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Fixed Beam thermowells 1
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I do not necessarily want to instrument anything at this time. I am preparing a spec. My client wishes all thermowells to be dimensioned in accordance with ASME PTC 19.3. The ASME standard mentions the fixed beam thermowell, which I and everyone else I asked, never heard of. Normally I would use a welded type of thermowell for high steam velocities.
Why do you think the recommendation in the ASME standard is flawed?
Why do you think the recommendation in the ASME standard is flawed?
It is a fixed beam design that extends across a pipe diameter and is supported at each end. it is the worst possible configuration for a temperature measurement, flow induced vibration issues, and thermal stresses.
High velocity designs are available but are rarely needed in the average process application.
PTC 19.3 may be revised, as thermowells satisfying its design criteria have failed. This is apparently the result of it being based on an incomplete and in some respects flawed, stress analysis.
The safest strategy is to use thermowells short enough to avoid flow induced resonance all together.
You might check with the April 2002 issue. There are related articles in Chemical Engineering Mag. and in the Oil & Gas Journal about that time.
At 300 ft/s, accurate temperature measurement is a problem because of aerodynamic heating, what are you trying to measure?
High velocity designs are available but are rarely needed in the average process application.
PTC 19.3 may be revised, as thermowells satisfying its design criteria have failed. This is apparently the result of it being based on an incomplete and in some respects flawed, stress analysis.
The safest strategy is to use thermowells short enough to avoid flow induced resonance all together.
You might check with the April 2002 issue. There are related articles in Chemical Engineering Mag. and in the Oil & Gas Journal about that time.
At 300 ft/s, accurate temperature measurement is a problem because of aerodynamic heating, what are you trying to measure?
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Aerodynamic heating is covered in a number of texts and generally requires empirical correction factors. Before you attempt to incorporate it in any spec you have to define what you mean by accurracy. My experience is that for power plant operations it won't be a concern.
We've covered the velocity ratings with a simple clause linked to PTC 19.3. It is safe for most steam applications but you have to comply with its ratings. Having said that 350 ft/s is a demanding service and you should be considering weld-in thermowells, with full penetration welds and u-dimensions less than an inch or so. To avoid sensor damage associated with the on-set of flow induced resonance you have to reduce the PTC velocity ratings of your well by more than half.
Good luck,
We've covered the velocity ratings with a simple clause linked to PTC 19.3. It is safe for most steam applications but you have to comply with its ratings. Having said that 350 ft/s is a demanding service and you should be considering weld-in thermowells, with full penetration welds and u-dimensions less than an inch or so. To avoid sensor damage associated with the on-set of flow induced resonance you have to reduce the PTC velocity ratings of your well by more than half.
Good luck,
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