Exhaust Gas Economizer (EGE) is a device used to generate steam by using waste
heat from the exhaust gas outlet casing of the main engine (diesel engine) and
considered as waste heat regenerative equipment onboard.
The device is made of a casing and the tubes (fin type or bare type) and arranged
either horizontally or vertically in the casing. Commonly, the tubes are arranged in
bundle or groups and can be removed panel to panel. The casing is reinforced so
that it can withstand the pulsing of exhaust gas from the main engine and the vibration of the ships. It has manholes and inspection holes to perform inspections
and cleaning maintenance as necessary. The casing is covered with heat insulators
and lagging.
A circulation line for feed water is provided either from a separate steam and water
drum or an attached drum. Two units of boiler circulating pumps are connected, one
as in operation unit and the other as stand-by unit. Steam generated is separated by
the steam drum and supplied directly to the applicable equipment.
7.6.1 Operation of Exhaust Gas Economizer
The EGE is started in operation when the main engine (diesel engine) is started to
use. Preparation is required before the main engine is to be used for sea passage
and when the engine will be stand still.
a. Start-up Operation
When the EGE is in cold condition, supply feed water to the EGE water side from
the auxiliary boiler by the following:
i. Open the following valves:
a. air vent valve and pressure gauge root valve of the EGE
b. suction valve of the boiler circulating pump
c. inlet and outlet valve of the auxiliary boiler drum to EGE
d. inlet and outlet valve of the EGE
ii. Make sure that the drain valve of the EGE is closed. Start the boiler circulating pump and gradually open the delivery valve until full open. Wait until water spouts at the EGE air vent then close the air vent valve.
iii. If the steam drum separator is not under pressure, open the air vent valve
(atmospheric valve) and operate the main engine. Close the air vent valve of the
separator drum when steam pressure exceeded to more than 0.1 MPa.
iv. When the main engine is already running and steam generation is increases,
open the steam outlet valve of the steam separator gradually to avoid water
hammer in the piping system.
b. Stopping
EGE operation stops when the main engine ceases its operation and no exhaust
gases passes to the tubes. In normal cases, the boiler circulating pump need not
to be stopped the operation. In such case that the boiler circulating pump should
be stop, such as carrying out maintenance and in unavoidable circumstances
carry out the following;
i. Operate the boiler circulating pump at least 30 minutes after the main engine is
totally stopped. It should continue to run until the EGE tubes temperature is less
than 150oC or equal to the steam temperature at the separator drum.
ii. Stopped the boiler circulating pump when there is no abnormal condition in the
exhaust gas side of the EGE, such as soot fire, temperature abnormalities, etc.,
iii. When the boiler circulating pump is stopped, close all the valves and open the air vent valve of the EGE water side.
7.6.2 Safety Devices
a. Safety Valve
The safety valve is provided to release excess pressure when the EGE steam
pressure exceeded the maximum working pressure during operation.
Normally, the safety valve popping pressure is 125% of the maximum working
pressure. The device is required to be overhauled and calibrated every dry
docking period.
b. High Temperature Alarm Sensor
High temperature alarm sensor is provided to warn the operator from possible
trouble of the EGE. The sensor alarm is commonly set to about 300 C temperature and alarm signal is given to a monitoring device in the engine control room.
7.6.3 Maintenance Procedures and Precautions
A suitable maintenance procedure should be applied to EGE exhaust gas side and
water side to maintain its working efficiency.
a. Cleaning by Soot Blowing
Carry out regular operation of the soot blowing device, i.e., twice a day, every
watch etc. and when the following condition is occurred:
i. When the main engine turbocharger was dry cleaned using marine gritt, the soot
will accumulate in the EGE tubes. Carry out EGE soot blowing immediately.
ii. When the main engine is operating at low load condition continuously, carry out
soot blowing procedures frequently. To increase the effect of soot removal,
operate the main engine load above 75% or more when soot blowing.
iii. Draft loss in the exhaust gas side has increased while the main engine is in
operation, the soot blowing frequency must be increased until a suitable water
washing procedure has conducted.
iv. A rise in the economizer gas outlet temperature indicates increased of
contamination in the heating surface and deterioration in heat exchange.
v. Operation of Automatic Soot Blower;
a.) Always keep the manual drain valve and the steam root valve in open
position. The control panel should be “on” position at all times while the EGE
is in used.
b.) In case of manual soot blower, open the drain valve at the soot blower piping
and open the steam root valve gradually to drain the water accumulated in the
piping. Close the drain valve when steam is coming out. Be sure that no water
remains in the soot blowing piping’s.
c.) Fully open the stop valve of the soot blower and operate the soot blower
handle to a set angle in an approximately one minute per unit. Do the same
steps in all soot blowers.
b. Water Washing of the Exhaust Gas Side Tubes
When soot blowing is not sufficient, water washing procedure of the exhaust gas
side must be conducted. Water washing should also be conducted regularly to
maintain the condition of the tubes and the performance of the EGE.
Carry out water washing procedures as follows:
a. When the draft loss is more than two times of the reference guide and the main
engine is affected.
b. Water Washing procedures:
i. When carrying out water washing, it is better to start when the EGE is still warm
as it has greater soot removing effect.
ii. Open the soot drain valve of the EGE dust catcher and make sure that it is not
clogged including the piping.
iii. Install canvass cover to the exhaust gas inlet to prevent washing water to flow to the turbocharger.
iv. Close all the manholes except on the top cover manhole to make the EGE soot
washing effective. When there are other doors open, the washing water might not
fall at the bottom due to the effect of ventilation.
v. Prepare sufficient amount of fresh water, and keep the supply pressure above 0.2
MPa or as per the maker’s instruction book.
vi. Spray fresh water at each side of the tubes and from top to bottom. The soot that is washed from the top maybe caught in the lower tubes. Operate the boiler
circulating pump momentarily to force dry the soot so that it can be effectively
removed. Damped soot will be dried by the boiler water circulating which
stimulates shrinkage; the soot is isolated from the heating surface and is easily
released.
vii. There are cases that the soot from the dust catcher cannot be washed away
effectively or clogging might occur in the drain piping. Neutralize the soot by using
sodium hydroxide (NaOH). As a guide, about 10 kg of sodium hydroxide (NaOH)
shall be put in every 1 m3 of the drainage in the drainage tank. To prevent the
drainage pipe from clogging and corrosion, carefully wash the drain pipe and the
drainage tank with fresh water.
c. Precautions to Avoid Soot Fire
Soot is the by products of combustion that is adhering to the EGE tubes. When
the soot has reached its combustible state, it can burn out and melt the EGE
tubes.
To prevent soot burning or soot fire:
i. Pay close attention to the operational conditions (draft losses, inlet/outlet
temperatures of the exhaust gas, etc.) of the EGE. Set proper intervals for sootblow
operations and water washing, and carry out such operations properly.
ii. After water washing of the EGE, secure enough time for drying (through the
operation of the boiler-water circulating pump) or remove as much soot as
possible from the heating tubes by conducting air blowing. Remove as much
dropped and accumulated soot as possible from the soot catcher.
iii. Generation of soot is effected by incomplete combustion of the main engine,
affected by the fuel quality and the combustion of lubrication. Operate the main
engine efficiently to minimize soot accumulation in the exhaust gas economizer.
d. Countermeasures to EGE Soot Fire (Actions to be done in case of soot fire)
i. Run both the boiler circulating pumps in parallel if the EGE exhaust gas
temperature becomes abnormally high.
ii. Stop or slowdown the main engine as soon as possible when the situations
permitted.
iii. In the initial stage, operate the soot blower to extinguish the fire.
iv. Arrange for personnel at the EGE area to check its temperature and operating
conditions. Make necessary preparations for the possible breakout of fire.
v. After confirming that EGE temperature has lowered, inspect all tube sections for
sign of burn-out.
vi. Conduct water washing of EGE tubes if necessary.
vii. If there are damage tubes, leaking water may have gone to main engine exhaust
gas passage. Check and ensure that no water is accumulated in the main engine
exhaust gas passage.
viii. Conduct repair of the damage tubes either by bypassing or blanking of plugs for the damage tubes.
e. Precautions against Low Temperature Corrosion
i. Bypassing of EGE Exhaust Gas Flow
When the ship is in maneuvering/main engine is at low load condition operate the
EGE exhaust gas bypass valve, if equipped. Bypassing the flow of exhaust gas to
the EGE prevents the accumulation of soot forming corrosive byproduct of
combustion that can cause low temperature corrosion to the tubes.
ii. Controlling of Boiler Circulating Water Quantity to the EGE
When the ship is in maneuvering/main engine is at low load condition the boiler
circulating water quantity to the EGE is controlled by a flow control valve (if
equipped). It increases/decreases the quantity of circulating water to the EGE, to
maintain suitable exhaust gas outlet temperature to a point that it will not fall
down to a temperature that may cause low temperature corrosion to the tubes.
