POWER October 2014 - 41

OPERATIONS & MAINTENANCE
2. Saving steps. The proper operation of important instruments and weather enclosure
heating systems at this plant can be confirmed by operators at ground level by the four red LED
lamps, instead of by climbing stairs multiple times each shift. Further, networked instruments
can often display an error message on the plant distributed control system in the event of an
enclosure heater failure, without adding any additional wiring. Source: POWER
3. Frozen pipes. Frozen water in pipes
and valves often caused extensive damage
during the February 2011 freeze, particularly to
water mains and fire protection water pipes.
Source: POWER
drum within predetermined limits to prevent
damage to the boiler. Damage could be caused
either by running the drum dry, resulting in
major damage due to overheating of drum and
tubes, or by allowing the level to rise too high,
which can result in carrying over slugs of water
into the steam tubes and possibly into the
steam supply lines to the turbine. " To the operator's
credit, the drum water controls were
placed into manual mode, and drum water levels
were monitored by operators with radios
reporting status to the control room.
At a different plant, once the unit tripped
and residual heat was lost in the water systems,
other instrumentation and sensing
lines froze, thereby preventing a unit restart.
Another plant reported the same problem,
though it was able to continue operation at
reduced load.
The solution to frozen instrumentation and
sensing lines (particularly, critical drum level,
feedwater flow, and steam flow transmitters)
must be holistic. For example, there must be
freeze protection in the main circuit breaker
panel that powers the instruments and the
panel board transformer, insulated and heated
drum level transmitter(s) enclosure(s), a
means to continuously monitor enclosure
temperature, and a means to confirm operation
of the heat trace cable used throughout
the system.
Heater strips in instrument cabinets were
often overcome by freezing temperatures,
though additional insulation and additional
heating strips often solved this problem. A
single point failure in the heating system can
cause a trip of the unit. Modern heat trace
cables are available that are prefabricated,
October 2014 | POWER
not easily broken, and have a self-regulating
heating element. Some users report installing
lights to give a visual indication of the box
temperature. Steady lit means the circuit is
on and the box is at set temperature, flashing
light means the circuit is on but the box is
below set temperature, and when the light is
out it means there is no power available to the
heater. This approach works well when the
enclosure is covered with an insulating blanket
and operators can see the lights at elevation
from ground level (Figure 2).
Pipes and Valves. Several plants reported
that frozen steam turbine drain lines prevented
unit restart until ambient temperatures
rose above freezing so the ice could melt. Another
unit experienced frozen deaerator level
control sensing lines that failed to signal the
need for makeup water, causing a plant shutdown
signal generated by low boiler feedwater
pump suction pressure.
Another utility attempted to start a unit
that was offline when the sub-freezing temperatures
hit, but frozen instrument lines prevented
the restart. Heat tracing of instrument
and sensing lines is commonly found at most
outdoor plants, though heat tracing is used to
prevent freezing, not to thaw frozen lines.
Users also reported problems with small
piping supports, because they can act as a heat
sink and cause the pipe to freeze even when
insulated. The same applies to external pipe
hangers that penetrate pipe lagging. Heat sinks
should be eliminated from plant piping.
Non-heat-traced pipes froze at several
plants, causing pipes to rupture (Figure 3),
including outside fire protection piping and
piping located near building ventilation louwww.powermag.com
vers
(thus rendering the fire protection system
inoperable). The National Fire Protection
Association (NFPA) has established clear fire
protection system freeze prevention requirements
(NFPA 13). From a safety viewpoint,
you should heat trace the water supply to the
safety showers located throughout the plant.
Ammonia lines to the selective catalytic
reduction system of a gas-fired plant must be
properly heat traced and insulated. A crack
in this line could easily become a significant
environmental issue if left undetected. The
same should be said for chemical lines used
for boiler water treatment and the boiler water
sample lines.
Compressed Air Systems. Compressed
air systems may also be at risk. The typical
southern climate plant uses compressed air
outside directly after the coalescing filter.
Plants built in northern climates use air dryers
to provide compressed air at a dew point
temperature below the minimum design temperature
to prevent freezing of instrument air
lines. Air compressor condensate vent lines
should also be heat traced, unlike the typical
southern plant. High moisture levels in the
instrument air often caused freezing of airoperated
valve actuators.
Redundant dew point monitors may be
used to continuously monitor compressed
air quality. Do not rely on automatic air tank
drain systems for moisture control. One utility
reported the failure of an automatic air
tank drain resulted in the contamination of
its instrument air system, which is very difficult
to clean. At least one plant reported
that moisture in the control air system caused
a pneumatically controller positioner on a
fan damper to fail, causing a complete unit
forced outage.
Other Plant Systems. Freeze protection
must also be considered for outdoor equipment
that will not be operated. For example,
a heat-recovery steam generator (HRSG) full
of water may need to be drained when exposed
to unexpectedly low temperatures. The
41

http://www.powermag.com

POWER October 2014

Table of Contents for the Digital Edition of POWER October 2014