2000 Hook-up Book

Steam Tracing

Steam Traps For Tracer Lines Almost any type of steam trap could be used to drain tracer lines, but some lend themselves to this application better than oth ers. The traps should be physically small and light in weight, and as they are often fit ted in exposed positions, they should be resistant to frost. The temperature at which the conden sate is discharged by the trap is perhaps the most important con sideration when selecting the type of trap. Thermo-Dynamic ® traps are the simplest and most robust of all traps, they meet all of the above criteria and they discharge condensate at a temperature close to that of steam. Thus they are especially suitable on those tracing applications where the holding back of condensate in the tracer line until it has subcooled would be unacceptable. Tracers or jackets on lines carrying sul phur or asphalt typify these applications where the tracer must be at steam temperature along its whole length. It must be remembered that every time a Thermo-Dynamic ® trap opens, it discharges conden sate at the maximum rate corresponding to the differential pressure applied. The instanta neous release rates of the steam flashing off the condensate can be appreciable, and care is need ed to ensure that condensate return lines are adequately sized

if high back pressures are to be avoided. Thus, the use of swept back or “y” connections from trap discharges into common headers of generous size will help avoid problems. Where the traps are exposed to wind, rain or snow, or low ambient temperatures, the steam bubbles in the top cap of the trap can condense more quickly, lead ing to more rapid wear. Special insulating caps are available for fitting to the top caps to avoid this, Fig. 33. In other non-critical applica tions, it can be convenient and energy efficient to allow the con densate to sub-cool within the tracer before being discharged. This enables use to be made of some of the sensible heat in the condensate, and reduces or even eliminates the release of flash steam. Temperature sensitive traps are then selected, using either balanced pressure or bimetallic elements. The bimetallic traps usually discharge condensate at some fairly constant differential such as 50°F below condensing tempera tures, and tend to give a continuous dribble of condensate when handling tracer loads, help ing minimize the size of condensate line needed. They are available either in maintain able versions, with a replaceable element set which includes the valve and seat as well as the bimetallic stack, or as sealed non-maintainable units as required. Balanced pressure traps nor mally operate just below steam temperature, for critical tracing applications, see Fig. 34. The trap is especially suitable where small quantities of condensate are produced, on applications where sub-cooling is desirable, and where the conden sate is not to be returned to the recovery system.

Steam

1/2" (15mm) OD

SYSTEM DESIGN

3/8" (10mm) OD 1/4" (6mm) Bore

Steam Trap

Figure 32 Typical Instrument Tracing

Important— Getting Rid of the Muck

Pipes delivered to the site may contain mill scale, paint, preserv ing oils, etc. and during storage and erection will collect dirt, sand, weld splatter and other debris, so that on completion, the average tracer line contains a consider able amount of “muck.” Hydraulic testing will convert this “muck” into a mobile sludge which is not adequately washed out by simply draining down after testing. It is most important that the lines are properly cleaned by blowing through with steam to an open end before diverting to the steam traps. Unless this is done, the traps will almost certainly fail to operate correctly and more time will be spent cleaning them out when the plant is commissioned.

Figure 33 Insulating Cap for Thermo-Dynamic ® Trap

Figure 34 Balanced Pressure Tracer Trap

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