thermowell selection and application notes

7/28/2019 Thermowell Selection and Application Notes

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Burns Engineering Thermowell Selection and Appl ication

Thermowell Selection and Application

If you are experiencing audio problemsplease call the teleconference number below

1-408-600-3600Access code 660 637 935

Bill Bergquist, Sr. Applications EngineerJ eff Wigen, National Account Manager J effBill

2


The thermowell used with your temperature sensor provides criticalprotection, access, and measurement integrity. Well shed some lighton how, why, and when to apply this important device.

Thermowell basics Purpose Types Process connection styles Materials, coatings and surface finish

Performance Wake frequency & strength Immersion length Time response

Custom designs Specifying

Definitions/codes Typical drawing Details Testing & certifications Options

Preventive Maintenance Selection Checklist

What Well Cover

3


Why use a thermowell?

Protect the temperature sensor from:

Excessive pressures

Excessive material velocities & viscosities

Corrosion

Erosion

Simplify maintenance

Sensor replacement without draining the system Eliminate potential for contamination system stays closed

Protection increases potential life of the sensor

Improve temperature measurements

Thermowell Basics - Purpose

Athermowellsmainpurposeistoprotectthe

temperaturesensortoinsurelonglifeand

accuratemeasurement.Maintenanceisaidedby

allowingthesensortoberemovedwithout

havingtodrainthepipeortank. Sensorchanges

canbedoneonthefly.


2/17

4


Thermowell types

Stepped

Tapered

Straight

Sanitary

Protection tube

Typically made from pipe

Usually not suited for RTDs

Thermowell Basics - Types

Thermowelltypesaredesignatedbythestyleof

stemorwettedportionofthewell. Protection

tubesaretypicallyusedforthermocouplesand

arenotsuitedforusewithRTDsbecausetheydo

notprovideenoughsupportforthesensor.

Mostthermowellsaremachinedfrombarstock

toinsure

integrity

for

high

pressure

applications.

Flangesarejoinedtoathermowellstemmost

commonlybyawelding process. Smallerwells

maybeconstructedfromtubingandhavethe

endweldedclosedandaprocessconnection

weldedon. Thesearetypicallyusedinlow

pressureapplications.

5


Thermowell Basics - Stem Types

Stepped

Tapered

Straight

Stepped,sometimesreferredtoasreducedtip,

wellsareusuallydiameterandstepdownto

diameter

to

improve

time

response.

Tapered

wellsareabalancebetweenstrengthandtime

response. Straightwellsarespecifiedwhere

increasedlifeisdesiredduetocorrosionor

erosion.

6


Connection styles

Flange ANSI B16.5

Van Stone

Socket weld

Weld-in

Threaded Sanitary flange

O-Ring

Thermowell Basics - Process Connections

Connectionstotheprocessareaccomplished

withtaperedpipethreadsoradaptationsofpipe

orsanitarytubingflangeconnections. Ablankor

blindflangeismodifiedtoacceptthethermowell

stemandtheneasilyattachestostandard

flanges.

Weldinstylesareusedforconvenienceorwhere

exposedthreadsmaycollectcontaminantssuch

asinfoodprocessingorpharmaceutical

production.

AspecialtytypeofconnectionincorporatesanO

ringtosealinsideasleeveweldedtoatank.

SometimesreferredtoasanIngoldfittingafter

thecompanythatinventeditforphandoxygen

sensors.


3/17

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Thesearejustafewoftheseveralstyles

available. Anyimaginablepipeortubing

connectionstyleeitherhasbeenorcouldbe

adaptedtoathermowellprocessconnection.

8


Sanitary Weld-In

Asanitaryweldinstylesuchasthisoneisusedin

sanitaryprocesstankswherecracksandcrevices

arenot

desired.

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Sanitary Flange

Thisisathermowellwithasanitaryflange

connectionwhichissometimesreferredtoasa

TriClampconnector. Itisthemostcommon

styleofsanitaryconnection. Thehighpolishand

largeradiusbetweenstemandflangeimprove

corrosionresistanceandmakecleaningeasier.


4/17

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Sanitary with Process & Instrument Flange

Anotherstyleofsanitarywellusesasanitary

flangeforattachmentofthetemperaturesensor.

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Sanitary Process and Instrument Flange

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Sanitary Easy Access Tri-Clamp

Notoolsarerequiredtoinstallorremovethe

sensorwhichmakesforeasiermaintenance.


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13


O-Ring

Themostcommonstyleofinstrument

connectionisaNPTfemalethreadasshown

hereonthisOringstylewell. Thisstylewell

installsintoasleeveweldedtoatankwhich

acceptsthewellandthethreadednutholdsitin

place. AtightfitinsuresthattheOringseals

securely.

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Process connection selection

Thread size Typically tapered pipe thread

Flange size and rating Gasket surface type

Raised

Flat

RTJ

Lag length Added thermowell length outside process to reach through

insulation

Brass or SS plug for test wells


Whenselectingaprocessconnectionthereare

severalthingstoconsider. Threadedconnections

arethe

easiest,

flange

connection

require

a

flangesize,pressurerating,andgasketsurface

style.

Laglengthisanextensionofthenonwetted

portionofthewelltoreachthroughinsulationor

otherobstaclestoprovidewrenchaccess.

Typicallyspecifiedforthreadedwells.

Testwellsthatsitemptyforlongperiodsoftime

shouldhaveaplugintheinstrumentconnection

topreventdebrisfromentering.

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Factors to consider

Define the conditions Corrosion/erosion

Temperature

Pressure

Fluid Properties

Match the pipe or tank materials Refer to corrosion charts

Materials

Materialselectioncanbeassimpleaschoosing

thesamematerialasthetankorpipewhereitis

tobeinstalled. Thisworksmostofthetimebut

thereareexceptions. Inthecaseofcorrosionor

erosionanobjectplacedintoaflowismore

susceptibletothoseeffectsandadifferent

materialwillbenecessarytoprovidelongerlife.

Therearecorrosionchartsavailablefrom

thermowellmanufacturersthatlistthepreferred

materialforchemicalsatdifferentconditions.


6/17

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Materials Corrosion

Condition MaterialChemical

Materialselectiontablesareavailablefor

numerouschemicalsandconditions. Notethat

concentrationandtemperaturehavean

influenceontherecommendedmaterial. The

higherthetemperatureand/orconcentrationthe

morecorrosivethechemicalcanbe.

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Corrosion

Photo courtesy of Alloy Engineeringhttp://www.thermowells.com/library.html

Choosingthewrongmaterialcanresultinfailure

ofthewell. Herethetiphascorrodedawayand

allowedthe

process

fluid

to

leak

inside

the

well.

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Corrosion


Thisisaviewinsidethetipsectionshowing

corrosionontheinsidefromtheleakingprocess

fluid.


7/17

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Materials

Cobalt-Chromium (Stellite) Cobalt-Nickel (Wallex) Tungsten Carbide

Silicon Carbide (Hexoloy) Alumina

Materials - Erosion

Photo courtesy of LisaDanielson

Particlessuspendedinafluidorpowdered

materialscancauseerosionofthewellover

time. Thereareseveralmaterialsavailableto

slowdowntheerosion.Mostarehardmaterials

thatareappliedthroughaweldingorspraying

process. Sometimesjustpickingthecorrect

thermowellmaterial

is

sufficient

to

minimize

the

erosionaffects.

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Materials Metals

Plastic

Ceramic

Surface coatings Teflon

Tantalum

Ceramic

Hard facing

Surface finish Improved corrosion resistance

Sanitary applications Electropolish

Passivation

Materials - Selection

Avarietyofmaterialscanbeusedtoconstruct

thermowells. Surfacecoatingscanbeusedto

improvecorrosion

and

erosion

resistance.

PlasticssuchasTeflonarehighlycorrosion

resistantand ceramicsofferhightemperature

capabilitiesandcanbeagoodchoiceforerosive

applications.

Tantalumisusedforacidresistanceandcanbe

appliedasasleeveoritcanbebondedtothe

surfaceofnearlyanythermowellstyle.

Improvingthesurfacefinishwillincrease

corrosionresistanceandmakeiteasiertoclean.

Fewerridges

and

crevices

means

asmaller

surfaceareaforthecorrodenttoattack.

Electropolishingorpassivatingthesurface

furtherincreasescorrosionresistance.

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Materials

Sometimeslessexpensivematerialsturnoutto

bethebestchoice. OriginallyaTeflonplated

metalwellwasusedformonitoringthe

temperatureofanacidicwasteproduct. Teflon

platingiseasilydamaged(thinkofyourfrying

pan)and

the

acid

quickly

ate

away

the

metal

well

causingittoleak. Thesolutionwastouse

standard3PVCpipetoconstructthewellwhich

wasabout10ftlongandattachedwithaflange

connection. Aninternalwellwasmadewith

PVCtohousethetemperaturesensorallowingit

tostickoutacoupleinchesforbettertime

response.


8/17

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Positive Material Identification (PMI)

PositiveMaterialIdentification(PMI)referstothe

identificationandanalysisofvariousmetalalloys

basedontheirchemicalcompositionin

nondestructivetesting(NDT).Measurement

resultsareshownintheformofelemental

concentrationinpercentageorbyspecificalloy

namesuch

as

SS316L

or

Inconel

625.

PMI

is

afieldtestingmethodmadepossiblebythe

portabilityofmostPMIanalyzers.These

instrumentsalsocanbeusedinthelaboratory

TheXRayFluorescenceTechniqueXRF

XRFinstrumentsworkbyexposingasampletoa

beamofXrays.Theatomsofthesampleabsorb

energyfromtheXrays,becometemporarily

excitedandthenemitsecondaryXrays.Each

chemicalelementemitsXraysataunique

energy.Bymeasuringintensityandcharacteristic

energyof

the

emitted

Xrays,

an

XRF

analyzer

can

providequalitativeandquantitativeanalysis

regardingthecompositionofthematerialbeing

tested.

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Measurement accuracy

Wake frequency and strength calculations ASME PTC 19.3 TW-2010

Immersion length Elbow well

Time response Heat transfer paste

Bore and probe diameter

Performance

Measurementaccuracycanbeadverselyaffected

byathermowellunlessafewprecautionsare

taken.Wakefrequencyandstrengthcalculations

areprobably

the

most

important.

These

calculationswilltellyouhowlongthewellcanbe

immersedintoyourprocessbasedontheflow

conditions(moreonthislater). Thishastobe

balancedwiththeaccuracyneedsofthesensor

tobeimmersedsufficientlytopreventstem

conductionorimmersionerror.

Finally, timeresponseisslowerwiththeaddition

ofathermowellandforprocessesthatchange

temperaturerapidlythiscanbeasignificanterror

source.


9/17

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ASME PTC 19.3 TW - 2010

The long awaited PTC 19.3 TW-2010 is a completely new standard that establishes

the practical design considerations for thermowell installations in power and

process piping. This code is an expanded version of the thermowell section

contained in the PTC 19.3-1974, and incorporates the latest theory in the areas

of natural frequency, Strouhal frequency, in-line resonance and stress evaluation.

ASME responded to changing industry demands for a more comprehensive setof thermowell evaluations. Key enhancements over the 1974 edition include:

Expanded coverage for thermowell geometry

Natural frequency correction factors for mounting compliance,

added fluid mass, and sensor mass

Consideration for partial shielding from flow

Intrinsic thermowell damping;

Steady state and dynamic stress evaluations

Improved allowable fatigue limit definition.

ASME PTC 19.3 TW-2010

Excerpted from ASME website:http://staging.files.asme.org/Catalog/Codes/PrintBook/25750.pdf

Thenewreleaseofthethermowellsectionofthe

PowerTestCodeincorporatethelatesttheoryfor

wakefrequencyandstrengthcalculations. If

youreapipingdesignerallthisprobablymeansa

lottoyou. Forsomeoneselectingathermowell

itmaybealittlelikelearninghowtobuilda

chainsawto

cut

down

atree.

Interesting

but

not

reallynecessary. Yourthermowellsupplierwill

providethecalculationsbasedyourprocess

conditionsanddesiredthermowellconfiguration.

Youneedtoknowthatyouwantthecalculations

donetothenew2010versionofthestandard.

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Von Krmn vortex street, by [http://www.mcef.ep.usp.br/staff/jmeneg/cesareo/Cesareo_HomePage.htmlCesareo de La Rosa Siqueira

Von Krmn Vortex

Whenacylindricalobjectisplacedinaflowthere

isadisturbancecreatedonthedownstreamside

thatoscillates

back

and

forth.

The

frequency

of

theoscillationincreasesastheflowrate

increases. Iftheresonantfrequencyofthe

thermowellmatchesthefrequencyofthe

vorticesthewellwillstarttovibrateandwillfirst

damagetheRTDandthenthewellwillfail.

Therearealotofgoodillustrationsofthis

phenomenaontheinternet. Asearchforvon

karmanorwakefrequencywillreturnseveral.

26


Thermowell Selection

Thisgraphshowstheacceptableflowratesfor

variousthermowelldesignsandadiameter

directimmersionRTD. Notethatashortincrease

inimmersionlengthhasalargeaffectonthe

allowableflowrate.


10/17

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Information Required

Thisistypicaloftheinformationthatisneeded

toperformthecalculations.

28


Stress Crack


Anexampleofwhatcanhappenifthewake

frequencyandstrengthcalculationsarenot

done.This

well

incorporates

asupport

part

way

downthestemtoprovidesupportbutitwasnt

enough. Acrackformedasoutlinedaroundthe

stem.

ASMEPTC19.3doesnotrecommendtheuseof

supportssuchasthis. Shorterimmersionor

largerdiameterarethepreferredsolutions.

29


Stress Crack


Alittlebetterviewofthecrackthatformed.


11/17

30


Immersion Length

25C

Direct immersion RTDs require 10 x probe diameter plus sensitive length toavoid stem conduction errors. Thermowells require about 4.5.

200C

Aruleofthumbforminimumimmersionlength

toavoidstemconductionerroris10timesthe

probediameterplusthesensitivelengthofthe

probe.MostRTDshaveasensitivelengthof

approximately1.0. SoforadiameterRTDthe

minimumrecommendedimmersionis3.5.

Thermowellsrequire

about

4.5

as

aminimum

forbestaccuracy.

31


Immersion Error for 200L05ANN135TT302

-5.00

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

0 2 4 6 8 10 12

Immersion Depth (Inches)

IndicatedError(%D

eltaT)

Ice Bath

100C OilB ath

200C OilB ath

Immersion Error

Athermowell/RTDassemblywasimmersedina

bathtodeterminethestemconductionat

variousdepths.

At

4.5

inches

most

of

the

error

hasdisappeared. Asyoucanseetheerroris

mostlyindependentofthebathtemperature

used. Theerrorisexpressedasapercentageof

thedeltaTbetweenambientandprocess

conditions.

32


Specialty Elbow Wells

Whenlinesizedoesntallowforalongenough

immersionlengthaspecialtythermowellsuchas

thiscanbeused. Notetheflarearoundthe

internalthermowellwhichavoidsflow

restriction.


12/17

33


Elbow Well Drawing

Forthoseapplicationsthatrequireathermowell,

theelbowstyleworkswellifsizedcorrectly. An

immersionlengthofatleast4formost

applicationsisnecessarytoprovideanaccurate

measurement. Shorterimmersionlengthsmay

workforsomeprocessfluidsandflowrates.

34


Installation in a Tee

Fluid

SENSOR INSTALLEDPERPENDICULAR TO FLOW

SENSOR INSTALLEDPARALLEL TO FLOW

SENSOR INSTALLEDPARALLEL TO FLOW

Good

Better

Best

Insmalldiameterlinesthereistypicallynot

enoughroomtogettheproperimmersionlength

toavoid

stem

conduction.

This

illustrates

three

methodsofmodifyingthepipingtoprovideextra

room. Theoneontherightisthebestmethod

becauseitprovidesfortheflowcomingdirectly

atthetipofthesensorforlessstressonthe

probeanditalsowashesoutthedeadlagportion

ofthetee.

35


Anti-Immersion?


Anotherinstallationmethodwhichwedont

recommend. Theresprobablyajokebehindthis

suchasitsforroomairtemperatureorit

makesanicehandhold. Likelyjustthe2nd

shift

crewhaving funwiththe1stshiftcrew. Inany

case,aremindertoalwaysdoublecheckyour

installation.


13/17

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Performance - Time Constant

Adding heat transferpaste cuts thesetimes nearly in half.

TimeConstantisdefinedasthetimeittakesfor

thesensortoindicate63.2%ofastepchangein

temperatureinwatermovingat3fps.

37


Specialty Fast Response Tip

Time constant is about 4 seconds

Afewspecialtywellssuchasthisonecan

improvethetimeconstantsignificantlywitha

smalldiameter

tip,

heat

transfer

paste,

and

a

specialdesignRTD.

38


Specialty Flow-Through Design

Thiswellprovidesprotectiontothesensorbut

allowsflowintothesensorforimprovedtime

response.


14/17

39


Specialty Large Diameter Elbow

A3tubesanitaryelbowthermowellisshown

hereandrevealstheinternalthermowellwhichis

3/8diameterandhasa.260borediameter.

40


Process connection P Immersion length U Lag length T Bore depth A usually overall length minus Tip diameter D Root diameter Q Stem style Instrument connection Material

Coatings

Bore diameter Surface finish Plug Testing Certifications

Specifying - Definitions/Codes

Standardvariablelettersareusedtodescribe

thermowelldimensions.

41


Specifying Typical Drawing

OfthesetheimmersionlengthUandthebore

depthAaremostcommonlyusedforspecifying

thelengthofthesensor.


15/17

42


Thread size

1/2 , 3/4, or 1 NPT

Socket weld

3/4 refers to O.D. of connection based on pipe size.

3/4 =1.05 OD of fitting

Flange 2.0 x 150 LB or class 150

1.5 x 300 LB

Sanitary flange

1.5 , 2, 2.5 etc.

Flange size refers to the tube size it is used to connect andnot the actual flange diameter. For example: 1.5 sanitaryflange measures 2.0 diameter

Specifying Some Need to Know Details

Someknowledgeofthevarioustypesofprocess

connectionsisrequiredtochoosethecorrect

size. ThreadedconnectionsareNPTandfairly

straightforward. Flangeconnectionsizesreferto

thesizepipeortubewhichtheyconnect.

43


A few of the available certifications: Hydrostatic internal or external pressure testing per ASTM Dye penetrant testing per ASTM X-ray examination per ASTM Material certification Wake frequency and strength calculations Mechanical Surface Finish Certification Canadian Registration Number (CRN) Material Certificate of Conformance No ADM (Animal Derived Material) Certification that no polishing compound was used Certification of NACE Approval Electropolishand Surface Finish Certification Positive material identification (PMI) Inspection Certificate (ISO 10474, EN 10204, DIN 50049) 3.1B Passivation Weld Map - location & numerical identification of every weld

Specifying - Testing and Certifications

Thesearejustafewofthepossibleteststhatcan

beperformedonathermowell. Ofthese,the

mostcommon

are

material

certification,

wake

frequencycalculations,andpressuretesting.

44


Some available options: Plug and chain

Marked with tag number (customer specific information)

Open ended thermowell

1/8 Diameter weep hole

Full weld penetration on flanged well, includes certification

British Standard Pipe process threads (BSP)

Deep Tap threads (for explosion proof applications)

Velocity collar (not recommended per the ASME standard)

Coatings for corrosion or erosion resistance

Flange facing options

Cleaned for oxygen service

Root and tip Q and D dimension options

Specifying - Options


16/17

45


Preventive maintenance

Thermowell

; Bore cleaning

; Heat transfer compound

; Product buildup on wetted portion

; Cracks in flange weld or leaky gasket

; RTD bottoms in well and spring loads

An ounce of prevention

Thermowellstypicallyneedonlyminimal

maintenance,keepingthemcleaninsideandout

isusuallyallthatisrequired. Heattransferpaste

shouldberenewedifthesensorisremovedfor

calibrationorreplacement.

46


Thermowell selection

Material selection

Corrosion

Erosion

Strength

Process connection style

Flange

Threaded

Welded

Sanitary

Stem style

Wake frequency and strength calculations

Immersion length

Surface finish

Time response

Thermowell Selection Checklist

47


Thermowell basics Purpose Types Process connection styles Materials, coatings and surface finish

Performance Wake frequency & strength Immersion length Time response

Custom designs

Specifying Definitions/codes Typical drawing Details Testing & certifications Options

Preventive MaintenanceSelection Checklist

Summary


17/17


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