pressure fundamentals

7/21/2019 Pressure Fundamentals

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Why Measure Pressure?

Process industries are organizations that transform one substance into another (e.g.,crude oil into gasoline, pulp into paper, steam into electricity). Four of the most commonreasons that process industries measure pressure follow:

Safety Process efficiency

Cost savings

Inferred measurement of other variables

SAFETYPipes, tanks, valves, flanges, and other equipment used with pressurized fluids in processindustries are designed to withstand the stress of a specific range of pressures. Accuratepressure measurement and precise control help prevent pipes and vessels from bursting.In addition, pressure measurement and control help minimize equipment damage, reducethe risk of personal injury, and prevent leaks of potentially harmful process materials intothe environment. Pressure measurement used to control the level and flow of process

materials helps to prevent backups, spills, and overflows.

PROCESS EFFICIENCYIn most cases, process efficiency is highest when pressures (and other process variables)are maintained at particular values or within a narrow range of values. Accurate pressuremeasurement can help sustain the conditions required for maximum efficiency. Forexample, the piece of paper on which these words are written was created from a pulpsolution put through a paper machine at a specific pressure. If the pressure had goneabove or below the set point (required range), the result would have been scrap instead ofa usable sheet of paper.

COST

SAVINGS

The equipment used to create pressure or vacuum in process industries (e.g., pumps andcompressors) uses considerable energy. Because energy costs money, precise pressuremeasurement can save money by preventing the unnecessary expense of creating morepressure or vacuum than is required to produce the desired results for a particularprocess.

INFERRED MEASUREMENT OF OTHER VARIABLESPressure measurements are frequently used to infer the measurement of other processvariables, such as the rate of flow through a pipe, the level of a fluid in a tank, the densityof a substance, or how two or more liquids in a tank interface. For example, if aconstriction is placed in a pipe, pressure will drop in a predictable way. By measuring the

pressure of fluid in a pipe before and after the constriction, the rate of flow through the pipecan be calculated. For a discussion on how pressure measurement can be used to inferthe values of other process variables.


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What is Pressure?

Pressure is the amount of force applied over a defined area. The relationship betweenpressure, force, and area is represented in the following formula:

Where:P = PressureF = Force

A = Area

If a force (due to physical contact) is applied over an area, pressure is being applied.Pressure increases if the force increases or the size of the area over which the force isbeing applied decreases.

Weight X and Weight Y in Figure 1.1 are applying different amounts of pressure to thesurface, even though the two weights are each 100 lb. Weight X has a base of 100 in 2.Therefore, the pressure being applied by Weight X is 100 lb of force being applied over anarea of 100 in2, or 1 lb/in2.

Weight Y is resting on a 1 in2 base. Therefore, the pressure being applied by Weight Y is100 lb of force being applied over an area of 1 in2, or 100 lb/in2.


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To better understand the relationship between force and area, think about the impact ofyour weight on a couch. The force in this case is produced by your weight, which, in thisexample, will remain constant. If you lie down on the couch, your weight is applied over alarge area and the cushions compress to a certain degree. If you stand on the couch on

one foot, your weight is applied to a much smaller area and the cushions compress muchmore. The force (your weight) is now being applied over a smaller areatherefore thepressure is increased.

Pressure Variables

The factors that influence the pressure of a liquid are different from the factors thatinfluence the pressure of a gas. Therefore, when measuring pressure, it is important to

understand the pressure properties of liquids and gases.

LIQUID PRESSUREThe pressure exerted by a liquid is influenced by three factors:

Depth of the liquid

Density of the liquid

Pressure on the surface of the liquid

Depth of a LiquidThe pressure at a point below the surface of a liquid increases as the depth of the liquidabove the measurement point increases. Pressure is affected by the depth, rather than thevolume, of a liquid. If other factors (e.g., density of the liquid and pressure on the surfaceof the liquid) are constant, the pressure at a depth of 10 ft in a large tank holding 5,000 galof water will be equal to the pressure at a depth of 10 ft in a smaller tank holding only 5 galof water.

An example from everyday life might be your experience of swimming five feet beneath thesurface of a swimming pool and swimming five feet beneath the surface of a large lake.Even though the lake contains a far greater amount of water, the pressure on your body ata five foot depth is not proportionately greater. The pressure in the lake and in the pool isthe same at a depth of five feet.


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Density of a Liquid

Density is the mass of a particular substance per unit of volume. A liquid with a greaterdensity has a greater mass per unit of volume. Liquids with greater densities will applymore pressure to a given area than liquids with smaller densities because higher-densityliquids are heavier per unit of volume. Variations in temperature cause liquids to expandand contract, which increases or decreases the volume of the liquid. When the volume of a

liquid changes, the density changes as well.

Density is often represented in terms of specific gravity. Specific gravity is the ratio of thedensity of a particular liquid to the density of water at the same temperature. Water has adensity of 1,000 kg/m3 at 60 F (15.6 C). Temperature is specified when giving a densityvalue because temperature affects density. The density of gasoline is 660 kg/m3 at 60 F(15.6 C). To calculate the specific gravity of gasoline, divide the density of gasoline by thedensity of water:

Because specific gravity is a ratio of densities, it does not change as units of measurechange. Therefore, the specific gravity of gasoline at 60 F (15.6 C) is always 0.66, evenif the density of gasoline and the density of water are expressed in a different unit ofmeasure (e.g., lb/ft3):

Pressure on the Surface of a Liquid

Pressure on the surface of a liquid is pressure that is exerted above a column of liquidbeing measured. In an open tank, atmospheric pressure (the pressure exerted by theEarthsatmosphere) is the pressure on the surface. If a gas is added to the top of acolumn of liquid in a closed tank, pressure on the surface would result. If there is a vacuum(space void of all pressure) above the liquid in a closed tank, a negative pressure on thesurface exists. In a closed-tank application, the pressure on the surface is called vessel

pressure. The effects of pressure on the surface must be taken into account to produce anaccurate pressure measurement.


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GAS PRESSUREUnlike a liquid, a gas will exert equal pressure on all parts of the container in which it isheld. Two factors affect the pressure exerted by a gas:

Volume of the container in which the gas is held

Temperature of the gas

Common practice in process industries is to refer to both liquids and gases as fluids.

Container Volume

The relationship between the pressure exerted by a gas and the volume of the container inwhich it is held is known as Boyleslaw. Because a gas can be compressed, the pressureof a gas increases proportionately as the volume of the container in which the gas is helddecreases. Conversely, if a set amount of gas is transferred to a larger container, thepressure will decrease in proportion to the increase in container volume.

Temperature of a Gas

The relationship between gas pressure and temperature is known as Charless law. Gas

pressure is affected by changes in temperature. As the temperature of a gas increases,the energy of the individual gas molecules increases as well. As a result, the gasmolecules collide with the vessel wall more frequently and with greater force, and thepressure exerted against the inside wall of the vessel increases.

If the volume of the vessel holding a gas and the amount of gas are unchanged, thepressure exerted by the gas on the vessel walls will change in proportion to changes in thetemperature of the gas.

Pressure UnitsPressure units can be divided into two categories: units of force over area and unitsreferenced to columns of fluid.

UNITS OF FORCE OVER AREAThe following are units of force over a defined area:

Pounds per square inch (psi)

Kilograms per square centimeter (kg/cm2)

Grams per square centimeter (g/cm2)1 g/cm2= 1/1,000 kg/cm2

Pascals (Pa or N/m2)N stands for Newton

Kilopascals (kPa)1 kPa = 1,000 Pa Bar1 bar = 100,000 Pa

Millibar (mbar)1 mbar = 1/1,000 bar

Pounds per square inch, or psi, are the units of pressure most commonly used in NorthAmerica. The other units of pressure listed above are most often used internationally.


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UNITS REFERENCED TO COLUMNS OF FLUIDThe following are units of pressure referenced to a column of fluid:

Inches of water (inH2O at 68 F [20 C])Most commonly used in the U.S.

Feet of water (ftH2O)

Meters of water (mH2O)

Millimeters of water (mmH2O)

Inches of mercury (inHg)

Millimeters of mercury (mmHg)

Atmosphere (atm)The pressure exerted by the Earthsatmosphere at sea level

Torr1 torr = 1 mmHg

Pressure units referenced to a column of fluid serve as a useful measure of pressure, eventhough they do not represent a force over a defined area. Because of gravity, a column offluid will exert a certain force (weight) downward and thus a certain predictable pressure.The higher a column of fluid, the greater the force exerted by that fluid. The more dense afluid, the greater the force exerted by that fluid. Units of measure must have staticvaluestherefore, fluid column height and fluid density must be specified when

representing pressure as a column of fluid.

1 inH2O is the amount of pressure applied by a one-inch column of water at 68 oF (20 oC).Because the temperature is specified, the density will remain constant and themeasurement unit fixed.

Another commonly used fluid for pressure measurement is the element mercury (Hg),often expressed as a pressure measurement in inches of mercury (inHg). 1 inHg is equalto the amount of pressure applied by a one-inch-high column of mercury with a density of13.5951 g/cm3. Again, because density is specified, the measurement unit remains fixed.Millimeters of mercury (mmHg) are also used to express pressure. 1 mmHg is the amountof pressure applied by a 1 mm high column of mercury with a density of 13.5951 g/cm3.

Units of pressure can also be expressed in atmospheres (atm). 1 atm is equal to thepressure exerted by the earths atmosphereat sea level. 1 atm is equal to 101.325 kPa, orapproximately 14.6959 psi. Torr is a unit of pressure based on atmosphere (1 Torr equals1/760 atm). One Torr is approximately equal to 1mmHg.


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CONVERTING UNITS OF PRESSUREProduct literature (e.g., manuals, product data sheets, product price lists) for eachpressure measurement instrument lists the pressure range within which that device can beeffectively and safely operated. However, the pressure units used in the product literaturemay not be the same as the units specified by people at the plant for his or her application.Therefore, unit conversions are often required to determine if a particular pressure-measurement device will meet the requirements of an application.

For example, imagine that a someone at the plant identifies 40 bar as the maximumamount of pressure a particular process produces. The user wants to know what rangeinstrument to use. The product literature lists pressure ranges in psi, so a conversion frombar to psi is necessary before a recommendation can be made. Units of pressure can beconverted using a conversion table, such as the table below, that shows the relationshipsbetween different units of pressure (e.g., how many bar equal 1 psi). To convert 40 bar topsi, look in the conversion table to find that one bar equals 14.5038 psi. Because you needthe psi value of 40 bar, multiply 14.5038 by 40 to obtain a value of 580.151 psi. Now youcan determine from the product literature that a specific Range of instrument is needed.


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Reference Pressures

Pressure-measurement devices differ in what they use as a reference pressure. Areference pressure is a pressure measurement that is compared to the measured pressureof the process material. The sensor is the part of a pressure measurement instrument that

physically reacts to pressure input. The side of the sensor that measures the pressure ofthe process material is called the high side of the instrument. The other side, or referencepressure side, is known as the low side of the instrument. Pressure-measurement devicescan be categorized according to the reference pressure from which they measure. Thethree reference pressures are:

Absolute

Gage

Differential

Absolute and gage devices measure the difference between the pressure of the processfluid and a reference pressure. Differential devices take two pressure measurements of theprocess fluid at different points and measure the difference between them.

ABSOLUTE PRESSUREAbsolute pressure measurements compare measured pressure to a perfect vacuum.Because no pressure reading can be less than a perfect vacuum, an absolute pressure-measurement device will never have a negative reading. The reference pressure of anabsolute pressure-measurement device (i.e., a perfect vacuum) never changes.

GAGE PRESSUREA gage pressure-measurement instrument uses the pressure of the surroundingatmosphere (approximately 14.7 psi) as a reference pressure. Changes in atmosphericpressure (such as those due to changes in the weather) cause the output of a gage sensorto change. Depending on the application, the output change may or may not be desirable.In process systems not open to atmosphere (e.g., a process in an unvented tank),pressures of the process material being measured could be less than the surroundingatmospheric pressure, which would result in a negative pressure reading.

DIFFERENTIAL PRESSURE

A differential pressure measurement uses a second process pressure as a referencepressure. Differential pressure measurements are often used to infer the rate of flowthrough a pipe by determining the pressure drop that occurs from one point in a system toanother, such as the drop that occurs across a filter in a pipe.


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For example, if a differential pressure (DP) instrument is installed so that the high side ofthe instrument measures the pressure on the upstream side of the flow element in a pipeand the low side of the instrument measures the pressure on the downstream side of theflow element, with the high side pressure at 12 psi and the low side pressure at 10 psi, thedifferential pressure is 2 psi.

Changes in atmospheric pressure do not affect the output of a differential pressure-measurement instrument because both measured pressure and reference pressure areequally influenced by exposure to the atmosphere.

DESIGNATING REFERENCE PRESSURES

The designator a for absolute, g for gage, and d for differential is often attached to the end

of pressure units to indicate the reference pressure or type of instrument being used.Thus, pressure measurements are usually represented as psig, psia, or psid rather than

just psi. Bar becomes bar g, bar a, or bar d.

Converting Absolute Pressure Measurements

An absolute pressure measurement registers the pressure of the surrounding atmosphereas part of the pressure reading, whereas a gage pressure measurement uses atmosphericpressure as its reference. Therefore, absolute values can be converted to gage values bysubtracting atmospheric pressure from the absolute pressure reading (Figure 1.2).


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For example, to find the gage value for an absolute pressure measurement device thatreads 34.7 psia and is surrounded by an atmosphere of 14.7 psia, use the followingequation:

To convert a gage value to an absolute value, simply reverse the process describedabove. Add atmospheric pressure to the gage value.

14.7 psia is slightly higher than the standard pressure value of 1 atm, which is 14.6959 psi.The value changes depending on the weather and the location of the instrument, butnormally does not vary more than a few tenths. For most applications, using a value of14.7 psia for atmospheric pressure is sufficient.


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Measurable Pressures

The three types of measurable pressures in the process control industry follow:

Head pressure

Static pressure

Vapor pressure

HEAD PRESSUREHead pressure, also known as hydrostatic pressure, is the pressure exerted by a columnof fluid (Figure 1.3). Head pressure is directly proportional to the specific gravity of the fluidand the height of the fluid column.

Depending on where the pressure transmitter is mounted, calculations must be performedto factor out errors and ensure the correct head pressure is seen by the instrument.Pressure-measurement instruments are often mounted above or below the tap, or point atwhich the process fluid is being measured.


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If the pressure transmitter is mounted below the tap, the column of fluid held in theconnection piping between the tap and the instrument will put additional pressure on thesensor of the instrument. If the distance from the tap to the pressure transmitter and thespecific gravity of the fluid are known, the added head pressure can be factored out of themeasurement.

If the pressure-measurement instrument is mounted above the tap, gravity will act on thecolumn of fluid, pulling it away from the instruments sensor and thus creating a negativehead pressure. A negative head pressure can also be calculated and then factored out ofa pressure measurement.


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STATIC PRESSUREStatic pressure, or line pressure, is the pressure exerted in a closed system. A closedsystem is a system that is sealed from atmosphere. An example of static pressure can befound in a common boiler system. As the water in the boiler is heated, pressure increases.The term line pressure is more commonly used in flow applications.

VAPOR PRESSUREVaporization is the transformation of a substance from a liquid state to a gas state (e.g.,

water to steam). The transformation occurs at a specific temperature for each liquid. Forexample, water turns to steam (boils) at 212 F (100 C). At the molecular level, thetransformation of a liquid into its gaseous form is simply an increase in the distancebetween the individual molecules of the substance due to an increase in energy (i.e.,heat). As the molecules move about faster and with more force, they occupy more space.

When pressure is applied to a substance, the molecules of the substance are pushedcloser together, and more energy is required to spread them apart. For this reason,increased pressure causes the boiling point of a liquid to rise. Conversely, a decrease inpressure causes the boiling point of a liquid to fall. For example, water boils at 212 F at ornear sea level, but at high altitudes where the atmospheric pressure is lower, water boils atless than 212 F.

The relationship between pressure, temperature, and the boiling point of a substance canbe plotted on a simple, two-axis graph. Figure 1.4 shows the vapor pressure curve. Eachsubstance has its own respective vapor pressure curve. The vapor pressure curve of oil,for instance, differs from the vapor pressure curve of glycerin.

Pressure-measurement instruments that use a fill fluid are sensitive to high temperatures.If the fill fluid of an instrument boils, the instrument will no longer give an accurate pressurereading. Therefore, the temperature and pressure conditions of fill fluids must remain suchthat the process fluid always falls within the liquid region of its vapor pressure curve.


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Inferring Non-Pressure Variables

Because there is a known relationship between pressure and density, pressure and level,and pressure and the flow of a fluid through a pipe, these non-pressure variables can beinferred from pressure measurements.

FLOWA common use of pressure measurement is to infer a fluidsflow rate through a pipe. As afluid flows through a pipe with a decreasing diameter, fluid velocity increases at a rateproportional to the decrease in pipe diameter. Bernoullisprinciple states that as a fluidspeeds up to bypass an obstruction, pressure drops. The pressure of the fluid flowingthrough a pipe will be greater on the upstream side of an obstruction in the pipe than onthe downstream side.

If pressure is measured before and after an obstruction in the pipe (e.g., a flow elementsuch as a venturi tube, flow nozzle, wedge, or annubar) the difference between the twomeasurements, or differential pressure, is proportionate to the flow rate of the fluid throughthe pipe.


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The flow equation used for DP flowmeters is based on Bernoullis equation, which showsthat flow rate (Q) is proportional (a) to the square root of differential pressure (DP):

LEVELIf specific gravity is known, then the level of a liquid in a container can be determined fromthe pressure measurement by rearranging the equation used for density calculation:

The units used to express height and pressure must be comparable. Remember thatpressure on the surface of a liquid can affect a pressure measurement. For example, if youare using a pressure measurement to infer the level of a tank open to the surroundingatmosphere, then the atmospheric pressure must first be subtracted from the pressurereading in order to obtain an accurate level calculation.

DENSITY MEASUREMENTPressure is equal to the height of the column of liquid being measured multiplied by thespecific gravity of the liquid. Therefore, if the height of the column is a known constant (asin the case of the distance between two pressure measurement points on a vessel), thedensity can be inferred from the pressure reading using the following equation:

Units of pressure are usually different than units of height. The equation requirescomparable units. Most pressure measurements used for density calculations aretherefore made in units based on referenced columns of fluid (e.g., inches of water). The


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height in the equation can also be expressed in inches, and the units will cancel eachother out of the equation:

The specific gravity value can be converted into mass per unit of volume units, such asgrams per cubic centimeter (gm/cm3).

Constant Level - Open Tank (vented)In open tank applications, level can be measured with either gauge or differential pressure(DP) transmitters, using the assumption that P = L*SG. If specific gravity needs to bemeasured, then the equation can be modified to SG = L/P. For this to work, the level muststay constant so that any pressure changes reflect the specific gravity change.This works well in a tank where there is no change in level, such as when it is desirable tomeasure a density change as a way of monitoring a reaction, or as the fluids temperaturechanges.

Constant Level - Closed Tanks (pressurized or vacuum), Wet Leg or RemoteSeal AssemblyIn applications where the condensation tends to occur in the dry leg, use a wet leg systemor remote seal assembly instead (Figure 4):


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Varying Level - Use of One TransmitterOften the density of a fluid must be determined in a vessel where the level is changing anda reference column is impractical. In this case, a differential pressure (DP) measurementcan be made. In this situation, the measured height is the distance between the high andlow pressure taps. The upper tap must be covered by the process fluid at all times. Theconnection between the upper tap and the transmitter can be either a wet leg or a remote

seal and capillary (Figure 5). In either case, the calibration must account for the referenceheight contributed by the wet leg or the capillary. This is done by subtracting the referenceheight times the Srefvalue from the process (between taps) height, times the processspecific gravity.

Varying Level - Use of Two TransmittersDensity can also be measured with two transmitters. With this arrangement, the output ofthe bottom transmitter can be used to provide both a level and, with a second transmitter,a density measurement (Figure 6).


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INTERFACE MEASUREMENTAn interface is the boundary between two immiscible (incapable of being mixed) fluids withdifferent densities (e.g., oil and water). An interface measurement finds the boundary

between two liquids stored in the same tank, each with a different density. For example,when oil and water occupy the same vessel, the oil floats on top of the water. The interfacebetween the two fluids is the upper level of the water and the lower level of the oil. If thedensity of both fluids is known, interface can be inferred from a pressure measurement.


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Pressure Gauges

All pressure measurements depend upon some portion of the instrument being physicallymoved by the pressure source being measured. Two types of pressure-measurementgauges are liquid column gauges and mechanical gauges. In a liquid column gauge, the

height of a column of liquid varies in response to applied pressure. Mechanical gaugeshave mechanical parts that move in response to applied pressure.

LIQUID COLUMN GAUGESBelow are two types of liquid column pressure gauges:

Barometer

Manometer

BarometerA barometer is a device that measures atmospheric pressure. A barometer consists of a

clear, hollow tube with one end blocked off. The tube is filled with liquid and set, with theblocked end pointing up, into a reservoir of fill liquid (typically mercury) (Figure 1.5).

When the tube is upright and longer than the column of liquid at atmospheric pressure,there is a void at the top of the tube. For example, a column of mercury is 29.9 in high(29.9 inHg) at a pressure of 1 atm. Therefore, the barometer tube must be longer than 30in.

The distance from the top of the liquid in the reservoir to the top of the liquid in thebarometer is the barometric (atmospheric) pressure. If atmospheric pressure changes, thelevel of the fluid in the tube changes as well.

Barometers are highly accurate. They are often used as a reference for calibrating otherpressure instruments. Barometers are also commonly used for weather forecasting.


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ManometerA derivation of the barometer is the manometer. A manometer is a clear, U-shaped tubepartially filled with fluid. One leg of the manometer is the reference side; the other leg isthe measured side. A pressure measurement is made by comparing the fluid levels of thecolumn in each leg of the manometer U (Figure 3).

If the reference side of the manometer U is open to atmospheric pressure, the manometerwill function as a gage instrument. If the reference side of the manometer U is sealed, a

vacuum exists above the fluid column on the reference side of the manometer. In thiscase, the manometer will function as an absolute instrument.


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MECHANICAL PRESSURE GAUGES

Mechanical pressure gauges have two basic parts:

Sensing device

Mechanical dial or indicator (connected to the sensing device; gives a pressurereading)

The most commonly used types of pressure-sensing devices are:

Bourdon tube

Bellows and capsules

Mechanical pressure gauges are still widely used in the process control industry.

Bourdon TubeBourdon tubesare curled, flexible tubes with one closed end. As fluid flows into a bourdontube, the tube straightens. As pressure increases, the tube straightens further. Whenpressure decreases, the tube springs back to its original shape. Several different metalsand other materials are used to make bourdon tubes.

Bourdon tubes come in four designs (Figure 1.7):

C shaped

Twisted

Helical

Spiral


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Figure 1.7b: Bourdon tube mechanism


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Bellows and Capsules

Bellows and capsules are pleated, flexible chambers that expand when filled with materialunder pressure. The individual chambers of a capsule are sealed so that only the firstchamber in the series is actually in contact with process pressure. A bellows is openinside.

Capsules tend to spring back to their original shape when the pressure is released.Bellows often require an external spring to push them back into shape. A referencepressure may also be applied to the outside of the bellows or capsule.


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Pneumatic Pressure Cells

A transducer is a device that converts a signal in one form into a signal in another form.Pneumatic transducers sense pressure and put out a stream of air in response. Pneumaticdevices consist of the following component parts:

Sensing deviceUsually a diaphragm, bourdon tube, or capsular element

Supply pressure nozzleA steady stream of air flows through the supply nozzle

FlapperConnected to the sensing device; directs more or less of the air flow fromthe supply nozzle to the output pressure nozzle

Output pressure nozzleReceives a stream of air (regulated by the flapper) fromthe supply nozzle and directs the air stream out of the instrument

PNEUMATIC TRANSMITTERS

Apneumatic transmitter is a device that, in response to input pressure, puts out aproportionate, standardized pneumatic signal.

The most common industry standard for the output of pneumatic transmitters is 315 psig.The pneumatic transmitter is calibrated so that when the process pressure is at its lowestacceptable point, the output of the transmitter is 3 psig. When the process pressure is atits maximum acceptable limit, the transmitter output is 15 psig. Pneumatic transmittersignals between 3 psig and 15 psig correspond to process pressures within the operatingrange.


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PNEUMATIC CONTROLLERS

Apneumatic controller is a device that, in response to an input pressure, sends apneumatic output to a relay. A relay is a device that sends a pneumatic impulse strongenough to physically accomplish some action (e.g., open or close a valve) to regulate the

process.

For example, as process pressure increases, a sensing device moves. Because thesensing device is connected to the flapper, the flapper also moves. The flapper is set tomove back and forth over the opening of the supply nozzle in a way that directs more airflow through the output nozzle. Therefore, as process pressure increases, the air pressureoutput of the pneumatic device increases proportionately. In response to the output signalfrom the pneumatic controller, a valve in a relay opens allowing more pneumatic pressureto be sent to a valve. The pneumatic pressure sent by the relay adjusts the setting of thecontrol valve in order to keep the process pressure within defined limits.

Electronic Pressure Transmitters

Electronic transmitters convert input pressure into a digital or electrical signal. Electronictransmitters have two basic parts:

Sensor

Electronics

Like a mechanical pressure gauges sensing device, the sensorof an electronic pressuretransmitter physically responds to changes in input pressure. The sensor converts the

physical movement into an electrical property, such as capacitance, voltage, inductance,or reluctance. The electronics part of the transmitter changes the output of the sensor intoa standard electronic signal.

The most widely used electronic signal in the process control industry is the four to twentymilliamp (420 mA) signal. When using the 420 mA signal, an electronic transmitter iscalibrated so that when the process pressure is at its lowest acceptable point, the output ofthe transmitter is 4 mA. When the process pressure is at its maximum acceptable limit, thetransmitter output is 20 mA. Transmitter outputs between 4 mA and 20 mA correspond toprocess pressures within the operating range.

Several types of sensors used with electronic pressure transmitters are listed below:

Variable capacitance Piezoresistive

Piezoelectric

Variable inductance

Variable reluctance

Vibrating wire

Strain gauge


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VARIABLE CAPACITANCECapacitance is the ability of a substance to hold an electrical charge. A capacitor is adevice consisting of two conductive plates aligned with one another but not makingcontact. The space between the plates is filled by an insulating medium known as adielectric. In the variable capacitance sensor of most pressure transmitters, the dielectric is

oil. Three factors affect the capacitance of a capacitor: Surface area of the plates

Insulating properties of the dielectric

Distance between the plates

In a variable capacitance pressure transmitter, the surface area of the plates and theproperties of the dielectric do not change. One of the capacitor plates, called a sensingdiaphragm, moves in response to the applied pressure. Because the sensing diaphragmneeds to be surrounded by the dielectric, it cannot be directly exposed to processpressure. Therefore, the sensing diaphragm is held by glass insulation in a sealedchamber. Usually, a fixed capacitor plate and an isolating diaphragm are on either side ofthe sensing diaphragm. The isolating diaphragms are actually in contact with process orreference pressures (Figure 1.8).


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OperationDuring operation, the isolating diaphragms and fill fluid on the process and reference sidesof the sensor transmit the process and reference pressure to the oil fill fluid. The fluid inturn transmits the process and reference pressure to the sensing diaphragm in the centerof the variable capacitance sensor. The sensing diaphragm deflects in response todifferential pressure across it. In gage pressure transmitters, atmospheric pressure istransmitted to the low side of the sensing diaphragm. In absolute pressure transmitters, areference pressure is maintained on the low side. The displacement of the sensingdiaphragm is proportional to the pressure. Capacitor plates on both sides detect theposition of the sensing diaphragm. The differential capacitance between the sensingdiaphragm and the other capacitor plates is converted electronically to an appropriatecurrent, voltage, or digital output signal.

Benefits and Limitations of Variable Capacitance DevicesVariable capacitance devices are very durable, accurate, vibration resistant, and can becalibrated to measure both large and small amounts of pressure. Variable capacitancedevices are, however, sensitive to changes in temperature.


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PIEZORESISTANCEResistance is the amount of opposition to the flow of electricity exhibited by a particularconductor. Apiezoresistor is a piece of metal or a semiconductor that exhibits a change inresistance when bent or stretched. In a piezoresistive transmitter, pressure is transmittedthrough a fill fluid from an isolating diaphragm to a piezoresistor (Figure 1.9). As the

piezoresistor is flexed by pressure, its resistance changes. The piezoresistor is usuallypart of a configuration of electrical conductors and resistors set up to measure changes inresistance called a Wheatstone bridge. The change in resistance is then converted by theelectronic components of the transmitter into a standard control signal, commonly amilliampere, voltage, or digital control signal.

Benefits and Limitations of Piezoresistive DevicesPiezoresistive devices are also very responsive to pressure changes and are highlyaccurate. Like variable capacitance devices, piezoresistive devices are sensitive tochanges in temperature.

PIEZOELECTRIC

The sensor of a piezoelectric transmitter consists of a pressure-sensing device, such as adiaphragm, connected to a piezoelectric crystal. Apiezoelectric crystal is a natural orsynthetic crystal that produces a voltage when pressure is applied to it. The piezoelectriccrystal produces a very small voltage, so the voltage is usually amplified and thenconverted by the transmitter electronics into a standard control signal, commonly a

milliampere, voltage, or digital control signal.

Benefits and Limitations of Piezoelectric DevicesPiezoelectric measurement devices are sensitive to changes in pressure, but are notparticularly good at measuring static pressure. Piezoelectric devices are temperature andvibration sensitive. Because of these limitations, piezoelectric technology is not widelyused in the process control industry.


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VARIABLE INDUCTANCEInductance is a process by which a body that has magnetic or electrical properties passesthose properties to another body without making contact. In a variable inductancetransmitter, a movable iron core is surrounded by a coil of wire. An alternating current (ac)input is run through the coil. The iron core is attached to a sensing device (e.g., diaphragm

or capsule). As the sensing device moves in response to pressure, the iron core moves inrelation to the surrounding coil. The change in position of the iron core causes aproportionate change in the inductance of the coil. The change in inductance can bemeasured and converted electronically to a standard control signal, commonly amilliampere, voltage, or digital control signal.

Fig. 1.10: Variable inductance

Benefits and Limitations of Variable Inductance DevicesVariable inductance devices are durable and not greatly influenced by vibration. They are,however, sensitive to fluctuations in temperature and to changes in the ambientelectromagnetic field.

VARIABLE RELUCTANCE

Variable reluctance transmitters also convert changes in inductance into a standardmilliampere, voltage, or digital control signal, but are configured differently than variableinductance transmitters. Two coils are placed on either side of a magnetic diaphragm. An

AC voltage is run through the coils, which turns the coils into electromagnets. As themagnetic diaphragm moves closer to one coil and farther from the other in response topressure, the inductance of the coils changes because of changes in the magnetic fieldaround the coils. The property of a body to create change in a magnetic field is calledreluctance.

Benefits and Limitations of Variable Reluctance DevicesBecause the basic operating principle is similar to that of variable inductance devices,variable reluctance devices exhibit similar characteristics. Variable reluctance devices aresensitive to fluctuations in temperature and in the ambient electomagnetic field.

VIBRATING WIREIf a current is passed through a wire that is in a magnetic field, the wire will vibrate.Changes in the tension with which the wire is held change the frequency of the vibration ofthe wire. If one end of the wire is attached to an elastic sensing device, such as adiaphragm, changes in pressure on the diaphragm will cause proportionate changes in the


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vibrational frequency of the wire. The frequency is measured and converted into astandard control signal, commonly a milliampere, voltage, or digital control signal.

Benefits and Limitations of Vibrating Wire DevicesBecause vibrating wire technology is dependent on measuring vibration, mechanicalvibration of the device can cause inaccurate outputs.

STRAIN GAUGEThe electrical property of resistance of a conductive substance changes as that substanceis stretched. In a strain gauge, one end of a wire is attached to an elastic sensing device,and the other end of the wire is secured in place. As the sensing device moves inresponse to changes in pressure, the wire is stretched or relaxed. The variations in strainon the wire cause measurable changes in the resistance property of the wire. Changes inresistance are electronically converted into standard control signals, such as milliampere,voltage, or digital control signals.

Some strain gauges use foil cemented to the back of a sensing device instead of wires. As

the amount of strain on the foil changes, resistance changes as well. The piezoresistivetransmitters are actually a form of strain gauge.

Fig. 1.11: Bonded foil strain-gage and Thin-film strain gage

Benefits and Limitations of Strain Gauge DevicesStrain gauges can be very sensitive to small changes in pressure, but the output of thestrain gauge (i.e., a change in resistance) is very small and requires amplification. Straingauges that are cemented to a sensing device can become partially detached andtherefore inaccurate. Strain gauges are also extremely sensitive to temperature changes.

pressure fundamentals

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