2000 Hook-up Book

Steam Tracing

The temperature of process liquids being transferred through pipelines often must be maintained to meet the requirements of a process, to prevent thickening and solidifica tion, or simply to protect against freezeup. This is achieved by the use of jacketed pipes, or by attach ing 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 rel atively small but the tracing system is often a major part of the steam installation, and the source of many problems. Many large users and plant contractors have their own inhouse rules for tracer lines, but the following guidelines may be useful in other cases. We have dealt only with external tracing, this being the area likely to cause difficulties where no existing experience is available. External tracing is simple and therefore cheap to install, and fulfills the needs of most processes. External Tracer Lines One or more heat carrying lines, of sizes usually from 3/8" up to 1" nominal bore are attached to the main product pipe as in Fig. 6. Transfer of heat to the product line may be three ways—by conduction through direct contact, by convec tion currents in the air pocket formed inside the insulating jacket, and by radiation. The tracer lines may be of carbon steel or copper, or sometimes stainless steel. Where the product line is of a particular material to suit the fluid it is carrying, the material for the tracer line must be chosen to avoid electrolytic corrosion at any contact points. For short runs of tracer, such as around short vertical pipes, or valves and fittings, small bore cop per pipes, perhaps 1/4" bore may be wound around the product lines as at Fig. 7. The layout should be arranged to give a continuous fall along the tracers as Fig. 9a rather

than Fig. 9b, and the use of wrap around tracers should be avoided on long horizontal lines. A run of even 100 ft. of 6 inch product line will have a total of about 500 to 600 ft. of wrap around tracer. The pressure drop along the tracer would be very high and the temperature at the end remote from the supply would be very low. Indeed, this end of the tracer would probably contain only condensate and the temper ature of this water would fall as it gives up heat. Where steam is present in the tracer, lifting the condensate from the multiplicity of low points increases the problems associated with this arrangement.

SYSTEM DESIGN

Lagging

Product

Aluminum Foil

Air Space

Tracer

Figure 6 Tracer Attached To Product Line

Figure 9 Continuous Fall On Wrap Around Tracer

9b

9a

Figure 10 Attaching Tracer To Line

Figure 10a Short Run Welds

Figure 7 Small Bore Tracing Wraped Around Vertical Product Line

Figure 10b Continuous Weld

Lagging

Product

Heat Conducting Paste

Figure 8 Clipping Tracer Around Bends

Tracer

Figure 10c Heat Conducting Paste

12