Titanic’s Prime
Mover – An Examination of Propulsion and Power
(Revised January 31, 2011)
By
As most us know, the
triple-screw steamer Titanic, and her sister ship Olympic, were
propelled by a combined machinery arrangement consisting of two reciprocating
engines and a single Parsons’ turbine. The reciprocating engines were of the
triple-expansion type with one high-pressure cylinder, one
intermediate-pressure cylinder, and two low-pressure cylinders. The Parsons’
turbine, which was fed by exhaust steam from the reciprocating engines, was a
low-pressure reaction type. Each reciprocating engine drove one 3-bladed wing propeller of 23½ feet diameter, one on
the port side of the vessel, and the other on the starboard side of the vessel.
The turbine engine drove a central propeller of 16.5 feet diameter located on
the ship’s centerline directly ahead of the rudder.[1]
The reciprocating engines were designed for 15,000 indicated horsepower (IHP)
each when running at 75 revolutions per minute. The turbine was designed to
develop about 16,000 shaft horsepower (SHP) when running at 165 revolutions per
minute. At those numbers of revolutions, the ship was expected to make 21
knots.[2]
Titanic’s propelling machinery was registered at 50,000 horsepower. Standard
“normal full revolutions” in service was considered to be 78 revolutions per
minute for a speed a little over 22½ knots. When running ahead at 83
revolutions per minute on her reciprocating engines, the entire power plant
would develop about 59,000 horsepower, of which 18,000 horsepower would be
contributed by the turbine.[3]
When carrying those number of revolutions, the ship would have made close to 24
knots through the water.
The arrangement of the
propulsion and power generating plants for these ships are shown in the diagram
below.
The aft most compartment is the turbine engine
room that contained the Parsons’ low-pressure turbine, thrust blocks for the
propeller shafts of the reciprocating engines, and the ship’s two main
condensers that turned exhaust steam back into boiler feedwater. Ahead of that
compartment was the reciprocating engine room with the two reciprocating
engines, the feedwater heaters, the ship’s refrigeration plant, and other
auxiliary equipment. Ahead of the engine rooms were six separate boiler rooms
and 11 stokeholds. All of these compartments were separated from each other by
transverse watertight bulkheads and protected by automatic drop-down watertight
doors.
Generating
Steam
Steam was generated in
Titanic’s boiler rooms located just ahead of the engine rooms on the
tank top level just above the ship’s keel. The ship carried 24 double-ended
Scotch boilers, each 15 feet 9 inches in diameter and 20 feet long with 6
furnaces, three on each end. The ship also carried 5 single-ended auxiliary
boilers, also 15 feet 9 inches in diameter but only 11 feet 9 inches long with
three furnaces each. Although these single-ended boilers could also be hooked
up to the ship’s main steam supply lines, they were generally used to power the
ship’s electric generating plant and other auxiliary engines while the ship was
in port. The total heating surface of Titanic’s boiler plant was 144,142
square-feet with a grate surface of 3,466 square-feet. All of the boilers were
constructed in accordance with the rules of the Board of Trade for a working
pressure of 215 pounds-per-square-inch (psi), and were tested to a pressure of
430 psi. They were arranged for working under natural draught conditions,
assisted by fans that blew air into the open stokeholds. With the reciprocating
engines running at 75 revolutions per minute and 24 double-ended boilers hooked
up, a supply rate of just over 260 lbs of steam per minute per boiler would be
produced.[4]
The Scotch marine
fire-tube boilers such as the ones used on the Titanic contained a large
quantity of water and took a long time to bring it up to pressure. Typically,
it took almost 12 hours from the time one of these boilers were lit until it
could brought on line at a working pressure of 215 lbs per square inch. A bank
of these boilers is shown in the photograph below.
The three furnaces at
the end of a Scotch boiler were in the form of corrugated fire-tubes. Each
furnace terminated in a combustion chamber surrounded by water. From the
combustion chamber the hot gases from the burning coals passed through a bank
of relatively small fire-tubes surrounded by water to the smoke uptake box on
the front face of the boiler. In the double-ended boilers, overall space was
saved since furnaces on opposite ends shared the same combustion chamber. To
prevent cold air hitting the combustion chamber’s opposite wall when a furnace
door was opened, a baffle of firebrick was typically installed in the middle of the chamber. The
diagram below shows a schematic of one of these double-ended Scotch boilers
viewed from the side.
The boiler rooms, also called sections,
were numbered 1 through 6. Boiler Room (BR) No. 1 was just ahead of the
reciprocating engine room and contained the five auxiliary single-ended
boilers. When lit, these boilers would be fired from stokehold No. 1 at the
forward end of the room. No. 2 boiler room was forward of No.1 and contained 5
double-ended boilers. The furnaces facing aft were fired from stokehold No. 2
and those forward were fired from stokehold No. 3. Ahead of BR No. 2 was BR No.
3 also with 5 double-ended boilers that were fired from stokeholds 4 and 5,
respectively. Similarly, ahead of BR No. 3 was BR No. 4 with stokeholds 6 and
7, and ahead of that was BR No. 5 with stokeholds 8 and 9. Ahead of BR No. 5
was BR No. 6 with stokeholds 10 and 11. However, due to the narrowing of the
hull as we approached the bow, BR No. 6 contained only 4 double-ended boilers
instead of the usual 5.
The Titanic had four elliptical-shaped
funnels, but only the three forward funnels were used to take up the waste
gases from the six boiler rooms.[5]
The funnel elliptical cross section measured 24 feet 6 inches by 19 feet 0
inches. Their average height above the casing was about 70 feet. The uptakes by
which the waste gases were conveyed to these funnels were united immediately
above the watertight bulkheads which separated boiler rooms No. 1 and 2 (J),
No. 3 and 4 (G), and No. 5 and 6 (E). The aft most of the three forward funnels
with boiler uptakes, the ship’s third funnel, had a transverse baffle in its
lower end that divided the funnel between boiler rooms No. 1 and No. 2. This
was in the portion between the top of the boiler uptakes and the top of the
funnel casing. The aft side of the funnel, serving No. 1 boiler room, had 5
individual flues each serving a single-ended boiler; while the forward side,
serving boiler room No. 2, had two flues that were offset to port with each
serving one of the two port-side double-ended boilers that could be operated separately
there. A large single combined starboard-side flue served the other three
double-ended boilers in BR No. 2. The ship’s second funnel also had a
transverse baffle in its lower end that divided it between boiler rooms No. 3
and No. 4. In this case, the aft side of the funnel, serving No. 3 boiler room,
had a combined flue serving all 5 double-ended boilers in that room. The
forward side, serving No. 4 boiler room, had 2 flues that were offset to
starboard with each serving one of the two starboard-side double-ended boilers
that could be operated separately there. A large single port-side flue served
the other 3 double-ended boilers in BR 4. The foremost funnel had only a single
transverse division that separated boiler room No. 5 from boiler room No. 6,
each served by a single combined flue.
The baffling was eliminated in
stages as the temperature of the gases dropped. Initially, there was
baffling that divided each bundle of smoke tubes from the next within the
same boiler end. Then there were separate uptake trunks from each boiler
end. Then, once these merge, interior baffles divided each boiler from the
next, and one boiler room from the next up to the top of the uptakes.
Then, in the lower part of the funnel, a transverse baffle dividing one
boiler room from the other plus the additional baffling as noted above
for the single-ended boilers and those double-ended boilers that could be
connected separately to the auxiliary steam supply line. The visible part of
the funnel above the casing had a single, non-divided inner flue which vented
all the gases from the two boiler rooms that it served.
For use at sea, there were automatic ash
ejectors equipped in boiler rooms No. 2 through No. 6, the boiler rooms that
contained the double-ended boilers. These were recessed into the coal bunkers
one located on the port side and the other located on the starboard side in one
of the section’s stokeholds. The specific locations of these automatic ash
ejectors on the Titanic were: BR No. 2 in stokehold 2, BR No. 3 in
stokehold 4, BR No. 4 in stokehold 7, BR No. 5 in stokehold 8, and BR No. 6 in
stokehold 10. These ash ejectors were worked from a duplex feed pump located in
a small pump room located off the ship’s centerline and recessed into the aft
port-side coal bunker in boiler rooms No. 2, 3, 5 and 6. In BR No.4, the pump
room was recessed into the aft starboard-side bunker since the recess in the
aft port-side bunker was used as a store space.[6]
To remove the ash, a trimmer would wheel it in a barrow to the nearest ash
ejector where he would dump it into a hopper. From there the ash would be
carried by a water jet up an inclined pipe and ejected well clear of the ship
above the waterline.
For port use, four ash hoists were used
to lift the ash in canvas bags to small rooms on E deck called “ash places” for
later disposal. These were located at the aft starboard side of BR No. 2, the
aft port side of BR No. 3, the forward starboard side of BR No. 4, and the aft
port side of BR No. 6.
Each boiler
room was separated from another by a transverse watertight bulkhead (WTB) that
ran as high as E deck 11 feet above the ship’s load waterline. The coal bunkers
feeding the stokeholds were arranged transversely on both sides of these
watertight bulkheads, and a watertight passage through the bunker space on the
tank top level to starboard of the ship’s centerline allowed access from one
section to another and was protected by a drop-down watertight door (WTD).
These watertight doors, as well as all the others on the tank-top level, could
be dropped by throwing a single switch on the ship’s navigating bridge, or
locally by a hand lever located near each door, or automatically by a float
under the floor plates should a compartment get flooded accidentally.
Steam from the boiler
rooms was carried by two main steam pipes which passed through the watertight
bulkheads to the reciprocating engine room. Shut-off valves were located at
three of these bulkheads, including the one at bulkhead K going into the engine
room. This was a quick-acting, balanced emergency valve fitted on each main
steam pipe so that the steam could at once be shut off in case of a rupture in
either one of the main pipes. On the after side of this bulkhead in the engine
room were the main steam stop valves, 24-inches
in diameter, each provided with a large separator and a cross connection. The
separators were used in saturated steam lines to separate and remove any
moisture formed because of heat losses. They worked by providing a series of
changes in the direction of the steam flow and included a large surface area to
intercept the droplets. The cross connection allowed either range of piping to
be used for either or both engines. The main stop valves were operated by hand
wheels and screws from the starting platform which was situated in the center
of the forward end of reciprocating
engine room near the bottom.
The Titanic’s coal bunkers were filled
through coaling ports located just above F deck. Although the major bunker
spaces ran transversely across the ship, they also filled some side space that
ran longitudinally between decks F and G. The ship’s coal bunker capacity was
over 6,600 long-tons, enough for 8 days of steaming at full speed.
For the purpose of specifying bunker capacities,
letter designations were given in the Harland & Wolff drawing office
notebook. These spaces and their designations are shown in the figure below.
Also shown are the location of the watertight bulkheads and their letter
designations,[7]
the locations of the watertight passages through the bunkers and their
watertight doors on the tank-top level, ash ejector recesses and ash hoist
locations, and the locations of the coaling ports on F deck.
The capacities in tons of Titanic’s
coal bunkers are given in the table below.
The simplified diagram
below shows a typical midship boiler room with its bunker spaces.
To work the 24
double-ended boilers while at sea during a four-hour watch period required 48
firemen, 20 trimmers, and 5 leading firemen, sometimes called leading stokers.
Each boiler room required 4 trimmers to work the coal and carry the ash to the
ejectors. That is because there were two stokeholds in each of these rooms, one
forward and the other aft, and each stokehold had two bunkers to work, one to port
and the other to starboard. To feed the boiler furnaces required from 8 to 10
firemen. One fireman was responsible for working one end of a given
double-ended boiler. Thus 10 firemen were needed in boiler rooms No. 2 through
No. 5 that had five double-ended boilers in each. In boiler room No. 6, with
only four double-ended boilers, only 8 firemen were needed. The men in each of
these double-ended boiler rooms were supervised by a leading fireman. If some
of the double-ended boilers were not lit, the extra personnel in a watch
section would be assigned other jobs like cleaning machinery, etc.
Each of the six boiler
rooms were equipped with an illuminated telegraph that received orders from a
single transmitting telegraph located in the engine room. The illuminated
colored orders read, from top to bottom, STOP (red), SLOW (blue), HALF (green),
and FULL (white).[8]
There was also a Kilroy’s stoking indicator
equipped in each of the 11 stokeholds. These were controlled by two stoking
regulators that were located in the engine room. The stoking indicator showed
which of the three furnaces was to be fired at a given end of a boiler at a
given time. Only one of the three furnaces on a given end would be opened at
any time, and an arrangement was made on the stoking regulators so that
corresponding furnaces on the other end of a double-ended boiler would never be
opened at the same time. The stoking regulator set the time that each furnace
was to be fired. It could be set to regulate the firing of the furnaces every 8,
9, 10, 12, 15, 20, 25, or 30 minutes, depending on how much steam was needed
and the number of boilers that were connected up at any given time.
A Closed Loop System
The purpose of Titanic’s
plant was to extract stored energy from coal and convert that energy into
useful work used to move the ship, power equipment, heat and light living
spaces, and refrigerate perishable food stores. For propelling the ship, the
heat energy released from the burning of coal in the ship’s boilers was
transferred into steam which was then sent to the ship’s engines where it was
converted into mechanical work turning the ship’s propellers. As with other
steam vessels of the time, Titanic’s power plant was a closed-loop
thermodynamic system that took exhaust steam from the engines and condensed it
back into fresh water, reheated it, and then feed it back to the boilers. A
schematic of the entire power plant arrangement is shown below.
High pressure saturated steam (red) from the
boilers at 215 pounds-per-square-inch gauge pressure (psig) at 394° F is fed to
the high-pressure (HP) cylinder on each reciprocating engine. Here the steam is
expanded down to 78 psig at 322° F (yellow) and then fed to the
intermediate-pressure (IP) cylinder where it is expanded down to 24 psig at
266° F (green). The steam at the output of the IP cylinder is then fed to a pair of low-pressure (LP) cylinders
which further expanded the steam. The exhaust steam from the output of the LP
cylinders from both reciprocating engines, now at 9 pounds-per-square-inch
absolute (psia) pressure and 188° F (light blue), is then directed to
changeover valves where it is led to the input of the Parsons’ reaction
turbine.
In the turbine the steam is further expanded
down to 1 psia at 102° F (blue) where it is then delivered to a pair of surface
condensers where the exhaust steam is condensed back into feedwater at a
temperature of about 70° F (very dark blue). The two condensers had a combined
cooling surface of 50,550 square-feet, and were designed to work under a vacuum
of 28 inches with cooling water at 60° F. Four gunmetal centrifugal pumps (not
shown in the schematic) were fitted for circulating sea water through these
condensers. Each pump had suction and discharge pipes of 29 inches in diameter,
and each pump was driven by a compound steam engine.
On the Titanic and Olympic, the
turbine engine was cut in only when the reciprocating engines were running
ahead at half speed (50 revolutions per minute) or higher. During maneuvering
operations or when going astern, the turbine would be bypassed as steam from
the reciprocating engines would be redirected by the changeover valves to
exhaust directly into the two main condensers.
A pair of dual-type Weir's air pumps drew off
the feedwater from each condenser as well as venting air and non-condensable
gases to the atmosphere, exiting with the condenser circulating water
discharge. The feed water was discharged from the air pumps through separate
feed lines to two 2,790 gallon feed tanks, one placed on each side of the ship
just abaft the bulkhead dividing the engine rooms.
From the feed tanks, the water drained into two 48 cubic-foot hotwell tanks,
one located on each side of the reciprocating engine room. From these tanks the
feedwater was drawn off by two pairs of Weir hotwell pumps, one pair working
each tank. From the hotwell pumps, the feed water was forced through two pairs
of feedwater filters, which removed grease, lubricating oil and other
contaminants. From the feed filters, the water flowed to a Weir "Uniflux"
horizontal-surface feedwater heater located on the forward transverse bulkhead
on the starboard side of the reciprocating engine room. Here the feedwater,
still under the pressure from the hotwell pumps, flowed through the tubes of the
heater. Meanwhile, exhaust steam at 5 psia from the four steam engines that ran
the ship’s electric dynamos was passed through the shell and around the tubes
of this surface heater where it was condensed by the cooler feedwater flowing
inside the tubes. This heat exchange process raised the feedwater temperature
from about 70° F to 140° F. A Weir's mono-type air pump handled the condensed
steam from the dynamo engines that came out of the heater, adding this water to
the feed supply.
Still under the pressure from the hotwell pumps,
the feedwater leaving the surface heater flowed upward to a Weir's direct
contact heater located high up at the level of D deck at the ship’s centerline
on the forward bulkhead of the reciprocating engine room. Here the feed water
passed through a spring-loaded valve and entered the interior of this heater
where it dropped through a conical dispersion plate causing it to fall in
droplets through a cloud of exhaust steam that was admitted from the ship's
many auxiliary engines that ran various pumps and the refrigeration equipment.[9]
This simultaneously raised the temperature of the feed water from 140° F to
230° F while condensing the incoming
exhaust steam from the auxiliary engines and adding it to the feed supply. The
direct contact heater also acted as the main controller for the return feed
pumps by way of an internal float control that automatically regulated the
steam supply to the feed pumps and the hotwell pumps, thereby causing these to
work in unison.
From the direct contact heater, the feedwater flowed down by gravity to four
pairs of Weir's main feed pumps, two pairs being located at floor level on both
sides of the forward end of the reciprocating engine room. The feed pumps
supplied boiler feedwater through a set of feed mains to the various boiler
rooms at a pressure greater than the working pressure of the boilers. These
pumps were connected to the feed mains through valve chests which allowed any
pump to feed any feedwater main. From there, the water was manually admitted to
the boilers, the level being carefully maintained to keep the boiler tubes and
furnaces submerged while maintaining the correct volume of space above the
surface of the water for the production of steam.
This entire process worked continuously as long
as the main plant was in operation. There was a much smaller auxiliary
condensing plant located on the starboard side of the reciprocating engine room
which was used to handle the auxiliaries while the ship was in port.
The ship was also equipped with a silent
blow-off from the main steam pipeline for use
as a "bleed line" to regulate the rise in steam pressure that would
otherwise occur when the plant was put on stand-by and the fires banked. This
was to prevent the safety valves from popping off which might otherwise
occur. The silent blow-off connection was never used
to "dump" a load of steam into the condensers. On Titanic
they led a silent blow-off line from near the starboard side stop valve to the
main LP outlet pipe of the starboard reciprocating engine. From there the
steam was sent to the main starboard side condenser when the change-over valves
were set as they would be with either the turbine or all main engines stopped.
By first expanding the high-pressure, high-temperature steam in this massive outlet
pipe, potential damage to the connections between the tubes and plates in
the condenser that might otherwise occur was avoided.
Despite being a close loop system, there were
always some loss of feedwater supply. Additional fresh water for the boilers
was carried in fresh-water tanks in the ship’s double bottom. These were
located under the reciprocating and turbine engine rooms and had a total
capacity of just over 1000 tons. Distilled fresh water could also be
re-supplied from three evaporators located near the aft watertight bulkhead on
the starboard side of the turbine engine room. If needed, each evaporator could
produce 60 tons of fresh water every 24 hours.
Auxiliary
Steam Supply and the Electrical Power Plant
In addition to the
main steam supply, there were separate steam connections to the pipe supplying
steam to all the auxiliary machinery on board the ship. The auxiliary machinery
included various pumps, the refrigeration machinery, the electrical plant, the steering engines, steam winches
and ash hoists. The auxiliary steam supply came from the five single-ended
boilers in BR No. 1, or from two port-side double-ended boilers in BR No. 2 and
two starboard-side double-ended boilers in BR No. 4. This auxiliary steam pipe
could also be cross connected to the main steam supply lines in the
reciprocating engine room, thus receiving steam from any boiler in the ship.
The ship had four main
electric dynamos with a capacity of 400 kilowatts each. They produced 100 volts
of direct current (DC). These dynamos, and the steam engines that drove them,
were situated in a separate watertight compartment located aft of the turbine
room. The 580 indicated horsepower steam engines that drove each dynamo were
vertical, three-crank, compound engines with one HP and two LP cylinders. They
ran at 325 revolutions per minute, and took in steam at 185 psig from two
separate steam supply pipes to which they could be cross connected. One pipe
was on the port side and was connected to the five single-ended boilers in BR
No. 1 and with the two port-side double-ended boilers in BR No. 2. The other
supply pipe, on the starboard side, was the auxiliary steam supply that was
connected to the five single-ended boilers in BR No. 1, and to the two
port-side double-ended boilers in No. 2, and the two starboard-side
double-ended boilers in No. 4.
In addition to these,
there was a separate steam pipe leading to a pair of emergency dynamo engines
situated on a platform 20 feet above the ship’s waterline on D deck on the aft
side of the turbine engine room casing. These emergency dynamos produced 30
kilowatts of electric power each at 100 volts DC. These sets supplied power to
500 incandescent lamps fitted throughout all passenger, crew and machinery
compartments, at the end of passages, near stairways, and on the Boat deck.
There were also change-over switches that enabled 5 arc lamps, 7 cargo and
gangway lamps, the ship’s navigation lights, the lights on the navigating
bridge (including the wheelhouse and chart rooms), the Marconi apparatus, and 4
electrically-driven boat winches all to be connected up to this emergency
circuit if needed. It was the practice to run these emergency dynamos every
night after sunset in case of an accident to the main electrical supply during
the night.[10]
Each emergency dynamo
was driven at 380 revolutions per minute by a two-crank, compound engine with
one HP and one LP cylinders. The emergency steam supply pipe that fed these
engines ran along E deck above the watertight bulkheads
and was arranged so it can take steam from the double-ended boilers in
any of the three boiler rooms No. 2, 3, or 5. As a backup in case of accident
to the main steam supply pipes, there was a connection that branched off of
this emergency supply pipe to the pumps in the engine room that were connected
to the bilges throughout the ship. There was also a cross connection to this
pipe so that steam reaching the engine room from any boiler in the ship could
be passed up to the emergency dynamos by opening two or three valves.
A schematic of the main and auxiliary steam
supply is shown in the diagram below.
The
Main Reciprocating Engines
Steam flow though Titanic’s
reciprocating engines is shown in the diagram below along with some of the
details of the reciprocating engine room. The engine cylinders, starting from
forward and working aft, were low pressure, high pressure, intermediate
pressure, and low pressure. In this arrangement, the low-pressure expansion
stage was split between two smaller LP cylinders rather than having one very
large LP cylinder. The combined effect of lower weight and four cranks on each
engine shaft instead of three helped reduce overall vibration.
The forward and aft LP
cylinders on these engines both had two slide valves with two ports and a common
chest. The twin slide valves of a given LP cylinder worked from the same
cross-head by a single set of double bar links and eccentrics. The HP cylinder
contained a single piston valve, and the IP cylinder contained two piston
valves that worked similarly to the twin slide valves of the LP cylinders. Each
set of links had its own separate adjustment to control the steam cutoff in the
cylinders.
These two
reciprocating engines would present mirror images to each other as they both
rotated in opposite directions when both were set to go ahead or astern.
Looking forward, the starboard engine rotated clockwise and the port engine
counter-clockwise when run ahead.
Some additional reciprocating engine details
are:
Reciprocating
Engine
|
|
|
Engine
type |
Inverted, Double-Acting, Triple-Expansion |
|
Number
and type of cylinders |
1 HP, 1 IP, 2 LP |
|
Engine
Weight |
1,000 tons |
|
Engine
Height |
30 feet |
|
H.P.
Cylinder Diameter |
54 inches |
|
I.P.
Cylinder Diameter |
84 inches |
|
L.P.
Cylinders Diameters |
97 inches |
|
Stroke |
75 inches |
The word
"Inverted" refers to the fact that the cylinders are positioned above
the crankshaft, a throwback to the early days of steam engines when the
cylinders were located below the crankshaft. The term “double-acting” refers to
the admission of steam alternately to both sides of a piston first pushing it
up, then pushing it down. The process is
shown in the simplified animation below.
The term
"triple-expansion" refers to the number of stages that the steam is
worked through a particular engine. There are also “double-expansion,” or more
commonly called “compound” engines, with only two stages of expansion, and
"quadruple-expansion" engines which have 4 stages of expansion. All
of these engine types have multiple pistons operating at different pressures
driving a common crankshaft. Steam from the boilers was first admitted to the
high pressure stage where it expands to a lower pressure as it forces the
piston up and down. From this stage the steam is exhausted into a manifold and
then to the inlet valve of another stage designed to operate at a lower
pressure. Again power is produced as the steam does work on that piston and
expands down to yet a lower pressure. This process continues through each stage
until the steam exhausts from the final stage where it is sent to a condenser,
or as in the case of the Olympic and Titanic, to a turbine engine
where it is expanded down further extracting more work from the steam.
The amount of work
done by a reciprocating steam engine is shown by what is called an indicator
diagram. This traces out pressure Vs. piston travel (which is proportional to
cylinder volume) during a complete engine cycle. The area enclosed by the
diagram represents the amount of net work that is done on the face of a piston.
Since these engines were double-acting, the total work done for one revolution
of given cylinder is the sum of the work for both ends of that cylinder, or
twice that shown on the diagram. For a triple-expansion engine, this must be
done for each cylinder and the results added to get the total work done. A
typical indicator diagram for a single cylinder is shown below.
There are four key
areas on the indicator diagram. The Admission point is where steam is first
admitted to the cylinder, the Cut-off point is where steam is cut off from the
cylinder and allowed to expand as the piston continues to move out, the Release
point is where the expanded steam is first allowed to exit the exhaust port,
and the Compression point is where the exhaust port is closed causing the
remaining steam in the cylinder to be compressed to a higher pressure just
before the admission point is reached again. The work done per revolution, the
area within the diagram curve, multiplied by the revolutions per minute of the
engine is an indication of the amount of power produced in the cylinder.
According to White Star Line rules, it was the responsibility of the Chief
Engineer to see that his staff were all familiar with the use of these
Indicator diagrams, and that the indicated power taken from them was
periodically compared with the expenditure of coal on board.[11]
A set of combined
indicator diagrams plotted to a common scale of pressure and cylinder volumes
for a triple-expansion steam engine using the working pressures that were used
on the Titanic is shown in the figure below.
Titanic’s
4-cylinder reciprocating engines were balanced on what was called the Yarrow,
Schlick, and Tweedy system. The crank throws were not arranged at 90-degree
intervals as one might assume. Instead, vibration was reduced by adjustment of
the relative crank angles and crank sequence being used. Beginning with the HP
cylinder piston at top dead center (TDC), the crank sequence and angles of the
engines were: HP at TDC, then a 106° rotation for the IP to TDC, then a 100°
rotation for the forward LP to TDC, then a 54° rotation for the aft LP to TDC,
then a 100° rotation for the HP to return to TDC.
This crank arrangement
is shown in the diagram below.
When maneuvering the
engines, the engineers would be at the controls on the bottommost platform,
called starting platform, located between the two engines near the forward LP
cylinder columns. It was their job to control the steam supply and the
direction of engine rotation. When entering or leaving port, the chief engineer
would also be on the starting platform along with two senior and three junior
engineers. The chief engineer would be in the center of the platform, midway
between the two reciprocating engines near the two hydraulic reversing engines
(called Brown’s engines) watching every movement of the telegraphs, and seeing
that all orders were being carried out. The two senior engineers worked the
controls for the two engines, and a junior engineer wrote down every order that
came down from the bridge and logged the time to the nearest minute off an
electric clock that was controlled by a master clock in the chart room on the
bridge. Two other junior engineers were there on the starting platform to
answer the engine-order telegraphs which were located about 12 feet apart on
the forward LP cylinder columns of the engines.
To maneuver a
triple-expansion steam engine, an engineer would open a throttle valve to
regulate the supply steam to the engine. The valves on the cylinders were
actuated by means of eccentrics driven by the crankshaft, there being two
eccentrics, one for ahead and the other for astern, connected to an expansion
link which operated a cylinder valve by way of a connecting rod. This was
called a Stephenson arrangement. With the expansion link full over to one side,
one of the eccentrics alone, ahead or astern, would actuate the cylinder valve.
With the expansion linkage at the middle point, both eccentrics would have
equal influence and the cylinder valve would not admit steam for either astern
or ahead operation even with the supply valve opened. At intermediate positions
of the expansion linkage, steam would be cut off from the cylinder at some
intermediate point in the piston stroke allowing steam to expand in the
cylinder to achieve maximum economy of steam usage for a desired indicated
power output. The diagram below shows some of the details behind a typical
Stephenson linkage arrangement.
An animation of the
motion of this linkage for an intermediate position of the expansion link for ahead
operation on the port reciprocating engine’s HP cylinder is shown below.
To control the
direction of the engine and set the cutoff point, the engineer at the controls
would have to move a “reversing” lever that would cause the expansion linkage
to move to an ahead or astern position. For large engines such as on the Titanic,
a hydraulic cylinder called a Brown’s engine would be used to actually move the
linkage. The movement of the linkage would be adjusted to control the desired
cutoff point that steam is admitted to the cylinders and thereby set the
desired degree of steam expansion within the cylinders. At full ahead speeds, a
cutoff point of about 40 to 45% of the piston stroke might be used.[12]
In an extreme
emergency, it was possible to reverse the engines from full ahead to full
astern. However, this was not an instantaneous process. In general, and with
the engineers ready at the controls, it would typically take from 10 to 20
seconds from the time the order was received in the engine room for the engines
to first come to a stop and first start to reverse their direction of rotation.
It then would take another 30 to 50 seconds before the engines would be backing
hard.[13]
The lifting of the changeover valves to redirect exhaust steam from the reciprocating
engines to the condensers directly, thus taking the turbine engine off line,
would take only about 10 seconds once the changeover-valve reversing lever was
swung over.[14]
Both reciprocating
engines were also equipped with Aspinall governors that worked with a Brown's
engine that controlled the reciprocating engine's throttle
valve in case it started to race such as in the case of a propeller
partially or completely breaking the surface of the water. It was also designed
so that in the cases of really severe racing, such as the dropping of a
propeller or a broken shaft, the governor would lock the throttle in the closed
position until the engine was stopped. The reciprocating engines were connected
to the propeller shafts by way of thrust shafts that contained 14 collars to
take up the thrust from the propeller shaft. On each side of these collars were
thrust rings in the thrust block that were fixed to the hull of the vessel. The
thrust blocks for the reciprocating engines were located just aft of the
forward watertight bulkhead in the turbine engine room.
The Turbine Engine
There was no throttle control for the turbine
engine. The turbine simply responded proportionally to the amount of exhaust
steam being received from the reciprocating engines. The changeover valves
ahead of the turbine did provide some form of speed control as there was an
automatic override whereby the turbine's Proell centrifugal governor actuated
the Brown's engine used to operate the two piston changeover valves. If the speed
of the turbine rotor exceeded 10% above the maximum number of revolutions
dialed in, the changeover valves would redirect steam
to the condensers until the speed of the turbine dropped below the preset
limit.
When the turbine engine was connected up it ran
at a rate of about 2.22 times that of the reciprocating engines. According to Olympic’s
Senior Second Engineer John Therle, when the reciprocating engines were making
80 revolutions per minute (rpm), the turbine was making between 175 and 180
rpm; and the maximum number of revolutions on the turbine was 190 rpm when the
reciprocating engines were run at their highest possible speed.[15]
The following diagram shows the relationship between the number of revolutions
on the turbine engine to the number of revolutions on the reciprocating engines
based on the above.
Steam flow through Titanic’s
turbine engine is shown in the diagram below. As mentioned before, during
maneuvering, going astern, or for speeds less than half-ahead (about 50
revolutions per minute on the reciprocating engines), the turbine engine would
be disengaged by actuating the changeover valves. This would redirect steam
from the input to the turbine and send it directly to the main condensers. The
redirected steam path is shown in the diagram below as dashed lines.
Some additional turbine engine statistics are:
Turbine
Engine
|
|
|
Engine
type |
Parsons’ Direct-Coupled, Low-Pressure Reaction |
|
Turbine
Weight |
420 tons |
|
Rotor
Diameter |
12 feet |
|
Rotor
Length |
12 feet 8 inches |
|
Input
Blade Length |
18 inches |
|
Output
Blade Length |
25.5 inches |
The turbine was a
direct-coupled type, meaning that its rotor was directly coupled to the
propeller shaft without any reduction gearing. It was also a multistage
reaction type turbine in which there were alternating rows of fixed blades and
moving blades. The moving blades projected radially from the rotor and were
mounted so that the spaces between the blades have the shape of nozzles. The
rows of fixed blades were of the same shape and also had spaces between the
blades with the shape of nozzles. These fixed blades were fastened to the
casing in which the rotor revolved. The fixed blades guided the steam into the
moving blades. The alternating rows of turbine blading increased in size along
the length of the turbine to make full use of the energy of the steam as it
expanded while passing through. On the Titanic this amounted to a 42%
increase in blade length from the input side of the turbine forward to the
output side aft.
A simple animation of
a reaction turbine is shown below.
The reaction turbine
is moved essentially by three forces. The first is the reactive force produced
on the moving blades as the steam increases in velocity as it expands through
the nozzle-shaped spaces between the blades. The second is the reactive force
produced on the moving blades when the steam changes direction. And the third
force is the push or impulse of the steam impinging upon the blades. The
turbine is moved primarily by the first two reactive forces, and only to a lesser
extent by the direct impulsive force.
The advantage of the
turbine was that it could take steam below atmospheric pressure and expand it
further down to a pressure just a little above a vacuum. The turbine was very
large and rotated relatively slowly, a little over twice the rotational
velocity of the reciprocating engines. This allowed it to be directly coupled
to the shaft of the central propeller without the need for reduction gearing.
A Quadruple Expansion System Overall
The Titanic, as
well as her sister ship Olympic, used this hybrid arrangement of
combined machinery. The low-pressure reaction turbine was a fourth stage of
expansion, extracting every ounce of power from the steam supplied from the
boilers. The result was an increase in fuel economy and overall horsepower. A
simple animation of the combined propulsion system used on Titanic is
shown below. This real-time animation models the operation of the reciprocating
and turbine engines closely, including the crank arrangement described above.
As a point of
comparison, the all-turbine equipped Cunard Line’s Lusitania and Mauretania
had a gross tonnage of about 70% that of Titanic.[16]
Yet, these two ships required a power plant that produced about 50% more power
to drive them at a design speed of only 4 knots faster.[17]
That is for a design speed of about 19% above the design speed of Olympic
and Titanic, the Lusitania and Mauretania had to generate
about 68,000 HP compared to 46,000 HP for Olympic and Titanic, as
well as having a corresponding increase in coal consumption.[18]
As a specific example,
Lusitania on her third transatlantic crossing westbound burned an
average of 1,090 tons of coal per day from Queenstown to New York at an average
crossing speed of 24.25 knots.[19] Olympic on her maiden voyage burned an
average of 629 tons per day during the crossing from Queenstown to New York at
an average crossing speed of 21.43 knots.[20] The difference is a 73% higher coal
consumption rate on Lusitania for a gain of only 2.8 knots (13%) in
average crossing speed for the smaller, all-turbine equipped vessel. Looking at the total coal consumed for the
Queenstown to New York crossing, the 31,600 gross-ton Lusitania burned
4,976 tons of coal, while the 45,300 gross-ton Olympic burned only 3,540
tons of coal; a total savings for the larger, hybrid-propulsion equipped vessel
of 29%.
Acknowledgement
I would like to thank both Mark Chirnside and
Scott Andrews for their consultation and review of this fascinating topic, and
the sharing of detailed information and knowledge with me over the past few
years.