ECONOMICAL SPEED – SLOW STEAMING / SUPER SLOW STEAMING
a) Full Speed
The ship’s speed achievable at the Normal Continuous Rating (NCR) of the main engine which is normally 85% of Maximum Continuous Rating (MCR), 90% of thermal load with the exception of some vessels where the engine maker has determined NCR as 90% MCR.
b) Slow steaming – Low load operation
The ship’s speed when the ship sails with technically the lowest output of the main engine and is in the range of 45% to 60% MCR.
c) Specific to MAN Diesel engines Slow steaming operations (Low load operation)
Operation of Main engine at 40 – 42 % Load. This RPM to be maintained is just one RPM higher than the normal RPM after the auxiliary blower cuts off. We shall call it Low load operation or slow steaming operations.
d) Super Slow steaming Operations
This is operating between 20 to 45 % engine loads. Engines with slide type fuel valves can operate on Long term Very Low Load operation or Super slow steaming operations.
Inspections of Scavenge air manifolds and exhaust gas passageway must be performed during long term operation under 45% loads.
SLOW STEAMING OPERATION
When slow steaming is carried out, the Master and Chief Engineer should follow the technical requirements mentioned below.
Though all modern fleet vessels are equipped with engines designed for operating on low load without any modifications, Chief Engineer must keep following disadvantages and risks associated with low load operation in mind to ensure safe operations.
- Increased fouling by soot in exhaust gas passages
- Unstable combustion at minimum limit low load
- In some cases, increased thermal load on the exhaust valve
- Increased running periods for auxiliary blowers
- Increased running periods for auxiliary machinery
- Possible increased hull and propeller fouling, especially in tropical
The minimum output of all main engines should remain as 50% MCR. During slow steaming, Master and Chief Engineer should fully understand the technical requirements instructed below:
Auxiliary blowers must be on Auto and cut off:
The Auxiliary blower(s) of main engine should not be operated i.e., scavenging air pressure should always be kept above the setting pressure at which auxiliary blower automatically stops. The scavenging air pressure will vary depending on the sea and hull conditions, etc. If there is any concern about the scavenging air pressure dropping below the setting pressure, the engine load should be increased appropriately.
Maintain Exhaust gas temperatures:
The exhaust gas temperature after the cylinders of main engine should be kept above 230 deg C and below 400 deg C to avoid poor combustions developed due to insufficient air supply. If there is any concern in this regard, the engine load should be increased appropriately.
Periodic increase of engine load:
The engine load should be periodically increased above 75% to 80% of MCR for at least 1 hour once a day in order to burn off any accumulated carbon deposits in the exhaust gas system. The load-up and load-down have to be done very carefully following makers guidelines, with monitoring main engine and turbo charger parameters to prevent malfunction due to build-up of deposits. It will be prudent to observe if the colour of smoke from funnel becomes clearer, if possible.
ME/ ME-C engines require a load increase of one hour every 48 hours, and Turbocharger grit cleaning must be conducted at the same time.
Economizer Soot Blowing:
Soot blow of the economizer and solid cleaning of the turbocharger of main engine should be carried out before decreasing the engine rpm either when the engine load is increased according to Point (3) or whenever the pressure drop across EGE exceeds the recommended upper limit. It should be considered that the vessel equipped with air type soot blowers should use the main air compressors to minimize the operation period of soot blow.
Fuel Viscosity:
Fuel viscosity at engine inlet should be maintained at the lower level of the recommended range as specified in the engine manufacturer’s instruction manual. The tracing steam lines of fuel lines should be in operation. In addition, feed rate of fuel oil into the purifier should be reduced as much as possible.
Central LT cooling water temperature:
In case of the vessels having central cooling system, the LT cooling water temperature should be kept at 36 deg C to maintain the optimum scavenging air temperature.
Air cooler Seawater outlet temperature:
In case of the vessels having sea water cooling system, the sea water temperature after the air cooler should be kept below 50 deg C to avoid deposits and blockages of air cooler tubes.
Supply Fuel additives if necessary:
It is recommended to use Fuel additive for the combustion improvement during the low load Operation. The process of supplying additives must be worked out with the owner and charterer as the case demands.
Jacket cooling water temperatures:
The Jacket Cooling Water (CW) temperature after the cylinders should be kept at following recommended temperatures as specified in the engine manufacturer’s instruction manual to avoid low temperature corrosion of combustion parts.
Cold corrosion has been observed in some cases at loads below 20% with too low cooling water temperature and high fuel sulphur content (above 3%)
Main Engine combustion condition:
Main Engine performance analysis checks are to be done.
The exhaust gas temperature after the cylinders should be kept above 250 deg C to reduce accumulation of unburnt oil and fouling of the exhaust gas manifold causing a fire and eventual over speeding damage to turbocharger. At the same time, it should be kept below 400 deg C to avoid poor combustion developed due to the insufficient air supply.
Exhaust Gas Economiser (EGE) Gas outlet temperature:
The exhaust gas temperature after the economizer should be kept above 180 deg C to avoid low temperature corrosion (Acid corrosion) of the economizer.
Cylinder Oil Feed rate:
In case of engines with mechanical cylinder oil lubricators, as the cylinder oil feed rate at part load is automatically decreased in proportion to the engine speed, cylinder oil dosage tends to be in excess against the reduction of engine load during low load operation. If excessive cylinder oil dosage is confirmed at scavenge port inspections, the cylinder oil feed rate can be reduced and adjusted manually during long term low load operation, this must be based on visual inspection carried out for cylinder liners, piston rings and piston crowns.
Piston rings sticking in ring grooves due to excess cylinder lube oil and/or unburnt fuel deposits, liner polishing and subsequent scuffing due to top/ring land coke deposits can be the result from wrong setting. Keep cylinder oil consumption down to the recommended level.
Keep Engine Room under manned Operations:
In order to detect abnormality of main engine and other machinery in engine room as early as possible during the low load operation, close observation on main engine and others should be made.
Particularly careful attention should be paid to irregular vibration and/or abnormal sound from any machinery part. Engine room will be operated under fully manned condition.
F.O. Analysis results affecting the decision for slow steaming:
If poor combustion fuel is detected, the owners and Charterers must be advised that slow steaming operation is difficult, and priority must be given to achieve safe navigation.
Scavenge space cleaning and inspection:
- Ensure that drain passages from the main engine scavenge box are clear. Carry out the drain procedure at least every 8 hours.
- Increase the frequency of air scavenging manifold and scavenge box space cleaning, as required.
- If feasible, inspect the scavenge spaces within 3 days of starting to slow steam,. Depending on the assessment, the frequency may be reduced or extended to a maximum of once per week.
- Scavenging air space may require increased cleaning due to excess cylinder oil and risk of blow-by. Risk of scavenging air receiver and piston underside space fires. Keeping a good cylinder condition prevents blow-by.
At the end of every low load voyage:
- Inspect EGE internals at the first opportunity. This may require increasing the frequency of EGE water washing.
- Inspect the air scavenging manifold at the first opportunity.
- Inspect the exhaust manifold and EGE. Pay special attention to signs of acid corrosion.
MONITORING OF COMPONENTS / PARAMETERS – SLOW STEAMING
The Master, Chief Engineer must monitor the Main Engine performance strictly during the slow steaming in accordance with the guidance given above.
After the slow steaming with the slowest speed is carried out, the Master and Chief engineer are requested to inspect each component and record the condition as mentioned below for safe operation and proper machinery maintenance. Period for inspection is specified below.
a) To carry out a visual inspection and cleaning (if necessary) of inside of the exhaust gas manifold and inlet grids of the turbocharger at the following intervals:
First time inspection:
At the first port after the commencement of low load operation.
Subsequent inspections:
(i) Three(3) months after the above inspection.
(ii) Six(6) months after the above inspection.
If any sign of abnormality and concern is observed, the inspection and cleaning.
Interval should be shortened .
b) If any sign of abnormality and concern is observed during routine maintenance of the exhaust valves and fuel valves (including injection test), the inspection and maintenance interval should be shortened.
c) To carry out a visual inspection of liners and piston rings along with ring gap measurement through the scavenging ports every month so as to monitor the condition and any sign of over lubrication.
d) To carry out a visual inspection of scavenging manifold and under piston spaces including flap valves every month so as to monitor the condition. The cleaning interval should be decided based on the actual condition to avoid fire in scavenging space due to accumulation of sludge & excessive cylinder oil and to minimize the risk of blow-by.
e) To monitor and record the exhaust gas pressure drop of economizer under the normal load every day, and moreover, to carry out a visual inspection of economizer inside every month so as to monitor the condition, in order to avoid soot fire of economizer due to accumulation of soot. Based on the result of this inspection, the water-washing should be planned and carried out carefully.
As initial data Chief Engineer must record the performance data of Main Engine at low load condition during slow steaming. The following interval is to be followed:
(i) Within one (1) week after the commencement of low load operation.
(ii) During the next laden and ballast voyage after the above measurement.
Note:
1) After the above initial measurements, the performance data should be taken at load as high as possible when the ship speed is increased in order to confirm the normal condition.
2) The performance data to be measured containing the following data:
- Ship speed (knots)
- M/E revolution (rpm)
- M/E output (%MCR) & thermal load (%)
- M/E fuel oil consumption (ton/day)
- M/E inlet fuel oil temperature (degree C) and viscocity (cSt)
- M/E cylinder oil consumption (g/ps/hr)
- M/E cylinder outlets exhaust gas temperature (degree C)
- M/E jacket cooling FW outlet temperature (degree C)
- M/E scavenging air pressure (bar)
- M/E scavenging air temperature (degree C) & dew point temperature (degree C)
- M/E T/C inlet & outlet exhaust gas temperature (degree C)
- M/E T/C revolution (rpm)
- Economizer outlet exhaust gas temperature (degree C)
- T/G power (kW) If Provided .
- D/Gs power (kW)
- D/Gs fuel oil consumption (ton/day)
- Boiler fuel oil consumption (ton/day) if used
- Dump valve opening (%)
- The exhaust gas pressure drop of economizer (mmAq)
Needless to say, continuous slow steaming with the slowest speed will require more attention from Chief Engineer and other engineers. Monitoring of concerned parameters and condition from both the engine control console and the machine side should be made more carefully.
ENGINE LOAD UP – SLOW STEAMING OPERATIONS
Load up of the Main Engine must be carried out once a day and the duration from 45% to 75% load must be 60 minutes. Please keep the load up 75% and above for minimum 30 minutes in order to clean the exhaust gas passage ways. Turbocharger dry cleaning should also be carried out when the engine load is increased. Soot blowing also is to be done then.
It is recommended that the engine load should not be increased too fast and never faster than above timings in order to prevent the deterioration of cylinder condition.
MONITORING OF COMPONENTS – SUPER SLOW STEAMING
First time inspection
After approx 24 hours since starting the Super Slow steaming – Very low load operation.
Inspection parts: All piston crowns (Ring lands, Combustion surface)
Scavenge air receiver including buffer space in all cylinder units. Please also take high resolution photos and examine in the computer by enlarging.
Monthly Inspections
Inspection parts. : All piston crowns (Ring lands, Combustion surface)
Scavenge air receiver including buffer space in all cylinder units. Turbo charger protection grids in Exhaust gas receiver. Exhaust gas economizer inspection and cleaning if required. Please also take high resolution photos and examine in the computer by enlarging.
ENGINE LOAD UP – SUPERSLOW STEAMING OPERATIONS
This routine is to be done every week .When it is necessary to increase the engine load significantly after an extended period of super slow steaming the following procedure is to be followed:
Load up from 20% to 45% Load duration must be 30 minutes.
Load up from 45% to 75% load duration must be 60 minutes .Please keep the load up 75% and above for minimum 30 minutes in order to clean the exhaust gas ways. Turbocharger dry cleaning should also be carried out when the engine load is increased. Soot blowing also is to be done then.
It is recommended that the engine load should not be increased too fast and never faster than above timings in order to prevent the deterioration of cylinder condition.
CYLINDER OIL FEED RATE
In case of engines with mechanical cylinder oil lubricators, as the cylinder oil feed rate at part load is automatically decreased in proportion to the engine speed, cylinder oil dosage tends to be in excess against the reduction of engine load during low load operation. If excessive cylinder oil dosage is confirmed at scavenge port inspections, the cylinder oil feed rate can be reduced and adjusted manually during long term low load operation, This must be based on the visual inspection carried out for cylinder liners, piston rings and piston crowns.
EXHAUST VALVE OVERHAUL
Pressure of exhaust valve stem sealing air is adjusted by scavenge air pressure on some engine types. Lower scavenge air pressure may increase the wear of exhaust valve stem seal ring. Therefore, time between overhaul for exhaust valve may be reduced in connection with the wear of exhaust valve stem seal ring. Also refer details concerning the sealing test of exhaust valve air cylinder, and ensure sealing is good.
The recommendations are to be considered as overall guidelines based on engine performance including the exhaust gas boiler in general.
SPARE AUXILIARY BLOWER – SUPER SLOW STEAMING
As a safety precaution, it is recommended for a vessel to has a spare auxiliary blower on board vessels which are doing super slow steaming for long durations, since the maintenance of auxiliary blowers may be necessary before the scheduled dry-docking.
