2000 Hook-up Book - Spirax Sarco

SYSTEM DESIGN
38
Steam Trap Selection
A full discussion of steam trap
functions are found in the companion
Fluid System Design
volume, “STEAM UTILIZATION.”
The material covers operation of
all types of traps, along with the
need for proper air venting and
trap selection. Traps are best
selected not just on supply pressure
and load requirements, but
after reviewing the requirements
of the application compared to
trap characteristics including discharge
temperature, air venting
capability, response to pressure
and load change, and resistance
to dirt, corrosion, waterhammer
and freezing conditions.
Answering these questions leads
to the selection of the most
appropriate generic type of trap
and the general recommendations
found in Table 11 reflect this.
This Selection Guide covers most
trap uses and the recommended
type can be expected to give satisfactory
performance.
Condensate removal was needed from 3 polyvinyl butyral
extruders at a pressure of 240 psi. Application required
that a consistent temperature be maintained the length of
the extruder to provide product quality in the melt. There
were nine sections per extruder.
The customer had used various brands of traps and
trap styles to drain the extruders. Most recently they used
a competitors bimetallic trap. They were experiencing
inconsistent temperatures throughout the length of the
extruder because the bimetallic traps subcooled the condensate,
which then backed up into the heat transfer area.
They were also experiencing high maintenance costs in
relation to these traps.
Steam Trap Sizing
Steam main drip traps shall be
sized with a 2 times safety factor
at full differential pressure. In
most cases, they will be 3/4” size
with low capacity orifice or smaller
unless otherwise shown on the
drawings and they shall be located
every 200 feet or less. Traps
for steam tracing shall be 1/4" to
1/2" size. They shall be located
every 100 feet or less. Radiator
traps shall be pipe size. Freeze
protection traps shall be 1/2" to
3/4" size unless otherwise noted.
Traps for equipment drainage
are sized with safety factors that
reflect the differences of the
HVAC and Process industries,
such as variations in actual
hydraulic head and material construction
of tube bundles. A
summary of these typical recommendations
are as follows:
HVAC Industry
• Non-modulating control systems
have traps selected with
a 2 times factor at full pressure
differential.
• Modulating control systems
with less than 30 psig inlet
pressure have traps selected
Case in Action: Polyvinyl Butyral Extruders
for full-load at 1/2 psi pressure
differential, provide 18 to
24" drip leg for condensate to
drain freely to 0 psi gravity
return. (With drip legs less
than 18", consult a Spirax
Sarco representative.)
• Modulating control systems
with greater than 30 psig inlet
pressure have traps selected
with a 3 times factor at full
pressure differential for all
preheat coils, and a 2 times
factor for others.
Process Industry
• Non-modulating control systems
have traps selected with
a 2 times factor at full pressure
differential.
• Modulating controls systems
with less than 30 psig inlet
pressure have traps selected
for full load at 1/2 psi pressure
differential, provide 18 to
24" drip leg for condensate to
drain freely to gravity return
at 0 psi. (With drip legs less
than 18", consult a Spirax
Sarco representative.)
• Modulating control systems
have traps selected with a 3
times factor at full pressure
differential.
Solution
Float & Thermostatic steam traps were recommended for
draining the extruders. This would give them immediate
condensate removal; therefore maintaining a consistant
temperature throughout the length of the extruder, providing
better control over product melt. Also, upon
recommendation, strainers were installed before the traps
to help keep dirt out, and cut down on maintenance cost.
Benefits
• Maintained consistent temperatures with existing equipment
because there is no condensate in the heat
transfer area.
• There is less maintenance cost due to the strainers
installed before the traps.