2000 Hook-up Book - Spirax Sarco

Steam
Steam Trap
1/2" (15mm) OD
3/8" (10mm) OD
1/4" (6mm) Bore
Figure 32
Typical Instrument Tracing
Important—
Getting Rid of the Muck
Pipes delivered to the site may
contain mill scale, paint, preserving
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 considerable
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
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 others.
The traps should be
physically small and light in
weight, and as they are often fitted
in exposed positions, they
should be resistant to frost. The
temperature at which the condensate
is discharged by the trap is
perhaps the most important consideration
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 sulphur
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 condensate
at the maximum rate
corresponding to the differential
pressure applied. The instantaneous
release rates of the steam
flashing off the condensate can
be appreciable, and care is needed
to ensure that condensate
return lines are adequately sized
Figure 34
Balanced Pressure Tracer Trap
Steam Tracing
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, leading
to more rapid wear. Special
insulating caps are available for
fitting to the top caps to avoid this,
Fig. 33.
In other non-critical applications,
it can be convenient and
energy efficient to allow the condensate
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 temperatures,
and tend to give a
continuous dribble of condensate
when handling tracer loads, helping
minimize the size of
condensate line needed. They
are available either in maintainable
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 normally
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 condensate
is not to be returned to the
recovery system.
17
SYSTEM DESIGN