Thermowell Calculation Guide V1.3 (1)

8/12/2019 Thermowell Calculation Guide V1.3 (1)

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ABB GroupMay 28, 2014 | Slide 1

Thermowell Calculation GuideIn accordance with ASME PTC 19.3TW-2010

Andrew Dunbabin March 2012


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Introduction

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

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

The 2010 standard includes significant advances in the knowledge of

thermowell 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 stress


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Failure of a thermowell


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 years

The thermowell was designed to ASME PTC 19.3 1974

The 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 polerippling a flag in the wind


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Frequency Ratio

Vortex shedding causes the

thermowell to vibrate.If this vortex shedding rate (fs)

matches the natural frequency

(fnc) 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


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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


Frequenc

y

Fn

Nominal lock-in

range

In line (drag) excitation Transverse (lift)

Fluid velocity


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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 setto 0.8. This was to avoid the critical resonance caused by the transverse (lift)

forces

The transverse

resonance band isabove the 0.8 limit

Following the inclusion of the in-

line (drag) forces, a secondresonance band may also need to

be avoided


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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 isthe theory used in the

calculation and should

not be estimated

without carrying out the

full evaluation.


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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 mmor 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: themaximum 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 theVon 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 ABBs Wake FrequencyCalculation


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Thermowell Types

STR/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:

Lsand bsare 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 keyinformation

The reason for suitability failure

can be found in the comments

section


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When a Calculation Fails

If 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 althoughthis 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 theonly 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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Thermowell Calculation Guide V1.3 (1)

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