asme ptc19.3 tw- 2010 thermowell design...

ASME PTC19.3 TW- 2010Thermowell design standard

© ABBMonth DD, YYYY | Slide 1

© ABB GroupJuly 26, 2013 | Slide 2

Introduction to thermowell stress calculation

ASME PTC 19.3 TW-2010 was written to replace ASME PTC 19.3-1974 followingsome catastrophic failures in non-steam service, these thermowells passedthe criteria laid out in 1974.

The 2010 standard includes significant advances in the knowledge ofthermowell behaviour. ASME PTC TW-2010 evaluates thermowell suitabilitynew and improved calculations including:

§ Various thermowell designs including stepped thermowells

§ Thermowell material properties

§ Detailed process information

§ Review of the acceptable limit for frequency ratio

§ Steady-state, dynamic and pressure stresses

Failure of a thermowell


§In 1995 a thermowell failed in the secondary coolant loop ofthe Monju fast breeder reactor in Japan.

§The failure closed the plant for 15 years

§The thermowell was designed to ASME PTC 19.3 1974

§The failure was found to be due to the drag resonance inducedon the thermowell by the liquid sodium coolant


Stresses on a Thermowell

Thermowells protect temperature sensors from direct contact with theprocess media. But once inserted into the process, the thermowell canobstruct flow around it, leading to a drop in pressure. Thisphenomenon creates low pressure vortices downstream of thethermowell.

These vortices occur at one side of thethermowell and then the other, which isknown as alternating vortex shedding.

Thermowell stress location

§ The thermowell is an unsupported beam and as such thestresses concentrate at the root of the stem



Frequency RatioVortex shedding causes the

thermowell to vibrate.

If this vortex shedding rate (fs)matches the natural frequency(fn

c ) of the thermowell, resonanceoccurs, and dynamic bendingstress on the thermowell greatlyincreases

The vortex shedding rate for the drag and lift must be calculated.

Forces created by the media in the Y plane (in-line with flow) are called drag andforces created in the X plane (transverse to flow) are called lift

X

Y

Flow Direction

Frequency Ratio LimitThe frequency ratio (fs / fn

c ) is the ratio between the vortex shedding rate and theinstalled natural frequency. In the old standard, the frequency ratio limit was setto 0.8. This was to avoid the critical resonance caused by the transverse (lift)forces

The transverseresonance band isabove the 0.8 limit

Following the inclusion of the in-line (drag) forces, a secondresonance band may also need tobe avoided

Frequency Ratio Limit

The frequency limit ratiois set at either 0.4 or 0.8.The criteria for whichlimit to use is defined inASME PTC 19.3 TW-2010and the theory issimplified below. This isthe theory used in thecalculation and shouldnot be estimatedwithout carrying out thefull evaluation.

Thermowells; when to perform a calculation§ A thermowell can be considered to

be at negligible risk if the followingcriteria are met:

§ Process media velocity is lessthan 0.64 m/s

§ Wall thickness is 9.55 mm ormore

§ Unsupported length is 610 mmor less

§ Root and tip diameter are 12.7mm or more

§ Maximum allowable stress is69 Mpa or more

§ Fatigue endurance limit is 21Mpa or more

§ For all other conditions it is advisedthat a calculation is performed


Thermowells; Assumptions and limits

§ A number of assumptions are made in theASME standard:

§ Surface finish of the thermowell willbe 32 Ra or better

§ The thermowell is solid drilled

§ There is no welding on the stem ofthe thermowell (other than theattachment to the flange)

§ That the flange rating and attachmentare in compliance with establishedstandards .

§ That the thermowell is within thedimension limits given in the standard

§ That any corrosion or erosion isallowed for


Thermowell; the pass criteria

§ There are four criteria for athermowell to pass evaluation toASME PTC 19.3 TW-2010§ Frequency limit: the resonance

frequency of the thermowell shallbe sufficiently high so thatdestructive oscillations are notexcited by the flow

§ Dynamic stress limit: themaximum primary dynamic stressshall not exceed the allowablefatigue stress limit

§ Static stress limit: the maximumsteady-state stress on thethermowell shall not exceed theallowable stress, determined by theVon Mises criteria

§ Hydrostatic pressure limit: theexternal pressure shall not exceedthe pressure ratings of thethermowell tip, shank and flange

§ All four of the criteria need to beevaluated and all four need to bepassed.


Implications to new projects andexisting assets


New Projects

§ New ASME PTC 19.3 TW-2010 standard is used andcertificates produced

§ The new possibility of having a frequency ratio limit of 0.4means tighter design constraints in a lot of cases

§ We must help to think around the application to provide asolution that satisfies both design standards and end userrequirements


Existing Assets

§ Majority will have been designed to 1974 standard

§ The new 0.4 frequency ratio means a lot of thermowells willnot pass the new standard

§ Re evaluation and re certification services are available

§ Operators will need to consider the implications when anexisting thermowell fails the new calculation

§ If process conditions change, for example increasing thethroughput on a part of plant will increase the flow ratesand this also can be evaluated and reported on


Example of evaluation report

§ Brownfield modification, new process conditions

§ Evaluation of 29 existing thermowells under existing andnew conditions

§ Only 6 passed the new standard under existing conditions!

§ Process limits defined and report given

§ Assistance in designing replacement thermowells© ABBMonth DD, YYYY | Slide 15


ABB’s wake frequency calculation tool


Thermowell TypesSTR/THREAD STR/SW STR/FLG STR/VAN STR/WELD

TAP/THREAD TAP/SW TAP/FLG TAP/VAN TAP/WELD

STEP/THREAD STEP/SW STEP/FLG STEP/VAN STEP/WELD

KEY: STR = STRAIGHT; TAP = TAPERED; STEP = STEPPED

THREAD = THREADED; SW = SOCKET WELD; FLG = FLANGED;

VAN = VAN STONE; WELD = WELD-IN


Dimension Details

Note:

Ls and bs are only applicable for step-shank thermowells


Calculation ReportProject and client details from the

Front Page are shown here

Input data from the Data Entrysheet is pulled through hereincluding the thermowell typeand material details

The calculated results are shownin either Metric or Imperial unitsas selected on the Front Page

Thermowell Suitability is the keyinformation

The reason for suitability failurecan be found in the commentssection


When a Calculation FailsIf a thermowell fails the evaluation, the design can be changed in the

following ways:

• Shorten the thermowell to reduce the unsupported length

• Increase the thickness of the thermowell root and tip

A velocity collar can be considered to reduce the unsupported lengthalthough this is not generally recommended. A velocity collar is used toprovide a rigid support to the thermowell and will work only if there is aninterference fit between the standoff wall and the collar.

Care must be taken to ensure the collar meets the standoff wall atinstallation and is not affected by corrosion. If a velocity collar is theonly viable solution, it is the responsibility of the operator to ensurethere is an interference fit between the standoff wall and the velocitycollar.

Example of Velocity collar

§ Last resort to replace surface measurement devices

§ Supply of thermowell and standoff as pairs

§ ABB inspector to examine post weld to ensure correct dia


Summary

§ The goal of all concerned with thermowells is simple

§ “to provide a safe and reliable product for the application”

§ To achieve this for what appears to be a simple metalcomponent is far from simple

§ The cost of not doing the work can have seriousconsiquences§ Loss of life

§ Loss of assets

§ Loss of production

§ Loss of reputation

§ Loss of liberty

§ The cost of doing it is trivial in comparison


asme ptc19.3 tw- 2010 thermowell design...

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