Velocity Ratings of Thermowells

In some cases, well failures are due, not to the effect of pressure, temperature, or corrosion, but to the vibrational effects to which they are subjected. Fluid, flowing by the well, forms a turbulent wake (the Von Karman Trail) with a definite frequency based on well dimensions and fluid velocity. If the natural frequency of the well equals the wake frequency, the well stem will vibrate to destruction and break off in the piping. It is, therefore, important that the well have sufficient design to prevent a frequency equality condition.

In the following tables, a recommended maximum velocity rating can be found for several standard well lengths and materials. To simplify the information, ratings given are based on operating temperatures of 1000° F. for wells made of Carbon Steel (C-1018) and Stainless Steel (304 and 316). Values for brass wells are based on 350° F. Operation limits for Monel wells are based on 900° F service. Slightly higher velocity is possible at lower temperatures. Single values appearing in the velocity tables may be considered safe for water, steam, air or gas. In shorter insertion lengths, consideration is given to the velocity pressure effect of water flowing at higher velocities. The values in parenthesis, therefore, represent safe values for water flow while the unbracketted value may be used for steam, air, gas and similar density fluids.

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