4, Fuel gas leak detection system
The HC concentration sensor in the ventilation system is responsible for detecting gas leaks and displaying it in the form of LEL% (Lower Explosive Limit) in the MOP. If there is a gas leak, when the concentration reaches 30-60% LEL, the ECS will only issue an alarm without changing the operating mode; When the concentration exceeds 60% LEL, the engine automatically switches to pure fuel mode and stops gas supply. Regarding the concentration of gas leaks, USCG has higher requirements. When sailing in USA waters, the parameter needs to be changed to 20-40% LEL alarm, and gas supply will be stopped if it exceeds 40% LEL. The HC concentration sensor can only detect system leaks, but cannot determine the specific leak point. In order to determine the specific location, it is necessary to use safe inert gas for detection, commonly nitrogen 10-300 (400 bar). The source of high-pressure nitrogen can be directly configured with high-pressure nitrogen cylinders to store nitrogen or equipped with nitrogen production devices, and then pressurized by a booster pump.
1. Detection method: After nitrogen leaks from the inner tube to the outer tube, the oxygen concentration between the double walled tubes will decrease. The oxygen concentration is measured through a dedicated detection port in the system using an oxygen concentration detector to analyze whether there is a leak. From Figure 2 of the gas auxiliary system, it can be seen that high-pressure nitrogen is distributed through the gas valve group, but the gas pipeline system is long and complex. During inspection, it is necessary to check section by section from the starting end of the valve group supply to the end (or in reverse). During the design of the system, detection tools and oxygen concentration measurement holes were reserved in the pipelines and cylinder heads for segmented inspection.
2. Leakage detection tools and oxygen meters. Leakage detection tools are tools used to block gas pipes in order to separate the gas pipelines that need to be tested. In order to adapt to different forms of inner tube shapes, different forms of tools have been designed. Before using the oxygen meter, measure the oxygen concentration in the surrounding environment and compare it with the oxygen concentration measured from inside the double walled tube. Figure 13 is a schematic diagram of the detection tool and oxygen analyzer.
Figure 13: Leak detection tools and oxygen meters
3. There are many specialized equipment in gas systems for leak detection, such as end covers, window valves, purge valves, release valves, cylinder heads, gas injection valves, and their installation holes. Their internal gas channels are relatively complex and require several different detection tools to be used in conjunction to accurately detect whether they have leaks.
4. Pipeline verification test: After disassembling and inspecting any components in the gas system, a pipeline pressure test is required to prevent leakage. For the tightness test of the inner gas pipeline, ECS provides an automatic testing program with an operation interface on MOP. Use 10 bar nitrogen and follow the interface prompts to confirm whether the pipeline pressure has decreased. The outer pipe is tested using 7 bar compressed air and checked for operation through the valve group on the ventilation system.
5, Servo hydraulic oil system
The hydraulic system of ME-C-GI mainly consists of HPS (Hydraulic Power Supply Unit), HCU (Hydraulic Cylinder Unit), Low Pressure Supply System, Seal Oil System, Fuel Gas Control Block, Drain Pipes, etc. It provides servo hydraulic oil and actuators for fuel injection, exhaust valve opening and closing, gas injection, and cylinder oil injection.
1. HPS unit is a system that provides servo hydraulic oil, mainly including filtering device, electric servo pump, servo pump with machine belt, safety accumulator module, high-pressure oil pipe, and oil collection pipe with leakage detection probe. The hydraulic oil comes from the engine system oil (or from an independent hydraulic oil tank).
2. The main function of the HCU unit is to perform specific operations for opening and closing fuel and exhaust valves, including distribution blocks, electronic fuel injection systems (ELFI+fuel booster+fuel valve), electronic exhaust valve execution systems (ELVA+exhaust valve actuator+air spring), etc.
3. The main component of the LPS (low pressure supply system) is the low-pressure system booster pump unit. The main purpose of designing LPS is to effectively remove air from the hydraulic components of the HCU unit and gas control module. Normally, it is to increase the pressure to 6 bar based on the oil pressure provided by the system oil pump.
4. Sealing oil system is a component that prevents high-pressure gas from leaking into the servo oil system. The components that pose this risk are window valves and gas injection valves. The sealed oil pump equipped with a safety module pressurizes the oil pressure to about 20-25 bar higher than the gas pressure from LPS, and enters from the gas adapter block on a certain cylinder head, connecting to other cylinders through internal pipelines. Ultimately, the sealing oil will be sprayed into the cylinder combustion chamber along with the gas for combustion, but its consumption is relatively low, about 0.135g/KWh. Figure 14 is a schematic diagram of the sealing oil system.
Figure 14: Schematic diagram of sealing oil system
5. The function of the hydraulic oil drain pipe is to collect ELWI ELGI, The hydraulic oil released from the blow off valve, vent valve, gas injection valve, and gas adapter block is discharged into the discharge chamber of the HCU unit and ultimately returned to the engine system oil circulation cabinet (or independent oil cabinet).
6. Gas injection control hydraulic system (Figure 15), the high-pressure oil generated by the hydraulic system is connected to the control unit of the gas injection device through port P2. The ELWI valve controls the action of the window valve, while the ELGI valve controls the action of the gas injection valve. The main valve cores of the blow off valve and vent valve are opened by servo hydraulic oil, allowing the gas between the accumulator chamber and the window valve to be released into the return pipe or muffler.
Figure 15: Hydraulic schematic diagram of gas injection control
6, ME-C-GI Engine Control System
The reliable and safe operation of dual fuel low-speed engines requires a lot of system support. In addition to the traditional ME-C control system, there are also systems related to the storage, supply, pressurization, safety protection, and control of the second fuel.
1. The traditional ME-C control system mainly includes the EICU unit (Engine Information Control Unit): the information exchange center, which is mainly connected to remote control, security, vehicle clocks, etc. ECU unit: speed control module. CCU unit (Cylinder control unit): The cylinder unit control module receives signals from the angle decoder (TACHO system) and achieves precise control of fuel injection and valve opening and closing through the control of FIVA. It also controls the cylinder injector and cylinder head start valve. ACU unit (Auxiliary control unit): controls servo oil pumps, auxiliary fans, etc. SCU unit (Scavenge air control unit): controls the scavenging system. CWCU unit (Cooling water control unit): controls the temperature of the cylinder liner cooling water according to the engine load.
2. The dual fuel ME-C-GI control system has four gas control units, namely GPCU Fuel Gas Plant Control Unit; Gas Auxiliary Control Unit (GACU) - Fuel Gas Auxiliary Control Unit; GPSU - Fuel Gas Plant Safety Unit; Gas cylinder safety unit GCSU - Fuel gas cylinder safety unit. Like the ME-C control system, these modules are composed of a multifunctional control board (MPC) and software. All modules in the ECS are a dual redundant network composed of Arc network, which has self checking function. Any module disconnection will be displayed in the MOP.
(1)GPCU unit function:
1) Control the inert gas system, receive inert gas pressure signals, HC sensor, open/close signals of inert gas supply valve and vent valve, and issue inert gas supply signals.
2) Send signals such as power failure, system failure, HC alarm, etc. to the alarm system.
3) Send the signal of gas combustion mode to the driver's console and the control panel at the engine side.
4) Receive operation signals from the ventilation system, flow switch signals, and control signals from the dry air valve to control the operation and stop of the ventilation system.
5) Receive the on/off signals of the gas return valve and gas release valve in the gas return system, and control the action of the gas return tank valve.
6) Receive the switch signal of the main gas valve in the gas valve group.
7) Receive signals on the completion status of gas supply preparation and gas supply operation in the gas supply system, and send signals to the gas supply system for gas supply operation or stop, as well as real-time gas load.
(2)GACU unit function: 1) Receive gas supply signals from the gas valve group and pressure signals from the gas passing through the valve group, as well as power failure signals from the valve group system. Receive gas preparation request signals and gas flow restriction signals from the gas supply system. Receive real-time gas flow, temperature, and calorific value parameter signals. 2) Send a gas pressure setting signal to the gas supply system (based on engine load).
(3)GPSU unit function: 1) Receive signals from the gas emergency stop buttons on the driver's console, central control console, and machine locations. 2) Receive signals from HC sensor A and safety flow switch in the ventilation system, and send dry air flow switch signals to the ventilation system. 3) Receive emergency stop signals from the security system and ELWI operable signals. 4) Receive the opening and closing signal of the gas return system vent valve, and send control commands for the vent valve action to the return system. 5) Receive the switch signal of the return gas pipeline test valve in the gas valve group and send the control signal of the test valve. 6) Receive the switch signal of the main valve in the gas valve group and send the control signal of the main valve. 7) Receive the switch signal of the vent valve in the gas valve group and send the control signal of the vent valve. 8) Receive pressure signals from the gas to the engine.
(4)GCSU unit function: Each cylinder of the engine is equipped with a GCSU unit #, which receives signals from HC sensor B in the ventilation system and controls components on the gas control block together with CCU #. CCU # controls the action of ELGI to provide precise timing for gas injection, while GCSU controls the action of ELWI, purge valve, and vent valve. Figure 16 is a schematic diagram of gas control.
Figure 16: Schematic diagram of gas control system
7, Conclusion: This article briefly introduces the composition and control principles of the MAN ME-C-GI dual fuel engine in terms of gas. Safety is the most important for the use of LNG combustible gases on ships. However, where does security come from? Safety comes from the careful design and manufacturing of engine manufacturers and shipyards, as well as the skilled operation and meticulous maintenance of crew members during operation. I think we can learn about ship management during the operation of dual fuel engines from the following three levels. Firstly, master the system composition and basic control principles, have a certain understanding and comprehension of network structure, functions of various modules, hydraulic units, cylinder control units, gas systems, sensor layout, etc., and be able to complete the daily operation of the engine; Secondly, a more in-depth study of the entire control system and engine operating conditions can enable proficient mastery of PMI system and COCOS-EDS system applications. By utilizing various theoretical data, charts, etc., comprehensive evaluation and analysis of ship engines can be conducted, problems can be identified in a timely manner, and appropriate adjustments can be made; Thirdly, it can quickly conduct comprehensive analysis and handling of various faults that occur. In a sense, if the first two levels are well mastered, the probability of engine failure under its management will decrease. The rapid comprehensive analysis of faults not only requires theoretical support, but also the accumulation of rich experience, which comes from the summary of previous cases and one's own careful experience in management. The MAN ME-C-GI engine adopts technologies such as EGRBP (Exhaust Gas Recirculation by Pass), EGRTC (EGR Turbo Cut Off), HPSCR (High Pressure Selective Catalytic Reduction), LPSCR (Low Pressure SCR) in Tier III technology, which mainly deals with NOX emissions from the engine exhaust to meet Tier III emission requirements. The addition of these devices makes the entire engine system more complex. From the perspective of ship management, there are many issues worth considering for dual fuel engine systems, such as the use of cylinder oil, gas consumption, cleaning and management of servo hydraulic oil, speed regulation of engine power, handling of system alarms, daily maintenance and management of gas systems, maintenance of MPC boards, insulation inspection of ECS, and maintenance of exhaust gas treatment systems. The rapid development of new technologies requires managers to keep up with the times, strengthen learning and communication, in order to adapt to the requirements of ship management in the new era.