Jumat, 31 Agustus 2007

Introduction of Process Instrumentation


Learning Outcome #1

Upon successful completion of this module, the student will be able to describe the importance of process instrumentation as it relates to industrial production.
Enabling Objectives for Learning Outcome #1
The student will be able to:
(a)     State the three most important operation criteria for process plant instrumentation.
(b)     State the five purposes of instrumentation in a process plant.
Learning Outcome #2
Upon successful completion of this module, the student will be able to demonstrate how simple open-loop and closed-loop control systems are arranged, and identify their major components.
Enabling Objectives for Learning Outcome #2
The student will be able to:
(a)    List and - describe the function of the following control system components: controller, measurement, final control element (FCE), and process.
(b)    Sketch the arrangement of the components in a typical control system.
(c)    Use ISA symbols relating to control systems.

INTRODUCTION
A process plant is an arrangement of processing units, such as heat exchangers, distillation columns, reactors, and absorbers, which are integrated in a systematic and rational manner.  The plant's overall objective is to convert raw material into desired products, by the application of energy, in a safe and cost-effective manner.  A process plant that is designed correctly should address the following criteria:
Safety
The safe operation of the process plant is of primary concern, and the design of the plant should allow for the safe containment of process pressures.  Construction materials should be able to withstand the designated temperatures and chemical concentrations.  In addition, a control system should be specified that is capable of maintaining the process parameters at the design operating points, providing for an orderly equipment shut-down, and providing fail-safe capability.
Production
In order to maximize the return on investment to the plant operators, the design of the process plant should be such that the quality and quantity of the desired product meets or even exceeds the original design expectations.  In order to accomplish this, the process plant must utilize a control system that is capable of providing the necessary production and quality control.
Environmental Standards
The process plant must comply with federal, provincial, and municipal environmental regulations concerning air pollution and plant effluent if the plant is to remain operational, and avoid punitive action by government pollution control agencies.  For example, the plant design should incorporate a method of treating the plant effluent prior to discharge.  Instrumentation can play a large part in pollution control., by monitoring and controlling the discharge of plant effluent.
These requirements demonstrate the need for accurate process measurement, and precise and safe control of process parameters.  This is accomplished by proper design and specification of the elements in each control loop, and also requires skilled installation, configuration, and tuning of each piece of equipment.

PROCESS MEASUREMENT INSTRUMENTATION
The importance of process measurement in a process plant cannot be over emphasized.  Current process control and quality control technology relies on accurate and repeatable measurements, in order to safely produce a return on investment consistent with the design of the process plant.
In general, measurement instruments cannot measure the process parameter directly.  Rather, the device measures some other parameter, and infers that that is the desired parameter.  Consider the following two examples:
* Suppose it is required to read the temperature of a room using an alcohol-in-glass thermometer.  It is clear that, in reality, the expansion and contraction of the alcohol is what is being measured relative to the room temperature.  The height of the alcohol column is, in turn, compared to a calibrated scale and a temperature measurement is inferred.
* If the steam pressure in a boiler drum is to be measured by using a common pressure gauge, in reality the mechanical deflection of the measuring element inside the gauge is what is being measured relative to the steam pressure.  This deflection of the measuring element is converted to angular motion,  a pointer is linked to the measuring element and compared to a calibrated scale as an. inferred pressure measurement.
This phenomenon of inferred measurement causes the instrument to carry out a conversion from one energy form to another; in the case of the room thermometer, from heat energy to mechanical energy.  This conversion process increases the complexity of the instrument and, consequently, increases the potential sources of error.
Fig. 1 illustrates a typical filled-system temperature measurement system.  The instrument utilizes the volumetric expansion and contraction of the f'illed medium, which in turn, deflects a spiral expansion element.  The deflection of the spiral element is able to indicate temperature on a calibrated scale via a linkage.

Figure 1: Filled System Temperature Measurement
An instrument of this type is subject to numerous sources of error, such as:
1. Incorrect immersion of the bulb in the measured medium.
2. Changes in ambient temperature which would expand or contract the capillary.
3. Changes in atmospheric pressure which would affect the spiral expansion element.
4. Differences in elevation between the bulb and the spiral expansion element, resulting in a change in hydraulic head which would cause the spiral expansion element to deflect based on elevation rather than changes in temperature.
5. Incorrect calibration of the linkage mechanism.
There are specific techniques used to compensate for all of these sources of error, but any errors remaining can compound to affect the overall accuracy of the instruments
The most common parameters measured on a process plant are sometimes referred to as the bigfour: pressure, temperature, level, and flow.  These account for about 80% of all measurements on a typical process plant.  The remaining 20% include analytical measurements, and measurements of density, humidity, and speed.
Recording Instrumentation
Process parameters such as temperature, pressure, and flow require continuous measurements in real time.  If review of the measurements is desired, provision must be made to capture the parameters with respect to time.  The recorder shown in Fig. 2 is a device used to accomplish this task, and may take many forms depending on the application.  The usual method is to inscribe the measurement of the parameter on a chart with respect to time.  These charts can be circular or linear, and may be driven by a timing mechanism.  The process parameter is recorded by a pen which leaves a trace on the chart. thus producing a historical record.  The duration of the record is a function of chart speed (time base) and length of chart paper.

Figure 2: Two-Pen Recorder
In addition to recording parameters on a continuous basis, it may be desirable to record process parameters on a time interval basis.  This method is used to store large numbers of process parameters at preprogrammed intervals.  A device called a data logger or historical trend unit is used for this purpose.  The data is compiled continuously and can be stored for days, months, or even years.  From this stored data, long-term trends can be established, and the data can be used to investigate plant failures or other problems.
Indication Instrumentation
To provide visual indication of process parameters to the plant operators, instrument indicators are used extensively in process operations.  The indicators can be the traditional circular dial types, or linear analog and digital displays.  Indicators may be field mounted, or panel mounted in a control room.
The circular indicators are calibrated and ranged to give a reasonably accurate indication of a wide range of process parameters.  The indicator mechanisms . are selected to match the parameter signal and the desired range of measurement, and use a pointer on a calibrated scale to indicate.  In recent years, circular gages have been superseded by linear indicators displaying the process parameter in the form of a bar against a calibrated scale, as shown in Fig. 3.

Figure 3: Dual Indicator
Digital display units, which give a numerical indication of the process parameter, are becoming increasingly common; some indicators may give the operator a choice of analog or digital indication, or may display both simultaneously,
Alarm Instrumentation
The purpose of alarm instrumentation is to detect process parameters that deviate outside predetermined limits.  An alarm instrument loop usually consists of a switch or contact closure device as a field device, and an end device such as a horn, light, or bell.  The alarm loop is normally digital, having two conditions corresponding to an alarm state and a non-alarm state.  The field devices are typically diaphragm-operated pressure switches for pressure/level applications, filled system switches for temperature applications, and mechanical switches to detect flow.  End devices are designed to alert the operator of an undesirable condition by audio/visual means.  The end devices are often concentrated into a panel, known as an annunciator, designed to alert the operator if a field device is actuated, and to indicate its location.
Fig. 4 shows a typical process alarm loop designed to activate when the steam pressure is too high.  The detector is a pressure switch that is activated by closing the switch contacts at a predetermined level.  The end device is an incandescent bulb.  It is possible to combine alarms to give warnings of varying degree.  For example, the alarm loop could be designed to give a warning of low pressure, and specified as a PAL (pressure alarm low); an additional switch could be included in the same loop, set to operate at a lower pressure, and known as a PALL (pressure alarm low-low).

Figure 4: Typical Discrete Process Alarm


PROCESS CONTROL SYSTEMS
The three requirements listed in the introduction - safety, production, and environmental responsibility - suggest the need for continuous monitoring of the operation of a process plant.  In addition, some form of external intervention or control must be applied to ensure that these operations criteria are met.
The operations criteria can be achieved through the specific arrangement of equipment like pumps, process vessels, and valves, and through human intervention by plant operators.
The three main objectives of control systems are:
1. To suppress the influence of external disturbances on a process plant.
2. To ensure the stability and safety of a process plant.
3. To optimize the productivity of a process plant.
Cancelling the Effect of Disturbances
An attempt to suppress the influence of disturbances is the most common objective of a control system, although it is not necessarily the most important one.  This control objective attempts to return the process parameter back to the setpoint as effectively as possible.  The control system is called upon to make changes to the process in order to negate or cancel the effects of any external disturbances.  Fig. 5 represents a process that is designed to maintain a set outflow temperature.

Figure 5: Hot Water Process
In this process, the set outflow temperature (T out) is subject to influences from outside the process.  The disturbances could be varying feed flow (F in) and/or feed temperature to the process.  An increase in flow (F in) would normally require an increase in steam flow (F st,) to compensate for the increase in heat required to maintain the outlet temperature (T out ) at the desired setpoint.  Similarly, a decrease in feed temperature (T in ) would require a corresponding increase of steam flow (F st,) to balance the process.
Process Stability
Ensuring the stability of a process is an important aspect of control.  Without stability control, the behavior of the process can range from virtually inactive, with very little control action taking place, to a process that is totally unstable.  An unstable condition affects safety of plant and personnel, in addition to causing poor productivity.  Some processes are naturally stable or self-regulating; others are naturally unstable, and therefore non self-regulating.
Fig. 6 describes a self-regulating level application.  As the level in the vessel rises, the liquid head increases, raising the pressure difference across the outlet nozzle.  The increased pressure differential across the nozzle results in a greater flow from the nozzle.  At some point, the outflow of material from the process will balance the inflow of material into the vessel, and the level will stabilize and remain at that point until the inflow changes.

Figure 6: Self-Regulating Level Process
An example of a non self-regulating process is depicted in Fig. 7, showing a vessel which has a constant outflow due to the positive displacement pump.  The pump delivery volume is constant and not dependent oil the liquid head.  Since the discharge from the positive displacement pump is constant, a steady state vessel level will exist only when the inflow to the vessel exactly equals the outflow.

Figure 7: Non Self-Regulating Process
If the inflow is allowed to deviate from the steady-state condition by however small an amount, the vessel will either overflow or run dry.  It is clear that a level control system is required in order to maintain the stability of the process.
The Feedback Control Loop
A fundamental part of any industrial control system is the feedback control loop.  It consists of a controller, measurement element, final control element, and the process, as illustrated in Fig. 8. All of the components in the loop are interconnected so that information is passed continuously around the loop.  This is classic closed-loop control and, as long as the loop remains closed, automatic feedback control will exist.


Figure 8: Feedback Control Loop
Feedback is in the form of information gained by monitoring the controlled variable, and comparing the controlled variable signal to the setpoint.  If the feedback causes the difference between the setpoint and the controlled variable to increase, then the feedback is positive.  This situation will cause increasing loop instability, a dangerous and undesirable condition.  However, under negative feedback control, the setpoint (usually a fixed value) and the controlled variable are continually compared and the error between the two diminishes.
If the flow of information around the loop is interrupted in any way, (as, for example, when the controller is placed on manual control), then the loop is said to be open and automatic feedback control ceases.  This situation is illustrated in Fig. 9.

Figure 9: Open Control Loop
1.     Controller
The principle function of a controller is to generate an output signal that will manipulate the final control element in such a way that the equilibrium of the loop will be maintained or restored.  The controller is normally the only component in the loop that is capable of counteracting the effect of disturbances on the process.  The controller can be divided into two sections.  The first section is designed to function as an error detector, where the setpoint is compared to the controlled variable and the direction of the error is established.  The second section manipulates that error by solving a control algorithm, in order to produce an output that is of the correct magnitude.,
Fig. 10 identifies the components of a typical feedback controller.  In performing the control function, the automatic controller acts upon the error (e) which is determined by comparing the setpoint (sp) with the controlled variable (cv).  No control action will take place if no error exists (e = 0).  However, if an error does exist (e=<>0), then the control algorithm will act on that error to produce a change in output (m).  The output is applied to the final control element, which changes the manipulated variable in an effort to reduce the error to zero.

Figure 10: Feedback Controller
The control algorithm is a mathematical equation that is solved in real time, to produce a continuous output of the correct magnitude to restore the process to equilibrium.  The controller adjustments shown in Fig. 10 are manual tuning adjustments that affect coefficients in the control algorithm; they are adjusted to configure the algorithm to generate the most effective output necessary to maintain and restore loop stability.
The direction of the output is determined in the sign of the error.  If the error is plus (+e), then the output of the controller (m) will increase as the error increases.  The converse is also true: as the error decreases, so the controller output will decrease.  The sign of the error (e) can be reversed by switching the inputs of the setpoint and the controlled variable at the error detector.  This switching capability is a standard function of any controller, and is used to ensure that the controller output moves the final control element in the correct direction to maintain or restore loop stability.
2.      Measurement
The primary function of the measurement component is to produce a signal that represents the value of the controlled variable.  This is achieved by the use of a transducer or transmitter that is designed to convert the physical properties of the controlled variable to a standard electrical or pneumatic signal that is acceptable to the controller.  As an example, the controlled variable may be a flowing liquid which is converted to a differential pressure by a head-type flow meter.  In turn, the differential pressure is converted to a standard analog signal by a transmitter.
Table 1: shows typical standard analog signals currently used in the process control industry.
3.     Final Control Element
The final control element (FCE) is the loop component that changes the value of the manipulated variable, and regulates the supply of energy or materials to the process.  The final control element responds to the changes in output (m) of the controller that are necessary to maintain or restore loop equilibrium.  The final control element is typically a valve, but may also be a variable speed conveyor belt, a louvre actuator, a variable speed pump, or some other device.  The.selection and application of the final control element is critical to the performance of the entire loop.  Regrettably, the importance of this component is often overlooked, to the detriment of loop performance.
4.      Process
The process component of the control loop is by far the most complex and is usually the part of the loop that the instrument designer cannot change.  All processes have three major characteristics: capacity, resistance, and dead time.  All three characteristics occur in every process in varying degrees, and all affect the controllability of the process.
Fig. 11 illustrates a single capacity process with minimum resistance, and assumes a measurement dead time of short duration.  This would be an easy process to control, requiring only a single simple control loop.

Figure 11: Single Capacity Process
Conversely, a process with multiple capacities, high resistance, and long dead time, similar to the one illustrated in Fig. 12, could be extremely difficult to control with a simple single control loop.  It may be necessary to apply a multi-loop control configuration in order to maintain acceptable loop stability.
Figure 12: Multi-Capacity Process
In practice, a process will contain multiple capacitance and resistance characteristics.  Depending upon their size and relationship to each other, these characteristics affect the controllability of the loop to varying degrees.  Dead time, also referred to as delay, transportation lag, and distance velocity lag, is rarely found in a pure form, but occurs in combination with capacity and resistance.
In addition to the three characteristics found in all processes (capacity, resistance, and dead time), some processes exhibit non-linear gain characteristics and loop interactions which greatly increase the complexity of the control schemes necessary to maintain loop stability.
Fig. 13 illustrates the loop reaction curves of two processes: one with single capacity as shown by the solid line, and the multi-capacity process shown by a broken line.  By observing the process reaction curves of each process, it is possible to predict the degree of difficulty in controlling each process.  AS the reaction time of a loop increases, as in the case of the multiple capacity process, the degree of difficulty to control and maintain loop stability also increases.

Figure 13: Control Loop Response
Little can be done to improve the controllability of a process once the physical plant is in place.  It is, therefore, necessary to specify and correctly tune all loop components, especially the controller, in order to balance the effects of the process upsets.  Knowledge of process characteristics, and the ability to implement an effective control strategy to maintain loop stability, are essential to the study of industrial instrumentation.


Inez Girlasa Pangestika




لسلام عليكم و رحمة الله و بركاته

Bayi Melenium, dan terlahir sesuai harapan yg didambakan, mendapat anak Perempuan ( Girl-asa ), proses kelahiran sangat cepat masuk jam 02:00 dinihari dan lahir kira-kira jam 03:00 dini hari ( itupun ditinggal ngopi ama temen ).
Pas Balik tanya keruang suster.....Pak Selamat ya udah lahir anaknya perempuan, berat 2.85 kg panjang 49cm.....????

Alhamdullilah....sekarang nich bocah sudah 7 Tahun....kelas 2 SD, Ada beberapa kelebihan dari anakku satu ini : Kalau Mamanya lagi masak rajin bener bantuin......ngelukis atau gambarnya juga bagus ….di photo2 juga gayanya pede buanget.....terus disekolah juga ga jelek2 amat masuk sepuluh besar......dan rajin bener ngasih makan ikan di aquarium……intress nya kayaknya miara hewan …..hewan piaraan apa aja dia suka…..tapi Mamanya ga suka…….!!!

Kekurangannya ya seperti anak2 kebanyakan : belajar kalo ga disuruh....ya pasti ga belajar.....mandi....kalau ga disuruh....ya ga bakalan mandi.....bangun...kalau ga dibangunin....ya ga bakalan bangun......shollat kalau ga disuruh shollat....ya ga bakalan sholat......Butttt...Maen kalaupun ga disuruh......pasti dia maen terussssss......!!!

Kebiasaan dari kecil yg susah dihilangkan, dari anakku satu ini tidur diatas dadaku….!!
Karena sewaktu kecil suka sekali dia tidur didadaku sambil kuelusin rambutnya atau kuelusin punggungnya sambil bersenandung hingga di terlelap/ tidur.

Kalau ditanya cita2nya, Inez kalo udah gede mo jadi apa…???…” Guru “ katanya…..!!!????






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