2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
18
Steam Tracing
A similar but maintainable
type intended for use on instrument
tracer lines, where the
physical size of the trap is important
as well as its operating
characteristics is shown in Fig. 35.
Just as the distribution of
steam is from a common header,
it often is convenient to connect a
number of traps to a common
condensate header and this simplifies
maintenance. As noted, the
discharge should preferably enter
the header through swept connections
and the headers be
adequately sized as suggested in
Table 8 (page 16).
Figure 35
Maintainable Balanced Pressure
Tracer Trap.
Case in Action: Product Steam Tracing with Temperature Control and Overheat Protection
During steam tracing project design, it was found that five
thousand feet of 2" product piping was to be traced with
150 psig steam. Product temperature was to be maintained
at 100°F, with maximum allowable temperature of 150°F
and a minimum allowable temperature of 50°F.
Of particular concern was the fact that the pipeline
would always be full of the product, but flow would be
intermittent. Overheating could be a real problem. In addition,
the tracing system had to be protected from freezing.
Solution
The 5,000 feet of product piping was divided into 30 separate
traced sections including: a cast steel temperature
regulator, a bronze temperature control valve used as a
high limit safety cutout, a sealed balanced pressure thermostatic
steam trap, a vacuum breaker, and pressure
regulators supplying steam to all 30 tracing sections. Each
section operates effectively at the desired temperature,
regardless of flow rate or ambient temperature.
These may be increased
where high pressures and traps
discharging condensate at near
steam temperature are used, or
decreased with low pressures
and traps discharging cooler condensate.
Temperature Control
of Tracer
Where it is essential to prevent
overheating of the product, or
where constant viscosity is
required for instrumentation,
automatic temperature control is
frequently used.
On many systems, the simplest
way to achieve control is to
use a reducing valve on the
steam supply to the tracer lines or
jacket. This can be adjusted in the
light of experience to give the correct
steam pressure to produce
the required product temperature.
Clearly this is an approximate
way to control product temperature
and can only be used where
the product flow is fairly constant.
Where closer control is required,
the simple direct acting tempera-
ture control often provides an
economic solution. This will give
close control and since it is not
necessary to provide either electric
power or compressed air, the
first cost and indeed the running
costs are low.
Benefits
• The chance of product damage from overheating is minimized
and steam consumption is reduced through
steam pressure reduction (150 psig to 50 psig) with the
pressure regulator.
• The product temperature is maintained at a consistent
set temperature, maximizing process control under all
flow conditions with the temperature regulator.
• Product damage from overheating is prevented through
use of the high limit safety cutout. The system will shut
down completely, should the temperature regulator overshoot
its set point.
• The tracing system is protected from freezing with the
sealed balanced pressure thermostatic steam trap discharging
to drain. Thorough drainage is also facilitated
by the vacuum breaker.