Heat Tracing for Impulse Lines
Impulse lines are the small-bore tubes connecting a process tap to a transmitter. They are the classic weak point of winter operation: short, uninsulated, often ignored, and capable of freezing in a way that produces a wrong reading rather than an obvious alarm.
Why impulse lines freeze first
- Small diameter → small thermal mass → cools quickly.
- Often partly uninsulated at the tap, transmitter and manifold.
- Dead-legs: no flow, so no heat carried from the process.
- Located high or exposed, where wind accelerates cooling.
- Condensate can collect at low points and freeze solid.
The failure mode is worse than a frozen pipe
A frozen impulse line does not always fail loudly. It can produce a wrong but plausible reading — the transmitter sees a trapped pressure that does not follow the process. Operators make decisions on wrong data, or the control loop misbehaves. That is why impulse-line tracing belongs to instrument integrity, not just to winter maintenance.
Design guidance
1. Trace the whole path, not just the tube
Include the process tap, root valve, impulse tube, manifold and transmitter housing in the thermal envelope. A traced tube entering an unheated transmitter is still a cold end.
2. Insulation is part of the system
Impulse lines need insulation as much as the tube needs heat. Insulation boxes around transmitters, with heaters where required, are usually the practical solution.
3. Prefer self-regulating cable for short runs
Short circuits with many bends suit self-regulating cable: it can be cut to length on site and tolerates tighter bends than MI cable.
4. Mind the temperature limit
Instrument lines may have temperature limits far below what the process could tolerate — check the transmitter and the tubing material, not just the process fluid.
5. Slope and drain
Where condensate can collect, slope the line and provide a drain. Tracing cannot fix a low point that fills with liquid.
6. Steam versus electric
Steam tracing on impulse lines often causes overheating and burnt instrument lines; electric tracing with a thermostat is easier to control. Conversion is common but requires checking the existing thermal design.
Checklist
| Item | Confirm |
|---|---|
| Scope | Tap, valve, tube, manifold, transmitter |
| Temperature limits | Tubing material and transmitter rating |
| Insulation | Boxes and line insulation complete |
| Cable | Self-regulating, correct voltage, cut to length |
| Control | Thermostat set point, alarms |
| Drainage | Slope and low-point drains |
| Documentation | Circuit schedule including impulse lines |
Common mistakes
- Tracing the tube but leaving the transmitter housing cold.
- No insulation on the tap or manifold.
- Steam tracing left uncontrolled, overheating the instrument line.
- Low points without drains.
- Impulse lines missing from the tracing circuit schedule, so nobody maintains them.
Instrument freeze protection?
Send the impulse line and transmitter details. We specify tracing and insulation as one system.
Request a Quote