Steam generators produce pressurized steam through boiling water inside metal tubes. Steam has many industrial uses and is generated in large power plants as well as smaller on-site systems. There are two main types of steam generators - those with a steam drum that separates steam and water, and once-through steam generators without a drum. Proper water treatment and boiler maintenance are required to prevent scale buildup and corrosion and ensure efficient steam generation.

1. Steam Generators
Lecture 6
by
Engr Mohammad ikhlaq khattak

2. Boilers are specialized mechanical devices designed to generate
pressurized steam. Steam is an essential raw material that drives
industry and commerce worldwide.
From the electricity used to power our homes and run our plants, to
the plastic and vinyl in our automobiles, the paints and wood in on our
homes, and the preparation of the food we eat, steam is used to make
our lives more comfortable and convenient.
Steam has several unique properties that make it suitable and
economically advantageous to perform this work.
The main operating cost associated with a boiler system is not the
water, but the energy involved in converting it to steam

3. A steam power plant consists of a boiler, steam turbine and generator,
and other auxiliaries.
The boiler generates steam at high pressure and high temperature.
The steam turbine converts the heat energy of steam into mechanical
energy.
The generator then converts the mechanical energy into electric
power.

4. Steam generation is the familiar process of boiling water to make
steam.
Thermodynamically, the heat energy used results in a change of phase
from liquid to gaseous state, i.e. from water to steam.
A steam generating system has to provide a continuous and
uninterrupted heat source for this conversion.

5. When the water converts to steam, dissolved solids are left behind to
concentrate in the boiler water. A portion of the solids must be
removed by blowdown to keep concentrations within desired ranges.

6. As the steam cools, it reverts back to a liquid, which is called condensate
(distilled water). Steam traps are used to efficiently collect the
condensate for return to the deaerator or feedwater tank.
At this point, condensate mixes with the fresh makeup water in a
deaerator or feedwater tank. This mixture now becomes the feedwater
that is pumped back into the boiler when necessary to compensate for
water losses that occur during steam production and blowdown.
It’s important to note that the pressure range of the boiler and the end
use of the steam dictate makeup water quality, which can range from
raw water, to softened water, to high purity de-mineralized water.

7. A simplified diagram of steam generation process. Aimed to provide
superheated steam to a turbine, the steam generation system is
basically composed of a recovery steam generator, a recirculation
pump, control valves and interconnection pipes

8. 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.

9. 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.
An 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.

10. 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.

11. Feedwater
Feed water 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:

12. 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.

13. 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.

14. High pressure steam generation
Technical and economic considerations indicate that the most efficient
and economic way of producing high pressure (HP) steam is to heat
relatively small diameter tubes containing a continuous flow of wat er.
Two inherently different boiling systems are used in order to
accomplish this task:
- The first system uses a steam drum or a fixed steam-water
separation point and
- The second system that does not use a steam drum, referred
to as once-through steam generator (OTSG).

15. Steam Drum
Perhaps the most widely used steam generation system is a steam
drum. In this system, the drum serves as the point of separation of
steam from water.
Subcooled water enters the tube to which heat is applied. As the water
flows across the tube, it is heated to the boiling point.
Consequently, bubbles are formed and wet steam is generated. In most
boilers, a steam-water mixture leaves the tube and enters the steam
drum.
The remaining water is then mixed with the makeup (replacement)
water, returned back to the heated tube and the process is repeated.

16. Once Through Steam Generator (OTSG)
Without the use of a steam drum, subcooled water also enters the
heated tube and gets converted to steam along the flow path (length of
tube).
The point where water turns into steam depends on the water flow rate
(boiler load) and heat input rate.
With close control of both flow rate and heat input rate, we can make
sure that all of the water is evaporated withing the tube and only steam
leaves the tube.
Therefore, there is no need for having a steam drum.

17. Figure- presents a schematic view of steam
generators with steam drum and without drum
(OTSG).

18. operates at 80 bars or higher

26. Boiler Options
Boilers come in a wide range of sizes and shapes and can be classified based on design,
operating pressure, and fuel type. There are four basic types of steam boilers:
• Firetube Boilers – Hot combustion gases pass through tubes running through a
sealed container of water. These boilers are typically used for process and heating
applications.
• Water Tube Boilers – Water is fed under pressure into boiler tubes surrounded by
hot combustion gases. These systems are typically found in large pharmaceutical,
petrochemical, and large manufacturing plants, as well as university campuses and
government buildings.
• High Heat Flux Steam Generators – Pumps feedwater through a single tube coil,
transferring heat from hot gases that convert the water to steam very rapidly. Their
simple and small construction along with easy operation and quick start up make
these systems ideal for use as auxiliary boilers and when working in limited spaces.
• Electric Boilers – Uses electricity rather than gas to heat hot water via a heating
element. These units are typically used when there is no fossil fuel available or in
areas that have very low electric power rates.

27. Water Treatment
Boiler systems are an exceptional value due to their long-life
expectancy. However, regular maintenance must be performed to
ensure trouble-free operation.
Scale deposits, excess blowdown, and condensate loss can reduce
boiler efficiency and increase operating costs.
Water treatment is a critical part of the preventative maintenance
program for any boiler system. These solutions protect boilers from
the harmful effects that may cause production losses or unexpected
shutdowns, insuring safe, reliable, and cost-effective steam production

29. A fire-tube boiler
The heat of the gases is transferred through the walls of the tubes by
thermal conduction, heating the water and ultimately creating steam.
Water tube boiler:Burning gasses travel
through the heat exchange passes of the
boiler, through the large open combustion
chamber and back through the narrower
paths of tubes. These gases heat the water
inside the tubes. As water circulates in the
tubes from the lower mud drum, up to
the steam drum.

45. • Thanks