Pressure Measurement Training Material

7/28/2019 Pressure Measurement Training Material

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GBIPL Pressure Measurement- 1 -

TABLE OF CONTENTS

1 Pressure Gauge Measurement.........................................................................................2

1.1 Reasons for measuring pressure.................................................................................2

1.2 Application of Pressure Gauges.................................................................................2

1.3 Principles of Pressure.................................................................................................21.4 Kinds of Pressure .......................................................................................................2

1.5 Effect of Atmosphere.................................................................................................3

1.5.1 Pressure units .....................................................................................................3

1.5.2 Pressure Gauges classifications .........................................................................3

1.6 Selection.....................................................................................................................4

1.6.1 Selection of mounting arrangement ...................................................................4

1.6.2 Selection of Measuring Element........................................................................4

1.6.3 Selection of Movement Mechanism. .................................................................4

1.6.4 Dial Scale ...........................................................................................................5

1.6.5 Diaphragm materials and wetted parts...............................................................5

1.6.6 Fill fluids in seal units........................................................................................51.6.7 Pointers ..............................................................................................................5

1.6.8 Gauge selection..................................................................................................5

1.6.9 Safety .................................................................................................................5

1.6.10 Definations of Comman Terms Used : .............................................................6

1.7 Tests ...........................................................................................................................6

Accessories ................................................................................................................7

2 FUNDAMENTALS OF PRESSURE MEASUREMENT ................................................7

2.1 REASONS FOR MEASURING PRESSURE...........................................................7

2.1.1 Providing Operating Information.......................................................................7

2.1.2 Providing Test Data ...........................................................................................7

2.1.3 Measuring Quantity ...........................................................................................7

2.1.4 Indicating Operational Readiness ......................................................................8

2.2 KINDS Of PRESSURE .............................................................................................8

2.2.1 Gauge Pressure...................................................................................................8

2.2.2 Absolute Pressure...............................................................................................9

2.2.3 Negative Pressure...............................................................................................9

2.2.4 Differential Pressure ........................................................................................10

2.2.5 Atmospheric Pressure ......................................................................................10

2.3 PRESSURE MEASURING UNITS ........................................................................10

3 FUNDAMENTALS OF PRESSURE SENSING ELEMENTS: .....................................11

3.1 BOURDON TUBE:-................................................................................................113.2 BELLOWS...............................................................................................................12

3.3 DIAPHRAGM:-.......................................................................................................13

3.3.1 SELECTION OF DIAPHRAGM ....................................................................14

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1 Pressure Gauge Measurement

Pressure is defined as the force per unit area. Such force always produces adeflection, a distortion, or some change in volume or dimension irrespective offorce is small or large. It is this resulting change to a sensing elementthat

provides the basic measurment and controlling movement for pressureinstruments.

1.1 Reasons for measuring pressure

a) Providing Operating Information to enable adjustments in control loopb) Providing test datac) Measuring Quantityd) Indication of Operational Readinesse) Measuring Force e.g. Hydraulic press

1.2 Appl ication of Pressure Gauges

Compressors, Chemical Processes, pipelines, pumps, refrigeration.Automotive test equipments,Refineries, Oil extraction, Ships, Medicalequipments, Vacuum systems, Boilers, Lubrication, Presses, etc

1.3 Principles of Pressure

Pressure is defined as force acting over a given area (p = f / a ) Liquid Columns : Head Pressure

Head Pressure depends on height of liquid column (H) & (D) density of liquid( p = H*D )

Force & Pressure

F = P * AAll three factors must be expressed in compatable units Manometers

Manometwrs are used for pressure measurement in terms of Head of liquidcolumn

1.4 Kinds of Pressure

1) Gauge pressure :Most common methodPressure is applied inside measuring element and atmospheric pressuresurrounds outer side of elementIndicated pressure is in excess of atmospheric pressure. This type is also

known as gauge pressure.(By custom, this is assumed as gauge pressureand " g " is omitted.)

2) Atmospher ic pressure is also reffered as Barometric pressure3) Absolute pressure : Atmospheric pressure is result of the weight of earth's

atmosphere and varies with altitude and prevailing weather conditions.Fixed reference point is established, at an atmospheric pressure equal to zero (fullvacuum) called Zero absolute pressure.Gauges may be constructed to use zero absolute pressure and indicated by letter " a"

4) Negative pressure :Pressure bellow the atmospheric pressure is termed asNegative pressure OR vacuum(It should be noted that there is No such thing as negative absolute pressure.)

5) Differential pressure : It is difference between two pressure measured at twopoints at same time.

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6) Conversion of gauge pressure to absolute pressure may be carried out byadding atmospheric pressure to gauge pressure

1.5 Effect of Atmosphere

Gauge indicating in terms of absolute pressure is not affected by changes in

atmospheric pressureBut gauges measuring in terms of gauge pressure ( or vacuum ) are affectedby change in atmospheric pressure.because indicated value is difference of internal pressure and externalatmospheric pressure.

Standard Atmospheric pressure : Atmospheric pressure at sea level =759.93mm Hg ( termed as 760 mm )

1.5.1 Pressure uni ts

a) Pascal : It is 1 Newton per square meter. It is not gravimetric unit, since forceof gravity does enter into its defination.Pascal is actually very small ,hence kilopascal is commonly used.Bar is another unit = 100 kpa

b) Liquid heads are measured in mm of water column OR mm of MercuryOther units may be used by using suitable conversion factors.

1.5.2 Pressure Gauges classif ications

1. By Function / Application Pressure Gauges used for measurement of 1 to 1000 bar pressure for

high repeatability Vacuum Gauges used for measurement of 1 bar or 760 mm HG

vacuum pressure for high repeatability Compound Pressure Gauges used for measurement of 1 to 40 bar

pressure of pumps on discharge lines. Duplex Gauges used as test gauges specially boiler tests Differential Pressure Gauges :used for measurement of flow Hygine Gauges used in food industry / pharma industry Dairy gauges used in milk processing industry Receiver Gauges used for receiving signals Test Gauges used for calibration standards Refrigerent Gauges they have equivalent temperature scale Hydraulic Gauges used on presses

Liquid level Gauges used for receiving signals Absolute Gauges Oxygen Gauges used for oxygen gas applications need proper cleaning of

oils and greases. Acetylene Gauges used for acetylene applications, need proper cleaning

of oils. Ammonia Gauges : they have equivalent temperature scale Chemical Seal Gauges used for protecting device used to isolate

pressure sensing element from corrosive, viscous,slurry applications . Diaphragm Gauges : used for very low pressure measurement with good

accuracy Capsule Gauges : used for very low pressure measurement with high

accuracy Draft Gauges

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Load Gauges Inline Gauges Pressure switch used for controlling automatic machine operations Glycerine filled Gauges used in high vibratins/ pulsations External Zero adjustment gauge

2. By Case size By size of Dial like :Dia. 50, Dia 100, Dia 150 ,Dia 250 etc

3. By Measuring Element Bourdon C type Bourdon Coil type Bourdon Helical type Spiral Bourdon Sensor Fused Quartz Helix Pressure Sensors Bellows Capsules

Diaphragms Pistons Manometers

4. By Material of Measuring Element Stainless steel SS 316 Ti Phosphor bronze Monel Beryllium copper Brass

5. By AccuracyAccuracy is conformity of gauge indication to accepted standard or true value.It is difference between indicated value and true value expressed as % ofgauge span.

1.6 Selection

1.6.1 Selection of mounting arrangement

1. Bottom entry local mounting2. Bottom entry surface mounting

3. Back entry local mounting4. Back entry flush panel mounting

1.6.2 Selection of Measuring Element

Chice depends on required performance and cost

1.6.3 Selection of Movement Mechanism.

Movement greatly magnifies the small linear motion of tip and converts it in torotary motion so as to produce full travel of indicating pointer. For thismovement should be very precision type and as free as possible of friction.e.

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1.6.4 Dial Scale

Frequency of graduation should bear a relationship to the grade of accuracy.Generally the value of the smallest graduation interval should be one to twotimes the numerical accuracy of the gauge.

1.6.5 Diaphragm materials and wetted partsDepending on Corrosive nature of process, sediments, viscosity and hazardousness of

process fluid

1.6.6 Fill flu ids in seal units

Silicon oil DC 200 upto 200oCSilicon oil DC 704 upto 340oCFlurolube / Halocarbon oil for oxidizing mediaEdible oil for food applcation

Tolune -40 up to 90oC

1.6.7 Pointers

Pointer length should be selected such that the tip of the pointer does notextend beyond the outer end of any graduation and should not be less thanthe inner edge of graduations. Should be bold enough to permit easy readingfrom distance.Knife edge pointers are used on high accuracy gauges. Micro pointers areused for precise positioning of pointer is desired.

1.6.8 Gauge selection

Following factors are normally to be considered:

Nature of pressurized medium, including its pressure range: Medium maybe corrosive, may solidify at atmospheric conditions Chemicalcompatibility, temperature compatibility, magnitude of pressure

Environmental conditions: Effect of temperature --- range shift, zero shift,max. Temperature limit

Mechanical shocks and vibrations, Humidity , corrosive atmosphere Method of connecting gauge to pressurized medium : strength

considerations, Correction for liquid head, installation, effect of leakage Safety considerations Size of gauge

Required accuracy of gauge Required type of case: dry, filled, solid front

1.6.9 Safety

Adequate safety results from intelligent planning & careful selection andinstallation of gauges into pressure system. User should inform all conditionspertinent to application &environment.Fire and explosions within pressure system can cause pressure elementfailure with very violent effects.a) Hydrogen embrittlement

b) Contamination of a compressed gas,c) Formation of acetylenes,d) Weakenening of joints

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e) Corrosionf) Fatigueg) Mechanical shocksh) Excessive vibrations

Safety recommendationsa) Operating pressure: use between middle half of scale (25% to 75%)b) Avoid use near zero pressure (less than 10% of span)c) Ensure compatibility with pressure mediumd) Ensure cleanlinesse) Use with liquefied gas systems use safety pattern (solid front)

1.6.10 Definations of Comman Terms Used :

Accuracy is conformity of gauge indication to accepted standard or true value.Accuracy rat ing :expressed in percent of actual output reading.Adujustment span : Means provided in an instrument to change slope of an

input-output curve.Adjustment Zero : Means provided in an instrument to produce parallel shift of

the input-output curve.Calibration report : A table or graph of the measured relationship of an

instrument as compared over its range against a standard.Calibration traceability : The relationship of the calibration of an instrument

through a step by step process to an instrument or group of instruments calibratedand certified by a national standardizing laboratory.

Dampening : The progressive reduction or suppression of oscillation in a deviceor system.

Hysteresis :That property of an element evidenced by the dependence of thevalue of the output ,for a given excursion of the input ,upon the history of priorexcursions and directions of the current traverse.

1.7 Tests

All tests are carried as per latest European Standard EN 837Visual checks for dimension, unit,range,process connection,dial markings,wetted

parts of material.

Calibration for Accuracy

Repeatability

Hysteresis Pointer adjustment

Over range protection

Vacuum protection

Effect of mounting position

Effect of high temperature

Effect of low temperature

Effect of storage temperature

Cyclic test

Vibration test

Endurance test Shock test

Proof pressure test

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Fatigue test

Energy release test (For Solid Front gauges only)

Disc blow out test

Helium leak test

Weather proof test

Leak test for accessories

Accessories

Gauge protector (Gauge saver) Snubber Built in snubber Isolation valves Two valve manifolds Syphons Valve with drain plug

2 FUNDAMENTALS OF PRESSURE MEASUREMENT

2.1 REASONS FOR MEASURING PRESSURE

There are many reasons why it may be necessary to measure the pressure of a gassuch as air or carbon dioxide or a liquid such as water or oil. In some applications, onlya rough indication of the pressure is needed, while in others the pressure may becritical, requiring an accurate measurement in order to avoid endangering personnel andequipment. The following paragraphs list several reasons for measuring pressure.

2.1.1 Providing Operating Information

Many products from the petrochemical industries require careful control of pressureduring the manufacturing process. While the actual control will probably be automatic, apressure gauge will give the operator a constant indication of the pressure so thatadjustments can be made to the control loop.

In some applications the normal pressure of the process may not be highlycritical, but it is necessary to know when the pressure exceeds some set limit. If thepressure becomes too high due to an abnormal condition in the process, it may cause

damage to instrumentation, pumps, or other equipment or even burst the pressurevessel. A continuous indication of the pressure will allow the operator to shut down theprocess or vent the system before any damage is done.

2.1.2 Providing Test Data

In the shop or laboratory, pressure measurements are often required as part of theevaluation and testing of materials or equipment. Generally such measurements mustbe made with a high degree of accuracy, requiring the use of a special class of gaugescalled test gauges.

2.1.3 Measur ing Quanti ty

The quantity of a gas stored in a tank is proportional to the pressure of the gas.Therefore, a pressure gauge when installed on a tank of known volume can be

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calibrated in terms of quantity, enabling the user to determine how much gas isconsumed by a particular process.

The quantity of liquid in a tank of known volume can be calculated by measuring thepressure of the liquid at the bottom of the tank. Gauges are furnished calibrated in termsof gallons of a specified liquid.

2.1.4 Indicating Operational Readiness

Sprinkler systems for fire protection and fire extinguishers are maintained underpressure, and a pressure gauge is installed in the sprinkler system or on the fireextinguisher to indicate the amount of the pressure. Gauges for this purpose could beeither analog or digital for displaying an operating zone so that the user can easilydetermine if the equipment is in fact operational.

2.2 KINDS Of PRESSURE

Pressure is measured using various reference points as zero, which results in what maybe called different kinds of pressure. The various terms used to describe these aregauge pressure, absolute pressure, negative pressure, differential pressure, andvacuum.

2.2.1 Gauge Pressure

The most common type of pressure is constructed so that the measured pressure isapplied to the inside of the measuring element (i.e., the bourdon) and atmosphericpressure surrounds the outside of the measuring element. Therefore, the pressure

indicated by the gauge will be the amount that the measured pressure is in excess ofthe surrounding atmospheric pressure. This kind of pressure is called gauge pressure,and its reference (ambient) atmospheric pressure. Atmospheric pressure is oftenreferred to as barometric pressure. The left-hand side of the chart illustrates a gaugepressure of 50 psi. To be strictly correct, this should be written as 50 psig to indicate it isgauge pressure.

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2.2.2 Absolute Pressure

Atmospheric pressure is the result of the weight of the earths atmospheric and variesdepending on the altitude and the prevailing weather. A fixed (unchanging) referencepoint is called zero absolute pressure. Gauges may be constructed to use zero absolutepressure as a reference point, in which case the indication would be in terms of a unit

such as psi absolute, which is abbreviated as psia. Fig introduces the term StandardAtmospheric Pressure and equates it to 14.696 psia. This pressure represents theweight of a column of air 1 sq. inch in cross section having a height that extends fromsea level to the outer limits of the earths atmosphere.

GAUGE PRESSURE ABSOLUTE PRESSURE

VAC

VAC

0 bar

ATMOSPHERIC

PRESSURE ATSEA LEVEL

0 bar760mm Hg(- 1.013 bar)

400mm Hg

(- 0.53 bar)

0mm Hg

0.5 bar

1.0 bar

1.5 bar

2.0 bar

2.5 bar

4.013 bara3.0 bar

DIIFERENTIAL PRESSURE

2bar

VACUUM

NEGATIVE

PRESSURE

GAUGEPRESSURE

ABSOLUTEPRESSURE

760mm Hg abs(1.013bara)

ZERO

ABSOLUTEPRESSURE

Based on standard atmospheric pressure 760mm Hg = 1.013bara

2.2.3 Negative Pressure

Negative pressure is a term intended to replace vacuum. Negative pressure, or vacuum,is a pressure below atmospheric pressure and therefore, like gauge pressure, uses thesurrounding atmosphere as the reference point. Because of this, the maximum negativepressure or vacuum that can be obtained is value numerically equal to the existing

ambient atmospheric pressure at which point absolute pressure will be zero. It iscustomary to express vacuum in terms of a liquid head (in Hg, cm H2O, etc.), whereasnegative pressure is usually expressed in force/area units, except that a minus sign is

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placed in front of the numerical value to indicate that the pressure is negative (-10 psi, -70 kPa, etc.). In both the cases the gauge indicates atmospheric pressure as zero, andthe numerical value of the indications increases as the negative pressure or vacuumapproaches zero absolute pressure. This is, of course, no such thing as negativeabsolute pressure, since the reference point for absolute pressure is zero atmospheric

pressure. A vacuum would be indicated if the absolute pressure was less than thebarometric pressure.

2.2.4 Differential Pressure

One other term is illustrated in fig. namely differential pressure. Gauges are available tomeasure and indicate the differences between two pressures, such as B (10 psi) and C(40 psi) in fig. the differential pressure gauge indicates the difference on a scale thatwould read zero when the two pressures are equal regardless of their individual values.

The letter d indicates difference, so that in this example the differential pressurebetween points B and C would be expressed as 30 psid. The zero point may be in thecenter of the scale to permit readings in one direction or the other, depending on whichpressure is greater. If one of the two pressures is always the greater of the two, the zeropoint may be at the beginning of the scale. Because such gauges read the differencebetween the two pressures, the distinction between the gauge and absolute pressure isirrelevant ; variations in atmospheric pressure will not alter the readings because bothinputs are identically affected by barometric changes.

2.2.5 Atmospheric Pressure

The pressure indicated by a gauge that measures in terms of absolute pressure is notaffected by changes in atmospheric pressure; such gases measure the pressure withrespect to an unchanging absolute zero reference. Gauges that measure in terms ofgauge pressure are affected by the change in atmospheric pressure because thepressure element of these gauges senses the difference in pressure between thatapplied to the exterior, which is usually atmospheric pressure. Therefore a decrease inatmospheric pressure surrounding the element has the same effect as an increase inpressure applied to the interior of the element. The importance of changes inatmospheric pressure depends on the required accuracy of indication and themagnitude of the pressure being measured.

2.3 PRESSURE MEASURING UNITS

Pressure and temperature are the most important physical variables in our world.Pressure is defined as a force (Fn) uniformly distributed over a given area (A) and actingupon it.

p = Fn/AThis force can be caused by liquids, gases or vapours or by solid bodies.

The force unit Newton is defined as

1N = 1 kg m/sq. secFrom this pressure unit Pascal (Pa) named after the French physicist Blaise Pascal isdirectly derived

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1 Pa = 1 N/ sq.m

These units are standardized by International standards ISO 1000-1973, the Germanstandard is DIN 1301.

1 Pascal corresponds to the pressure uniformly exerted upon an area in which the forceof 1 Newton is vertically applied to the area of 1 sq. m

The special name for the tenth of a mega pascal (MPa) is the Bar.

1 bar = 1,00,000 Pa = 0.1 MPa

The unit Bar named after the French Physicist Bar proved to be very practical since itcorresponds to order of magnitude at atmospheric pressure. For this reason, theGerman DIN standards use the bar almost exclusively. Internationally this unit isbecoming increasingly used.

Previous units in use were

Technical atmosphere1 kg/sq.cm = 0.981 bar.

Physical atmosphere1 atm = 1.013 bar.

Column of water1 mm WG = 0.0981 mbar.

3 FUNDAMENTALS OF PRESSURE SENSING ELEMENTS:

Mechanical methods of measuring pressure have been known for centuries. U-tubemanometers were among the first pressure indicators. Originally, these tubes weremade of glass, and scales were added to them as needed. But manometers are large,cumbersome, and not well suited for integration into automatic control loops. Therefore,manometers are usually found in the laboratory or used as local indicators. Dependingon the reference pressure used, they could indicate absolute, gauge, and differentialpressure.

The different pressure measuring elements used are:

1. Bourdon Tube2. Bellows3. Diaphgram

3.1 BOURDON TUBE:-

A Bourdon tube is C-shaped and has an oval cross-section with one end of the tubeconnected to the process pressure (Figure 3-1A). The other end is sealed andconnected to the pointer or transmitter mechanism. To increase their sensitivity,Bourdon tube elements can be extended into spirals or helical coils (Figures 3-1B and 3-1C). This increases their effective angular length and therefore increases the movement

at their tip, which in turn increases the resolution of the transducer

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3.2 BELLOWS

A bellows is a cylindrical device that is usually ribbed and very flexible. Bellows areusually made of phosphor, bronze, or brass. A pair of matched bellows can be used to

measure absolute pressure or differential pressure. Usually one end of the bellows isattached to the transducer case and the other end if free to move. When pressure isapplied to the inside of the bellows, it will expand, causing the free end to move. Themovement of the movable end is measured in order to identify the amount of pressure.

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3.3 DIAPHRAGM:-

A diaphragm is a mechanical pressure transducer that is used to detect slight changesin pressure. A diaphragm is a single disk that is exposed to a process. Since processpressure is exerted over a relatively large area, this instrument is more sensitive to smallchanges in pressure. This principle is based on the basic pressure equation, P = F/A. Ifa small pressure is exerted over a large area, the force will be relatively large, and willcause the diaphragm to flex. A mechanical device, such as a pin, rod, or bar, is usuallyconnected to the diaphragm so that the amount of flex can be measured in order todetermine the amount of pressure exerted.

Diaphragm capsules are frequently used as isolation devices.

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3.3.1 SELECTION OF DIAPHRAGM

There are various types of diaphragm available like corrugated, flat and capsules. Theavailable diaphragm materials are stainless steel and spring steel. Depending upon thetype of application the appropriate diaphragm should be used.In the case of thediaphragm, the reference magnitude is the elastic deformation S of the diaphragm,which is fixed around its periphery and loaded on the side. This deformation isconverted by known methods into a rotary travel. Due to their shape and positiondiaphragms have a high mechanical resistance and are less shock-sensitive. In mostcases, they are made of steel or of stainless steel. The diaphragm calculation is moredifficult than that for the bourdon tube. HARINGX developed a method, which can be

reduced to a simple formula, using nomograms. The starting point is the deflection oftheflat,peripherallyclampedplate

S = 3/16 * (R)/(E*d) * p

With E = E/(1- )

Which clearly shows the importance of the parameters radius r anddiaphragm thickness d.

For small deflections, flat diaphragms show linear characteristic for pressure p andtravel s. For this reason they are mostly used for electrical and generally capacitivesensing methods. For a pressure measuring instrument with mechanical indication

measuring travels between 1 and 3 mm are needed. In this case, the nonlinearity of aplane diaphragm would be extremely disturbing. Useful deflections with a good linearitycan be obtained if the diaphragm has concentric corrugations.

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Pressure Measurement Training Material