ASME PTC19.3 TW- 2010 Thermowell design standard © ABB Month DD, YYYY | Slide 1
Introduction to thermowell stress calculation 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
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Thermowell material properties
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Detailed process information
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Review of the acceptable limit for frequency ratio
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Steady-state, dynamic and pressure stresses
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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 the process media. 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.
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Thermowell stress location §
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The thermowell is an unsupported beam and as such the stresses concentrate at the root of the stem
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 media 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.
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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 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.
Thermowells; when to perform a calculation §
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A thermowell can be considered to be at negligible risk if the following criteria are met: §
Process media velocity is less than 0.64 m/s
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Wall thickness is 9.55 mm or more
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Unsupported length is 610 mm or less
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Root and tip diameter are 12.7 mm or more
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Maximum allowable stress is 69 Mpa or more
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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 ASME standard: §
Surface finish of the thermowell will be 32 Ra or better
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The thermowell is solid drilled
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There is no welding on the stem of the thermowell (other than the attachment to the flange)
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That the flange rating and attachment are in compliance with established standards .
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That the thermowell is within the dimension limits given in the standard
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That any corrosion or erosion is allowed for
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Thermowell; the pass criteria §
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There are four criteria for a thermowell to pass evaluation to ASME 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 flow
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Dynamic stress limit: the maximum primary dynamic stress shall not exceed the allowable fatigue stress limit
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Static stress limit: the maximum steady-state stress on the thermowell shall not exceed the allowable stress, determined by the Von Mises criteria
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Hydrostatic pressure limit: the external pressure shall not exceed the pressure ratings of the thermowell tip, shank and flange
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All four of the criteria need to be evaluated and all four need to be passed.
Implications to new projects and existing assets © ABB Month DD, YYYY | Slide 12
New Projects
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New ASME PTC 19.3 TW-2010 standard is used and certificates produced
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The new possibility of having a frequency ratio limit of 0.4 means tighter design constraints in a lot of cases
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We must help to think around the application to provide a solution that satisfies both design standards and end user requirements
Existing Assets
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Majority will have been designed to 1974 standard
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The new 0.4 frequency ratio means a lot of thermowells will not pass the new standard
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Re evaluation and re certification services are available
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Operators will need to consider the implications when an existing thermowell fails the new calculation
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If process conditions change, for example increasing the throughput on a part of plant will increase the flow rates and this also can be evaluated and reported on
Example of evaluation report
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Brownfield modification, new process conditions
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Evaluation of 29 existing thermowells under existing and new conditions
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Only 6 passed the new standard under existing conditions!
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Process limits defined and report given
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Assistance in designing replacement thermowells
ABB’s wake frequency calculation tool
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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; © ABB Group July 26, 2013 | Slide 17
VAN = VAN STONE; WELD = WELD-IN
Dimension Details
Note: Ls and bs are only applicable for step-shank thermowells © ABB Group July 26, 2013 | Slide 18
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 July 26, 2013 | Slide 19
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
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Increase the thickness of the thermowell root and tip A velocity collar can be considered 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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Example of Velocity collar
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Last resort to replace surface measurement devices
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Supply of thermowell and standoff as pairs
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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”
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To achieve this for what appears to be a simple metal component is far from simple
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The cost of not doing the work can have serious consiquences
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Loss of life
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Loss of assets
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Loss of production
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Loss of reputation
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Loss of liberty
The cost of doing it is trivial in comparison
