Instrumentation for process measurement and control pdf


 

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Instrumentation For Process Measurement And Control Pdf

mentals needed in process control and instrumentation. The discussion of the basic principles underlying pressure measurement has been expanded to include. Process. Measurement and Analysis. VOLUME I. Bela G. Liptak. EDITOR-IN- CHIEF Dedicated to you, my colleagues, the instrument and process control. instrumentation and control engineers at P & I Design Ltd. This experience is based on design and Accurate flow measurement is a key element in process productivity. medical-site.info 3.

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What are the interview questions asked to an instrumentation and control engineer? What are interview question for instrumentation engineering? What are you doing if you worked as an Top Instrumentation Engineering? What … Instrumentation engineering interview questions and Instrumentation engineering interview questions and answers for freshers and experienced - List of Instrumentation engineering questions with answers that might be asked during an interview Instrumentation Engineer Interview Questions Glassdoor.

Top Interview Questions. Sort: Relevance Popular Date. How long will you work with the firm? Dunn McGraw-Hill Electronics and Communication Engineering - Measurements Explain what is instrumentation? A positioner is a device put into a valve to ensure that it is at a correct position of opening as per the control signal. Instrumentation engineering interview questions and Tips and tricks for What are interview question for instrumentation engineering? Here are top interview questions for your job interviews: Question.

The functionof the integral reset modeis to: 8. Opposechangein measurement b. Automaticallyadjustthe controller's gain c. Eliminate offset d. It is alsoa conditionoflife on this planet: we live at the bottom of an atmosphericocean that extendsupward for manymiles. This massof air has weight, and this weight pressingdownwardcausesatmosphericpressure. Water, a fundamentalnecessityof life, is suppliedto most of us underpressure.

In the typical processplant, pressureinfluencesboiling point temperatures, condensingpoint temperatures,processefficiency, costs, and other important factors. The measurementand control of pressure,or lack of it-vacuum-in the typical processplantis critical. Instruments are availableto measurea wide rangeof pressures. How theseinstruments function is the subjectof this chapter.

What Is Pressure? Pressure is often defined in terms of "head. We want to find the pressure in the boUom ofthe column. The weight ofthe column mar be calculated by first finding the volume of water. Water weighs So the weight of 23 cubic feet will be 23 times The area ofthe base is 1 square foot, or 12 inches times 12 inches, or square inches. The pressure equals 1, In practice, we find that only the height ofthe water confits. It mar be present in a small pipe or beneath the surface of a pond.

In any case, at a depth of 23 feet, the pressure will amo unt to approximately 10 pounds per square inch. If in your home the water pressure is 50 pounds per square inch and the system uses a gravity feed, the water tank, or reservoir, holds the water at a height of 50 divided by 10, or 5 times 23 equals feet above the point where the pressure measurement is made.

Head and pressure, then, mar mean the same thing. We must be able to convert from one to the other. You mar encounter reference to inches of mercury for pressure measurement.

Mercury is Therefore, ahead ofmercuryexerts apressure Because it is hazardous, mercury no longer is used commonly in manometers. The head or pressure terms cited thus far are called, collectively, "gauge pressure. Gauge pressure makes no allowance for the fact that on earth we exist under a head of air, or an atmosphere. The height of this head of air varies with elevation, and also to some degree with weather conditions. If rou ride an elevator from the bottom to the top floor of a tall building, rou will likely feel your ears "pop.

A simple method of measuring atmospheric pressure would be to take a length of small diameter 0. Fill the tube entirely with mercury and temporarily seal the end. Invert this end into a deep dish of mercury and remo ve the seal. The result will be a column of mercury as shown in Figure with some space remaining at the top. Atmospheric pressure on the surface of the exposed mercury will balance the height of mercury in the tube and prevent it from running out of the tube.

The height of the mercury above the level in the dish is, then, a measure of atmospheric pressure. At sea level, this would amount to approximately When the etfect of the atmosphere is included in our measurement, we then must use absolute pressure gauge pressure plus atmospheric pressure. Units ot Measurement Every major country has adopted its own favorite units of measurement.

The United States has traditionally employed the English system. However, international trade has made it necessaryto standardize units of measurement throughout the world.

Fortunately, during this standardization, there has be en rationalization ofthe measurementsystem. The force of common usage is so strong that the familiar English system will undoubtedly persist for many years, but the changeover is definitely underway.

The time will soon corne when process industries will deal exclusively with SI units. Pressure Measurement Perhaps the area that has caused the most concern in the change to SI units is pressure measurement. The new unit of pressure, the pascal, is unfamiliar even to those who have worked in the older CGS centimetre, gram, second metric system.

Once it is accepted and understood, it willlead to a great simplification of pressure measurement Cromthe extremes of full vacuum to ultrahigh pressure.

It will reduce the multiplicity ofunits now common in industry to one standard that is compatible with other measurements and calculations. To understand the pascal and its relationship to other units of pressure measurement, we must return to a basic understanding of pressure.

As noted previously, pressure is force per unit area. In the English system, the distinction between mags and force became blurred with common usage of terms such as weight and mags. We live in an environment in which every object is subject to gravity. Every object is accelerated toward the center of the earth, unless it is restrained. The force acting on each object is proportional to its mags. However, in faci, force and mass, as quantities, are as different as apples and pears, as the astronauts bave observed.

A numbec of schemes bave be en devised to overcome ibis problem. For example, a quantity called the pound-force was invented and made equal to the force on a mass of one pound under a specified acceleration due to gravity. The very similarity between these two units led to more confusiott. The pascal, by its definition, removes all these problems.

The Pascal The SI unit of pressure is defined as the pressure or stress that arises when a force of one newton N is applied uniformly over an area of one square metre m2. This pressure has been designated one pascal Pa. This is a small unit, but the kilopascal KPa , 1, pascals, and the megapascal MPa , one million pascals, permit easy expression of common pressures.

The definition is simple, because gravity has been eliminated. The pascal is exactly the same at every point, even on the moon, despite changes in gravitational acceleration. In SI units, the unit offorce is derived from the basic unit for mass, the kilogram kg , and the unit of acceleration metres per second per second, mls2. At thai time, the SI unit was called the "newton per square metre. The use ofthe millibar in meteorology lent weight to the acceptance of the bar. The kilopascal kPa , 1, pascals, equals 0.

The megapascal mPa equals psi and is convenient for expressing high pressures. The pascal may be regarded as a "measuring gauge," the size ofwhich has been defined and is constant. This gauge can be used to measure pressure quantities relative to absolute vacuum.

Used in this way, the results will be in pascal absolute. The gauge may also be used to measure pressures relati ve to the prevailing atmospheric pressure, and the results will be pascal gauge. Ifthe gauge is used to measure the difference between pressures, it becomes pascal differen- tial. The use of gauge pressure is extremely important in industry, since it is a measure ofthe stress within a vesseland the tendency offtuids to leak auto It is really a special case of differential pressure measurement, inside versus outside pressure.

Where there is any doubt about whether a pressure is gauge, differential, or absolute, it should be specified in full. However, it is common practice to shaw gauge pressure without specifying, and to specify by saying "absolute" or "differential" only for absolute or differential pressures.

The use ofthe millibar in meteorology lent weight to the acceptance of the bar. The kilopascal kPa , 1, pascals, equals 0. The megapascal mPa equals psi and is convenient for expressing high pressures. The pascal may be regarded as a "measuring gauge," the size ofwhich has been defined and is constant. This gauge can be used to measure pressure quantities relative to absolute vacuum. Used in this way, the results will be in pascal absolute.

The gauge may also be used to measure pressures relati ve to the prevailing atmospheric pressure, and the results will be pascal gauge. Ifthe gauge is used to measure the difference between pressures, it becomes pascal differen- tial. The use of gauge pressure is extremely important in industry, since it is a measure ofthe stress within a vesseland the tendency offtuids to leak auto It is really a special case of differential pressure measurement, inside versus outside pressure.

Where there is any doubt about whether a pressure is gauge, differential, or absolute, it should be specified in full. However, it is common practice to shaw gauge pressure without specifying, and to specify by saying "absolute" or "differential" only for absolute or differential pressures.

The use of "g" as in psig is disappearing, and the use of "a" as in psia is frowned upon. Neither g flor a is recognized in SI unit symbols. However, M is recognized for differential pressure in all units. The "standard" gravitational acceleration is 9. This is an arbitrary figure selected as a near average of the actual acceleration due to gravity found all over the earth.

The following are typical values at different places: This is oflittle practicat importance in industrial applications. However, with some transmitters being sold with a rated accuracy of: Gravity-Dependent Units Units such as psi, kglcm2, inches of water, and inches of mercury Hg are all gravity dependent. The English unit pounds per square inch psi is the pressure generated when the force of gravity acts on a mass of one pound distributed aveT one square inch. Consider a dead weight tester and a standard mass of one pound which is transported around the earth's surface: The same applies to units such as inches of water and inches of mercury.

The force at the bottom of each column is proportional to the height, density, and gravitational acceleration. Dead weight testers are primary pressure standards. They generate pressure by applying weight to a piston that is supported by a fluid, generally oil or air. By selecting the weights and the cross-sectional aTea of the piston, the pressure generated in any gravity field can be calculated. Therefore, dead weight testers are gravity dependent.

For accurate laboratory work, the gravity under which the tester was calibrated and that at the place of use must be taken into account. Similarly, the pressure obtained by a certain height of fluid in a manometer depends on density and gravity.

Factors given in the conversion tables in the Appendix, it should be noted, deal with units of force, noi weight. Dead weight testers will be discussed in more detaillater in ibis chapter. Gravity-lndependent Units While gravity plays no part in the definition of the pascal, it has the same value wherever it is measured.

Units such as pounds-force per square inch and kilogram-force per square centimetre are also independent of gravity because a specific value of gravitational acceleration was selected in defining these units. Under equal gravity conditions, the pound-mass and pound-force are numerically equal which is the cause of considerable confusion. Under nonstandard gravity conditions the usual case , correction factors are required to compensate for the departure from standard.

It should be noted that the standard value of actual gravity acceleration is llot recognized as such in the SI unit system, where only the SI unit of acceleration of one metre per second per second is used. In the future, only the measured actual gravity at the location of measurement G will be used when gravity plays a part in the system under investigation. The pascal is a truly gravity-independent unit and will be used to avoid the presently confusing question of whether a stated quantity is gravity dependent.

Pressure Standards Now let us consider the calibration standards that are employed with pressure-measuringinstruments and the basic instruments that are used to measure pressure. It mar help to look at the ways in which the standards for pressure calibration are established. You will recalI that head is the same as pressure. A measure of head, then, can be a dependable measure of pressure.

Perhaps the oldest, simplest, and, in many respects, one of the most accurate and reliable ways of measuring pressure is the liquid manometer.

Figure shows a differential manometer. When only a visual indication is needed and static pressures are in a range that does Dot constitute a safety hazard, a transparent tube is satisfactory.

When conditions for the visual manometer are unsuitable, a variety of ftoat-type liquid manometers are often employed. Simple U-tube manometer.

The simplest differential gauge is the liquid-filled manometer: It is often used to calibrate other instruments. The most elementary type is the U-gauge, which consists of a glass tube bent in the form of a U, or two straight glass tubes with a pressure connection at the bottom. In the more advanced designs, vertical displacement of one side of the manometer is suppressed by using a chamber of large surface area on that side.

Figure shows such a manometer. If the area ratio is in the vicinity of 1, to 1, the displacement in the large chamber becomes quite small and the reading on the glass tube will become extremely close to true inches or true millimetres.

The large side would have to be of infinite area for the reading in the glass tube to be exact. This problem is sometimes overcome with a special calibration of the scale. However, if the glass tube becomes broken and must be replaced, the scale must be recalibrated. A more common and quite reliable design features a zeroing gauge glass as shown in Figure The scale mar be adjusted to zero for each differential pressure change, and the reading mar be taken from a scale graduated in actual units of measurement arter rezeroing.

Well or reservoirmanometer. Well manometerwith zeroingadjustment. Incline manometer tubes, such as those shown in Figure , will give magnified readings, but must be made and mounted carefully to avoid errors due to the irregularities ofthe tube. It is also essential that the manometers be precisely positioned to avoid errors due to level.

Still other types of manometers for functions other than simple indication, including those used with high pressure and hazardous fluids, employ a float on one leg of the manometer. When reading a manometer, there are several potential sources of error. One is the effect of gravity, and another is the effect of temperature on the material contained within the manometer.

Correction tables are available which provide the necessary correction for the conditions under which the manometer is to be read. Perhaps even more important is the meniscus correction Figure A meniscus surface should always be read at its center-the bottom, in the case of water, and the top, in the case of mercury. To be practical, gravity and temperature corrections are seldom made in everyday work, but the meniscus correction, or proper reading, must always be taken into account.

Inclined manometer. Readinga manometer. The dead weight tester is shown in Figure The principIe of a dead weight is similar to thai of a balance.

Gravity acts on a calibrated weight, which in tom exerts a force on a known area. A known pressure then exists throughout the fluid contained in the system. This fluid is generally a suitable oil. Good accuracy is possible, bot requires thai several factors be well established: Perhaps the most important part of the procedure is to keep the piston floating.

This is accomplished generally by spinning the weight platform. An accuratetestgaugemar beusedwith hydraulic pumpin a similarsetup. Still another type of dead weight tester is the pneumatic dead weight tester.

This is a self-regulating primary pressure standard. An accurate calibrating pressure is produced by establishing equilibrium between the air pressure on the underside of the ball against weights of known mass on the top.

A diagram of an Ametek pneumatic tester is shown in Figure In this construction, a precision ceramic ball is ftoated within a tapered stainless steel nozzle.

A ftow regulator introduces pressure under the ball, lifting it toward the annulus between the ball and the nozzle. Equilibrium is achieved as soon as the ball beginsto lift. The ball ftoats when the vented ftow equals the fixed ftow from the supply regulator. This pressure, which is also the output pressure, is proportional to the weight load.

During operation, the ball is centered with a dynamic film of air, eliminating physical contact between the ball and the nozzle. The regulator sensesthe change in ftow and adjusts the pressure beneath the ball to bring the system into equilibrium, changing the output pressure accordingly.

Thus, regulation of output pressure is automatic with change of weight mass on the spherical piston or ball. Plant Instruments That Measure Pressure Directly Thus far in tbis chapter we bave been concemed with the detinition of pressure, and some of the standards used bave been described. In the plant, manometers and dead weight testers are used as standards for comparison and calibration.

The working instruments in the plant usually include simple mechanical pressure gauges, precision pressure recorders and indicators, and pneumatic and electronic pressure transmitters. A pressure transmitter makes a pressure measurementand generates either a pneumatic or electrical signal output thai is proportional to the pressure being sensed.

We will discuss transmitters in detaillater in ibis chapter. Now we will deal with the basic mechanical instruments used for pressure measurement, how they operate and how they are calibrated. When the amount of pressure to be measured is very small, the following instruments might be used.

Bell Instrument This instrument measures the pressure difIerence in the compartment on each gide ora bell-shaped chamber.

Instrumentation for Process Measurement and Control, Third Editon - PDF Free Download

Ifthe pressure to be measured is gauge pressure, the lower compartment is vented to atmosphere. Ifthe lower compartment is evacuated, the pressure measured will be in absolute units.

If the difIerential pressure is to be measured, the higher pressure is applied to the top of the chamber and the lower pressure to the bottom. The bell chamber is shown in Figure Pressure ranges as low as Oto 1 inch Oto Pa of water can be measured with this instrument.

Calibration adjustments are zero and span. The difficulty in reading a manometer accurately to fractions of an inch are obvious, ret the manometer is the usual standard to which the bell difIerential instrument is calibrated.

The bell instrument finds applications where very low pressures must be measured and recorded with reasonable accuracy. Slack or Limp-Diaphragm The slack or limp-diaphragminstrument is used when very small pressuresare to be sensed.

The most commonapplicationofthis gauge Fig. Bell instrument. The range of this type instrument is CromO to 0. To make this instrument responsive to very small pressures, a large aTea diaphragm is employed.

This diaphragm is made of very thin, treated nonporous leather, plastic, or rubber, and requires an extremely small force to deftect it. A spring is always used in combination with the diaphragm to produce a deftection proportional to pressure. Let us assume the instrument shown in Figure is being used to measure pressure. The low-pressure chamber on the top is vented tothe atmosphere.

The pressure to be measuredis applied to the high-pressure chamber on the bottom. This causesthe diaphragm to move upward. It Fig. Slack or limp-diaphragm instrument. This instrument is calibrated by means of zero and span adjustments. The span adjustment allows the ridged connection to the spring to be varied, thus changing the spring constant.

The shorter the spring the greater the spring constant; thus, a greater force is required to deftect it. As the spring is shortened, a higher pressure is required for full deftection of the pointer. The zero adjustmentcontrols the spring's pivot point, thereby shifting the Creeend of the calibrated spring and its attached linkages.

Thrning the zero adjustment changes the pointer position on the scale, and it is normally adjusted to read zero scale with both the high and low pressure chambers vented to the atmosphere. This instrument is extremely sensitive to overrange, and care should be taken to avoid this problem.

The other long-term difficulty that can develop is damage to the diaphragm. The diaphragm mar become stiff, develop leaks, or become defective, which results in error. These difficulties mar be observed by periodically inspecting the diaphragm. Pressure Gauges In the process plant, we frequently find simple pressure ganges scattered throughout the process and used to measure and indicate existing pressures.

Therefore, it is appropriate to devote some attention to the operation of a simple pressure gauge. The most common of all the pressure ganges utilizes the Bourdon tube.

The Bourdon tube was originally patented in In bis patent, Eugene Bourdon stated: An increase of interna! As pressure is applied internally, the tube straightens out and returns to a cylindrical Corm.

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The excursion of the tube tip moves linearly with internal pressure and is converted to pointer position with the mechanism shown.

A Bourdontube. B Typical pressuregauge. Ifthe Bourdon tube is overranged, that is, pressure is applied to the point where it can no longer return to its original shape, the gauge mar take a new set and its calibration becomes distorted. Whether the gauge can be recalibrated depends on the extent ofthe overrange. A severely overranged gauge will be ruined, whereas one that has been only slightly overranged mar be recalibrated and reused.

Most gauges are designed to handle approximately 35 percent of the upper range value as overrange without damage. Typically, a gauge will exhibit some amo unt of hysteresis, that is, a difference due to pressure moving the tube in the upscale direction versus the spring characteristic of the tube moving it downscale.

A typical gauge mar also exhibit some amount of drift, that is, a departure from the true reading due to changes over a long period in the physical properties of the materials involved. All of these sources of error are typically included in the manufacturer' s statement of accuracy for the gauge. A typical Bourdon tube gauge, carefully made with a Bourdon tube that has been temperature-cycled or stress-relieved, will bave an accuracy of: A carefully made test gauge will bave an accuracy of: The range of the gauge is normally selected so that it operates in the upper part of the middle third of the scale.

The gauge must be vertical to read correctly. A typical pressure gauge is shown in Figure B. Liquid or Steam Pressure Measurement Whena liquid pressureis measured,the piping is arrangedto prevent entrappedvaporswhich mar causemeasurement error.

If it is impossible to avoid entrappingvapors, vents should be provided at all high points in the line. When steampressureis measured,the steamshouldbe prevented from enteringthe Bourdontube.

Otherwise,the hightemperaturemar damagethe instrument. Ifthe gaugeis belowthe paint of measurement, a "siphon" a singleloop or pigtail in a vertical plane is provided in the pressureline to the gauge. A cock is installed in the lme betweenthe loop and the gauge.

In operation,the looptraps condensate,preventing the steamfrom enteringthe gauge. Seals and Purges If the instrument is measuring a viscous, volatile, corrosive, or extremely bot or cold fluid, the use of a pressure seal or a purge is essential to keep the fluid out of the instrument. Liquid seals are used with corrosive fluids. The sealing fluid should be nonmiscible with the measured fluids. Pulsation Dampener If the instrument is intended for use with a fluid under pressure and subject to excessive fluctuations or pulsations, a deadener or damper should be installed.

This will provide a steady reading and prolong the life of the gauge. Two other elements that use the Bourdon principIe are the spiral Figure and helical Figure The spiral and helical are, in effect, multitube Bourdon tubes. Spirals are commonly used for Fig. The higher the pressure to be measured, the thicker the walls ofthe tubing Cromwhich the spiral or helical is constructed.

The material used in the construction mar be bronze, beryllium copper, stainless steel, or a special NiSpan C alloy. Spirals and helicals are designed to provide a lever motion of approximately 45 degrees with full pressure applied. If this motion is to be translated into pen or pointer position, it is common practice to utilize a four-bar linkage, and this necessitates a special calibration technique. If, instead of measuring gauge pressure, it is necessary to read absolute pressure, the reading must make allowances for the pressure of the atmosphere.

This mar be done by utilizing an absolute double spiral element. In tros element, two spirals are used. One is evacuated and sealed; the second has the measured pressure applied. The evacuated sealed element makes a correction for atmospheric pressure as read on the second element.

Thus, the reading can be in terms of absolute pressure, which is gauge plus the pressure of the atmosphere, rather than gauge. An absolute double spiral element of this type mar be used to measure pressures up to psi or Kpa absolute.

This element is shown in Figure Metallic Bellows A bellows is an expandableelementmade up of a series of folds or corrugationscalled convolutions Figure Wheninternalpressure is appliedto the bellows, it expands. Becausea sizablearcais involved. In some instruments, the bellows is placed in a sealed can and the pressure is applied externally to the bellows.

The bellows will then compress in a fashion similar to the expansion just described. Such an arrangement is shown in Figure A variety of materials are used to fabricate bellows, including brass, beryllium copper, copper nickel alloys, phospher bronze, Monel, steel, and stainless steel.

Brass and stainless steel are the most commonly used. Metallic bellows are used from pressure ranges of a few Fig.

Bellows receiver unit. Bellows in sealedcan.

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A bellows will develop many times the power available from a helical, spiral, or Bourdon tube. A bellows is typically rated in terms of its equivalent square inch area. To create a linear relationship between the excursion of the bellows and the applied pressure, it is common practice to bave the bellows work in conjunction with a spring, rather than with the spring characteristic of the metal within the bellows itself.

Each bellows and spring combination has what is called a spring rate. The springs used with the bellows are usually either helical or spiral. Typically, the spring rate of the helical spring is ien times or more thai of the bellows material itself. Using a spring with a bellows has several advantages over relying on the spring characteristics of the bellows alone.

The calibration procedure is simplified, since adjustments are made only on the spring. Initial tension becomes zero adjustment and the number of active turos becomes span adjustment. A spring constructed of stable material will exhibit long-term stability thai is essential in any component. When a measurement of absolute pressure is to be made, a special mechanism employing two separate bellows mar be used.

It consists of a measuring bellows and a compensating bellows, a mounting support, and an output lever assembly Figure The measuring and compensating bellows are fastened to opposite ends of the fixed mounting support: The motion of ibis movable plate is a measure of the difference in pressure between the two bellows. Since the compensating bellows is completely evacuated and sealed, ibis motion is a measure of pressure above vacuum, or absolute pressure applied to the measuring bellows.

This movable plate is attached to the output lever assembly which, in turo, is linked to the instrument pen, or pointer. In many applications, the bellows expands very little, and the force it exerts becomes its significant output.

This technique is frequently employed in force-balance mechanisms which will be discussed in some detail in Chapter 8. Pressure Transmitters Signal Transmissions In the process plant, it is impractical to locate the control instruments out in the plant near the processoIt is also true thai mostmeasurements are Dot easily transmitted from some remote location.

Pressure measiIrement is an exception, but if a high pressure measurement of some dangerous chemical is to be indicated or recorded several hundred feet from the point of measurement, a hazard will be created. To eliminate ibis problem, a signal transmission system was developed. In process instrumentation, ibis system is usually either pneumatic air pressure or electrical.

Becauseibis chapter deals with pressure, the pneumatic, or air pressure system, will be discussed first. Later it will become evident thai the electrical transmitters perform a similar function. When a transmission system is employed, the measurementis converted to a pneumatic signal by the transmitter scaled from O to percent ofthe measured value. This transmitter is mounted close to the point of measurement in the processo The transmitter output-air pressure for a pneumatic transmitter-is piped to the recording or control instrument.

The standard output range for a pneumatic transmitter is 3 to 15 psi, 20 to kPa; or 0. These are the standard signals that are almost universally used. Let us take a closer look at what this signal means. Suppose we have a field-mounted pressure transmitter that has been calibrated to a pressure range of psi to psi When the pressure being sensedis psi 69 kPa , the transmitter is designed to produce an output of 3 psi air pressure or the approximate SI equivalent 20 kPa.

When the pressure sensedris es to psi This signal is carried by tubing, usually V4-inchcopper or plastic, to the control room, where it is either indicated, recorded, or fed into a controller. The receiving instrument typically employs a bellows element to convert this signal into pen or pointer position. Or, if the signal is fed to a controller, a bellows is also used to convert the signal for the use of the controller. The live zero makes it possible to distinguish between true zero and a dead instrument.

The top scale signal is high enoughto be useful without the possibility of creating hazards. Pneumatic Recorders and Indicators Pressure recorders and indicators were described earlier in this chapter. Pneumatic recorders and indicators differ only in that they always operate from a standard 3 to 15 psi or 20 to kPa signal. The indicator scale, or recorder chart, mar be 1abeledO to 1, psi.

This would represent the pressure sensed by the measuring transmitter and converted into the standard signat that is transmitted to the receiver. Because the scale is labeled in proper units, it is possible to read the measuredpressure. A typical pneumatic indicator is shown in Figure top and its operation mar be visualized by studying Figure bottom, right. The input signal passes through an adjustable needle valve to provide damping, then continues to the receiver bellows.

This bellows, Fig. A spring opposing the bellows provides zero and span adjustments. Regulating the amo unt of spring used its effective length provides a span adjustment. Setting the initial tension on the spring provides a zero adjustment.

The force plate is connected to a link thai drives the pointer arbor assembly. Changing the length ofthe link by turning the Dut on the link provides an angularity adjustment.

The arm connecting the pointer and the arbor is a rugged crushed tube designed to reduce torsional effects. A takeup spring is also provided to reduce mechanical hysteresis. Overrange protection is provided to prevent damage to the bellows or pointer movement assembly for pressures up to approximately 30 psi or about kPa.

A pneumatic recorder also uses a receiver bellows assembly as shown in Figure top. This receiver provides an unusually high torque due to an effective bellows area of 1.

The input signal tirst passesthrough an adjustable damping restrictor. The output of this signat is the input to the receiver bellows assembly. The receiver consists of a heavy duty impact extruded aluminum can containing a large brass bellows working in compression. The large effective area of the bellows assures an extremely linear pressure-topen position relationship. The motion, created by the input signal, is picked up by a conventionallink and transferred to an arbor.

Calibration is easily accomplished by tuming zero, span, and angularity adjustments on ibis simple linkage system. All calibration adjustments are accessible through a door Figure on the gide of the instrument and mar be made while instrument is in operation. The arm interconnecting the pen and the arbor incorporates a rugged tubular member to reduce torsional effects and takeup spring to reduce mechanical hysteresis.

Overrange protection is provided to prevent damage to the bellows or pen movement assembly for pressures up to 30 psi or about kPa. Mechanical Pressure Seals Application A sealedpressuresystemis usedwith a pressuremeasuringinstrument to isolate corrosive or viscousproducts, or products that tend to solidify, from the measuringelementand its connectivetubing.

The seat itself mar take many forms, depending on process conditions, but consists of a pressure sensitive flexible member, the diaphragm, functioning as an isolating membrane, with a suitable method of attachment to a process vessel or line. Principie or Operation Process pressure applied to the flexible member of the seal assembly forces some of the filling fluid out of the seal cavity into the capillary tubing and pressure measuring element, causing the element to expand in proportion to the applied process pressure, thereby actuating a pen, pointer, or transmitter mechanism.

A sealed pressure system offers high resolution and rapid response to pressure changes at the diaphragm. The spring rate ofthe flexible member musi be low when compared with the spring rate of the measuring element to ensure thai the fill volume displacement will full stroke the measuring element for the required pressure range. A low-diaphragm spring rate, coupled with maximum fill volume displacement, is characteristic of the ideal system.

SeaIconnected to 6-inch pressure gauge. The flexible member ofthe seal should ideally accommodate any thermal expansion of the filling medium without perceptible motion of the measuring element. A very stiff seal member, such as a Bourdon tube, combined with a low-pressure high-volume change element, will produce marked temperature effects Crom both varying ambient or elevated process temperatures. Sealsconnectedto differentialpressuretransmitter.

It should be noninjurious to the diaphragm and containing parts, and should Dot cause spoilage in the event ofleakage. Silicone-based liquid is the most popular filling fluid. The system is evacuated before the filling fluid is introduced.

The system musi be completely filled with fluid and free from any air pockets thai would contract or expand during operation, resulting in erroneous indications at the pen or pointer or in an output signat. The degree of accuracy of any filled pressure system depends on the perfection of the filling operation. Calibration Techniques The procedure for calibration of a pressure instrument consists of comparing the reading of the instrument being calibrated with a standard.

The instrument under calibration is then adjusted or manipulated to make it agree with the standard. Success in calibration depends Dot only on one' s ability to adjust the instrument, but on the quality of the standard as well.

Field Standards Field standards must be reasonably convenient to use and must satisfy the accuracy requirements for the instrument under calibration. A inch water column, for example, is extremely accurate but Dot practical to set up out in the plant. For practical reasons, we find that most field standards are test ganges. The test gaugeis quite similar, in most cases, to the regular Bourdon ganges.

However, more care has gone into its design, construction, and calibration, making it very accurate. A good quality test gauge will be accurate to within:: This is adequate for most field use. Under some conditions, a manometer mar be used in the field. This usually occurs when a low-pressure range is to be calibrated and no other suitable standard is readily available. Portable Pneumatic Calibrator All of the ingredients required to perform a calibration bave been combined into a single unit called a portable pneumatic calibrator.

The portable pneumatic calibrator will accurately apply, hold, regulate, and measure gauge pressure, differential pressure, or vacuum. The gauge case mar be evacuated to make an absolute pressure gauge.

The pointer in the operation of the gauge in this calibrator differs in that it makes almost two full revolutions in registering full scale, providing a scale length of 45 inches.

This expanded scale makes the gauge very easy to read.

The calibrator is available in seven pressure ranges SI unit ranges. Of these, three are sized for checking 3 to 15 psi or 20 to kPa pneumatic transmission instrumentation. The calibrator is shown in both picture and schematic form in Figures and The air switching arrangement makes it possible to use the calibrator as both a source or signal and a precise gauge for readout.

This portable pneumatic calibrator is manufactured by Wallace and Tieman, Inc. For GaugePressure: For DifferentialPressure: Low-testpressureis appliedto the casethroughS. For AbsolutePressure: For Vacuum: For Positiveand NegativePressures: A receiving instrument is then employed to convert the signat into a pen or pointer position or a measurement input signat to a controller. The Foxboro Model llGL force-balance pneumatic pressure transmitter is an example of a simple, straightforward device that performs this service Figure Before its operation is discussed, the operation of two vital components must be described: These two mechanisms are found in nearly every pneumatic instrument.

ModelllGM pressure transmitter is a force-balance instrument that measures pressure and transmits it as a proportional 3 to 15 psi or 20to kPa pneumatic signa!. Flapper movement of only six ten-thousandths of an inch 0. This small pressure change applied to the pneumatic amplifier or relay becomes an amplified change of 3 to 15 psi or 20 to kPa in the amplifier output.

It is shown in a cross-sectional view in Figure Pneumatic Relay A relay is a pneumatic amplifier. Like its electronic counterpart, the function of the relay is to convert a small change in the input signal an air pressure signal to a large change in the output signal.

Typically, a 1 psi or 7 kPa change in input will produce approximately a 12 psi or 80 kPa change in output.

The input signal nozzle pressure enters the Telar through another port and acts on the diaphragm. As the input signal increases, the stem pushes against a ball valve which in turo moves a flat spring, allowing the supply of air to enter the Telar body. Further motion of the stem valve causes it to close off the exhaust port.

Thus, when the input pressure increases, the stem exhaust valve closes and the supply valve opens; when the input decreases, the stem valve opens and the supply valve closes. This varies the pressure to the output. Principie ot Operation The 11GM Pneumatic Transmitter Figure is a force-balance instrument that measures pressure and transmits it as a proportional 3 to 15 psi pneumatic signal 20 to kPa. The pressure is applied to a bellows, causing the end of the bellows to exert a force through a connecting bracket on the lower end of the force bar.

The metal diaphragm is a fulcrum for the force bar. The force is transmitted through the ftexure connector to the range rod, which pivots on the range adjustment wheel. Any movement of the range rod causes a minute change in the clearance between the ftapper and nozzle.

The output pressure which is established by this balance is the transmitted signal and is proportional to the pressure IlPplied to the measurement bellows. Ifthe pressure be measured is high, such as 5, psi or MPa, a different sensing to element is employed. The pressure being measured is applied to a Bourdon tube. This pressure tends to straighten the tube and causes a horizontal force to be applied to the lower end of the force bar.

The diaphragm seat serves as both a fulcrum for the force bar and as a seal for the pressure chamber. The force is transmitted through a ftexure connector to the range rod, which pivots on the range adjustment wheel. Any movement of the rangerod causes a minute change in the clearance between the ftapper and nozzle.

The output pressure, which establishes the force-batancing, is the transmitted pneumatic signal that is proportionat to the pressure being measured.

This signat is transmitted to a pneumatic receiver to record, indicate, or control. The calibration procedure for this transmitter is the same as for the low pressure transmitter, but the clllibration pressure is developed with a dead weight tester. For safety, oil or liquid, never air, should be used for high pressure calibrations. Pressure measurements found in some applications require that absolute, rather than gauge pressure, be determined.

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