1. Why Impulse Lines Freeze First

Impulse lines (sensing lines) are typically small-bore tubing (1/2" or less). They contain a very small mass of fluid, meaning they have low Thermal Inertia. Unlike a large 10" process pipe which might take days to freeze after flow stops, a 1/2" tube can freeze in minutes.

Furthermore, impulse lines are often "Dead Legs"—there is no flow. The fluid is stagnant. Without flow to replenish heat, the fluid loses energy to the environment until it reaches ambient temperature. If ambient is below freezing, ice plugs form, blocking the pressure signal to the transmitter. This leads to loss of control, false readings, or safety trips.

2. Insulation Delays, Tracing Prevents

Insulation (Lagging): Reduces the rate of heat loss. It buys you time. A well-insulated line might take 4 hours to freeze instead of 15 minutes. However, insulation alone cannot prevent freezing indefinitely if the line is stagnant and ambient is below freezing. Eventually, $T_{fluid} \to T_{ambient}$.

Heat Tracing (Active Heating): Adds energy back into the system to replace what is lost. Electric Heat Tracing (EHT) or Steam Tracing maintains the tube temperature above a setpoint (e.g., 5°C). This is the only way to guarantee protection for stagnant lines in freezing climates.

3. The Wind Chill Factor

Convection heat transfer ($h$) is heavily dependent on air velocity. A 20 mph wind can increase heat loss by 3x to 5x compared to still air. This calculator uses a correlation for flow across a cylinder to estimate the convective coefficient based on wind speed. Always design for the "Worst Case" wind + "Worst Case" temperature.

4. Electric vs. Steam Tracing

• Electric (Self-Regulating): Most common. The cable resistance increases as it gets hot, reducing power output. This prevents overheating (to a degree). Easy to control with thermostats.
• Steam Tracing: Using small copper tubes wrapped around the impulse line. Extremely powerful heat output, often too much. Can boil the fluid in the impulse line ("flashing"), causing measurement errors. Requires steam traps and maintenance. Best for heavy viscous fluids (tar/bitumen) requiring high maintain temps.

5. The Vulnerable Manifold

The 3-valve or 5-valve manifold is a large chunk of metal that acts as a cooling fin. It often freezes before the tubing does. Best Practice: Use a "Winterization Enclosure" (Hot Box) that covers both the transmitter and the manifold, containing the heat from the tracing.