3DG J34 Pressure Measurement Test

8/8/2019 3DG J34 Pressure Measurement Test

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Electronic documents, once printed, are uncontrolled and may become outdated.Refer to the electronic documents in BecRef for current revisions.

Bechtel Confidential Bechtel Corporation 2002. Contains confidential and/or proprietary information to Bechtel and its affiliated companies which shall not be

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BECHTEL CORPORATIONENGINEERING - CONTROL SYSTEMS

ENGINEERING DESIGN GUIDEPRESSURE MEASUREMENT

3DG-J34-00001, Revision 002 2002 August 05Reason for Revision: Re-Issued for UsePrepared by: B. ParkesChecked by: M. BeneferApproved by: A. P. DiMartino

INTRODUCTION

This Design Guide covers the requirements for pressure measurement and the application ofpressure measuring instruments.


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Electronic documents, once printed, are uncontrolled and may become outdatedRefer to the electronic documents in BecRef for current revisions.

Bechtel Confidential Copyright Bechtel 2002. All rights reserved.

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CONTENTS

INTRODUCTION 1

1.0 PURPOSE 3

2.0 CODES AND STANDARDS 3

3.0 PRESSURE MEASUREMENT 4

4.0 PRESSURE GAUGES/INDICATORS 6

5.0 PRESSURE TRANSMITTERS 19

6.0 PRESSURE SWITCHES 22

7.0 PRESSURE INSTRUMENT INSTALLATION 24

8.0 WORK PROCESS 25

9.0 REFERENCES 28


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1.0 PURPOSE

The purpose of this Design Guide is to guide the instrument engineer in the selection,specification and installation of pressure measuring devices.

Pressure measuring devices are divided into three (3) categories: pressure indicator/pressuregauges, pressure transmitters and pressure switches.

In the industrial world, pressure measuring devices are not only used to sense pressure but alsoto measure inferentially level and flow rate. These inferential measurements are usually madewith differential pressure measuring devices.

This design guide, 3DG-J34-0001, pertains to pressure and differential pressure sensingdevices. The applications of pressure and differential pressure instruments in the measurementsof flow rate and level are described in detail in design guides 3DG-32-0001 (Flow Measurement)and 3DG-J33-00001 (Level Measurement) respectively.

2.0 CODES AND STANDARDS

ASME B40.100a Addenda to ASME B40.100 1998 for Pressure Gauges and GaugeAttachments which incorporates ASME B40.1 2000, ASME B40.2 2000,ASME B40.5 2000, ASME B40.6 2001and ASME B40.7.

ASME B1.20.1 Pipe Threads : General Purpose

ASME PTC 19.2 Part 2 : Pressure Measurement Instruments and Apparatus

API 551 Process Measurement Instrumentation (Section 7 Pressure)

ISO 228-1 Parallel Pipe Threads (G)

EN 472 Pressure Gauge Vocabulary

BS 6134 Pressure and Vacuum Switches

BS EN 837-1 Part 1 : Bourdon Tube Pressure Gauges - Dimensions, Metrology,Requirements and Testing

BS EN 837-2 Part 2 : Selection and Installation Recommendations for Pressure Gauges

BS EN 837-3 Part 3 : Diaphragm and Capsule Pressure gauges - Dimensions, Metrology,Requirements and Testing

Note: British Standard BS EN series of standards are equivalent to CEN EN series of standardsand carry the same number e.g. : BS EN 837-1 is the same as CEN EN837-1.


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3.0 PRESSURE MEASUREMENT

3.1 CATEGORIES

Pressure measuring devices are logically divided into the following categories:

Pressure Gauges/Indicators

Differential Pressure Gauges/Indicators

Pressure and Differential Pressure Transmitters

Pressure Switches

3.2 CLASSIFICATIONS OF PRESSURE AND ENGINEERING UNITS

3.2.1 Classifications of Pressure

The four classifications of pressure are:

Gauge Pressure

Absolute Pressure

Vacuum (Negative Pressure)

Differential Pressure

This demarcation of various types of pressure developed because to sense each type of

pressure requires a different type of pressure measuring device.

In order to convert measurements between gauge and absolute pressure the value of theatmospheric pressure at sea level is added or subtracted to the given measurement.

absolute pressure = gauge pressure + atmospheric pressure

e.g.

psia = psig + 14.696 (1 Atmosphere based on 60F at sea level)

gauge pressure = absolute pressure - atmospheric pressure

e.g.

psig =psia - 14.696 (1 Atmosphere based on 60F at sea level)

Note: Atmospheric conditions should be corrected for the site or plant location.


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Differential pressure is the difference in pressure between two locations. It is calculated bysubtracting the higher pressure from the lower pressure. Differential pressure units are psi andboth high/low pressure points must be measured in the same units either absolute or gauge

pressure.

3.2.2 Engineering Units

The engineering units that are utilized to describe pressure are numerous. Listed below are themost popular English (Imperial) and metric units in use.

English (imperial) units for pressure:

Pounds per square inch (psi)

Inches of water (in H2O)

Inches of mercury (in Hg)

In H2O and in Hg must be corrected for temperature (generally 60F since most liquids tend toexpand on increasing temperature)

Metric units for pressure:

Bar (bar)

Millibar (mbar)

Kilograms per square centimeter (kg/cm2)

Pascal (Pa) or Kilo Pascal (kPa)

Millimeters of mercury (mm Hg)

Mllimetres of water (mm H2O)

Conversion factors for these units related to the Pascal are as listed in Table 1.


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Table 1

Common Pressure Conversion Factors

Unit Symbol No of Pascals

Pounds per square inch psi 6894.76 Pa

inches of mercury in Hg 3386.39 Pa

millimeters of mercury mm Hg 133.322 Pa

inches of water in H2O 249.0889 Pa

bar bar 1 x 105

Pa

millibar mbar 100 Pa

kilograms per squarecentimeter

kg/cm2

0.98067 x 105

Pa

4.0 PRESSURE GAUGES/INDICATORS

In order to select, specify, and install the most suitable pressure gauge for any given applicationthe Instrument engineer must be knowledgeable with all the various aspects of pressure gauges:

Function

Case Type

Accuracy

Specific Type of Application

Measuring Element

There are also liquid-filled gauges and many accessories are available that can enhance theperformance of a pressure gauge.

4.1 FUNCTIONS OF PRESSURE GAUGES

Pressure gauges are specifically designed and fabricated and have specific indicating scales tomeet the following pressure measurement requirements:

Measure vacuum


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Measure gauge pressure

Measure differential pressure

4.1.1 Pressure Gauges

The common practice in instrumentation is that when the phrase Pressure Gauge is used italways implies that the instrument will measure gauge pressure, where 0 (zero) on the indicatingscale denotes ambient or atmospheric pressure (0 psig)

4.1.2 Vacuum Gauges

A vacuum gauge indicates negative pressure, usually having a span of 0 to 30 in Hg which isapproximately the maximum vacuum (0 psia) that can be attained.

4.1.3 Compound Gauges

A compound gauge is capable of indicating pressure above and below ambient pressure, that is,positive and negative pressure. The positive pressure portion of the scale is usually calibrated inPa, bar or psi and the negative pressure or vacuum portion in psi or in Hg.

4.1.4 Dual Scale Gauges

A dual scale gauge has a single pressure element, the dial of which contains a basic pressurescale and one or more additional concentric scales graduated in equivalent values of a differentpressure unit or other parameters related to the basic pressure.

4.1.5 Retard Gauges

In a retard gauge, the measuring element moves freely through only a portion of its pressurespan, usually the lower portion. At a definite point in the upper portion of its pressure span, therate of the remaining motion per unit of pressure change is reduced.

4.1.6 Surpressed Scale Gauges

A gauge having a suppressed scale shows only the upper portion of the total pressure range. Atpressures below the minimum shown on the scale, the indicating pointer is not actuated by theelastic chamber and remains inactive at the minimum point of scale until the pressure risesabove the minimum value.

A special gauge called a receiver gauge is an example of one having a suppressed scale. Areceiver gauge is designed to indicate the output from a pneumatic transmitter and calibratedwith reference to transmitter output. This type of gauge has a suppressed scale of 3-15 psig.The dial can be graduated in units related to the transmitter and can be in temperature,pressure, flow or level. Frequently this type of gauge has a dual scale, the first is the transmittedsignal and the second the quantity of the measured parameter.


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4.2 ENCLOSURE/CASE CATEGORIES

Pressure gauges are supplied in a wide variety of case types and are described by suchcharacteristics as size, method of mounting, location of pressure connection, and generalconstruction of case and ring.

4.2.1 Size

Case sizes are available in standard sizes varying from 40 to 405mm (1-1/2" to 16") diameter.These diameters refer to the approximate inside diameter of the case at the dial level. Thestandard sizes are : 40mm (1 1/2"), 50mm (2"), 65mm (2 1/2"), 90mm (3 1/2"), 115mm (4 1/2"),150mm (6"), 215mm (8 1/2"), 305mm (12") and 405mm (16")

4.2.2 Method of Mounting

Several common methods of mounting are used. These include:

Stem or direct mounting

Surface mounting

Flush mounting

4.2.3 Location of Connection

The pressure connection, in most instances, projects radially from the bottom of the case at the6 oclock position, or from the back of the case on the geometrical center, or below the center.Connections are generally provided with a pipe thread of 1/8", 1/4", or 1/2" NPT. Otherconnections are possible and this will depend on the country and standard that is applicable.

4.2.4 Case and Ring Construction

The two broad categories in this classification are cast (or molded) cases and drawn, sheetmetal cases. Generally, the lowest cost gauges are made using drawn, sheet metal cases withfriction, slip, or threaded rings. Higher quality gauges usually have more sturdy cases of moldedplastic or cast metal, with threaded rings, bayonet rings or hinged rings. Cast metal cases aremost commonly aluminum or cast iron, and plastic cases are usually molded from a high-impactcompound.

A safety pattern gauge has a blowout plug or blowout back to relieve in the case of element

failure. This type of gauge has a solid wall that separates the dial from the sensing element.

Oxygen and acetylene gauges must be safety pattern gauges and generally a safety patterngauge should be chosen unless the gauge is in low pressure service and a helical bourdon tubeis used as the measuring element.


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The lens is usually of shatter proof glass and the dial has black figures on a white background.The pointer zero should be capable of being adjusted without removing the pointer from its shaft.

4.3 ACCURACY CLASSIFICATIONS

The accuracy of an instrument is defined as the difference between the true value (based on apressure standard) and the indicated value expressed as a percent of the reading or span.

% Accuracy of reading = (indicated value-true value)*100/true value

% Accuracy of span = (indicated value-true value)*100/span

ASME Standard No. B40.100a specifies and grades the accuracy of pressure gauges. In Table2 the most common commercially available of these grades are listed.

Table 2

Grades of Accuracy as Defined in ASME B40.100a (Permissible Error % of Span)

Accuracy Grade First 25% of Scale Middle 50% of Scale Last 25% of Span

4A 0.1 0.1 0.1

3A 0.25 0.25 0.25

2A 0.5 0.5 0.5

1A 1.0 1.0 1.0

A 2.0 1.0 2.0

B 3.0 2.0 3.0

C 4.0 3.0 4.0

D 5.0 5.0 5.0

When specifying the accuracy for a gauge the application and importance of measurement tothe operator should be considered as the cost for the gauge is related to the accuracyspecification.

Pressure gauge manufacturers frequently refer to the above listed accuracy grades in theirtechnical literature. There are similar equivalent accuracy grades for other internationalstandards. When the ASME standard does not apply then reference to the equivalent nationalstandard for the instrument accuracy grade should be made.


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4.4 SPECIFIC APPLICATIONS

4.4.1 Ammonia Gauges

The ammonia gauge has steel or stainless steel internal parts to withstand the corrosive effectsof ammonia. It has a dial with two (2) scales; one for pressure and the other for thecorresponding temperature of saturated ammonia. Materials such as copper, brass and silverbrazing alloys should not be used and the gauge should have the inscription "AMMONIA" OR"NH3" printed on the dial.

4.4.2 Oxygen Gauges

Oxygen gauges are fabricated under very high cleanliness conditions so that no organiccontaminants are present that might react with oxygen and cause an explosion. The gauge mustbe marked in red on the dial with the inscription "OXYGEN SERVICE - USE NO OIL".

4.4.3 Hydraulic Gauges

Hydraulic gauges are specifically constructed for high-pressure service where water or a noncorrosive liquid is the pressure medium. They incorporate a special link that is designed toprotect the gauge mechanism against a sudden release of pressure.

4.4.4 Acetylene Gauges

Acetylene gauges are designed to indicate acetylene pressures and any other gases havingsimilar properties. Materials should be selected suitable for use with the gas pressures beingmeasured and the dial should have the inscription "ACETYLENE" engraved on the front.

4.5 MEASURING ELEMENTS

As briefly mentioned previously, all pressure gauges use some type of elastic element thatconverts the measured pressure into a proportional motion. Through suitable gears and levers,this motion is transferred into a corresponding motion of the pointer to give an indication of themeasured pressure. The degree of accuracy and reliability of such indications depend on thetype of measuring element used.

Three basic types of measuring elements are commonly used in pressure gauges:

Bourdon elements

Bellows elements

Diaphragm elements

Table 3 covers the pressure ranges normally associated with each of these elements.


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The following is a brief description and principle of operation of each of the elements listedabove. Refer to Reference No. 3 or equivalent text book for details pertaining to a detaileddescription and principle of operation of each of these pressure measuring elements.

Bourdon Elements

A Bourdon element is the most commonly used element for pressure gauges and is a curvedmetal tube closed at one end and fixed to a pressure source at the other. Application of pressureat the fixed end results in movement at the free end as the tube cross section deforms. Bourdontubes come in a C shape, spiral or helix depending on the pressure being measured.

Bellows Elements

A bellows element expands when pressure is applied to the inside, actuating an indicator,transmitter or controller. Bellows elements are usually used for low pressure less than 10 psigand for vacuum ranges. In pneumatic instruments the bellows usually operates in the range 3 to15 psig.

Diaphragm Elements

The diaphragm sensor is a thin flexible metal disc. Pressure to one side of the disc causes adeflection that actuates the indicator, transmitter or controller. Diaphragm elements are used tomeasure very low pressures and vacuums. One common application is furnace draft pressures.

4.5.1 Low Pressure Applications

Diaphragm and capsule type measuring elements are used forpressure sensing for thesame amount of pressure change the diaphragm element deflection increases 16 fold ifthe diameter is doubled.

Slack diaphragms are large diameter pressure measuring elements fabricated of syntheticmaterials like Buna-N or Viton. They are used for very low pressure/draft pressure anddifferential pressure measurements on boiler furnace, ducts, and HVAC ducts.

In many low pressure applications care must be taken to assure that hysteresis of the measuringelement does not eventually become a problem, particularly when the sensing/measuringelement is installed in a pressure switch and the set point is in the very low pressure range (seeFigure 1). If it is anticipated that the low pressure sensing element in a pressure switch may havehysteresis problems then consider using a pressure transmitter and configure the alarm/switchset point in the DCS.


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Table 3

Range for Measuring Elements

Element Application Minimum Span

(Commonly Supplied)

Maximum Span

(Commonly Supplied)Bourdon Pressure 0 - 1 bar 0 - 12 psi

0 - 4000 bar0 - 60,000 psi

Vacuum 0 - 750 mm Hg0 - 30 in Hg

0-750 mm Hg0-30 in. Hg

Compound 750 mm Hg - 0 - 1 bar0 - 30 in Hg - 0 - 15 psi

0 - 750 mm Hg - 0 - 16 bar0 - 30 in Hg - 0 - 300 psi

Bellows Pressure 0 - 25 mm Hg0 - 1 in Hg

0 - 6 bar0 - 100 psi

Vacuum 0 - 25 mm Hg0 - 1 in Hg

0 - 750 mm Hg0 - 30 in Hg

Compound Any span greater than 25mm(1 in) Hg Any total span less than 6bar (100 psi)

Metallic Pressure 0 - 250 mm H2O0 - 10 in H2O

0 - 0.8 bar0 - 10 psi

Diaphragm Vacuum 0 - 250 mm H2O0 - 10 in H2O

0-750 mm Hg0-30 in Hg

Compound Any span greater than250mm (10 in) H2O

0 - 750 mm Hg - 0 - 0.8 bar0 - 30 in Hg - 0 - 10 psi

4.6 PRESSURE GAUGE ACCESSORIES

There are many devices available that maybe used in conjunction with pressure gauges toimprove their ability to withstand adverse environmental conditions and to broaden theirusefulness. Listed below, and briefly described in subsequent paragraphs, are some of theseaccessories, which are sometimes referred to by the generic term gauge savers.

Diaphragm Seals

Pulsation Dampers/Pressure Snubbers

Siphons

4.6.1 Diaphragm Seals

Diaphragm seals should be used under the following circumstances:

Pressure medium would corrode socket and/or sensing element.

Pressure medium might freeze or solidify inside the pressure gauge.


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Pressure medium is highly viscous or a slurry that would erode and/or plug the sensingelement.

Pressure medium is a hazardous (toxic and/or radioactive).

High temperature process steams that exceed the maximum temperature rating of theinstrument.Capillary tubing and a diaphragm seal can also be used with pressure transmitters. Invibrating service diaphragm seals should be remotely mounted.

Some applications require that the lower housing of the diaphragm seal be welded to theprocess connection. Under this circumstance the lower housing material of construction must besuitable for welding to the process connection. The diaphragm and exposed housing or bottompart of seals shall be of materials of construction as required by the process. Process connectionof bottom bowl shall be 1/2" minimum, except on slurries that shall be 1" minimum, but is usually1 1/2" or 2". Filling material in diaphragm seals should be compatible with the process fluid.Diaphragm seals shall have a clean out plug (flushing connection) on the process side.

4.6.2 Pulsation Dampers/Pressure Snubbers

Rapidly pulsating pressure and short-term pressure surges will produce abnormal wear on themovement bearings and gear teeth and rapidly destroy the accuracy of the gauge. In addition,the indicating pointer may oscillate so rapidly that taking an accurate reading is impossible and, ifthe pulsation is a large percentage of the span, the bourdon will be subject to early fatiguefailure. It is the function of pulsation dampers (or dampeners) to eliminate or at least reducethese adverse effects.

Pulsation dampers are available in many forms, but all operate on the principle of restricting therate of fluid flow into the pressure element, and therefore, the rate at which the pressure canchange.

Several of these devices are briefly described below.

A threaded check is installed in the pressure port of the gauge.

A needle valve, installed between the gauge and the pressure source, is an excellentpulsation dampener. This valve can also act as the block and/or isolation valve.

A snubber using a metal disc available with different grades of porosity.


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4.6.3 Siphons

A siphon (a pipe coiled in the shape of a pigs tail) is a heat exchanger (ambient air cools theprocess fluid) that serves two functions:

Prevent steam or other condensable vapors from entering the pressure gauge. This isdesirable because, for example, saturated steam at 100 psig pressure is at a temperature of337F. At that temperature, well above the normally maximum recommended temperature of120F for pressure gauges, the soldered joints of the pressure element (usually a bourdontube) will fail.

Trap the condensate from draining away, thus effecting a seal through which incoming vapormust pass.

There are more modern versions of the old style pigtail siphon which provides a thermal barrierbetween hot vapors and the pressure measuring device.

4.6.4 Case Pressure Relief Devices

On rare occasions the pressure element (bourdon, diaphragm, bellows) of a pressure gauge willrupture. Under this unlikely circumstance it is important to assure that the pressure elementrupture will not cause a subsequent pressure gauge enclosure rupture.

Pressure gauge manufacturers usually build-in some standard method for providing casepressure relief into their gauges. However, as a precaution, the engineer should specify that ameans for providing case pressure relief is available.

Probably the most common case pressure relief method is to provide a large hole on the back of

the pressure gauge enclosure. Since large openings are generally objectionable unless coveredor closed in some way, loose fitting grommets are used to cover these holes. The grommet willbe ejected due to internal case pressure buildup. Care must be taken that ejection of thegrommet is not obstructed by the pressure gauge installation arrangement or that the grommet ispainted shut.

4.6.5 Over Range Protection

In some cases it may be required to protect the gauge from overpressure in excess of thegauge over range. Two common methods are overload stop and gauge protector .

The gauge protector is essentially a relief unit that normally allows the gauge tosee the line pressure and isolates the gauge from the line pressure when the processpressure exceeds a maximum.


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4.7 LIQUID-FILLED PRESSURE GAUGES

Liquid filled pressure gauges are pressure indicators having enclosures that are sealed andfilled with a clear viscous fluid. They are not to be confused with pressure gauges withdiaphragm seals in which the sensing element, usually a bourdon tube, is filled with a viscous

fluid.

The use of liquid filled gauges has greatly increased in recent years. They should be consideredfor use for the below listed reasons:

The fluid will dampen destructive rapid motion of the internal gauge components caused bypressure pulsations, mechanical shock, and vibration.

The fluid will protect the internal gauge components from corrosion damage, an importantconsideration for service in locations where the general environment is corrosive.

The fluid will act as a lubricant, thus increasing the life of the pressure gauge

If the gauge is located in an area subject to internal condensation, liquid fill will ensurereadability at all times

4.8 SELECTION OF PRESSURE GAUGES & TRANSMITTERS

Although this section pertains to the selection criteria for pressure gauges it also applies to alarge extent to the selection process of pressure transmitters. (Refer to Section 5.0 fordescription of pressure transmitters.)

When specifying a pressure gauge (or transmitter) the following factors must be taken Into

consideration:

The selection of a pressure gauge for any given application is a relatively easy process. Whenspecifying a pressure gauge the following factors must be taken into consideration:

Physical and chemical properties of the pressure medium

Temperature of the pressure medium

Pressure range to be measured

Accuracy requirement

Maximum pressure due to upset condition

Each of these factors is considered in the following paragraphs, and, equipped with this and theother information on gauge components and accessories contained in other chapters of thisdesign guide, the user should be able to select a gauge for a specific application that will operateproperly with minimum maintenance.


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4.8.1 Nature of the Pressurized Medium

Because the measuring element (elastic chamber) of a pressure gauge is exposed directly to thepressurized medium, needless troubles can be avoided by being knowledgeable about thenature of the medium. It may be corrosive, it may solidify at normal atmospheric temperatures,

or it may contain tars or solids that will leave deposits inside the elastic chamber. Suchconditions can impair the performance of the gauge or make it completely inoperative.

The pressure-containing envelope must be corrosion-resistant with respect to the pressurizedmedium. Published tables listing the corrosion compatibility between various chemicals andmaterials of construction should be consulted in order to specify and use the most suitablemeasuring element material. If none of the standard materials is suitable, a diaphragm sealshould be used in order to obtain the required corrosion resistance. (Refer to Section 4.6.1)

4.8.2 Temperature of the Pressure Medium

The measuring element in a pressure gauge is sometimes fabricated with copper alloy materials

that are soldered together. This solder cannot withstand sustained temperatures above 150F.Therefore, the temperature of the measured medium inside the bourdon tube should not exceed120F, thus providing a 30F margin of safety. If this cannot be accomplished, a siphondescribed in detail in Section 4.6.3, should be installed between the process and the pressuregauge. If a siphon or other means of cooling cannot be used, some Manufacturers will fabricatepressure gauges with silver brazed or welded joints (the preferred method). These gauges arecapable of withstanding temperatures above 150F to an absolute maximum of 250F. However,once 120F is exceeded other problems such as range and zero shift exist. Therefore, onlyunder extraordinary circumstances should a pressure gauge itself be allowed to reachtemperatures above 120F.

4.8.3 Pressure Range to be Measured

Probably the most common application mistake is to specify a pressure gauge range that ismuch too high for the intended service. The magnitude of the pressure will determine thepressure range selected. The ASME standard B40.100a recommends that the range of apressure gauge be selected so that the operating pressure occurs in the middle half of the scale,that is, between 25 and 75% of the span. As a guide, the full-scale pressure of the gauge shouldbe twice the operating pressure. By doing so, fatigue life will be improved and a margin providedin the event the operating pressure exceeds its intended value. Occasional application ofpressure up to the maximum range of the gauge will not be detrimental. Under mostcircumstances the Manufacturers range that is based on the ASME standard should be

selected. Special ranges are costly.

Typical ASME standard ranges are listed in Table 4.


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Table 4

ASME B40.100a Standard Pressure Ranges

psi kPa bar

0/3, 0/5, 0/10, 0/15, 0/30, 0/60,0/100, 0/160, 0/200, 0/300,0/600, 0/800, 0/1000, 0/1500,0/2000, 0/3000, 0/4000,0/5000, 0/6000, 0/10,000,0/15000, 0/20000, 0/30000,0/40000, 0/60000, 0/80000,0/100000

0/1, 0/1.6, 0/2.5, 0/4, 0/6, 0/10,0/16, 0/25, 0/40, 0/60, 0/100,0/160, 0/250, 0/400, 0/600,0/1000, 0/1600, 0/2500,0/4000, 0/6000, 0/10000,0/16000, 0/25000, 0/40000,0/60000, 0/100000, 0/160000,0/250000, 0/400000,0/600000

0/0.01, 0/0.016, 0/0.025,0/0.04, 0/0.06, 0.1, 0.16,0/0.25, 0/0.4, 0/0.6, 0/1.0,0/1.6, 0.2.5, 0/4.0, 0/6.0, 0.10,0.16, 0/25, 0.40, 0/60, 0/100,0/160, 0/250, 0/400, 0/600,0/1000, 0/1600, 0/2500,0/4000, 0/6000

4.8.4 Accuracy Requirement

Under normal circumstances most pressure gauges are not required to be very accurate.Pressure measurements that are required for trending or control purposes are better provided bypressure transmitters (not pressure indicators/gauges) that have a much higher accuracy.

Most applications require that the pressure gauge be reliable and sturdy. Unfortunately, pressuregauges - like many other instrumentation devices - tend to be more fragile as their accuracyincreases. Thus, the specifying engineer must balance the requirement of installing asturdy/reliable device with the wish to have this pressure gauge to be as accurate as possible.

ASME B40.100a lists the various grades of accuracy. Refer to Table 2 for an excerpt fromASME.

In Table 2 the accuracy for various grades of pressure gauges are listed. Most pressure gaugesinstalled are to ASME B40.100a grade 2A, having an accuracy of 0.5 % of span. Mostreceiver gauges (see Section 4.1.7) are also ASME B40.100a grade 2A.

Most pressure gauges used for calibration purposes are ASME B40.100a grade 3A, 0.25% ofspan. On the other hand, the instrument engineer should be aware that pressure gaugesfurnished as an accessory with a package unit frequently are the cheapest the package vendorcan obtain. These are sometimes called Commercial Gauges, and they have ASME B40.100a

grade B accuracy, 3/2/3% of span. This means that the first and fourth 25% of span have 3%accuracy while the second and third 25% (middle 50%) span has an accuracy of 2%.

4.9 DIFFERENTIAL PRESSURE GAUGES

Differential Pressure measurements are not only used to measure the pressure drop acrossvarious in-line components such as filters, screens, etc., but their most frequent application is inthe inferential measurement of level and flow rate. Almost all differential pressure


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gauges/indicators are so-called motion balance sensors that employ two bellows. The basiccomponents of the unit are the high and low pressure chambers, the range spring, and the driveassembly to transfer bellows motion to the readout pointer. The bellows in both chambers andthe passage between them is liquid-filled. When the unit is installed, the pressure in the highpressure chamber compresses the bellows, so that liquid flows from it into the low side bellows.

When the low pressure (or range) bellows expand, they exert a force against the range spring,which determines the span of the instrument. The linear motion of the range bellows moves thedrive lever, mechanically transmitting a rotary motion through the sealed torque tube assembly tothe indicator.

The bellows differential gauge is usually used where the process being measured has a veryhigh static pressure and the measured differential pressure is very low. It is possible for higherrange differentials to use bourdon tubes (see the Ashcroft range of differential pressure gauges).

4.10 APPLICATION NOTES

As a minimum, pressure gauges are normally installed at the locations listed below. Additional

gauges shall be installed if required for the proper operation of a unit.

On each pressure or vacuum vessel.

On the discharge of each pump, compressor and blower, and located inside of any block orcheck valve.

On the suction and discharge of each stage of a multistage reciprocating compressor.

Across in-line filter for delta P indication.

On the controlled side of all pressure regulating devices.

As tell-tales on rupture disc/relief valve combinations.

At each field pressure switch or blind pressure transmitter.

Normally pressure gauges should be 4-1/2 inch diameter, 1/2 inch NPT bottom connection,molded plastic case, white laminated phenol dials with black graduations. Process gaugesshould be solid front with blowout back. Movement may be either rotary geared stainless steel orcam and roller type. Accuracy is normally 0.5% of full scale. (ASME B40.100a, Grade 2A)Bourdon tube, socket, and tip shall be stainless steel or shall be made of suitable alloys forspecific services.

For pressure less than 1 barg (15 psig), and for differential pressure service, the sensing unitmay be a bellows, bellows displacement type mercury-less manometer or force balancemanometer. Force balance manometers should not be used in pulsating service. Slackdiaphragms should also be considered. (refer Section 4.5.1).


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Pulsation dampeners should be considered on all pulsating services such as reciprocatingpumps and compressors. For other applications where severe service from pulsing pressure isanticipated, pulsation dampeners may be specified at the instrument engineers discretion.Dampening should be provided by either pulsation dampeners or liquid filled gauge bodies withthe latter being preferred. (Reference 4.6.2 and 4.7).

Steam coil pigtails or siphons should be used on all gauges in steam and condensable vaporservice so as to maintain temperature of gauge below 120F. (Reference 4.6.3).

5.0 PRESSURE TRANSMITTERS

Pressure transmitters are devices that measure and then transmit a signal which is proportionalto the pressure sensed/measured to other locations. Most pressure transmitters presentlyinstalled in a process plant are the electronic Smart type. However, there are some applicationswhere a pneumatic pressure transmitter is more advantageous to use than an electronictransmitter. For example at an unmanned platform where there is no power and the process gascan be used for instrument air.

The most important and fundamental principle of pressure measurement is: Every pressuremeasurement is actually a differential Pressure measurement.

Namely:

Gauge Pressure = Measured Pressure - Atmospheric Pressure.

Absolute Pressure = Measured Pressure - Vacuum.

Differential Pressure= Measured Pressure #1 - Measured Pressure #2.

Therefore, gauge, absolute, and differential pressure transmitters function according to the sameoperating principle, and they actually have the same physical appearance.

5.1 ELECTRICAL/ELECTRONIC PRESSURE TRANSMITTERS

There are a number of standards for a signal transmission system, however the most widelyaccepted and implemented is 4-20 mA dc.

Many different methods of obtaining the transmission signal from a pressure transmitter arepresently employed. For example:

Capacitance

Strain-Gage

There are also many other measurement principles employed in commercially availablepressure transmitters. Among these are LVDT (Linear Variable Differential Transformer),


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reluctance, potentiometer, and rheostat. These methods are normally not employed in thepower/process industries.

For principles of operation of these transmitters consult the literature of their manufacturers.

5.1.1 Smart Pressure Transmitters

Currently, the transmitter of choice is the Smart microprocessor-based type. They are onlyslightly more expensive than the non-Smart type but offer the advantage of simplified fieldcalibration and an increase in accuracy to +/-0.1%.

Smart transmitters contain a microprocessor that can improve sensor performance and/oraccess to remote communications through a hand-held interface device, control system, or both.The microprocessor improves sensor performance in two ways. First, it can store input/outputcurves to compensate for sensor output errors caused by factors outside the process, such asambient or sensor temperature. Second, it can perform math calculations that condition sensoroutput. A simple software selection, for example, allows users to convert differential pressure

transmitters from square root to linear output.

The microprocessor can also improve the performance of a pressure transmitter bycharacterization of its calibration curve.

Smart transmitters are currently available using proprietary manufacturers standards or the morewidely accepted use both analog and digital protocol as follows:

4 - 20 mA dc (HART)

Profibus

Foundation Fieldbus

5.1.2 Wiring of Pressure Transmitters

As mentioned previously, the standard electrical transmission signal is 4-20 mA dc. This signal isgenerated by the pressure transmitter in a 2-wire configuration. In the 2-wire configuration, thepower supply is connected serially to the process loop. As the process pressure varies, itchanges the output current. The transmitter is calibrated to give a 4-20 mA dc signalcorresponding to the full span of pressure measurement.

The fieldbus transmitters can be wired in a variety of different arrangements including bus with

spur topology, daisy chain topology and tree topology.

The preferred arrangement is using a tree topology and wiring the digital transmitters through asingle junction box. Manufacturers are promoting special fieldbus junction boxes that have all therequired termination and end of line terminators pre assembled and wired. The power supply forthe two wire fieldbus system is located in the segment. The number of transmitters per segmentwill vary depending on whether intrinsically safe or explosion proof design is adopted.


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5.1.3 Span and Zero Adjustments of Pressure Transmitters

The zero and span of modern electronic pressure transmitters can be adjusted very easily. Thisis particularly helpful when a differential pressure transmitter is used to measure tank level. Thisfeature is described in detail in Design Guide 3DG-J33-0001: Level Measurement.

5.2 PNEUMATIC PRESSURE TRANSMITTERS

The standard pneumatic output signal is 3-15 psig. In order for the transmitter to be able toprovide this signal, it must be supplied with instrument air having a pressure of at least 20 psig. Afilter-regulator combination should be installed in the instrument air supply line, immediatelyupstream of the transmitter. The transmitted pneumatic output signal can be indicated on areceiver type pressure gauge as described in Section 4.1.6 of this design guide.

Consult the literature of a pneumatic pressure transmitter Manufacturers for details regardingtheir principal of operation.

5.3 APPLICATION NOTES

Pressure and differential pressure transmitters are normally solid state or microprocessor basedelectronic two-wire transmitters powered by the receiving system.

Transmitters can be supplied with either analog or digital integral or separate local indicatorsdepending on the type of transmitter. Where separate indicators are provided these are usuallylocated at a control valve bypass and used to monitor the process pressure when the bypassvalve is being used for manual control.

Integral local indicators make the requirement for a local pressure gauge next to the transmitter

redundant. Local pressure gauges should be limited to applications where positive local pressureindication is required (see Section 4.10). Local indicators should be scaled in engineering unitsfor digital transmitters because of the ease with which range changes can be made. When usingan analog transmitter select a range of 0 to 100% and use a scale factor, which can easily bechanged if the transmitter calibrated range has to be modified.

Transmitters must be suitable for and certified for the electrical area classification and shouldhave weatherproof cases to NEMA 4X or IP 65 as a minimum.

Transmission signals are 4-20 mA dc for electronic transmitters, 4-20mA dc with digital overlayfor HART and digital for fieldbus.

Ranges normally start at zero but elevated narrow span ranges are to be used where greatercontrol accuracy is required.

Standard accuracy is normally 0.1% of calibrated span.

Body and element should be a minimum of 316 stainless steel for pressure and differentialpressure transmitters.


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Smart transmitters should be considered, or prohibited by industry regulatory authorities forexample in the nuclear industry. Smart transmitters are microprocessor-based instruments andcome in a variety of industry protocols, HART, proprietary manufacturers protocol, Profibus andFoundation Fieldbus. When using fieldbus transmitters make sure that the device is compatiblewith the Host. In the case of Foundation Fieldbus, transmitters should have a FF tickmark

approval.

Process connections should be at least 1/2" NPT and instrument should be equipped with asocket or yoke for mounting on a 2" pipe. A yoke is not required if instrument is mounted on avessel or process line. In Europe the connections are G1/2"B which is a parallel thread to ISO228-1.

Electrical connections should be 1/2" NPT or 20 mm ISO metric depending on the requirementfor the project.

When required by the physical properties of the process fluid, a factory sealed filled system withdiaphragm shall be selected. If a capillary is used it shall be protected with 316 SS armor.

Differential pressure transmitters shall be rated for maximum static pressure of the process andshall be able to withstand over range pressure equal to the meter body rating.

6.0 PRESSURE SWITCHES

When a measured pressure reaches a pre-determined maximum or minimum, it is frequentlydesirable to turn on an annunciator light, sound an alarm, start or stop a pump, or open/close avalve. These actions can be provided by means of pressure switch. A pressure switch employsthe same elastic measuring elements (bourdon, bellows, diaphragm, etc) used in pressuregauges described previously in this design guide. Electric switching assemblies are attached to

the measuring element.

Both indicating and blind pressure switches are available.The current and voltage rating of pressure switches must be carefully considered.If the current carrying capacity of a certain switch is not large enough for the device tobe controlled (e.g. a large heater) the switchingcircuit should include an auxiliary relay/interposing relay/contactor that does have sufficientcurrent-carrying capacity.


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Pressure switches have a limited life-time. The measuring elements metal can fatigue andarcing, burning, and chattering will age the electrical contacts.

Users and Manufacturers of pressure switches have standardized on the following pressureswitch terminology:

The adjustable range is the pressure range within which the actuation point (set point) can beadjusted.

The set point is that pressure which actuates the switch to open or close the electrical circuit(depending on how the switch is wired). It may actuate either on increasing or decreasingmeasurement at that point. Set point accuracy defines the ability of a pressure switch tooperate repetitively at its set point.

The dead band or differential is the difference between the set point and the reactivationpoint. If the switch set point and the normal pressure are close together and the dead band isto large then this can prevent the switch from resetting when the pressure returns to normal.

Tolerance is usually referred to as the repeatable accuracy of the reactivation point.

These definitions are shown schematically on Figure 1.

Pressure switches should be selected to allow accurate process setting and capability towithstand maximum upset pressure. The engineer must also ascertain that the pressure switchmeets hysteresis and repeatability requirements. Pressure switch set point should be in themiddle third of the range.

Pressure switches should be DPDT (Double Pole Double Throw) with internally adjustable set

point. A DPDT switch provides more options and flexibility than a SPDT (Single Pole DoubleThrow) switch. Switch contacts should be hermetically sealed and rated for the circuit service oroperated device. For Intrinsically Safe and low current circuits the switch contacts should beplated with a noble metal such as gold or silver to minimize oxidation. Pressure switches shall befurnished with terminal strips. Minimum rating should be NEMA 4 (IP 65) with otherclassifications as required.

It is now becoming more popular to use a transmitter instead of a switch. Transmitters offersadvantages such as wider pressure ranges, easier set point changes, available diagnostics(smart transmitters), failure indication, no need for separate junction boxes and separatemulticores. Standardizing on transmitters also reduces spares inventories.


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Figure 1

Pressure Switch Terminology

7.0 PRESSURE INSTRUMENT INSTALLATION

Pressure measuring devices such as gauges, transmitters and switches must be connected to

and isolated from the process line. For correct measurement of the line pressure it is essentialthat good instrument engineering principles are followed:

Instrument IsolationPressure tapping points on the line should be on the top for horizontal lines and on the side forvertical lines and provided with a suitable piping isolation valve (root valve), generally 1/2" or3/4" is provided. The root valve is supplied by the Piping discipline and it is advisable to agreeat the beginning of a project with the Piping discipline the type and size of connection that isprovided at this valve so that the instrument hook-up matches.

In addition the pressure instrument is fitted with an instrument valve(s) or manifold valve for

isolation, draining or venting.

Installation GeneralWherever possible pressure instruments should be located as close to the tapping point aspossible unless temperature or vibration prohibits this. Direct mounting reduces installationand connections, which are potential sources of leakage. Generally the impulse lines are run


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in tube however for very high pressure (~ANSI 2500#) pipe is popular, particularly if the pipingspecification is a welded specification where threaded connections are prohibited.

When using tube or pipe for instrument hookups it is important to ensure that the thickness ofthe tube and schedule of the pipe is suitable for the design pressures of the piping system.

In gas service always mount the instrument above the tapping point and slope the impulse lineso that it drains back the main process line.

In liquid, steam and condensable vapor service mount the instrument below the tapping pointso that the impulse line is full of the process fluid. In low pressure applications the height ofthe liquid column has to account for in the calibration of the instrument.

With high viscosity liquids use a capillary of short a length as possible.

8.0 WORK PROCESS

8.1 PROCESS DATA

Process data is the responsibility of the Process or Mechanical engineer depending on the officeand business unit. The Instrument/control engineer needs the process data to specify thepressure instrument. Information required from Process/Mechanical includes the normal,maximum, design, alarm set pressures and normal/maximum temperatures.

8.2 MATERIALS OF CONSTRUCTION

Generally element materials are stainless steel as a minimum. The materials of construction for

the element or wetted parts of the pressure instrument is obtained by reference to the projectPiping specification or Material Selection Guide. Exotic materials such as monel and therequirement for NACE (materials for sour service) are selected on the basis of these documents.For hydrogen service special care should be taken in the selection of element materials toprevent permeation, gold plated diaphragms are often used.

8.3 DATASHEETS

Instrument datasheets are required for all pressure instruments. Refer to Design Guide 3DG-J21-00008 for information on datasheets. Datasheets completion generally utilizes a computersystem such as InTools. InSpec or CAIES.

As a minimum the following data is required to specify the basic types of pressure instruments.Refer to the specific instrument datasheets for full details of specification requirements.

Pressure Gauge

Tag number


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Service

Process fluid

Maximum temperature

Maximum pressure

Normal operating pressure

Scale range

Element

Element material

Accuracy

Over-range protection

Dial size

Safety Pattern

Accessories, Diaphragm seal, Snubber, Siphon

Process connection/rating

Pressure Transmitter

Tag number

Service

Process fluid

Maximum temperature

Maximum pressure


Scale range

Body material

Element material

Blind/indicating


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Electrical entry

Enclosure type

IS or explosion proof

Output

Pressure Switch

Tag number

Service

Process fluid

Maximum temperature

Maximum pressure


Scale range

Body material

Element material

Blind/indicating


Electrical entry

Enclosure type

IS or explosion proof

Switch action

Set point high

Set point low

Contact rating


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9.0 REFERENCES

API RP 551 Pressure Measurement Instrumentation, Section 4 Pressure

ISA Industrial Measurement Series - Pressure

Instrument Engineers Handbook Volume 2 Process Control Bela G Liptak

Documents

3DG J34 Pressure Measurement Test