Related Resources: Fluids Hydraulics Engineering

Flow Induced Vibration of Thermowells

Fluids Engineering and Design

Flow Induced Vibration of Thermowells
David S. Bartran
Jerry M. Kinseya
Robert Schappelle
Richard Yee
63 Pages

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Preface

The ASME Power Test Code, PTC 19.3 (October 25, 2000) currently establishes the design criteria for thermowells in steam service and is widely used as the starting point for many thermowell designs. It is the basis of the published ratings shown in Fig. 1. The code model is based on a quasi-static analysis of a prismatic beam and includes contributions due to hydrostatic pressure, fluid drag, and vortex shedding. While this model is readily modified to include other fluids and even mechanical damping, it is formally restricted to sub-critical, i.e. subresonant,conditions. This is an essential restriction in high pressure steam service, but it leads to the application difficulties described for long thermowells.

To better understand the design issues involved in relaxing the requirement for sub-critical operation and improving the rating method for practical thermowells, the simplest possible thermowell model is considered. This model which permits an exact solution and demonstrates how resonance(s) evolve with flow. It also removes the mystery of thermowell stress, at least for the practicing instrument engineer, and serves as a benchmark for approximate methods.

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