The commercial use of heat transfer compounds like steam in jacketed pipes is to keep the line hot. A steam-jacketed pipe is where the core on inner pipe containing the product fluid is surrounded by larger pipe called the jacket. Steam is carried in the annular region between the two pipes. The heat transfer coefficient for this system is very high. Typically, a jacketed system will maintain the product near the steam temperature (when there is little or no flow in the pipeline). On the negative side, the construction cost of a jacketed system is extremely high in both materials and labor. The system is very expensive to maintain. Leakage or failure of the system is difficult to locate. Where failure does occur, it is frequently due to leakage at the wall of the core pipe allowing steam to pass into the process material and vice versa. The removal of the material plugging the jacket is an expensive repair job. The usual solution is a complete replacement of the piping system. In addition the possibility of cross contamination could cause complete loss of the product. In some processes, cross contamination may be a critical hazard.
Ideally jacketed lines should be constructed in no more than 6 meters lengths and the condensate removed from each section. Steam should enter at the highest end so that there is a natural fall to the condensate out let. When it is considered impractical to trap each length, a number of lengths up to a total of 24-30 meters approx may be formed together in moderate climates, but in extremely cold parts of the world 12 meters should be maximum. Always avoid connecting solely through the bottom loop. This can only handle the condensate free of steam. Although in most cases 15 mm condensate outlet will be adequate, it is usual to make this the same size as the steam connection as it simplifies installation.
The temperature of the process liquids being transferred through pipelines often must be maintained to meet the requirements of a process, to prevent thickening and solidification, or simply as an anti-frost measure. This is achieved by the use of jacketed pipes, or by attaching to the product line one or more separate tracer lines carrying a heating medium such as steam or hot water. The steam usage may be relatively small but the tracing system is often a major part of the steam installation, and the source of many of the problems. Many large users and plant contractors have their own inhouse rules for tracer lines, but the following guide-lines may be useful in other cases.
Sizing of External Tracers: The tracing or jacketing of any line normally aims at maintaining the contents of the line at a satisfactory working temperature under all conditions of low ambient temperature with adequate reserve to meet extreme conditions. On some exposed sites, with an ambient still air temperature of say -18oC the effect of a 24 km/h wind will be to lower the temperature to an equivalent of -38oC. Even 0oC in still air can be lowered to an effective -16oC with a 30 km/h wind. Such circumstances which must be taken into full consideration when studying the tracer line requirements. Most of the sizing of external tracers is done by rule of thumb. Rule of thumb practices are generally based on the experiences of a certain company on a particular process and do not necessarily apply elsewhere. There are also widely differing opinions on the layout: some say that multiple tracers should all be below the
center line of the product line whilst other say with equal conviction that it is perfectly satisfactory to space the tracers equally around the line.