Thermowell Wake Frequency Calculation Guide

(simplified uniform beam): ( I = \frac\pi (d_root^4 - d_bore^4)64 ) (assume bore dia 6 mm → 0.006 m) ( I ≈ 8.9 × 10^-8 , m^4 ) ( m_metal = \rho_m × \textcross-sectional area ≈ 8000 × 7.85e-4 = 6.28 , kg/m ) ( m_added = 1000 × 4.91e-4 = 0.49 , kg/m ) ( m_total = 6.77 , kg/m )

determines the vortex shedding frequency and compares it to the thermowell’s natural frequency to ensure safe operation. 2. Governing Physics 2.1 Strouhal Number (St) The dimensionless Strouhal number relates vortex shedding frequency to flow velocity and thermowell diameter: thermowell wake frequency calculation

If the frequency of these vortices coincides with the natural frequency of the thermowell, occurs. This can cause large-amplitude vibrations, leading to fatigue failure, fracture, and loss of process containment. (simplified uniform beam): ( I = \frac\pi (d_root^4

Industry standards (ASME PTC 19.3 TW) require: [ f_w \le 0.8 f_n \quad \textor \quad f_w \ge 1.2 f_n ] (i.e., at least 20% separation margin) Step 1: Gather Input Data | Parameter | Symbol | Typical Value | |-----------|--------|----------------| | Fluid density | ρ | 1000 kg/m³ (water) | | Fluid velocity | V | 5 m/s | | Tip diameter | d_tip | 0.025 m (1 inch) | | Root diameter | d_root | 0.038 m (1.5 inch) | | Unsupported length | L | 0.15 m | | Thermowell material | - | 316 SS | | Modulus of elasticity | E | 193 GPa | | Material density | ρ_m | 8000 kg/m³ | Step 2: Calculate Reynolds Number [ Re = \frac\rho V d_tip\mu ] (μ = dynamic viscosity; for water at 20°C ≈ 1e-3 Pa·s) This can cause large-amplitude vibrations