Due to the downturn in the shipping market and the increasingly stringent emission control of main engines, the cost of Marine fuel is rising.
In order to save fuel costs, ship charterers usually require the main engine to slow down, but slow down will inevitably lead to increased sailing time, rental costs, so we must find a balance between fuel costs and rental.
The author received an instruction from the charterer on a certain voyage:
On the premise of not changing the technical parameters, the best speed of the main engine is found out for decision reference.
First, the main engine speed down sailing optimal speed decision
After consulting the relevant literature, the author found that most of the data mentioned the algorithm of ship economic speed and its consideration factors;
Consult the main engine manual, you can also get the relationship between fuel consumption and speed of the main engine.
The ship's main engine condition and ship condition change during actual operation, so these calculations may not be accurate.
A series of tests are carried out to determine the optimum speed of the main engine when it is sailing at reduced speed.
1. Test data
The main engine of the ship is MAN B&W 6S60MC low-speed diesel engine, using IFO 380 fuel, the maximum continuous operating speed (MCR) is 89r/min, the power is 10316 kW, and the usual continuous operating speed (NCR) is 82.6 r/min.
The test voyage was from Hay Point to Singapore, with a distance of 3937 n mile. The ship was in good sea condition with 73000t full load and an average draft of 13.8m.
The last dry-dock repair of the ship has been about 1.5a since delivery.
2. Benefit analysis
When only rental and fuel costs are considered, and no other factors are taken into account, the formula for calculating the total voyage cost at different engine speeds is MRPM = T(ND + FDPF.O) (1)
Where: MRPM is the total cost of the voyage, USD;
T is the sailing time of the voyage, d;
ND is the daily rental of the vessel in USD /d;
FD is the daily fuel consumption of the ship, t/d, which consists of the daily fuel consumption of the main engine and the daily fuel consumption of the auxiliary engine (1.6t /d).
PF.O is the current fuel price, USD /t.
By comparing the total cost MRPM at different engine speeds with the total cost at rated speed (82.4 r/min), the profitability at that speed can be obtained.
3. Feasibility
Referring to the host manual, it can be seen that when the host speed is 72.3r /min, the working load is 48% of the rated load.
If the main engine runs under this load for a long time, the auxiliary blower must not be started during operation, otherwise it will easily lead to impact load on the power grid and shorten the working life of the auxiliary blower.
In addition, it is necessary to ensure that the steam pressure of the exhaust gas boiler is large enough so that the oil boiler does not start, otherwise it will lead to additional fuel consumption.
According to the test, when the speed of the main engine is 72.3r /min, the speed of the supercharger is 9100 r/min, the scavenging pressure is 0.088MPa, and the pressure of the auxiliary blower is set at 0.043MPa to start and 0.062MPa to stop, which can ensure that the auxiliary blower does not start. The steam pressure of the exhaust gas boiler is maintained at 0.5MPa, and the oil-fired boiler is not started.
Therefore, the host speed is set to 72.3r /min, which is feasible.
Second, the problem caused by long-term low load of the host
(1) Cylinder lubrication.
The problem of cylinder lubrication must be considered when the main engine runs at low load for a long time.
One view is that the lower speed of the main engine will inevitably make the lubrication effect worse, and the oil supply of the cylinder should be increased to ensure the necessary lubrication. The other view is that the total amount of sulfur entering the cylinder will be reduced when the sulfur mass fraction of the fuel is certain, and the cylinder oil consumption should be reduced.
These two views need further analysis.
(2) Serious carbon accumulation.
The long-term low-load operation of the main engine and the deterioration of combustion will inevitably lead to serious carbon accumulation in the combustion chamber, exhaust valve, supercharger, boiler, etc., especially the carbon accumulation of the exhaust valve and boiler will lead to the deterioration of the main engine working condition.
(3) Steam pressure.
The long-term low-load operation of the main engine will inevitably lead to the reduction of the steam pressure of the exhaust gas boiler, and heavy oil is usually burned during normal navigation, so there must be enough steam.
If the steam pressure is below the set value, it will cause the oil-fired boiler to run, adding additional fuel consumption.
(4) Auxiliary blower.
The reduction of the speed of the main engine will inevitably reduce the speed of the supercharger, resulting in a reduction of the scavenging pressure.
When the scavenging pressure is lower than the set value, the auxiliary blower will run intermittently, resulting in an increase in generator load, and an impact load on the power grid at the moment of start-up, which will lead to the loss of power of the whole ship.
Low scavenging pressure will also worsen combustion.
Third, the countermeasures of long-term low load of the host
1, adjust the cylinder oil filling rate
(1) The basic set value of the cylinder oil filling rate. The ship adopts Jensen mechanical oiler. According to the instructions of the main engine manufacturer, the injection rate is set at MCR
QMCR= 24BSPMCR /(1000Cρ) (2)
Formula:
QMCR is the set fuel injection rate, L /d;
BS is the basic setpoint, set at 1.1g /(kW·h) as directed by the owner;
PMCR is the output power when MCR, when the host speed is 89.0 r/min, the power is 10316 kW;
C is the number of cylinders, this ship is 6 cylinders;
ρ is the cylinder oil density, the ship uses BN 70 cylinder oil,
The density is about 0.9343kg /L at 15 ℃.
According to the calculation, the filling rate QMCR = 48.6L /d.
(2) Oil filling rate changed with the speed of the main engine.
Keep the basic set value unchanged, the cylinder oil filling rate will decrease proportionally with the decrease of the main engine speed, the calculation formula is
Q part = QMCRn part /nMCR(3)
If the main engine speed is set to 72.3r /min, then the cylinder oil injection rate Q72.3 = 39.5L /d, and the daily cylinder oil consumption is QD=6Q72.3 = 236.8L /d, which is consistent with the actual measured value.
(3) Adjustment of filling rate with load change.
At present, some MAN B&W models are equipped with cylinder oil lubricator systems that are adjusted with MEP(mean effective pressure), and the cylinder oil filling rate is adjusted proportioned in the range of 40% to 100% MEP.
If the main engine is not equipped with this system, then you can manually adjust the fuel rate.
The calculation formula of the filling rate adjusted according to MEP is
Q part = QMCRP part /PMCR(4)
In the formula:
Part P is the average effective pressure under partial load. According to the test, it is 10.7MPa when the host speed is 72.3r /min.
PMCR is the average effective pressure at MCR, which, according to the test, is 17.9 MPa.
According to the MEP adjustment method, when the host speed is 72.3r /min, the cylinder oil consumption QD is 174.6L /d, that is, the cylinder oil consumption can be adjusted from 236.8L /d to 174.6L /d, but not less than 40% of the basic set value (19.4L /d).
In fact, a more conservative approach was adopted on board to adjust the cylinder oil consumption from 240 L /d to 200 L /d.
After arriving at the port, according to the inspection of the scavenging port, the oil injection rate is still large, and it can be appropriately reduced.
Obviously, it is beneficial to reduce the operating cost of the ship.
2. Reduce carbon accumulation
In order to reduce carbon accumulation, prevent combustion chamber corrosion, and ensure good combustion, the sweep temperature is increased to 41 ~ 43 ℃ as far as possible;
The cylinder liner cooling water temperature is set to a high limit of 82 ~ 83 ℃ (85℃ alarm), which is conducive to combustion and reduce corrosion;
Shorten the maintenance cycle of the exhaust valve and the injector, especially the atomization effect of the injector is poor when running at low speed, and it must be maintained frequently to ensure good combustion;
The main engine is accelerated to 82 r/min twice a day for 1h operation in order to clean the ash accumulation in the exhaust channel;
When the main engine accelerates, the soot blowing of the exhaust gas boiler is strengthened.
At the same time, the exhaust gas end of the supercharger is dry cleaned, and the pressure air end is washed to reduce the carbon and ash accumulation of the supercharger.
3. Control steam consumption
Control steam consumption and close steam pipes of equipment that do not need heating, including the accompanying steam pipes of some fuel lines;
Close the heating valve of the oil tank as much as possible to ensure the heating requirements of the fuel system of the main and auxiliary engines and avoid the start of the oil boiler.
4, control the minimum scavenging pressure
Control the minimum load of the main engine, especially under light load conditions, do not let the scavenging pressure lower than the starting set value of the auxiliary blower.
If necessary, the auxiliary blower can be placed in manual control mode, or lower the starting setting of the auxiliary blower, but it may lead to incomplete combustion, and the exhaust should be closely monitored. Strengthen the daily inspection and maintenance of auxiliary blowers.
Iv. Concluding remarks
In the low-load operation of the main engine, in order to obtain the best working conditions, you can also adjust the compression ratio, injection advance Angle, etc., but the actual operating ship can not adjust the main engine parameters at will, and the ship owner is not willing to run the main engine for a long time at low load.
In fact, with the consent of the owner of the ship, the ship implemented an economic speed of 72 r/min from this voyage, and the results of the operation of several voyages showed that the ship's operating parameters were maintained in good condition.