realizable. The dispersed phase must be redistributed every 5–7 ft. Packed towers are
   not satisfactory when the surface tension is more than 10 dyn/cm.
6. Sieve tray towers have holes of only 3–8 mm dia. Velocities through the holes are kept below 0.8 ft/sec to avoid formation of small drops. At each tray, design for the redistribution of each phase can be provided. Redispersion of either phase at each tray
   can be designed for. Tray spacings are 6–24 in. Tray efficiencies are in the range of 20–30%.
7. Pulsed packed and sieve tray towers may operate at frequencies of 90 cycles/min and amplitudes of 6–25 mm. In large diameter towers, HETS of about 1 m has been observed. Surface tensions as high as 30–40 dyn/cm have no adverse effect.
8. Reciprocating tray towers can have holes 9/16 in. dia, 50–60% open area, stroke length 0.75 in., 100–150 strokes/min, plate spacing normally 2 in. but in the range 1–6 in. In a 30 in. dia tower, HETS is 20–25 in. and throughput is 2000 gal/(hr)(sqft). Power requirements are much less than of pulsed towers.
9. Rotating disk contactors or other rotary agitated towers realize HETS in the range 0.1–0.5 m. The especially efficient Kuhni with perforated disks of 40% free cross section has HETS 0.2 m and a capacity of 50 m3 =m2 hr.

Instrumentation Reference Book, Fourth Edition

Instrumentation Reference Book, Fourth Edition
Butterworth-Heinemann | 2009 | ISBN: 0750683082, 0080941885 | 928 pages | PDF | 16,3 MB

Instrumentation embraces the equipment and systems used to detect, track and store data related to physical, chemical, electrical, thermal and mechanical properties of materials, systems and operations. While traditionally a key area within mechanical and industrial engineering, it also has a strong presence in electrical, chemical, civil and environmental engineering, biomedical and aerospace engineering.

The discipline of Instrumentation has grown appreciably in recent years because of advances in sensor technology and in the inter-connectivity of sensors, computers and control systems. In turn, this has meant that the automation of manufacturing, process industries, and even building and infrastructure construction has been improved dramatically. And now with remote wireless instrumentation, heretofore inaccessible or widely dispersed operations and procedures can be automatically monitored and controlled.

he new 4th edition of this already well-established reference work, will reflect these dramatic changes with improved and expanded coverage of the both the traditional domains of instrumentation as well as the cutting edge areas of digital integration of complex sensor/control systems.


Thoroughly revised, with up-to-date coverage of wireless sensors and systems, as well as nanotechnologies role in the evolution of sensor technology

Latest information on new sensor equipment, new measurement standards, and new software for embedded control systems, networking and automated control

Three entirely new sections on Controllers, Actuators and Final Control Elements; Manufacturing Execution Systems; and Automation Knowledge Base

Up-dated and expanded references and critical standards


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Rules Of Thumb : Evaporators

1. Long tube vertical evaporators with either natural or forced circulation are most popular. Tubes are 19–63 mm dia and 12–30 ft long.
2. In forced circulation, linear velocities in the tubes are 15–20 ft/sec.
3. Film-related efficiency losses can be minimized by maintaining a suitable temperature gradient, for instance 40–458F. A reasonable overall heat transfer coefficient is 250 Btu/(h)(ft2).
4. Elevation of boiling point by dissolved solids results in differences of 3–108F between solution and saturated vapor.
5. When the boiling point rise is appreciable, the economic number of effects in series with forward feed is 4–6.
6. When the boiling point rise is small, minimum cost is obtained with 8–10 effects in series.
7. In countercurrent evaporator systems, a reasonable temperature approach between the inlet and outlet streams is 308F. In multistage operation, a typical minimum is 108F.
8. In backward feed the more concentrated solution is heated with the highest temperature steam so that heating surface is lessened, but the solution must be pumped between stages.
9. The steam economy of an N-stage battery is approximately 0.8N lb evaporation/lb of outside steam.
10. Interstage steam pressures can be boosted with steam jet compressors of 20–30% efficiency or with mechanical compressors of 70–75% efficiency.

Chemical Process Equipment: Selection and Design

Chemical Process Equipment: Selection and Design
Publisher: Butterworth-Heinemann | ISBN: 0750693851 | edition 1988 | PDF | 773 pages | 25 mb

The most comprehensive and influential book on chemical process equipment ever written, fully revised and updated to bring the chemical engineer into the 21st century!

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Rules Of Thumb : Drying Of Solids

1. Drying times range from a few seconds in spray dryers to 1 hr or less in rotary dryers and up to several hours or even several days in tunnel shelf or belt dryers.
2. Continuous tray and belt dryers for granular material of natural size or pelleted to 3–15 mm have drying times in the range of 10– 200 min.
3. Rotary cylindrical dryers operate with superficial air velocities of 5–10 ft/sec, sometimes up to 35 ft/sec when the material is coarse. Residence times are 5–90 min. Holdup of solid is 7–8%. An 85% free cross section is taken for design purposes. In countercurrent flow, the exit gas is 10–208C above the solid; in parallel flow, the temperature of the exit solid is 1008C. Rotation speeds of about 4 rpm are used, but the product of rpm and diameter in feet is typically between 15 and 25.
4. Drum dryers for pastes and slurries operate with contact times of 3–12 sec, produce flakes 1–3 mm thick with evaporation rates of 15–30 kg/m2 hr. Diameters are 1.5–5.0 ft; the rotation rate is 2–10 rpm. The greatest evaporative capacity is of the order of
   3000 lb/hr in commercial units.
5. Pneumatic conveying dryers normally take particles 1–3 mm dia but up to 10 mm when the moisture is mostly on the surface. Air velocities are 10–30 m/sec. Single pass residence times are 0.5– 3.0 sec but with normal recycling the average residence time is
   brought up to 60 sec. Units in use range from 0.2 m dia by 1 m high to 0.3 m dia by 38 m long. Air requirement is several SCFM/lb of dry product/hr.
6. Fluidized bed dryers work best on particles of a few tenths of a mm dia, but up to 4 mm dia have been processed. Gas velocities of twice the minimum fluidization velocity are a safe prescription. In continuous operation, drying times of 1–2 min are enough, but batch drying of some pharmaceutical products employs drying times of 2–3 hr.
7. Spray dryers are used for heat sensitive materials. Surface moisture is removed in about 5 sec, and most drying is completed in less than 60 sec. Parallel flow of air and stock is most common. Atomizing nozzles have openings 0.012–0.15 in. and operate at pressures of 300–4000 psi. Atomizing spray wheels rotate at speeds to 20,000 rpm with peripheral speeds of 250–600 ft/sec. With nozzles, the length to diameter ratio of the dryer is 4–5; with spray wheels, the ratio is 0.5–1.0. For the final design, the experts say, pilot tests in a unit of 2 m dia should be made.

High Pressure Chemical Engineering

High Pressure Chemical Engineering
Publisher: Elsevier Science | ISBN: 0444824758 | edition 1996 | PDF | 728 pages | 14,8 mb

This present volume contains the text of all contributions (oral and posters), except for the four invited papers, which were presented at the 3rd International Symposium on High Pressure Chemical Engineering on October 7-9, 1996. The symposium was divided into three major sections, namely
- Chemical reaction engineering, - Separation processes and phase equilibria - Plant, apparatus, machinery, measurements, control.

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Rules Of Thumb : Drivers And Power Recovery Equipment

1. Efficiency is greater for larger machines. Motors are 85–95%; steam turbines are 42–78%; gas engines and turbines are 28–38%.
2. For under 100 HP, electric motors are used almost exclusively. They are made for up to 20,000 HP.
3. Induction motors are most popular. Synchronous motors are made for speeds as low as 150 rpm and are thus suited for example for low speed reciprocating compressors, but are not made smaller than 50 HP. A variety of enclosures is available, from weather-proof to explosion-proof.
4. Steam turbines are competitive above 100 HP. They are speed controllable. They are used in applications where speeds and demands are relatively constant. Frequently they are employed as spares in case of power failure.
5. Combustion engines and turbines are restricted to mobile and remote locations.
6. Gas expanders for power recovery may be justified at capacities of several hundred HP; otherwise any needed pressure reduction in process is effected with throttling valves.
7. Axial turbines are used for power recovery where flow rates, inlet temperatures or pressure drops are high.
8. Turboexpanders are used to recover power in applications where inlet temperatures are less than 10008F.