2000 Hook-up Book

Steam Tracing

Sizing of External Tracers The tracing or jacketing of any line normally aims at maintaining the contents of the line at a satisfac tory working temperature under all conditions of low ambient tem perature with adequate reserve to meet extreme conditions. Remember that on some exposed sites, with an ambient still air temperature of say 0°F, the effect of a 15 mph wind will be to lower the temperature to an equivalent of -36°F. Even 32°F in still air can be lowered to an effective 4°F with a 20 mph wind—circumstances which must be taken into full con sideration when studying the tracer line requirements. Details of prevailing condi tions can usually be obtained from the local meteorological office or civil air authority. Most of the sizing of external tracers is done by rule of thumb, but the problem which arises here is what rule and whose thumb? Rules of thumb are generally based on the experiences of a cer tain company on a particular process and do not necessarily apply elsewhere. There are also widely differing opinions on the lay out: some say that multiple tracers should all be below the center line of the product line while others say,

Jacketed Lines Ideally jacketed lines should be constructed in no more than 20 ft. 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 outlet as Fig. 12a. When it is considered impractical to trap each length, a number of lengths up a total of 80-100 ft. approx. may be joined together in moderate climates, but in extremely cold parts of the world 40 ft. should be the maxi mum. See Fig. 12b. Always avoid connecting solely through the bottom loop. This can only handle the conden sate and impedes the free flow of steam as Fig. 12c. As a general guide, see Table 7. Although in most cases 1/2" condensate outlet will be ade quate, it is usual to make this the same size as the steam connec tion as it simplifies installation. External Tracers In horizontal runs, the steam will generally flow parallel to the prod uct line, but as far as possible, steam should enter from the high end to allow free flow of the con densate to the low end, i.e. it should always be self-draining. It is generally considered preferable to fit one tracer on the bottom of the line as Fig. 13a, two tracers at 30° as Fig. 13b, three tracers at 45° as Fig. 13c. Where multiple 1/2" tracers are used, they should be arranged in loop fashion on either side of the product line, as Fig. 14. In ver tical lines, the tracers would be spaced uniformly, as Fig. 15a & b. The maximum permissible length of tracer will depend to some extent on the size and initial steam pressure, but as a general guide 3/8” tracers should not exceed 60 ft. in length and the limit for all other sizes should be about 150 ft. Bends and low points in the tracer, as Fig. 16a should always be avoided. For example, if it is necessary to carry a tracer line round a pipe support or flange,

with equal conviction, that it is per fectly satisfactory to space the tracers equally around the line. Then there are those who will endeavor to size their tracers from 3/8", 1/2", 3/4" or 1" and even larger pipe: while another school of thought says that as tracers have only minute contact with the product line it will give much more even distribution of heat if all trac ers are from 1/2" pipe in multiples to meet the requirements. This does have the added advantage of needing to hold a stock of only one size of pipe and fittings rather than a variety of sizes. For those who like to follow this idea, Table 6 will be useful for most average requirements. Type A would suffice for most fuel oil requirements and would also meet the requirement of those lines carrying acid, phenol, water and some other chemicals, but in some cases spacers between the product line and steam line would be employed. The steam pressure is impor tant and must be chosen according to the product temper ature required. For noncritical tracing Types A & B (Table 6) a steam pressure of 50 psi would generally be suit able. For Type C, a higher pressure and a trap with a hot discharge may be required.

SYSTEM DESIGN

Table 6: Number of 1/2 " (15mm) Tracers Used with Different Sizes of Product Lines Type A Type B Type C Noncritical Noncritical Critical General frost protection or Where solidification may When solidification may where solidification may occur at temps between occur at temps between occur at temps below 75°F 75-150°F 150-300°F

Product Line Size

Number of 1/2"

Number of 1/2"

Number of 1/2"

Tracers

Tracers

Tracers

1"

1 1 1 1 1 2 2 2 2 2

1 1 1 1 2 2 2 3 3 3

1 2 2 3 3 3 3 6 8

1 1 /

2 "

2" 3" 4" 6" 8"

10"-12" 14"-16" 18"-20"

10

14