© ABB Group May 3, 2023 | Slide 1

Thermowell Calculation GuideIn accordance with ASME PTC 19.3 TW-2010

Andrew Dunbabin March 2012

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IntroductionASME PTC 19.3 TW-2010 was written to replace ASME PTC 19.3-1974

following some catastrophic failures in non-steam service, these thermowells passed the criteria laid out in 1974.

The 2010 standard includes significant advances in the knowledge of thermowell behaviour. ASME PTC TW-2010 evaluates thermowell suitability new 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 stress

Failure of a thermowell

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In 1995 a thermowell failed in the secondary coolant loop of the Monju fast breeder reactor in Japan.The failure closed the plant for 15 yearsThe thermowell was designed to ASME PTC 19.3 1974The failure was found to be due to the drag resonance induced on the thermowell by the liquid sodium coolant

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Stresses on a Thermowell

Thermowells protect temperature sensors from direct contact with a process fluid. But once inserted into the process, the thermowell can obstruct flow around it, leading to a drop in pressure. This phenomenon creates low pressure vortices downstream of the thermowell.

These vortices occur at one side of the thermowell and then the other, which is known as alternating vortex shedding. This effect can be seen in the example of a flag pole rippling a flag in the wind

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Frequency RatioVortex shedding causes the

thermowell to vibrate.

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

c ) of the thermowell, resonance occurs, and dynamic bending stress on the thermowell greatly increases

The vortex shedding rate for the drag and lift must be calculated. The in-line frequency (parallel to flow) is 2x the transverse frequency.

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

X

Y

Flow Direction

Induced Frequencies

Where the induced frequency meets the natural frequency of the thermowell the amplitude of vibration increases rapidly

The drag frequency induced is twice that of the lift frequency induced. As such it meets the natural frequency of the thermowell at half the fluid velocity of the lift induced frequency

The drag forces are smaller than the lift forces and under certain special conditions may not be significant.

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Resonance “lock in”

Both lift and drag resonance tends to “lock in” on the natural frequency

The low damping of thermowells exaggerates this effect

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Fn

Nominal lock-in range

In line (drag) excitation Transverse (lift)

Fluid velocity

Frequency Ratio Limit

The frequency ratio (fs / fnc ) is the ratio between the vortex shedding rate and the

installed natural frequency. In the old standard, the frequency ratio limit was set to 0.8. This was to avoid the critical resonance caused by the transverse (lift) forces

The transverse resonance band is above the 0.8 limit

Following the inclusion of the in-line (drag) forces, a second resonance band may also need to be avoided

Frequency Ratio Limit

The frequency limit ratio is set at either 0.4 or 0.8. The criteria for which limit to use is defined in ASME PTC 19.3 TW-2010 and the theory is simplified below. This is the theory used in the calculation and should not be estimated without carrying out the full evaluation.

Thermowell stress location

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

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

be at negligible risk if the following criteria are met:

Process fluid velocity is less than 0.64 m/s

Wall thickness is 9.55 mm or more

Unsupported length is 610 mm or less

Root and tip diameter are 12.7 mm or more

Maximum allowable stress is 69 Mpa or more

Fatigue endurance limit is 21 Mpa or more

For all other conditions it is advised that a calculation is performed

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Thermowells; Assumptions and limits

A number of assumptions are made in the standard:

Surface finish of the thermowell will be 32 Ra or better

The thermowell is solid drilled There is no welding on the stem of the

thermowell (other than the attachment to the flange)

That the flange rating and attachment are in compliance with established standards .

That the thermowell is within the dimension limits given in the standard (table 4-1-1 and 4-2-1)

That any corrosion or erosion is allowed for

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Thermowell; the pass criteria

There are four criteria for a thermowell to pass evaluation to PTC 19.3 TW-2010

Frequency limit: the resonance frequency of the thermowell shall be sufficiently high so that destructive oscillations are not excited by the fluid flow

Dynamic stress limit: the maximum primary dynamic stress shall not exceed the allowable fatigue stress limit

Static stress limit: the maximum steady-state stress on the thermowell shall not exceed the allowable stress, determined by the Von Mises criteria

Hydrostatic pressure limit: the external pressure shall not exceed the pressure ratings of the thermowell tip, shank and flange

All four of the criteria need to be evaluated and all four need to be passed.

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Introduction to ABB’s Wake Frequency Calculation

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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

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Dimension Details

Note:

Ls and bs are only applicable for step-shank thermowells

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Calculation Report Project and client details from the

Front Page are shown here

Input data from the Data Entry sheet is pulled through here including the thermowell type and material details

The calculated results are shown in either Metric or Imperial units as selected on the Front Page

Thermowell Suitability is the key information

The reason for suitability failure can be found in the comments section

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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 (A and B)

A velocity collar can be added to reduce the unsupported length although this is not generally recommended. A velocity collar is used to provide a rigid support to the thermowell and will work only if there is an interference fit between the standoff wall and the collar.

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

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