2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
12
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 solidification,
or simply to protect against
freezeup. 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 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 convection
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 copper
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 temperature
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.
Figure 7
Small Bore Tracing
Wraped Around
Vertical Product Line
Figure 8
Clipping Tracer Around Bends
Product
Tracer
Lagging
Aluminum
Foil
Air Space
Figure 6
Tracer Attached To Product Line
Figure 9 Continuous Fall On Wrap Around Tracer
9a 9b
Figure 10 Attaching Tracer To Line
Figure 10a Short Run Welds
Figure 10b Continuous Weld
Lagging
Heat
Conducting
Paste
Product
Tracer
Figure 10c Heat Conducting Paste