Andrew Dunbabin March 2012
Thermowell Calculation Guide
In accordance with ASME PTC 19.3
TW-2010
© ABB Group
April 8, 2015 | Slide 1
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 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
© ABB Group
April 8, 2015 | Slide 2
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
© ABB Group
April 8, 2015 | Slide 3
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
© ABB Group
April 8, 2015 | Slide 4
Frequency Ratio
X
Vortex shedding causes the
thermowell to vibrate.
Y
Flow Direction
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
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
The vortex shedding rate for the drag and lift must be calculated. The in-line frequency
(parallel to flow) is 2x the transverse frequency.
© ABB Group
April 8, 2015 | Slide 5
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.
© ABB Group
April 8, 2015 | Slide 6
Frequency
Resonance “lock in”

Both lift and drag resonance tends to “lock in” on the
natural frequency

The low damping of thermowells exaggerates this effect
In line (drag) excitation
Transverse (lift)
Fn
Nominal lock-in
range
Fluid velocity
© ABB Group
April 8, 2015 | Slide 7
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
Following the inclusion of the inline (drag) forces, a second
resonance band may also need to
be avoided
The transverse
resonance band is
above the 0.8 limit
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

© ABB Group
April 8, 2015 | Slide 10
The thermowell is an unsupported beam and as such the
stresses concentrate at the root of the stem
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
© ABB Group
April 8, 2015 | Slide 11
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
© ABB Group
April 8, 2015 | Slide 12
Thermowell; the pass criteria

© ABB Group
April 8, 2015 | Slide 13
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.
Introduction to ABB’s Wake Frequency
Calculation
© ABB Group
April 8, 2015 | Slide 14
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;
© ABB Group
April 8, 2015 | Slide 15
VAN = VAN STONE; WELD = WELD-IN
Dimension Details
Note:
Ls and bs are only applicable for step-shank thermowells
© ABB Group
April 8, 2015 | Slide 16
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
© ABB Group
April 8, 2015 | Slide 17
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 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.
© ABB Group
April 8, 2015 | Slide 18
© ABB Group
April 8, 2015 | Slide 19