D TYPE BOILER
INFORMATION SHEET
BOILER CONSTRUCTION
Information Sheet Number 62B-203
INTRODUCTION
The D-type boiler has been installed in U.S. Navy ships since 1950. Whether 600 psi or 1200 psi, D-type boiler construction is basically the same with a few exceptions, such as number of fuel oil burners and overall size and volume.
REFERENCES
(a) Boilers NSTM Chapter 221
(b) Fireman NAVEDTRA 10520 Series
(c) Boiler Technician 3&2 NAVEDTRA 10535B
(d) Principles of Naval Engineering 10788 H
(e) Boiler Operation and Maintenance Manual
NAVSEA 0951-LP-022-6010
INFORMATION
A. Main propulsion boilers provide steam to the main propulsion turbines
and auxiliary services in order to supply all shipboard steam systems in
accordance with demand. (refer to Figure 1). It is designated as
a D-type boiler because of the relative positions of the drums and side
header which form the letter D. All D-type boilers are designated
as uncontrolled superheat boilers because all the steam generated by the
boiler must pass through the superheater. Superheater outlet temperature
is a result of the combustion gas flow in proportion to the total amount
of steam flow through all ranges (0 - 120%). The design characteristics
ensure that the temperature will stabilize at set point. The degree
of superheat is calculated by subtracting steam drum temperature from the
actual reading on the superheater outlet temperature gage. In this
lesson we will examine the components of the boiler and then we will bring
everything together by describing how water and steam is generated and
circulated through the boiler. The flow path of combustion gases
through the boiler will also be discussed.
D TYPE BOILER
C. The steam drum is located at the top of the boiler to provide an
upper reservoir for the water covering the generating tube bank.
Water is distributed from the steam drum to the lower drums and headers
by pipes called downcomers. Generated steam is also collected and
is separated from the water in the steam drum. Boilers are also equipped
with safety valves to relieve excessive pressure. The valves are
located on the steam drum and superheater outlet. They are designed
to relieve sufficient pressure to safely steam the boiler at 120% with
boiler steam stop valves closed (refer to Figure 3). These valves
are discussed in detail in lesson 62B-206.
D. Air vents or air cocks are installed on top of the steam drum to
expel air from inside the steam drum during cold boiler light-off or when
filling the boiler. The air vents or air cocks are shut when the
boiler starts generating steam.
SAFETY VALVE CONFIGURATION
E. The internal fittings in the steam drum help distribute the water
evenly throughout the drum, separate the generated steam from the water
and remove moisture from the steam before it leaves the boiler (refer to
Figure 4).
1. Lower baffle plates or apron plates Separate the
incoming feedwater and generated steam and direct the steam to the separators.
2. Primary separators (cyclone separators) Separate
most of the water from the steam by giving it a cyclone or rotary motion
so that the water particles are expelled from the steam by the centrifugal
forces. These separators are vertically mounted in the steam drum
so that the steam rises out the top and the water falls back into the steam
drum.
3. Secondary separators (chevron dryers) Remove additional
moisture from the steam by changing the direction of steam flow several
times. The steam passes on but the moisture cannot make the direction
change with the steam. These separators are mounted above the primary
separators and direct steam to the dry box which collects the steam at
the top of the steam drum, directing it to the steam outlet piping to the
superheater.
4. Feedwater leaves the economizer and enters the
boiler through the internal feed pipe and becomes "boiler water."
Perforations along the side of the feed pipe allow water to be distributed
evenly throughout the steam drum (refer to Figure 4)
STEAM DRUM INTERNALS
5. Since suspended solids may accumulate on the surface
of the water in the steam drum, there must be means of removing them.
The surface blow pipe is used to remove these light suspended solids from
the surface of the water and to reduce the total dissolved solid content
of the boiler water. Suspended solids usually consist of oil, salt
contaminants, or excessive treatment chemicals which can cause foaming
on the water surface. Dissolved solids usually consist of salt contaminants
and treatment chemicals that are in solution.
F. The D-type boiler uses the principle of accelerated natural circulation
to circulate water through the boiler. To enable this principle to
work, relatively cool water will naturally circulate through large diameter
pipes to distribution points low in the boiler. The downcomers are
these large diameter pipes connecting the steam drum with the water drum
and lower headers to ensure proper circulation by delivering water from
the steam drum to the water drum and lower headers. The downcomers
are located between the inner and outer air casing to protect them from
the direct radiant heat of the furnace.
G. The water drum is located at the bottom of the boiler below the
main generating bank and acts as a lower reservoir of water for distribution
to the main generating bank. Also, this large drum serves as a collection
point for solids (sludge) that precipitate to the bottom that are removed
by bottom blowdown.
H. The sidewall header is located along the furnace sidewall connecting
sidewall tubes from the furnace floor to the steam drum. It distributes
water to the sidewall tubes and provides another blowdown point for sludge
removal. The sidewall tubes are two inch tubes which protect the
boiler sidewall refractory from the direct heat of combustion and generate
a small amount of steam.
I. The lower rearwall header is located along the furnace rearwall
from the furnace floor to the steam drum or upper header to provide a lower
junction for rearwall tubes. It distributes water to the rearwall
tubes and provides yet another blowdown point for removal of sludge.
The rearwall tubes are two inch tubes which protect the boiler rearwall
refractory from the heat of combustion and generate some steam.
NOTE: By using wall tubes, more of the heat in the furnace
is absorbed by water and less refractory material is required, thereby
increasing boiler efficiency and reducing the boiler weight.
J. The upper rearwall header is often called the "floating header"
because of its free-standing design. It is located along the rearwall
of the furnace roof to provide an upper junction for the rearwall tubes.
It collects the steam generated in the rearwall tubes and direct it to
the steam drum through riser tubes.
K. Riser tubes are large tubes located above the furnace roof to provide
a connection between the upper rearwall header and the steam drum.
L. Superheater screenwall tubes help protect the superheater from direct
radiant heat of the furnace. The screen tubes consist of two to three
staggered rows of two inch tubes which are usually connected from the steam
drum to the water drum. Some boilers have a screenwall header installed
parallel to the superheater along the furnace floor as a lower connection
and a blowdown point for sludge.
M. The steam passes through the superheater picking up sensible heat
(about 300-400° F) which increases the energy of the steam, allowing
it to perform more work. The superheater is composed of superheater
headers which distribute steam to the superheater tubes or elements and
direct it from the inlet to outlet piping. These headers and elements
can be either vertically or horizontally mounted. (Refer to Figure 5).
N. The bulk of the steam generated by the boiler is formed in the main
generating bank because it has the largest heating surface. This
is a large group of one inch tubes which run from the water drum to the
steam drum and are located behind the superheater.
O. Since these boilers are uncontrolled superheat and the plant is
designed to use lower temperature steam in many applications to help reduce
construction and maintenance costs, the steam needed for these services
must pass through a desuperheater. The desuperheater is a multi-pass
tube bundle which is located in the water drum in most boilers. There
are several boilers which have the desuperheater in the steam drum.
As the superheated steam passes through the tube bundle, it gives up heat
to the boiler water in the water drum.
P. The boiler is protected from the high temperatures of combustion
by the refractory. Refractory lines the inside surface of the inner
casing enclosing all of the furnace area and extending to the outer row
of generating tubes. There are several different types of refractory
which work together to protect the boiler.
1. Firebrick is a heavy casted refractory used as
the outer layer of refractory and is exposed to the direct flames of combustion.
It has poor insulating qualities, but it will withstand direct flame contact.
2. Insulating brick is a lightweight casted refractory
used between the insulating block and firebrick. It has good insulating
properties, but it will not withstand direct contact with flame.
3. Insulating block is a pressed fiber material used
next to the inner casing. It has the highest insulating properties
of the various refractory, but it will not withstand direct contact with
flame.
4. Burner tiles are preformed refractory used to
form burner cones around where the burner assembly protrudes into the furnace.
Burner tiles are a specially shaped, heavy casted refractory used next
to the insulating brick around the burner openings. They have poor
insulating qualities, but they will withstand direct flame contact.
5. High temperature castable refractory is used to
fill in gaps in refractory or where shaping is needed to cover irregular
shaped items. It is used to patch refractory or to smooth uneven
areas between brickwork. It is packaged dry and must be mixed with
water prior to use, very much like cement or plaster.
6. Baffle tiles are a specially shaped refractory
made of silicon carbide for use in some boilers to form baffles on superheater
screen tubes. These baffles direct the flow of combustion gases across
the superheater to help maintain the temperature within design parameters.
7. When the refractory is installed and stacked,
it must be held in place. Anchor bolts are used for preformed refractory.
The anchor bolts are connected to the inner casing to support and retain
the refractories in position.
8. Since the boiler expands and contracts with heating
and cooling, expansion joints are built into the refractory to allow for
the thermal expansion and contraction.
Q. Since the boiler expands and contracts as it heats up and cools
down, sliding feet are installed to allow the boiler to move easily.
The feet are located below the boiler, usually under the front end of the
sidewall header and water drum. There is a greased phosphor bronze
friction plate on which these feet will move. The planned maintenance
system (PMS) requires lubricating the sliding feet every month. Some
newer ships have permalube sliding feet which never require lubrication.
Failure of the sliding feet to move can cause cracks in the air casing
and can cause header handhole plug leaks. Movement indicators are
installed on the sliding feet which have to be checked prior to light off,
during warm up, and after the boiler is on line to ensure positive movement
of the sliding feet. Each time sliding feet are checked the results
should be logged in the fireroom operating log. (Refer to figure 6)
SLIDING FEET CONFIGURATION
Figure 6
R. The boiler is enclosed by casings which provide an airtight boundary
from the boiler furnace up through to the stack area. The inner casing
encloses the boiler fireside area to the base of stack to provide an airtight
lining between the combustion air space and furnace to contain the products
of combustion within the boiler and support the refractory materials.
The outer casing encloses the entire boiler from the bilge to the stack
to provide double encasement so the boiler air pressure is not affected
by the fireroom atmosphere. The combustion air flows through this
space between the inner and outer casing and is directed to the air registers.
The stack is located above the boiler economizer and extends to above the
superstructure to carry boiler combustion products safely away from the
ship. The inner stack or smoke pipe provides a path for combustion
gases to the atmosphere, the outer stack supports the inner stack and provides
a space to receive incoming combustion air to the boiler and protects personnel
from the hot inner stack surfaces.
S. The fireroom watch team must be able to monitor the exhaust gases
to help maintain a clear smoke free stack. Smoke indicators and periscopes
are installed to allow monitoring of the stack gases leaving the boiler.
The smoke indicator is an electro-mechanical device and the periscope is
an optical device. All ships have periscopes and many have electro-mechanical
smoke indicators or stack gas analyzers. These devices are located
above the economizer at the base of the stack so that combustion gases
leaving the boiler must pass through its line of sight or the sensing element.
From monitoring the stack gases, the combustion process can be adjusted
for maximum efficiency or a casualty situation can be detected (Refer to
Figure 7)
TYPICAL PERISCOPE CONFIGURATION
T. Maintaining proper boiler water level is one of the most critical
aspects of boiler operation. To be able to monitor this critical
parameter water level indicators are installed. There are two types,
a direct reading gage glass mounted on the steam drum and a remote water
level indicator. The gage glass gives a direct measurement of the
steam drum water level. The remote water level indicator gives an
inferential indication of the steam drum water level.
1. The direct reading gage glass may be isolated
or removed for maintenance if necessary but, at least two remote water
level indicators must be installed and working. (Refer to Figure 8).
REMOTE WATER LEVEL INDICATOR
Figure 8
2. There are usually two remote water level indicators
in the fireroom for each boiler. They are located on the lower level
and the BTOW/console station. There is also a remote indicator located
at the throttle station for the engine served by that boiler.
3. There are high and low water alarms installed
in the remote water level indicators. They are set to alarm when
the steam drum water level reaches 7 inches above normal or 6 below normal
inches on most steam ships.
U. Because air casing fires sometimes occur in the boiler, a steam
smothering system is installed between the inner and outer casings.
This piping comes from the 150 psi desuperheated steam system and is perforated
to allow the steam to fill the casing and smother the fire. The piping
is located at the lower portion of the casing under the furnace floor and/or
brickpan. Steam can be admitted to the furnance by filling the air
casing and then opening the air registers. (Refer to figure 9).
STEAM SMOTHERING CONFIGURATION
Figure 9
V. Once the steam flowing through the superheater is what keeps it from
overheating, there must be a means of providing a flow prior to any steam
pressure forcing a flow. This means is called the superheater protection
steam system. The inlet is connected to the steam drum steam outlet
piping and outlet is connected to the desuperheater outlet piping. This
arrangement provides steam flow through the superheater during light-off
and securing. It is also the entry point for the steam used to provide
a steam blanket lay-up. The steam bled off the boiler to provide
the flow is routed to the auxiliary exhaust system. When the boiler is
being secured, this system needs to be aligned to prevent over-pressurization
of the boiler because it is still generating steam. This system is
commonly called the superheater bleeder. (Refer to figure 10)
SUPERHEATER PROTECTION SYSTEM
Figure 10
W. Fuel oil burners are located on the boiler front and extend into
the furnace to provide a means of firing the boiler. Depending on
boiler design two to six burners are installed in the boiler. (Refer to
Figure 11).
BURNER ASSEMBLY
Figure 11
1. The burners deliver fuel and air to the boiler
furnace in the proper mixture to obtain optimum combustion. The two
main components of an oil burner are the atomizer assembly and the air
register assembly. The atomizer divides the fuel oil into very fine
particles, the air register admits combustion air to the furnace and promotes
mixing of the air and the fuel oil spray.
2. The types of atomizers used on ships are straight
mechanical, steam, and vented plunger, as described below.
3. In straight mechanical atomization, all the oil
pumped to the atomizer is sprayed into the furnace. The firing rate
of this type of burner is controlled by varying the supply fuel oil pressure
and changing sprayer plate sizes.
4. In steam atomization, steam is used to help atomize
the oil into minute particles and to project a cone shaped spray of atomized
oil into the furnace.
5. The vented plunger type atomizer is designed to
permit a wide range of operation using the straight mechanical pressure
atomization principle without the need to change sprayer plate sizes or
use steam atomization. It is found in 1200 psi boilers.
X. Because the combustion gases leave ash or soot deposits on the tube
surfaces which inhibit efficient heat transfer, the soot must be removed.
Soot blowers use steam to blow soot off of the tube surfaces. In
addition to acting as an insulator, this soot forms sulfuric acid when
it becomes wet and eventually corrodes the tube metal.
1. Boilers have varying numbers of soot blowers but,
there are two basic types, rotary and stationary. They use unreduced
desuperheated steam as the motive force which is reduced in the element
by an orifice to 300 psi for rotating units and approximately 150 psi for
stationary units. Using relatively hot steam at a reduced pressure
minimizes moisture in the steam which can lead to erosion or acid corrosion.
2. The rotary type of soot blower has multi-nozzle
elements. The soot blower head steam valve is actuated by a cam when
the element is rotated. The element can be turned by a crank, chain,
or an air or electric motor. Steam is admitted from the head into
an element which incorporates uniformly spaced nozzles/holes to evenly
distribute steam along the area covered. (Refer to figure 12)
ROTARY SOOT BLOWER
Figure 12
3. The stationary type of soot blower usually has
one or two rows of nozzles directed to the area immediately near the drums
of the boiler. Steam is admitted for a short duration by a manually
or power actuated stop valve.
4. The soot blower element must be kept cool and
clean during operation. To accomplish this, a small amount of air
is piped into the element through a small air line. This air is called
scavenging air and comes from the combustion air that pressurizes the boiler
air casing. This allows a small amount of air to enter the soot blower
element to keep it cool and clean. There is a check valve installed
in this line to prevent steam from entering the air casing during soot
blower operation.
5. The operation of soot blowers is called "blowing
tubes." Tubes are blown on the following minimum occasions:
a. After leaving port
b. Before entering port
c. After making heavy smoke
d. Once each week when steaming
NOTE: EOSS requires tubes to be blown prior to securing
a boiler, if possible.
Y. Since the boiler water chemistry control systems, Chelant or Coordinated
Phosphate, both settle sludge to help maintain water purity, there must
exist a means to remove this sludge. The bottom blowdown system is used
to remove sludge from the water drum and the lower headers. The surface
blowdown system is used to remove suspended particles in the water and
provide a means of changing out the water in the boiler to lower the conductivity
and dissolved substances. Both systems share a common piping arrangement
with an overboard guarding valve and overboard discharge valve. (Refer
to figure 13)
SURFACE/BOTTOM BLOWDOWN PIPING
Figure 13
1. Surface blowdowns are conducted on a steaming
boiler as needed to maintain boiler water within the proper chemical control
limits. The Chelant system also uses an occasional scum blow to maintain
limits.
2. A boiler is bottom blown only when it is secured.
Never bottom blow a steaming boiler since this could cause a loss of natural
circulation and boiler damage. Boilers shall be secured and bottom
blown every 360 steaming hours if the Chelant treatment system is installed
or every 168 if the Coordinated Phosphate (COPHOS) system is in use.
There are many conditions that require a boiler to be bottom blown.
For a complete listing refer to NSTM 221 or 220 volume II.
B. Knowing the water/steam flow through the boiler is critical to understanding
the interrelationships of the boiler components. A summary follows:
1. The flow begins when feedwater enters the economizer
inlet header and flows through the economizer tubes to the outlet header
picking up approximately 100-200° F of sensible heat to about 350°F.
2. Feedwater leaving the economizer enters the steam
drum via the internal feed pipe. The feed pipe distributes the water evenly
along the length of the steam drum.
3. Boiler water then flows over the baffle plates
to the ends of the drum where the cooler more dense water flows down through
the downcomers to the water drum and lower headers.
4. Water in the water drum and lower headers is distributed
to the various generating tubes to replace the water being generated into
steam.
5. As water rises through the tubes, it is exposed
to the combustion gases through the tube walls increasing the water temp
which decrease it's density, allowing it to continue the flow upward.
6. Continuing upward, a portion of the water changes
to steam and enters the steam drum under the baffle plates. The water-steam
mixture is guided to the primary separators which separate the excess water
from the steam by centrifugal motion. Separated water falls
back to the steam drum above the baffle plates.
7. Steam exits the primary separator and enters the
secondary separators where the rapid changes in direction of flow causes
it to give up more moisture. Moisture removed drains back to the
steam drum above the baffle plates.
8. All accumulated steam is directed to the dry box.
The quality of saturated steam leaving the steam drum is designed to be
99.75% moisture free.
9. Steam flows through the saturated steam line to
the superheater inlet header. It makes four passes through the superheater
increasing steam temperature 300-400° F to approximately 850° F
and then exits through the outlet header.
10. Superheated steam leaves the outlet header and
a portion is routed to the desuperheater inlet based upon system demand.
All remaining steam goes through the main steam stop to the main steam
system to provide superheated steam to the main engine turbines, ship's
service turbine generators (SSTG), and on some ships, the main feed pumps
(MFP).
11. The steam that passes through the desuperheater
which is submerged in the water drum, gives up superheat to the surrounding
water. Steam leaving the desuperheater passes through the auxiliary
steam stop to provide steam to all auxiliary system demands.
12. A summary of the combustion gas flow follows:
a. Combustion gases flow from the furnace through
the screenwall tubes, superheater tubes, main generating bank, and economizer.
The hot combustion gases then pass through the inner stack and finally
exit the stack to the atmosphere. (Refer to figure 1)