INFORMATION
A.Main
Steam System Components
1. The main steam system (Figure 1) delivers
high thermal energy steam for electrical, main power generation, and is the
steam source for the auxiliary steam system which distributes steam directly,
or reduced, to all other steam equipment throughout the ship. Approximately 75%-80% of all steam produced
goes to the main steam system with the remaining 20%-25% going to the auxiliary
steam system.
2. Main steam piping is supported by spring
hangers which are similar to a coil spring on a car. Sway braces are installed to absorb lateral movement from vibration, movement caused by operating
equipment, and ship's motion. The
spring hangers and sway braces also allow the piping to expand and contract when
steam is admitted or secured. Where
piping goes through bulkheads, bulkhead expansion joints are installed to allow
for piping expansion and contraction while maintaining watertight integrity.
3. In split or twin propulsion plants, there
are two separate, but essentially identical steam generating systems. On platforms with a single plant, there is
one basic steam system. In split plant
ships, these systems are designed to be cross connected or aligned so that an
individual system may be used to supply all steam requirements of the ship, or
segmented with each system supplying parts of the steam requirement.
TYPICAL MAIN STEAM SYSTEM
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Figure
1 |
4. The primary isolation valve for the boiler
is the "steam stop" (see Figure 2).
There is one for main steam (called the "main steam stop") and
one for auxiliary steam from the desuperheater (called the "auxiliary
steam stop"). These steam stop
valves can be operated locally and from the DC deck in order to secure the
boiler in an emergency such as a major steam leak or fire in the space. Some ships have steam stops with air motors
which are designed only to close the steam stops remotely from the boiler operating
station and the DC deck. In all cases,
the stops are opened manually whether or not air motors are installed.
BOILER
STEAM STOP
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Figure 2 |
5. The main engine guarding valve(s) (Figure 3)
is the last valve before the main engine throttle valves. This valve(s) is a flexible wedge gate valve
and is usually installed above the main engine. Some ships have only one main engine guarding valve for both the
HP turbine and the LP turbine's astern element. In these installations, the piping splits off to the HP and
astern element downstream of the guarding valve. Other ships have two main engine guarding valves; one for the HP
turbine and one for the LP turbine astern element. The guarding valve in the single valve design and the guarding
valve to the HP turbine in the two valve design may have a pneumatically
operated, close only motor installed.
These valves may be pneumatically operated from the throttle station in
an emergency, but they are normally operated locally with the valve
handwheel. If the system has a separate
ahead and astern guarding valve, there is usually a separate cable type remote
operator for the astern element guarding valve. The main engine guarding valve functions to prevent unintentional
main propulsion turbine turning or heating in case of a leaking nozzle control
valve. Should the throttle valve
linkage fail with the throttle open, the guarding valve is used to secure the
engine or serve as an emergency throttle.
MAIN
ENGINE GUARDING VALVE
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Figure 3 |
6. The main engine steam strainer is located
after the guarding valve. Its primary
function is to strain any foreign particles from the main steam system to
prevent damaging the main turbines. On
some single shaft ships, the steam strainer is designed with the capability to
function as an in-line desuperheater using main feed as the cooling
medium. Should a ship so designed
suffer a casualty to the HP turbine, steam can be admitted directly to the LP
turbine for propulsion. However, main
steam is too hot for the LP turbine blading and must have the temperature
reduced by the in-line desuperheater.
7. The ahead throttle valve controls steam
admission to the HP turbine. The ahead
throttle valve handwheel is located in front of the gage board. The operator can observe the operations of
the engine by use of the gages and tachometer, and control the engine with the
throttle valve. The throttle valve
controls the opening and closing of the nozzle control valves (generally
multiple nozzle control valves), which are actuated by a lever arm and internal
lifting beam assembly (Figure 4).
STEAM CHEST
AND OPERATING MECHANISM
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Figure
4 |
8. The astern throttle valve controls steam
admission to the astern turbine element.
The astern throttle valve handwheel is located in front of the gage
board. The operator can observe the
operation of the engine by use of the gages and tachometer, and control the
engine speed with the throttle valve.
9. Bulkhead cut-out valves are installed to
isolate steam within the compartment.
On single shaft ships with separate firerooms and engine rooms, closed
bulkhead cut-out valves would prevent steam from flowing into the
engineroom. Cross connect valves
connect steam systems on twin plant ships and allow flexibility in plant
alignment. With these valves, steam may
be aligned for split plant or cross connected operation.
The valves may be
aligned to isolate steam line sections for maintenance, but allow continued
plant operation by rerouting steam from its normal path. On twin plant ships, these valves are
generally located on the after bulkhead of the forward machinery room and the
forward bulkhead of the after machinery room.
On these type platforms, the EOSS clearly determines which system is
aligned cross connected, and which system is aligned split out. This alignment affects the casualty control
actions of watchstanders.
10. SSTG root steam valves provide a means of
isolating idle SSTGs. These valves are
also used for warming-up the SSTG piping by cracking them slightly off their
seats.
11. Superheater header outlet piping has
pressure gages and thermometers for local and remote indications. Remote main steam pressure and steam
temperature gages are located at the engine room control station on the
throttle board.
12. This system is protected from
over-pressurization by the superheater dump valve, a boiler safety valve which
is lifted by a pilot valve on the steam drum or by actual main steam pressure.
B. REACTIONS TO LOAD/FLUID DEMANDS
1. The working pressure of the main steam
system is a function of plant design, equal to either steam drum setpoint or
superheater outlet pressure. The boiler
ABC system is designed to maintain the system at setpoint at all times in
response to changes in demand.
2. Starting equipment or changing the main
engine throttle valve's position affects system pressure. Opening main engine throttle valves or
starting equipment increases steam demand and initially decreases main steam
system pressure. The boiler ABC system
senses this pressure drop and automatically increases the boiler firing rate to
increase the steam generation rate to bring pressure back to setpoint.
3. The opposite happens when a decrease in
steam demand occurs. If equipment is
stopped or the main engine throttles are shut, steam demand decreases and
because the boiler is still producing steam at the previous demand rate, the
system pressure increases. The boiler
ABC system senses this and automatically decreases the boiler firing rate.
4. The main engine throttles have the biggest
influence on the system because of the amount of steam used in the main
engine. If the throttles are opened too
quickly, main steam pressure will significantly decrease as will steam drum
pressure. If steam drum pressure gets
too low, natural circulation will cease and tube damage can occur.
To prevent this
condition, the BTOW will secure the boiler(s) when steam pressure falls to a
given pressure (nominally 510-520 psig).
This situation whereby a boiler is secured because of a throttleman's
actions is referred to as, "dragging a boiler off the line". Should the throttleman close the throttles
too quickly, main steam pressure will increase faster than the ABC system can
reduce the boiler firing rate and boiler safeties can lift. To prevent either event from occurring, the
throttleman should answer all bells in accordance with the acceleration and
deceleration tables provided for each ship.
C. SYSTEM INTERRELATIONSHIPS
1. The main steam system supplies steam to the
600 psi auxiliary desuperheated steam system by passing steam through the
desuperheater located in the boiler's water or steam drum. All steam starts out as main steam and then
is desuperheated to make auxiliary steam.
This desuperheated steam is then reduced in pressure for use in other
systems. Changes in demand from these
systems create a change in demand of main steam.
2. The main steam system supplies steam for the
SSTGs for electrical power generation.
Electrical load changes affect the demand for main steam.
D. FLOW PATH OF MAIN STEAM
1. Flow path of main steam from the boiler
superheater to the ahead throttle valve.
a. Superheater
b. Main steam stop
c. Boiler guarding valve
d. Bulkhead cut-out valves (if applicable)
e. Main steam strainer
f. Main engine guarding valve
g. Ahead throttle
2. Flow path of main steam from the boiler
superheater to the astern throttle valve.
a. Superheater
b. Main steam stop
c. Boiler guarding valve
d. Bulkhead cut-out valves (if applicable)
e. Main steam strainer
f. Main engine guarding valve
g. Astern throttle valve
3. Flow path of main steam from the boiler
superheater to the SSTG throttle
a. Superheater
b. Main steam stop
c. Boiler guarding valves
d. Bulkhead cut-out valves (if applicable)
e. SSTG guarding valve
f. SSTG root steam valves
g. SSTG steam strainer
h. SSTG trip throttle valve