Thermowell Calculations for Common Installations

Rigidly Supported, Pipe Mounted, Flanged Nozzle Installations

Modal Analysis of Thermowell and Sensor Acceleration Stresses, and calculation of the Thermal Response Time and Measurement Error (Conduction, Convection, Radiation Effects) with Flow Conditions.

Perfect? No! But the calculation has proven useful in Risk Assessments of existing and planned installations and in Explaining Documented Failures. Traditional US units or S.I. units for user selected fluids and materials of construction.

Not for Amateurs, as the user will need to be familiar with common designs and installations as well as the nature of flow induced vibration and resonance. Ideal for in-plant risk assessments.

Suitable for Windows XP through Windows 10.

The calculations are intended to assist safe thermowell selection or assessment of failure risk and to facilitate design selection, including measurement error and response time. That said, all use is the responsibility of the user.

The calculations have been tested in evaluating actual failed designs, published finite element calculations, published lab, and flow tests. That said, any glitches or errors reported by the user will be addressed and advisory stakement or revised code re-posted as required.

If selected these apps will take into account the drag crisis identified in documented failures, particularly in liquid services. At issue is the onset of nonlinear fluid structure interaction, so the calculations can only represent a worse case estimate for risk assessments. The drag crisis does occur in compressible fluids, but only significant when tip deflections exceed 1%-2% of tip diameter.

Since pipe and flange mounted installations include dynamic ovalization of the pipe wall, you will find that additional modes are introduced. These are due to pipe wall ovalisation and the fact that the support stiffness in the flow direction is greater than that in the transverse direction. This results in extremely complex mode structure especially in the case of flanged nozzle installations. These are identified as lift “L” or drag “D” modes followed by the mode number designation, 1,2,3.

As always, avoidance of the lowest order, severe resonance is recommended. At a minimum a 20% lock-in avoidance of resonance is recommended at the maximum flow conditions.

Acoustic resonances in the piping, often adjacent to large compressors can lead to significant pipe wall deformations, with consequential sensor/thermowell damage, and in extreme cases loss of process containment. Relocation of the measurement is often the only effective option if skin temperature measurements of the pipe wall are not acceptable.

Measurement error and tip response times are practically independent of the thermowell length, as long as the pipe is insulated, even where tip exposure to flow is marginal. Inhomogeneous flows may require multiple probes, not longer ones.

Also note that the tip acceleration often governs thermowell selection since the sensors are commonly rated for tip accelerations 10 G’s for RTDs and 20 G’s for ruggedized thermocouples sensors. At severe levels of flow-induced resonance, tip accelerations can often exceed 200 G’s, that will destroy sensors and even accelerometers used in documented flow tests.

Thermowell Stress Calculations

1. Thermowells per PTC 19.3TW (2016) Designs for Structurally Rigid Support, includes Fluid Properties, Metal Selections, and so on. (Specify P, T, Flows, Pipe Size, etc.). Resonant Conditions verified by published shop tests. The thermal response and measurement error are also calculated. Less than 1 min runs, for three bending modes. Screen display and, if selected a “rtf” report summary is written to the folder where the app is located.

No Charge under Fair Use, on Registration.

“TWRigidSupport.exe”

2. Pipe mounted and rigidly supported thermowells per PTC 19.3TW Designs includes Fluid Properties, Metal Selections, and so on, with or without vortex shedding strakes. Resonant Conditions benchmarked against published shop and finite element tests. The thermal response and measurement error are also calculated. Less than 2 min runs for three bending modes used in the modal analysis. A summary of the calculations is presented on screen and, if selected a “rtf” report summary suitable for saving or printing is written to the folder where the app is located.

No Charge under Fair Use, on Registration.

“TWPipeMounted.exe”

3. Advanced code for flanged, pipe mounted, and rigidly supported thermowells with or without strakes. Shell flexibility is taken into account on the basis of shell theory in conjunction with the 1D discrete element calculations of the thermowell installation.

Bench-marked against finite element codes and actual flow tests test (various publications). Typical runs 5 minutes for three bending modes used in the modal analysis. The thermal response and measurement error are also calculated. Screen display and, if selected a “rtf” report summary suitable for Notepad or WordPad, is written to the folder where the executable is located.

No Charge under Fair Use, on Registration.

“TWFlgMtd.exe”

All apps were developed on the basis of the theory and documented failures in the reference section.

The executable identifed will be sent using a Microsoft OneDrive Link, login not required.

Dave Bartran, Ph.D., P.E.(Ret)

References:

1. “Flow induced vibration of thermowells”, ISA Trans., 38, 1999, pp. 123-132.
2. “Static and dynamic stresses of practical thermowells”, ISA Trans., 39, 2000, pp. 133-142.
3. “Thermowell design and selection”, Hydrocarbon Processing, Nov. 2001, pp. 99-100.
4. “Are your thermowells safe?”, TAPPI Journal, April 2002, pp.10-14.
5. “Thermowell integrity in pipeline services”, Oil & Gas Journal, April 2002, pp. 60-65.
6. “Critical Frequency Estimates for Thermowells Covered Under ASME PTC 19.3,” NIST Internal Report, NISTIR 7407, April 2007.
7. “Natural Frequencies of Plate Supported Thermowells,” (2015) ASME J. Press. Ves. Tech. 137(2), 024502, DOI: 10.1115/1.4028703.
8. “Support Flexibility and Natural Frequencies of Pipe Mounted Thermowells,” (2015) ASME J. Press. Vess. Tech., 137, 041201, DOI:10.1115/1.4028863
9. “The Drag Crisis and Thermowell Design”, (2018) J. Press. Vessel Tech. 140(4), 044501, DOI: 10.1115/1.4039882.
10. “Modal Analysis of Flange Mounted Thermowells”, (2019) ASME J. Press. Vessel Tech. 141(6): 064502, DOI: 10.1115/1.4044602.

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