ABB Group * | Slide *Thermowell Calculation GuideIn accordance with ASME PTC 19.3 TW-2010Andrew Dunbabin March 2012

ABB Group * | Slide *

ABB Group * | Slide *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 thermowellsThermowell material propertiesDetailed process informationReview of the acceptable limit for frequency ratioSteady-state, dynamic and pressure stress

ABB Group * | Slide *

Failure of a thermowell

ABB Group * | Slide *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

ABB Group * | Slide *

ABB Group * | Slide *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 * | Slide *

ABB Group * | Slide *Frequency RatioVortex 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 increasesThe 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

ABB Group * | Slide *

Induced FrequenciesWhere the induced frequency meets the natural frequency of the thermowell the amplitude of vibration increases rapidlyThe 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 frequencyThe drag forces are smaller than the lift forces and under certain special conditions may not be significant.

ABB Group * | Slide *

ABB Group * | Slide *

Resonance lock inBoth lift and drag resonance tends to lock in on the natural frequency The low damping of thermowells exaggerates this effect

ABB Group * | Slide *

ABB Group * | Slide *

Frequency Ratio LimitThe 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 LimitThe 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 locationThe thermowell is an unsupported beam and as such the stresses concentrate at the root of the stem

ABB Group * | Slide *

ABB Group * | Slide *

Thermowells; when to perform a calculationA thermowell can be considered to be at negligible risk if the following criteria are met:Process fluid velocity is less than 0.64 m/sWall thickness is 9.55 mm or moreUnsupported length is 610 mm or lessRoot and tip diameter are 12.7 mm or moreMaximum allowable stress is 69 Mpa or moreFatigue endurance limit is 21 Mpa or moreFor all other conditions it is advised that a calculation is performed

ABB Group * | Slide *

ABB Group * | Slide *

Thermowells; Assumptions and limitsA number of assumptions are made in the standard:Surface finish of the thermowell will be 32 Ra or betterThe thermowell is solid drilledThere 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 * | Slide *

ABB Group * | Slide *

Thermowell; the pass criteriaThere are four criteria for a thermowell to pass evaluation to PTC 19.3 TW-2010Frequency limit: the resonance frequency of the thermowell shall be sufficiently high so that destructive oscillations are not excited by the fluid flowDynamic stress limit: the maximum primary dynamic stress shall not exceed the allowable fatigue stress limitStatic stress limit: the maximum steady-state stress on the thermowell shall not exceed the allowable stress, determined by the Von Mises criteriaHydrostatic pressure limit: the external pressure shall not exceed the pressure ratings of the thermowell tip, shank and flangeAll four of the criteria need to be evaluated and all four need to be passed.

ABB Group * | Slide *

ABB Group * | Slide *

ABB Group * | Slide *Introduction to ABBs Wake Frequency Calculation

ABB Group * | Slide *

ABB Group * | Slide *Thermowell TypesKEY: STR = STRAIGHT; TAP = TAPERED; STEP = STEPPEDTHREAD = THREADED; SW = SOCKET WELD; FLG = FLANGED;VAN = VAN STONE; WELD = WELD-IN

ABB Group * | Slide *

ABB Group * | Slide *Dimension Details

Note:Ls and bs are only applicable for step-shank thermowells

ABB Group * | Slide *

ABB Group * | Slide *Calculation Report Project and client details from the Front Page are shown hereInput data from the Data Entry sheet is pulled through here including the thermowell type and material detailsThe calculated results are shown in either Metric or Imperial units as selected on the Front PageThermowell Suitability is the key informationThe reason for suitability failure can be found in the comments section

ABB Group * | Slide *

ABB Group * | Slide *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.

ABB Group * | Slide *

ABB Group * | Slide *

ABB Group * | Slide *

************