There are three types of steam tracing:
In this installation, we look at the External Method.
In the external method, pipe or tubing of small bore carrying steam is either wrapped around the product line or vessel, or else parallel runs are placed outside the product line, with the transfer of the heat taking place through the pipe wall. This is the more common method of the three, as it is the simplest and cheapest, it’s easy to repair or alter, and there is no possibility of cross-contamination between the product and the steam or condensate. The disadvantages are that the heat transfer rate between the tracer and the pipe is often unpredictable, consequently there is usually a slow heat up rate if the product is allowed to cool, and a high temperature drop is required between the tracer and the pipe, which often results in uneven temperature distribution.
The illustration shows the unusual manner in which the external tracer is applied. Tests have shown that there is a slight advantage to placing the tracer line below the product line when this method is used, but this is seldom done, due to the easier maintenance if the tracer is above the product line.
If more than one tracer line is used on the outside of the pipe, then it is recommended that they be on different traps. This will ensure that some heating will still take place if one trap becomes clogged or stuck in the closed position.
Also, for more uniform temperatures, two parallel tracer lines along a product pipe should, if possible, be fed in opposite directions so that a drop in temperature along the tracer lines will cancel each other out and maintain a more uniform total effect on the product line.
When insulation is placed over the traced line, it requires larger diameter insulation (often 1″ larger or more than normally would be used). This often leads to odd sizes being required, which in turn increases the price of the insulation.
On external tracing, if there is too much heat transfer to the product, it is usual to place some blocking between the tracer and the product line, or else place a thin layer of insulating paper between them. It is essential that the tracer be spaced uniformly along long pipe runs, and it should be fastened properly so as to permit some movement, without danger of it working loose or pulling away from the product line.
Excerpts from: Basics of Industrial Steam Utilization
Next: Internal Steam Tracing
Harry Woebkenberg, Vice President Marketing / Jordan Valve Product Manager