Introduction to heat trace
Heat tracing is the generic term which refers to the application of heat to a pipeline or vessel to prevent freezing, to thaw frozen fluid, to maintain viscosity or temperature of a fluid, or for other reasons such as to keep components from separating or gas from condensing. The term originated with the placement of steam lines adjacent to process or transport lines—the transporting pipeline was “traced” with the steam line. Cold regions operations increase the demand for freeze prevention and viscosity maintenance. Freeze protection is especially important, since freezing can damage pipes and equipment.
Heat tracing can utilize the heat given off by a hot fluid line near or touching a pipeline. Or it can use the electrical resistance of materials to produce heat, either in the pipeline itself or in a cable or pipe which is in contact with or placed inside the pipeline
Internal tracing and hea jacketing are expensive and complicated ways to use steam for heat tracing and are therefore rarely used. A pipe with internal steam lines is hard to clean, and both internal tracing and jacketing become complicated in the presence of valves or any other geometrical irregularities (Kohli 1979).
An external heat tracing system consists of headers, the supply line, traps, and usually a line to return the condensate to the source. Steam tracing is available at temperatures up to 370 °C (700 °F) but it is usually not used above 200 °C (400 °F) because of the high pressures involved.
All heat tracing systems except steam are capable of close temperature control. Thermostats are usually used with freeze protection systems, and can be used with steam. A thermostat is an on-off controller with a temperature sensor. The power is activated when the ambient temperature falls below a set point.
For close temperature control, there are sophisticated controls and temperature sensors available, such as analog control devices and thermistors or thermocouples. Analog controllers vary power input to a system as temperatures vary; they are usually reliable when installed properly. Because they moderate the power used for heat tracing, they are often greater energy-savers than thermostats. Even though analog controls are more expensive than thermostats, they are sometimes used with freeze-protection systems for reliability and economy. IEEE Standards 622-1979 and 622A-1984 cover the design and installation of electric pipe heating and control systems.
Saturated steam exists at unique combinations of temperature and pressure. If the pressure is reduced the steam becomes superheated, but the superheat in heat tracing applications is rapidly dissipated. For this reason, the basic control of steam heat tracing temperature is a pressure-reducing valve, and precise control of temperature is difficult to achieve with this method. In addition, uneven contact between the steam line and product pipe results in uneven distribution of heat. This effect becomes more significant if the steam temperature is quite different from the fluid maintenance temperature. Because of tailing, uneven distribution of heat, and the use of pressure reducing valves, temperatures of steam tracing methods typically will vary around a desired temperature by ± 5 1/2 °C (10 °F) below ground or ± 11 °C (20°F) above ground (Clough 1984).
Economic comparisons were found in the literature reviewed. Most of them show that steam is significantly more expensive to install and maintain than electrical resistance heat tapes, even though the energy cost is lower. This is be‑