Steam Systems - Waste Heat Boilers
September 17, 2001
Introduction
A boiler is a
vessel in which water is continuously vapourized into steam by application of
heat. A primary objective in designing a boiler is to provide for the
greatest possible efficiency in absorption of heat. Other objectives are
production of pure steam and safe, reliable operation.
Boilers in
acid plants are waste heat boilers cooling gases from sulphur furnaces,
regeneration furnaces and between catalyst beds.
Types of Boilers
There are
basically two types of boilers in common use:
1) Firetube
2) Watertube
Firetube Boilers
In firetube boilers
the hot gases are confined within tuibes arranged in a bundle within a shell.
Water circulates on the outside of these tubes. Water enters the boiler
from the steam drum through downcomers. As the water changes to steam it
rises to the top of the boiler and exits through risers to the steam drum.
Firetube boilers in acid plants are generally natural circulation boilers.
If the source of gas
is a sulphur or regeneration furnace, the inlet gas vestibule and the inlet
tubesheet will be protected by refractory and the individual tubes by ceramic
ferrules. In addition, the front of the boiler will be offset or located
at right angles to the furnace to prevent direct radiation from the furnace on
the tubesheet.
Watertube Boilers
In watertube boilers the
hot gases flow across the outside of the tubes and water is circulated within
the tubes. Water tube boilers are generally natural circulation boilers.
The inlet vestibule will be refractory lined if the source of gas is a sulphur
or regeneration furnace. In addition, the front of the boiler will be
offset or located at right angles to the furnace to prevent direct radiation
from the furnace flame on the tube bank. In a water tube boiler a
separate superheater section can be built into the same 'box' as the boiler
section.
Steam Drum
The steam
drum serves two primary functions:
1)
Separate the steam from the steam/water mixtures that return from the boiler.
2) Provide a holdup capacity of water to feed the boilers.
There are essentially
2 stages of separation in a steam drum. The primary stage of separation is
the coarse one of removing the bulk of the water from the steam. This is
achieved by providing adequate space above the normal steam drum level for
steam/liquid disengagment.
The second stage is removal of water droplets from the steam. This stage
is important if the steam is to be superheated and used in turbines.
Carryover from the steam drum contains high levels of dissolved solids.
These solids will be deposited in the superheaters, reducing heat transfer and
on turbine blades, reducing efficiencies. Knitted wire meshpads are
typically used to minimize carryover from the steam drum.
When boilers are specified, a steam purity 1 ppm TDS (total dissolved solids) is
usually specified to ensure minimum carryover.
Boiler Screens
The waste acid decomposed in a regeneration furnace contains solid residues
broadly known as ash. This ash remains molten at the high operating
temperatures of the furnace. When the gas enters the boiler, the molten
ash will deposit on the relatively cooler outside surface of the tubes.
Boiler screens are a
feature found in firetube boilers. A boiler screen in a set of widely
spaced tubes at the front of the boiler. The wide spacing provides gas
passages wide enough to prevent plugging. The boiler screen helps to keep
the downstream superheater and convection bundles relatively free of deposits.
The boiler for
ICI/KMC Taiwan contains 2 screen bundles. The tube spacing in the screen
bundle is 8.75 inches compared to 5.0 inches in the superheater bundle and 4.38
inches in the convection bundles. The tubes in the screen bundle are in a
staggered pattern whereas the tubes in the other sections are arranged inline.
Boiler Feed Water Quality
The requirement for boiler feed water quality is determined by the steam
pressure and end use of the steam. In general, the higher the operating
pressure of the system, the higher the quality of BFW required. Also, if
the steam is superheated and used in a turbo-generator, high quality BFW is
required to generate high quality steam. Appendix B contains a chart
listing recommended BFW qualities for various boiler operating pressures.
From previous Chemetics jobs, high quality demineralized water is used when the
steam is used in a turbo-generator.
Boiler Blowdown
Boiler feed water,
regardless of the type of treatment used still contains measurable
concentrations of impurities. Other sources of contaminants are from
condensate and boiler water treatment chemicals.
In a boiler,
essentially pure water vapour is discharged from the boiler leaving behind the
impurities entering in the feed water. The net result is a steady increase
in the concentration of the impurities in the boiler water. To maintain
reliable boiler operation the concentration of each component of the boiler
water must be limited to certain maximums. This is accomplished by blowing
down water from the boiler.
There are two
separate blowdown points in every boiler, a continuous and intermittent point.
The continuous blowdown is used to regulate the level of contaminants in the
boiler water and is usually located on the steam drum. The intermittent
blowdown is used to remove accummulated settled solids in relatively stagnant
areas of the boiler. There can be several intermittent points usuallly
located at the low points of the boiler.
Large blowdowns
represent high energy losses and high cost of preparing and treating fresh
makeup that is concentrated to such a limited degree. Therefore, there is
a balance between the amount of blowdown and the cost of additional water
treatment processes.
Boiler Bypasses
A bypass around the boiler is often required to control the gas outlet
temperature. This is not a requirement for a boiler in a regeneration
plant since there is no need to control the gas temperature entering the gas
cleaning system. However, in a sulphur burning plant where the
boiler is located before bed 1 or in between converter beds there is a need to
accurately control the gas temperature. Where the gas temperatures
are high, the bypasses must be refractory lined right to the point where the hot
and cold gas mix together. The high temperatures and refractory lining
create a mechanical problem in designing a bypass valve with a tight seal.
From a process point of view, if the leakage through the bypass is not taken
into account when the boiler is specified, the result may be an undersized
boiler. Figure 2 illustrates the problem when gas bypassing is not
taken into account. The quantity of bypass that will occur will be
dependent on the design and quality of construction of the bypass valve. A
conservative estimate of the total gas bypassing the boiler is 5%. If
bypassing is possible, it should be accounted for in the flowsheet and
specification of the boiler.
Boiler Exit Valve
Boilers installed in
sulphur burning plants should have a bypass valve and a valve located at the gas
outlet. The outlet valve is used during plant preheating to force hot
combustion gases through the open bypass directly to the converter beds.
Failure to install an outlet valve could result in prolonged preheat periods or
the inability to obtain the desired bed temperatures prior to firing sulphur.
Boiler Trim
Boiler trim is a general term referring to the accessories required for the
boiler. These include blowdown valves, level gauges, pressure gauges,
relief valves, transmitters, etc. These are generally supplied by the
vendor with the boiler as a complete package.
The boiler is generally equipped with multiple level instruments. A level
transmitter is used inconjunction with a controller to control level in the
steam drum. A separate level instrument installed in a standpipe is used
to initiate interlocks and shutdowns. This level instrument is usually a
series of capacitance probes whose lengths corresponds to the various levels in
the steam drum. A third instrument is generally a Clark-Reliance 'Eye-Hye'
which provides level indication in the form of indicator lights. Each
light corresponds to a specific level, usually inches of level above or below
the normal operating level. The indicator is usually located near the
manual blowdown valves so the operator can monitor the level as the boiler is
blown down. A fourth level instrument commonly installed on a steam drum
is a level sightglass.
