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Steam Generation
Steam is generated in main generation plants, and/or at various
process units using heat from flue gas or other sources. Heaters
(furnaces) include burners and a combustion air system, the boiler
enclosure in which heat transfer takes place, a draft or pressure
system to remove flue gas from the furnace, soot blowers, and
compressed-air systems that seal openings to prevent the escape
of flue gas. Boilers consist of a number of tubes that carry the
water-steam mixture through the furnace for maximum heat transfer.
These tubes run between steam-distribution drums at the top of
the boiler and water-collecting drums at the bottom of the boiler.
Steam flows from the steam drum to the superheater before entering
the steam distribution system.
Heater Fuel
Heaters may use any one or combination of fuels including refinery
gas, natural gas, fuel oil, and powdered coal. Refinery off-gas
is collected from process units and combined with natural gas
and LPG in a fuel-gas balance drum. The balance drum provides
constant system pressure, fairly stable Btu-content fuel, and
automatic separation of suspended liquids in gas vapors, and it
prevents carryover of large slugs of condensate into the distribution
system. Fuel oil is typically a mix of refinery crude oil with
straight-run and cracked residues and other products. The fuel-oil
system delivers fuel to process-unit heaters and steam generators
at required temperatures and pressures. The fuel oil is heated
to pumping temperature, sucked through a coarse suction strainer,
pumped to a temperature-control heater, and then pumped through
a fine-mesh strainer before being burned. In one example of process-unit
heat generation, carbon monoxide boilers recover heat in catalytic
cracking units as carbon monoxide in flue gas is burned to complete
combustion. In other processes, waste-heat recovery units use
heat from the flue gas to make steam.
Steam Distribution
The distribution system consists of valves, fittings, piping,
and connections suitable for the pressure of the steam transported.
Steam leaves the boilers at the highest pressure required by the
process units or electrical generation. The steam pressure is
then reduced in turbines that drive process pumps and compressors.
Most steam used in the refinery is condensed to water in various
types of heat exchangers. The condensate is reused as boiler feedwater
or discharged to wastewater treatment. When refinery steam is
also used to drive steam turbine generators to produce electricity,
the steam must be produced at much higher pressure than required
for process steam. Steam typically is generated by heaters (furnaces)
and boilers combined in one unit.
Feedwater
Feedwater supply is an important part of steam generation. There
must always be as many pounds of water entering the system as
there are pounds of steam leaving it. Water used in steam generation
must be free of contaminants including 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:
| 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. 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. |
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