Boiler Control System

Boiler Control System
The boiler control panel provides operation, control and interlock devices
required for the safe running of the boiler. This control panel directs the
performance of all functions required for automatic operation of the boiler and
provides a central control point for manual operation. The control system also
features a network of alarms which give warning if an abnormality occurs
during boiler operation.
In the event of a serious abnormality occurring, which would make it unsafe
for the boiler to continue in operation, the boiler automatic control system
shuts down the boiler in an emergency mode, by immediately shutting down
the fuel oil supply to the boiler.
Control Panels
ECR Console Remote Indication Panel
This indication panel is installed in the engine control room and it mimics the
monitoring systems and main controls found at the boiler and boiler side
control panel.
The ECR console has the following items:
Drum level indicator
Steam drum pressure indicator
Smoke indicator
Emergency stop switch
Burner run lamp
Lamp test switch
Boiler Side Boiler Control Panel
This control panel is installed at the boiler side. It contains the system power
supply unit, the sequence control for operation of the burner, the automatic
boiler controller and various necessary relay units.
The following alarms are mounted on the control panel :
Electrical AC power source failure
Burner start sequencer inactive
Manual trip
FD fan trip
Pilot pump abnormal
Drum level low-low
Atomising pressure low
Ignition fail
Flame fail
Flame eye abnormal
Burner piston valve abnormal
FO pressure low-low
Control air pressure low
Burner Control System
The boiler control panel (BCP) operates in a number of functions associated
with the boiler including the boiler management system (BMS), automatic
combustion control (ACC), and feedwater control (FWC). There are two
boilers and they are controlled on a master/slave basis with one of the boilers
being designated the master by the control system and the other the slave.
Under normal circumstances the master boiler would operate to supply the
ship's steam requirements but if it cannot meet demand the slave boiler is
started and comes under the control of the boiler control system. The boilers
may also operate in conjunction with the economiser; usually this means that
the economiser is operating at sea and one of the boilers is selected to act as
the water supplier and the steam collector for the economiser. The boiler
would operate if the economiser could not maintain steam pressure for any
reason.
The boiler control system controls the remote, manual and automatic
operations of one single-throat burner which is provided in the roof of the
boiler. This unit contains a programmable sequence control, which operates the
furnace purge, pilot burner and the automatic operation of the burner piston
valve. This is done by linking up with the boiler protective system and the
ACC. In addition, it transmits the automatic adjustment commands of
combustion air quantity and fuel oil quantity to the ACC for the start/stop of
the burner. Combustion control is at the heart of boiler operation because if
anything goes wrong with the boiler, its water supply or the combustion
system, fuel must be shut-off and that will prevent any problem becoming
more severe.
There are three boiler operating modes, one is the 18k mode (steam supply at
18kg-cm2), the second is the 7k mode (steam supply at 7kg/cm2) and the third
is the IGS mode (inert gas system in operation).
In port during cargo discharge the 18k mode would be selected but, at sea only
the 7k mode would normally be required with the oil fired boiler installation
providing support for the waste heat economiser. In some cases, intermittent
oil firing on the boiler may be needed to maintain steam pressure and the boiler
control panel would organise that. Selection of 18k mode or 7k mode is
executed by the changeover switch and selection of a particular mode auto-
matically changes the set point on the pressure indicator control (PIC).
Procedure for the Preparation of Boiler Control System
a) Turn on the power switches of the boiler control panel.
b) Check the action of each pilot lamp and buzzer using the buzzer
and lamp test switch on the control panel.
c) Supply air to all the control devices.
d) Reset the boiler interlock alarm.
e) Check that all alarm lamps are out.
Operating Method
There are three burner operating modes, AUTO (Automatic), MAN (Manual)
and HARD MAN (Hard Manual) mode. The burner is usually operated in the
AUTO mode and the MAN modes are only used in an emergency when the
AUTO mode cannot function. The HARD MAN mode allows for reposition-
ing of the burner switches and the MAN mode is for operating on manual.
Selection of Operating Modes
HARD MAN Mode
When operating in this mode, the operator must always be at the boiler and
able to monitor the situation and provide manual intervention at the control.
The following interlocks are effective:
Drum water level low-low
Flame monitor (pilot burner and main burner)
Operating Procedure
a) Check that the boiler and burner are in operating condition.
b) Start the fuel oil pump and the forced draught fan in the MAN
mode.
c) Turn the burner switch from the OFF position to the HARD MAN
position.
d) Set the fuel pressure controller and the air controller to the MAN
mode.
e) Purge the furnace. To do this the air controller should be manually
operated and the forced draught fan inlet vane should be fully
opened. The furnace must be purged for at least 3 minutes.
f) Check that the fuel temperature is within the required range.
g) Manually operate the air controller and the fuel oil pressure
controller and set the forced draught fan inlet vane and the fuel oil
control valve to the ignition open position.
h) Open the sub-door for the HARD MAN operating switches. Set
the pilot burner switch to the MANU ON position and ignite the
pilot burner. If the pilot burner has not ignited after 15 seconds the
pilot burner switch should be reset to AUTO and the process
started over again from item e) above, 'Purge the furnace'.
i) With the pilot burner burning correctly the main burner is ignited.
The fuel oil valve switch is set to MANU ON and the main burner
should ignite. If the main burner does not ignite after a period of
10 seconds the fuel oil valve switch should be reset to AUTO and
the procedure started over again from item 'e', above 'Purge the
furnace'.
j) With the main burner operating correctly the pilot burner switch
is reset to AUTO.
(Note ! When extinguishing the main burner the fuel oil valve switch must be
returned to AUTO.)
When burner purging is required, the burner purging switch must be used and
this must be set to MANU ON.
Fuel Oil Temperature Bypass
The fuel oil temperature by-pass switch is inside the sub-door. In this bypass
mode the starting interlock for fuel oil temperature low alarm is bypassed. It is
used when starting the boiler in the cold condition when steam is unavailable
for heating. When using 'A' grade heavy oil, the switch is set to BYPASS.
AUTO Mode
This is the mode which will normally be used. All operations, including the
commands for ignition and extinction, are operated automatically.
a) Set the fuel oil pumps, the forced draught fan and the controllers
to the AUTO mode.
b) Turn the burner switch from the OFF to the AUTO position.
When stopping this switch must be returned to the OFF position.
The following sequence of events must be accomplished for main burner
ignition.
a) When the burner switch is moved to the AUTO position the
program timer starts.
b) After a delay of 5 seconds the forced draught fan starts and the
atomising steam valve opens.
c) After a further 20 seconds the forced draught inlet vane starts to
move to the fully open position in order to purge the furnace. It
takes approximately 30 seconds to become fully open.
d) The purge timer (2P) commences when the burner switch is
moved to the AUTO position and the time set on this timer is 60
seconds. After the 60 seconds has elapsed, the forced draught fan
inlet vane starts closing gradually to the ignition position. At this
point the furnace will have been effectively purged.
e) The pilot burner is ignited 35 seconds after item 'd'.
f) When the flame of the pilot burner is detected by the flame eyes,
the electrical igniter stops sparking.
g) The main fuel oil valve opens 5 seconds after the pilot burner is
ignited.
h) The pilot burner is extinguished 15 seconds after it has been
ignited (item 'e').
i) The program timer stops at the lock-in position (graduation 85)
5 seconds after the pilot burner is extinguished.
j) If there is an ignition failure or a flame failure the burner control
goes into an extinction sequence.
k) The extinction sequence commences when the burner CUT
INTERLOCKS have actuated (following item 'j' above) or if the
burner switch is turned from the AUTO to the OFF position.
1) The program timer starts from the lock-in position.
m) The pilot burner is ignited 2 seconds after item 1) above.
n) The burner purge valve opens and the burner is purged for about
6 seconds. The burner is purged when the interlock is normal and
the pilot burner lights up.
o) The post purge period commences and the forced draught fan inlet
vane starts to open to its fully open position when the burner
completes it purge cycle.
p) After the set time (60 seconds) on the post-purge timer is
completed, the forced draught fan inlet vane starts closing to the
ignition position which takes about 30 seconds.
q) The forced draught fan stops after 30 minutes.
(Note ! If flame failure occurs during normal operation, it is important that the
cause of the failure is investigated before any attempt is made to restart the
boiler.)
Automatic Combustion Control
This system automatically regulates the fuel and air supply to the furnace in
order to maintain a preset steam pressure in the boiler. Regulation of the fuel
supply is accomplished by means of the air operated fuel control valve whilst
the air supply is controlled by means of the inlet vane of the forced draught fan.
Fuel supply is automatically cut off in the event of forced draught fan failure
or due to high or low water level in the boiler drum. The ACC system is
electro-pneumatically operated.
The automatic combustion control (ACC) system employed is of
a fuel oil pressure/air pressure measuring type.
The ACC is held at predetermined ignition position until the
burner is ignited.
After the burner is ignited the combustion rate is fixed at the
ignition position until the boiler pressure reaches the predeter-
mined pressure of 5 kg.cm2. This period is called 'steaming'.
When the steaming period is completed, the ACC system goes to
AUTO RUNNING and the combustion rate can be adjusted by the
system.
During the steaming period the combustion rate can be changed
by operating the fuel oil pressure controller in the MANU mode.
The air/fuel ratio can be changed by up to + 20% by altering the
air ratio dial. This dial would normally be set at the position 1.
MAN Mode.
This mode is selected to allow for manual starting of the boiler in order to see
that all stages are completed correctly. It is used when the boiler would
normally be operated automatically as a means of checking the start-up
procedure. The procedure is the same as for HARD MAN start but no switches
behind the HARD MAN sub-door are changed.
Feedwater Control.
This controller is of the two element type; both have proportional and integral
(P+I) control. It measures the steam flow rate and also the water level in the
boiler and adjusts the feedwater supply in line with changes in these values.
The P+I operation is performed when comparing signals from two separate
systems. One of the signals is generated by the difference between the water
level set and that detected in the steam drum of the boiler. The other is from
comparison between the detected steam flow rate from a steam flow
transmitter and the operating signal to the feedwater control valve. In ECON
(economiser) mode the water level change due to the ship's rolling and
pitching is accounted for by consideration of the moving average level in the
steam drum.

Master-Slave System.
The two boilers are independent units, but the control system is designed so
that they operate as a dedicated pair with one being the master boiler and the
other the slave. This means that the master boiler supplies the ship's steam
requirements until the demand exceeds its capacity and then the slave boiler
commences operation. The slave boiler will have been kept in a state of
readiness for such a situation and would have been at the same temperature as
the master boiler in order to avoid delays in wanning through.
a) The master boiler is controlled by actuation of the ON/OFF
switch and the pressure indicator so that the steam pressure of an
individual boiler will retain its set point. The actuation is executed
in the same way as the normal one boiler system.
b) When steam demand on the master boiler exceeds 80% of its
capacity, the slave boiler is switched on. It is switched off when
the steam demand is 30% of the total capacity of the two boilers.
The combustion rate for the slave boiler is the same as for the
master boiler, as the master boiler is exercising overall control.
c) In addition to the ON/OFF actuation of the slave boiler from the
master-slave controller, the slave boiler can also perform its own
ON-OFF actuation of combustion due to the pressure in the boiler.
The slave boiler retains overriding control of combustion due to
the pressure within the boiler and that pressure is the slave
boiler's own set pressure and is independent of the pressure
setting for the master boiler.
d) The low limiter actuates for 15 minutes after the slave boiler has
been started in order to prevent the ON/OFF hunting phenomenon
at the slave boiler.
e) The pressure in the slave boiler needs to exceed 11 kg-cm2 in
order for the master-slave configuration to operate.
f) In addition to the master-slave operation it is possible to select the
PARA mode where both boilers can be turned off independently.
In this case the boilers would be operating together each under its
own independent control and not as a master-slave pair.
Safety and Control System.
Incorporated in the safety and control system are operating functions, or sub-
systems, which react automatically to a change in condition outside of the pre-
set range. Failure of most sub-systems produces a visual alarm on the main
control panel and may also produce an audible alarm. In the majority of cases,
failure of a sub-system requires manual resetting of the cut-out before the sub-
system can be restarted. This provides protection for personnel on the ship and
for the boiler installation, as the reason for a sub-system failure can involve
more than that particular sub-system.


Inert Gas Topping-up Mode.
The inert gas system (IGS) topping-up mode is used in order to allow the boiler
to operate on a minimum load so that the IGS may function correctly. A
minimum boiler load of 25% is required so that the flue gases will contain no
more than 5% oxygen; the flue gas flow at minimum rate will be 10,300m3/h
at a temperature of 5°C. The following items will be interlocked, and hence not
operable, in this mode.
Boiler minimum load is limited to 30% or greater if dumping of steam
is operating.
The IGS running lamp is illuminated
Bypass of the FO burner auto stop
Steam supply valve to soot blowers is interlocked to FULL CLOSE

Boiler Alarm and Trips.
Description.

Emergency Mode.
The boilers may be operated in emergency mode when the burner sequencer is
inoperative.
a) Start the FD fan then fully open the FD fan inlet vane and perform
the furnace purge for 3 minutes.
b) Ensure that the FO temperature is at the specified level,
equivalent to 15cSt.
c) Set the FO control valve and FD fan inlet at IGNITION OPEN
positions respectively.
d) Light off the pilot burner with care and do not exceed 15 seconds
ignition time.


e) Ensure that the pilot burner has ignited and open the FO piston
valve to allow oil to the main burner.
f) Do not keep the FO piston valve open for longer than 10 seconds.
g) If the main burner fails to ignite, the furnace must be purged prior
to a repeat attempt at ignition.
(Note ! During an emergency operation a careful watch must be kept on the
boiler at all times.)
FO Temperature Bypass
When burning HFO during emergency mode this bypass must be operated.
Steam press. 7kg-cm2 mode (start-stop) 6.5 kg-cm2 - 9.5 kg-cm2
Slave boiler start enabled 11.0 kg-cm2
Slave boiler (start-stop) 12.6 kg-cm2 - 4.6 kg-cm2

 
 
Sootblowers
Auxiliary Boiler Sootblowers
No. of sets :

Two fitted to each boiler

Sootblowing has to be carried out at regular intervals to ensure that the heat
transfer surfaces are kept clear of deposits, as these retard heat transfer and can
constitute a fire hazard.
Two sootblowers are fitted to each boiler and should be operated daily when
boilers are in use, bearing in mind the position of the vessel and any local
legislation concerning pollution and clean air. They should be operated when
leaving port prior to shutting down the boiler. The sootblowers are fitted with
an air purge connection, the air being supplied from the discharge of the forced
draught fan. This purge or sealing air keeps the nozzles clear during boiler
operation and provides a seal at the air sealed wall boxes to prevent the escape
of boiler exhaust gas into the machinery space. Non-return valves prevent
steam from entering the air lines.
The sootblowers are only to be operated when the available steam pressure
exceeds 8 kg/cm2. An isolating valve, located in the steam line between the
sootblower steam supply valve (T25V) and the sootblower distribution line,
will only be opened by its associated pressure switch if the steam pressure
exceeds 8 kg/cm2.
Before operation, request permission from the bridge and notify the bridge on
completion.
Procedure for the Operation of the Auxiliary Boiler Sootblowers
a) The boiler should be on a minimum of 50% of full load and the
forced draught fan operating at a high rate during the sootblowing
period. The steam pressure must exceed 8 kg/cm2.
b) With the drain open slightly (No.l boiler valve T29V and No.2
boiler valve T28V), open the steam stop valve (T25V) to the
sootblower header.
c) When the pipeline is warmed sufficiently, shut the drain valve and
open the stop valve fully.
d) Operate the sootblower by turning the handwheel in a clockwise
direction. The sootblower cleans the boiler heating surfaces by
impacting steam from a row of nozzles set along the length of the
sootblower element. A cam and trigger arrangement, incorporat-
ed in the sootblower head, regulates the steam arc issuing from
the nozzles as the sootblower element rotates. This ensures
optimum cleaning of the tubes.
e) Operate the top sootblower first, followed by the bottom one. The
top blower should be operated again. The system is then shut
down and the drain valve opened.
WARNING
Do not operate the auxiliary boiler sootblowers during inert gas operations.