Hydrocarbon Processing - December 2021 - 36

Process Optimization
One key purpose of steam tracing is to maintain temperatures of
process fluids and, in critical cases, to keep product in a process
fluid transfer line from freezing (or solidifying). The heat load is
typically about the same as the radiant, convective and conductive
heat loss from the line. As lines are usually well insulated,
the heat loss and resulting condensate loads may be minimal.
This may present challenges and also opportunities for steam
trap selection, particularly when there are loops or lifts that flow
steam and condensate upward.
Steam tracing can generally be categorized as high temperature
or low temperature. In critical high-temperature applications,
condensate backup is undesirable. A steam trap with an
instantaneous and continuous discharge with a tight shutoff
may be preferred for maintaining the lowest lifecycle costs, and
thermodynamic disc traps or balanced-pressure thermostatic
traps may be acceptable for lowest-initial-cost installations.
Copper tracing lines experience corrosion over time and may,
therefore, present plugging difficulties. This scenario is best
mitigated by using a thermodynamic trap with enlarged ports
and short passages, or, for low-temperature applications, a bimetallic
steam trap. Bimetal traps with a low set temperature
can also be used for instrument enclosures to mitigate damage
to the instrument by excess temperatures.
Steam turbines. Among the most common steam equipment
applications in a petrochemical facility are steam turbines
providing power to pumps, compressors or power generators.
Turbines are very sensitive to condensate in the steam supply
and to condensate backup in the governor casing, body or exhaust
lines. Consequently, the best choice for this application
is a steam trap with instantaneous and continuous discharge,
such as a float and thermostatic design. However, thermodynamic
disc traps may be used with appropriate drainage pockets
and drain lines. Bimetal steam traps should never be used
because of the possibility of condensate backup or steam loss.
Turbine condensate drainage should be carefully considered
to ensure that all appropriate points on the upstream steam supply,
governor, casing and exhaust lines are all properly drained.4
A slug-tolerant separator should be considered on the steam
supply line immediately before critical turbines. Traps on the
exhaust should be appropriately sized for the turbine's efficiency
and operation, considering that the exhaust steam may be
significantly wet or may contain a substantial condensate load.
Process heating equipment. Process heating equipment
may include applications such as reboilers, heat exchangers, air
heaters, evaporators, concentrators, dryers and aboveground
storage tank coils. These applications may have various control
strategies and operating conditions that may affect the range
of steam pressures and flowrates. In some cases, a steam trap
may not work correctly due to a stall condition.6,7
This may occur
when the ideal steam pressure in the equipment for steadyTABLE
1. Recommended trap type according to application-Legend: First choice (1), second choice (2) and third choice (3)
Service/application
Tracing lines9
Low temperature,
< 200°F
High temperature,
> 200°F
Jacketed pipe
Saturated steam main:
≥ 20, ≤ 650
Superheated steam-
all services6
: ≥ 20, ≤ 650
Flare stack steam lines
Steam to control valve
Control valve outlet
to flare stack
Turbine supply lines
Turbine casing drains:7,8
≥ 20, ≤ 650
Turbine exhaust steam
lines: > 0, ≤ 150
Process heaters:
≤ 200 supply
Stall condition
No stall condition
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Process
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36 DECEMBER 2021 | HydrocarbonProcessing.com
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Free float and
thermostatic Thermodynamic
Bimetal
adjust-temp
Tracing
Balanced
pressure
thermostatic
Non-electric
pump/trap
Non-electric
pump
Level pot
with outlet
control
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Hydrocarbon Processing - December 2021

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