Technical Manual - Section 3 (Safety Hazards)

minerals and dissolved impurities that can damage the system
or affect its operation. Suspended materials such as silt,
sewage, and oil, which form scale and sludge, must be
coagulated or filtered out of the water. Dissolved gases,
particularly carbon dioxide and oxygen, cause boiler
corrosion and are removed by deaeration and treatment.
Dissolved minerals including metallic salts, calcium,
carbonates, etc., that cause scale, corrosion, and turbine blade
deposits are treated with lime or soda ash to precipitate them
from the water. Recirculated cooling water must also be
treated for hydrocarbons and other contaminants.
Depending on the characteristics of raw boiler feedwater,
some or all of the following six stages of treatment will be
applicable:
(1) Clarification
(2) Sedimentation
(3) Filtration
(4) Ion exchange
(5) Deaeration
(6) Internal treatment
HEALTH AND SAFETY CONSIDERATIONS
Fire Protection and Prevention
The most potentially hazardous operation in steam generation
is heater startup. A flammable mixture of gas and air can
build up as a result of loss of flame at one or more burners
during light-off. Each type of unit requires specific startup
and emergency procedures including purging before lightoff
and in the event of misfire or loss of burner flame.
Safety
If feedwater runs low and boilers are dry, the tubes will
overheat and fail. Conversely, excess water will be carried
over into the steam distribution system and damage the
turbines. Feedwater must be free of contaminants that could
affect operations. Boilers should have continuous or
intermittent blowdown systems to remove water from steam
drums and limit buildup of
scale on turbine blades and superheater tubes. Care must be
taken not to overheat the superheater during startup and
shut-down. Alternate fuel sources should be provided in the
event of loss of gas due to refinery unit shutdown or
emergency. Knockout pots provided at process units remove
liquids from fuel gas before burning.
Health
Safe work practices and/or appropriate personal protective
equipment may be needed for potential exposures to
feedwater chemicals, steam, hot water, radiant heat, and
noise, and during process sampling, inspection, maintenance,
and turnaround activities.
PRESSURE-RELIEF AND FLARE SYSTEMS
PRESSURE-RELIEF SYSTEMS
Pressure-relief systems control vapors and liquids that are
released by pressure-relieving devices and blow-downs.
Pressure relief is an automatic, planned release when
operating pressure reaches a predetermined level. Blowdown
normally refers to the intentional release of material, such as
blowdowns from process unit startups, furnace blowdowns,
shutdowns, and emergencies. Vapor depressuring is the rapid
removal of vapors from pressure vessels in case of fire. This
may be accom-plished by the use of a rupture disc, usually set
at a higher pressure than the relief valve.
SAFETY RELIEF VALVE OPERATIONS
Safety relief valves, used for air, steam, and gas as well as for
vapor and liquid, allow the valve to open in proportion to the
increase in pressure over the normal operating pressure.
Safety valves designed primarily to release high volumes of
steam usually pop open to full capacity. The overpressure
needed to open liquid-relief valves where large-volume
discharge is not required increases as the valve lifts due to
increased spring resistance. Pilot-operated safety relief valves,
with up to six times the capacity of normal relief valves, are
used where tighter
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