Influence of Ambient Temperature Conditions Main engine operation of MAN B&W two-stroke engines

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1 Influence of Ambient Temperature Conditions Main engine operation of MAN B&W two-stroke engines

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3 Contents Introduction... 5 Chapter Temperature Restrictions and Load-up Procedures at Start of Engine... 5 Start of warm engine normal load-up procedures... 5 Start of cold engine exceptional load-up procedures... 6 Preheating during standstill periods... 6 Jacket cooling water systems with a built in preheater... 7 Preheater capacity... 7 Chapter Engine Room Ventilation... 8 Air... 8 Air supply... 9 Air pressure Chapter Ambient Temperature Operation and Matching Standard ambient matched engine Non-standard ambient matched engine Design recommendations for operation at extremely low air Closing Remarks... 17

4 4 Influence of Ambient Temperature Conditions

5 Influence of Ambient Temperature Conditions Main engine operation of MAN B&W two-stroke engines Introduction Diesel engines used as prime movers on ships are exposed to the varying climatic conditions that prevail in different parts of the world, and must therefore be able to operate under all ambient conditions from winter to summer and from arctic to tropical areas. As the variations on the surface of the sea are rather limited, the diesel engine will not normally be exposed to really extreme s. However, the changes that do occur in the ambient conditions will, among other things, cause a change in the specific fuel oil consumption, the exhaust gas amount and the exhaust gas of the diesel engine. These changes are already described in our Project Guides and will therefore not be discussed in this paper. Also the scavenge air, compression and maximum firing pressures of the diesel engine will change with climatic changes and, at very low ambient air s, unrestricted engine operation requires adjustments of individual engine parameters. This paper describes our recommendations of load-up procedures on engine start up, the supply of ventilation air to the engine room and engine operation under normal, high and extremely low ambient conditions. The paper is divided into three chapters which, in principle, may be read independently of each other. Thus, Chapter 3 is more or less a copy of our paper Ambient Temp air as a common parameter. The three chapters are entitled: Temperature Restrictions and Loadup Procedures at Start of Engine Engine Room Ventilation Ambient Temperature Operation and Matching Chapter 1 Temperature Restrictions and Load-up Procedures at Start of Engine In order to protect the engine against cold corrosion attacks on the cylinder liners, some minimum restrictions and load-up procedures have to be considered before starting the engine. Below stated load-up procedures are valid for MAN B&W two-stroke engines with a cylinder bore greater or equal to 80 cm, and may with benefit also be applied for engines with a smaller bore. However, if needed, the existing loadup programme recommendation (from 90% to 100% in 30 minutes) is still valid for engines with bore sizes from 70 cm and down. Note: The below recommendations are based on the assumption that the engine has already been well run in. Start of warm engine normal loadup procedures As a summary, the load-up procedures recommended for normal start of engine are shown in Fig. 1. Recommended start of engine at normal engine load operation Fixed pitch propellers Normally, a minimum engine jacket water of 50 o C is recommended before the engine may be started and run up gradually from 80% to 90% Start of warm engine (normal load-up procedures) Required jacket water at normal start of engine: minimum 50 o C FPP: CPP: Recommended start of engine 1. at normal engine load operation A. Run up slowly minimum temp. 50 o C B. Run up slowly, (minimum 30 min) C. Run up slowly, (minimum 60 min) Fixed Pitch Propeller Controllable Pitch Propeller FPP From 0% up to 80% SMCR speed CPP From 0% up to 50% SMCR power FPP From 80% up to 90% SMCR speed CPP From 50% up to 75% SMCR power FPP From 90% up to 100% SMCR speed CPP From 75% up to 100% SMCR power 2. at normal very low engine load operation A. Run up slowly If normally 10% to 40% engine low load operation (slide fuel valves needed) extra slowly load-up procedure is recommended: minimum 30 min from 10% to 40% load and minimum 60 min from 40% to 75% load Fig. 1: Temperature restrictions and load-up procedures at normal start of engine Influence of Ambient Temperature Conditions 5

6 of specified MCR speed (SMCR rpm). during 30 minutes. SMCR = Specified Maximum Continuos Rating. Start of cold engine (exceptional load-up procedures) Required jacket water at start of cold engine: minimum 20 o C FPP: CPP: Fixed Pitch Propeller Controllable Pitch Propeller Recommended start of engine at normal engine load operation A. Run up slowly Minimum temp. 20 o C FPP From 0% up to 80% SMCR speed CPP From 0% up to 50% SMCR power rpm = revolutions per minute B. Run up slowly, Minimum (minimum 30 min) temp. 50 o C FPP From 80% up to 90% SMCR speed CPP From 50% up to 75% SMCR power For running-up between 90% and 100% of SMCR rpm, it is recommended that the speed be increased slowly over a period of 60 minutes. C. Run up slowly, (minimum 60 min) FPP From 90% up to 100% SMCR speed CPP From 75% up to 100% SMCR power Fig. 2: Temperature restrictions and load-up procedures at start of cold engine in exceptional cases Controllable Pitch Propellers Normally, a minimum engine jacket water of 50 o C is recommended before the engine may be started and run up gradually from 50% to 75% of specified MCR load (SMCR power) during 30 minutes. For running-up between 75% and 100% of SMCR power, it is recommended that the load be increased slowly over a period of 60 minutes. Recommended start of engine at normal very low engine load operation For engines normally running at 10% to 40% engine low load operation an extra slowly load-up procedure is recommended compared with above described load-up procedures, and is also shown in Fig. 1. Start of cold engine exceptional load-up procedures As a summary, the load-up procedures recommended for exceptional start of cold engine are shown in Fig. 2. Fixed pitch propellers In exceptional circumstances where it is not possible to comply with the abovementioned normal recommendations, a minimum of 20 o C can be accepted before the engine is started and run up slowly to 80% of SMCR rpm. Before exceeding 80% SMCR rpm, a minimum jacket water of 50 o C should be obtained before the above-described normal start load-up procedure may be continued. Controllable Pitch Propellers In exceptional circumstances where it is not possible to comply with the abovementioned normal recommendations, a minimum of 20 o C can be accepted before the engine is started and run up slowly to 50% of SMCR power. Before exceeding 50% SMCR power, a minimum jacket water of 50 o C should be obtained before the above described normal start load-up procedure may be continued. The time period required for increasing the jacket water from 20 C to 50 C depends on the amount of water in the jacket cooling water system, and on the engine load. Preheating during standstill periods During short stays in ports (i.e. less than 4 5 days), it is recommended to keep the engine preheated, the purpose being to prevent variations in the engine structure and corresponding variations in thermal expansions, and thus the risk of leakages. The jacket cooling water outlet should be kept as high as possible (max C), and should before start up be increased to at least 50 C, either by means of the auxiliary engine cooling water, or by means of a built in preheater in the jacket cooling water system, or a combination of both. 6 Influence of Ambient Temperature Conditions

7 Jacket cooling water systems with a built in preheater For two different jacket water preheater systems, A and B, the positioning of a preheater in the jacket cooling water system is shown schematically in Figs. 3 and 4, respectively. Preheater Preheater pump Preheater bypass For system A, the circulating water flow is divided into two branches, one going through the engine and one going through the cooling water system outside the engine. As the arrows indicate, the preheater water flows in the opposite direction through the engine, compared with the main jacket water flow. As the water inlet is at the top of the engine, the engine preheating is more effective in this way. Diesel engine Fig. 3: Preheating of jacket cooling water system System A Jacket water main pumps Direction of main water flow Direction of preheater circulating water flow For system B, the preheater and circulating pump are placed in parallel with the jacket water main pumps, and the water flow direction is the same as for the jacket cooling water system. Preheater Preheater pump Preheater bypass In both cases, the preheater operation is controlled by a sensor after the preheater. Preheater capacity When a preheater is installed in the jacket cooling water system, as shown in Figs. 3 and 4, the preheater pump capacity, should be about 10% of the jacket water main pump capacity. Based on experience, it is recommended that the pressure drop across the preheater should be approx. 0.2 bar. The preheater pump and the jacket water main pump should be electrically interlocked to avoid the risk of simultaneous operation. Diesel engine Fig. 4: Preheating of jacket cooling water system System B Jacket water main pumps Influence of Ambient Temperature Conditions 7

8 The preheater capacity depends on the required preheating time and the required increase of the engine jacket water. The and time relationship is shown in Fig. 5. The relationship is almost the same for all engine types. If a increase of for example 35 C (from 15 C to 50 C) is required, a preheater capacity of about 1% of the engine s nominal MCR power is required to obtain a preheating time of 12 hours. Temperature increase of jacket water Fig. 5: Preheating of diesel engine When sailing in arctic areas, the required increase may be higher, possibly 45 C or even higher, and therefore a larger preheater capacity is required. The curves in Fig. 5 are based on the assumption that, at the start of preheating, the engine and engine room are of equal s. It is assumed that the will increase uniformly all over the engine structure during preheating, for which reason steel masses and engine surfaces in the lower part of the engine are also included in the calculation. The results of the preheating calculations may therefore be somewhat conservative. Chapter 2 Engine Room Ventilation In addition to providing sufficient air for combustion purposes in the main engine, auxiliary diesel engines, fuel fired boiler, etc., the engine room ventilation system should be designed to remove the radiation and convection heat from Preheater capacity in % of nominal MCR power o C 1.50% 1.25% 1.00% 0.75% hours Preheating time The increase and corresponding preheating time curves are shown for the different preheater sizes indicated in % of nominal MCR power the main engine, auxiliary engines, boilers and other components. A sufficient amount of ventilation air should be supplied and exhausted through suitably protected openings arranged in such a way that these openings can be used in all weather conditions. Care should be taken to ensure that no seawater can be drawn into the ventilation air intakes. Furthermore, the ventilation air inlet should be placed at an appropriate distance from the exhaust gas funnel in order to avoid the suction of exhaust gas into the engine room. Major dust and dirt particles can foul air coolers and increase the wear of combustion chamber components. Accordingly, the air supplied to the engine must be cleaned by appropriate filters. The size of particles passing through the air intake filter should not exceed 5µm. An example of an engine room ventilation system, where ventilation fans blow air into the engine room via air ducts, is shown in Fig. 6. Air Measurements show that the ambient air intake (from deck) at sea will be within 1 to 3 C of the seawater, i.e. max. 35 C for 32 C seawater, and max. 39 C for 36 C seawater. Measurements also show that, in a normal ventilation air intake system, where combustion air is taken directly from the engine room of a ship, the engine room is normally C 8 Influence of Ambient Temperature Conditions

9 Air inlet Air outlet Engine room ventilation fans Air inlet This means that the turbocharger suction air will not be higher than about = 42 C (ref. 36 C S.W.), say 45 C. For arctic running conditions, a ducted air intake system directly to the turbocharger can be an advantage in order to maintain sufficiently high s for the crew in the engine room. With a ducted air intake, the turbocharger s intake air may be assumed to be approximately equal to the ambient outside air. Air supply AE AE ME AE In the case of a low speed two stroke diesel engine installed in a spacious engine room, the capacity of the ventilation system should be such that the ventilation air to the engine room is at least 1.5 times the total air consumption of the main engine, auxiliary engines, boiler, etc., all at specified maximum continuous rating (SMCR). ME: Main engine AE: Auxiliary engines Main ducts for supply of combustion air Fig. 6: Engine room ventilation system higher than the ambient outside air. This difference is even higher for winter ambient air s, see Fig. 7. In general, the engine room should never be below 5 C, which is ensured by stopping one or more of the air ventilation fans, thus reducing the air supply to and thereby the venting of the engine room. Since the air ventilation ducts for a normal air intake system are placed near the turbochargers, the air inlet to the turbochargers will be lower than the engine room. Under normal air conditions, the air inlet to the turbocharger is only 1 3 C higher than the ambient outside air. As a rule of thumb, the minimum engine room ventilation air amount corresponds to about 1.75 times the air con sumption of the main engine at SMCR. Accordingly, 2.0 times the air consumption of the main engine at SMCR may be sufficient. On the other hand, for a compact engine room with a small two stroke diesel engine, the above factor of 1.5 is recommended to be higher, at least 2.0, because the radiation and convection heat losses from the engine are relatively greater than from large two stroke engines, and because it may be difficult to achieve an optimum air distribution in a small engine room. This means that the average air in a ventilated engine room will not be lower than 5 C and not higher than = 51 C, say 55 C (ref. 36 C S.W.), as often used as maximum for design of the engine room components. To obtain a correct supply of air for the main engine s combustion process, about 50% of the ventilation air should be blown in at the top of the main engine, near the air intake to the turbochargers, as shown in Fig. 6. Influence of Ambient Temperature Conditions 9

10 Engine room T and difference T o C Fig. 7: Engine room Otherwise, this can have a negative effect on the main engine performance. Thus, the maximum firing pressure will be reduced by 2.2% for every 10 C the turbocharger air intake is raised, and the fuel consumption will go up by 0.7%. Furthermore, a correct air supply near the turbochargers will reduce the deterioration of the turbocharger air filters (from oil fumes, etc., in the engine room air), and a too draughty engine room can be avoided The engine room TER and the engine room/ambient air difference T are shown as functions of the ambient air T ER Amb. air temp. Tamb. amb TER T = TER - Tamb. o C Moreover, a sufficient amount of air should be supplied to areas with a high heat dissipation rate in order to ensure that all the heat is removed, for instance around auxiliary engines/generators and boilers. Ventilation ducts for these areas are not shown in Fig. 6. In the winter time, the amount of air needed to remove the radiation/convection heat from the engine room may be lower. Air pressure The air in the engine room should have a slightly positive pressure, but should not be more than about 5 mm WC (Water Column) above the outside pressure at the air outlets in the funnel. Accommodation quarters will normally have a somewhat higher over pressure, so as to prevent oil fumes from the engine room penetrating through door(s) into the accommodation. The ventilation air can be supplied, for example, by fans of the low pressure axial and high pressure centrifugal or axial types. The required pressure head of the supply fans depends on the resistance in the air ducts. All ventilation air is normally delivered by low pressure air supply fans which, to obtain sufficient air ventilation in all corners of the engine room, may require extensive ducting and a pressure head as stated below. Low pressure fans, p = mm WC For further information, please consult engine room ventilation standard ISO 8861: 1998 (E). 10 Influence of Ambient Temperature Conditions

11 Chapter 3 Ambient Temperature Operation and Matching Standard ambient matched engine Standard unrestricted service demands For a standard main engine, the engine layout is based on the ambient reference conditions of the International Standard Organization (ISO): ISO :2002(E) and ISO 15550:2002(E): ISO ambient reference conditions Barometric pressure: 1,000 mbar Turbocharger air intake : 25ºC Charge air coolant : 25ºC Relative air humidity: 30% With this layout basis, the engine must be able to operate in unrestricted service, i.e. up to 100% Specified Maximum Continuous Rating (SMCR), within the typical ambient range that the ship is exposed to, operating from tropical to low winter ambient conditions. The above tropical ambient relative humidity of 60% at 45ºC is theoretically the absolute limit at which it is possible for humans to survive. The corresponding wet bulb is 36.8ºC. MAN Diesel & Turbo has never measured levels above 50% at 45ºC, and humidity levels above standard tropical ambient conditions will never occur. When applying the central cooling water system which, today, is more commonly used than the seawater system, the corresponding central cooling water/scavenge air coolant is 4ºC higher than the seawater, i.e. equal to 36ºC. The winter ambient reference conditions used as standard for MAN B&W two-stroke engines are as follows: Winter ambient reference conditions Barometric pressure: 1,000 mbar Turbocharger air intake : 10ºC Cooling water : 10ºC (minimum for lub. oil cooler) Relative air humidity: 60% 10 C (to the scavenge air cooler), the specific fuel oil consumption (SFOC) will increase by approx. 2 g/kwh, see Fig. 8. Any obtained gain in reduced electric power consumption, therefore, will be more than lost in additional fuel costs of the main engine. The above ISO, tropical and winter ambient reference conditions are used by MAN Diesel & Turbo for ships, and MAN B&W two-stroke engines comply with the above rules. MAN B&W engines matched according to the above rules are able to operate continuously up to 100% SMCR in the air range between about -10 and 45ºC. Often the engine room is mistaken for being equal to the turbocharger air intake. However, since the air ventilation duct outlets for a normal air intake system are placed near the turbochargers, the air inlet to the turbochargers will be very close to the ambient outside air. Under normal air conditions, the air inlet to the According to the International Association of Classification Societies (IACS) rule M28, the upper requirement, normally referred to as tropical ambient reference conditions, is as follows: IACS M28 (1978): Tropical ambient reference conditions Barometric pressure: 1,000 mbar Air : 45ºC Seawater : 32ºC Relative air humidity: 60% Shipyards often specify a constant (maximum) central cooling water of 36 C, not only for tropical ambient conditions, but also for winter ambient conditions. The purpose is to reduce the seawater pump flow rate when possible, and thereby to reduce the electric power consumption, and/or to reduce the water condensation in the air coolers. However, when operating with 36 C cooling water instead of for example SFOC g/kwh Turbocharger air intake : 10 C 2 g/kwh 36 C C.W 10 C C.W % SMCR Engine shaft power Fig. 8: Influence on SFOC of the cooling water (scavenge air coolant) Influence of Ambient Temperature Conditions 11

12 turbocharger is only 1 3 C higher than the ambient outside air. The classification society rules often specify an engine room air of 0-55ºC as the basis for the design of the engine room components. The 55ºC is the used when approving engine room components. This, however, must not be mistaken for the above tropical air intake of 45ºC specified when related to the capacity or effect of the machinery. In recent years, owners/shipyards have sometimes required unrestricted service on special maximum ambient s higher than the tropical ambient s specified by IACS M28. In such cases, the main engine has to be special high matched, as described later in this paper. Furthermore, operation in arctic areas with extremely low air s has also sometimes been required by owners/ shipyards, and the measures to be taken are also described later in this paper. Operating at high seawater with standard matched engine An increase of the seawater and, thereby, the scavenge air has a negative impact on the heat load conditions in the combustion chamber. Therefore, all MAN B&W two stroke engines for marine applications have an alarm set point of 55 C for the scavenge air for protection of the engine, as described later. For a standard ambient matched engine operating at an increased seawater existing in some inland, gulf, bay and harbour areas, the maximum power output of the engine should be reduced to an engine load resulting in a scavenge air below the level of the scavenge air alarm. Nevertheless, the engine s obtainable load level will in all cases be much higher than required to ensure a safe manoeuvrability (4 6 knots) of the ship even at an extreme seawater of for example 42 C. When sailing in, for example, the harbour area during manoeuvring, the engine load will normally be relatively low (15 30% SMCR), and the corresponding scavenge air will then only be slightly higher than the scavenge air coolant. Therefore, a seawater as high as for example 42 C in harbour areas is not considered a problem for the main engine, and a special matching is not needed under these operating conditions. In general, when sailing in areas with a high seawater, it is possible to operate the standard ambient matched main engine at any load as long as the scavenge air alarm limit is not reached. If the alarm is activated, the engine load has to be reduced. Non-standard ambient matched engine If unrestricted loads are desired in a range different from the standard, different matching possibilities are available. Engine matching for non-standard air conditions Usually, higher or lower turbocharger air intake s may result in lower or higher scavenge air pressures, respectively, and vice versa. An increase of, for example, 5ºC of the tropical air from standard 45ºC to special 50ºC will result in a too low scavenge air pressure at 50ºC. However, the pressure reduction can be compensated for by specifying a correspondingly higher (turbocharger) scavenge air pressure at ISO ambient reference conditions. This involves that the engine, instead of being matched for the ISO-based design air of 25ºC, has to be matched for the = 30ºC turbocharger air intake. The original ISO-based heat load conditions will then almost be obtained for this higher design air. The principles for standard and special high (or low) ambient air matched engines are shown in Fig. 9. At the other end of the air range, the increase of 5ºC of the design air intake will involve a too high scavenge air pressure when operating at -10ºC. Operation below = -5ºC will then only be possible when installing a variable exhaust gas bypass valve system for low air s, as described later. Fig. 9 may in a similar way also be used to explain a special low matched engine. For example, if the standard tropical air needed is reduced by 10ºC, from 45ºC to 35ºC, the engine matching design 12 Influence of Ambient Temperature Conditions

13 Turbocharger air intake Max. Special design Special Low matched engine Min. Special tropical ISO based design layout Lowest ambient air Max. 45 C Standard design Temperature ISO 25 C Min. -10 C Standard ISO matched engine Normal tropical ISO design layout For engine loads higher than 30% SMCR a low scavenge air coolant is recommended (Giving low SFOC and low scav. air press.) Normal min. ambient air Possible low ambient air exhaust gas bypass for operation under extremely low ambient conditions Max. Special design Special High matched engine Min. Special tropical ISO based design layout Low ambient air exhaust gas bypass will be needed below min. Lowest ambient air Up to 100% SMCR running is not allowed Up to 100% SMCR running is allowed Up to 100% SMCR running only allowed when low ambient exhaust gas bypass (C1+2) is installed Fig. 9: Principles for standard and special high (or low) ambient air matched engines Influence of Ambient Temperature Conditions 13

14 air can be reduced to perature, which has a negative impact 25 ˉ 10 = 15ºC. on the combustion chamber s. Therefore, for all marine applications, an alarm set point of 55ºC for the This involves that the exhaust gas will increase by about 16ºC scavenge air is applied for compared with a standard ISO matched engine, whereas the protection of the engine. SFOC will increase. The standard marine scavenge air cooler is specified with a maximum 12ºC difference between the cooling Engine matching for high tropical seawater conditions water inlet and the scavenge air outlet For long time operation in an area with at 100% SMCR, which gives a maximum scavenge air of 36 + high tropical seawater s, the following should be observed. 12 = 48ºC for the scavenge air cooler layout and, accordingly, a margin of 7ºC An increase in the seawater and, thereby, of the scavenge air limit of 55ºC. to the scavenge air alarm coolant will involve a similar increase in the scavenge air tem- matched engine matched engine A difference of 8ºC is considered to be the lowest possible difference to be used for a realistic specification of a scavenge air cooler. Accordingly, the 48 ˉ 8 = 40ºC is the maximum acceptable scavenge air coolant for a central cooling water system, see the principles for layout of the scavenge air cooler in Fig. 10. The demand for an increased tropical scavenge air coolant (central cooling water) of up to 40ºC, therefore, can be compensated for by a reduced design difference of the scavenge air cooler. This can be obtained by means of an increased water flow and/or a bigger scavenge air cooler. Temperature C Standard air cooler design Special air cooler design Standard 55 C Scavenge air limit Scavenge air limit Max. 55 C 52 Standard 48 C Maximum scavenge air at 100% SMCR Maximum scavenge air at 100% SMCR Max. 48 C 44 Standard 36 C Standard 32 C Standard basis 25 C Standard tropical scavenge air coolant Standard tropical seawater ISO based scavenge air coolant ISO design layout High tropical scavenge air coolant High tropical seawater High scavenge air coolant ISO based design layout Max. 40 C Max. 36 C Max. 29 C Up to 100% SMCR running is not allowed (scavenge air) Up to 100% SMCR running is allowed (scavenge air) Up to 100% SMCR running is allowed (scavenge air coolant/central cooling water) Up to 100% SMCR running is allowed (seawater) 22 Fig. 10: Principles for layout of scavenge air cooler for standard and special high scavenge air coolant (illustrated for a central cooling water system) 14 Influence of Ambient Temperature Conditions

15 Design recommendations for operation at extremely low air When a standard ambient matched main engine on a ship occasionally operates under arctic conditions with low turbocharger air intake s, the density of the air will be too high. As a result, the scavenge air pressure, the compression pressure and the maximum firing pressure will be too high. In order to prevent such excessive pressures under low ambient air conditions, the turbocharger air inlet should be kept as high as possible (by heating, if possible). Furthermore, the scavenge air coolant (cooling water) should be kept as low as possible and/or the engine power in service should be reduced. However, for an inlet air below approx. ˉ 10ºC, some engine design precautions have to be taken. Main precautions for extreme low air operation With a load-dependent exhaust gas bypass system (standard MAN Diesel & Turbo recommendation for extreme low air operation), as shown in Fig. 11, part of the exhaust gas bypasses the turbocharger turbine, giving less energy to the compressor, thus reducing the air supply and scavenge air pressure to the engine. For the electronically controlled ME engine (ME/ME-C/ME-B), the load-dependent bypass control can be incorporated in the Engine Control System (ECS) as an add-on. Engine load, fuel index and scavenge air pressure signals are already available for the ME software and, therefore, additional measuring devices are not needed for ME engines. In general, a turbocharger with a normal layout can be used in connection with an exhaust gas bypass. However, in a few cases a turbocharger modification may be needed. Exhaust gas receiver Exhaust gas bypass The exhaust gas bypass system ensures that when the engine is running at part load at low ambient air s, the load-dependent scavenge air pressure is close to the corresponding pressure on the scavenge air pressure curve which is valid for ISO ambient conditions. When the scavenge air pressure exceeds the read-in ISO-based scavenge air pressure curve, the bypass valve will variably open and, irrespective of the ambient conditions, ensure that the engine is not overloaded. At the same time, it will keep the exhaust gas relatively high. The latest generations of turbochargers with variable flow, e.g. the VTA (Variable Turbine Area) system from MAN Diesel & Turbo, can replace the variable bypass and ensure the same scavenge air pressure control. Air intake casing Exhaust gas system Scavenge air receiver B D2 Diesel engine D1 1 2 C1+2 Scavenge air cooler Turbine Turbocharger Compressor B Exhaust gas bypass valve Controlled by the scavenge air pressure C1+2 Control device Ensures that the load-dependent scavenge air pressure does not exceed the corresponding ISO based pressure D Required electric measuring device D1 Scavenge air pressure D2 Engine speed and engine load Fig. 11: Standard load dependent low ambient air exhaust gas bypass system Influence of Ambient Temperature Conditions 15

16 Other low precautions Low ambient air and low seawater conditions come together. The cooling water inlet to the lube oil cooler should not be lower than 10 C, as otherwise the viscosity of the oil in the cooler will be too high, and the heat transfer inadequate. This means that some of the cooling water should be recirculated to keep up the. Furthermore, to keep the lube oil viscosity low enough to ensure proper suction conditions in the lube oil pump, it may be advisable to install heating coils near the suction pipe in the lube oil bottom tank. Ships with ice class notation For ships with the Finnish Swedish ice class notation 1C, 1B, 1A and even 1A super or similar, all MAN B&W two stroke diesel engines meet the ice class demands, i.e. there will be no changes to the main engines. However, if the ship is with ice class notation 1A super and the main engine has to be reversed for going astern (Fixed Pitch Propeller), the starting air compressors must be able to charge the starting air receivers within half an hour, instead of one hour, i.e. the compressors must be the double in size compared to normal. For other special ice class notations, the engines need to be checked individually. The exhaust gas bypass system to be applied is independent of the ice classes, and only depends on how low the specified ambient air is expected to be. However, if the ship is specified with a high ice class like 1A super, it is advisable to make preparations for, or install, an exhaust gas bypass system. Increased steam production in wintertime During normal operation at low ambient s, the exhaust gas after the turbochargers will The following additional modifications of the standard design practice should be considered as well: Larger electric heaters for the cylinder lubricators or other cylinder oil ancillary equipment Steam production kg/h 6S60MC-C7/ME-C7 2,500 SMCR = 13,560 kw at 105 r/min 2,000 Air intake : 0 C Cooling water : 10 C Surplus steam Total steam production, with exhaust gas bypass Cylinder oil pipes to be further heat traced/insulated 1,500 Total steam production, without bypass Upgraded steam tracing of fuel oil Steam consumption pipes Extra steam needed 1,000 Increased preheater capacity for jacket water during standstill Different grades of lubricating oil for 500 turbochargers Space heaters for electric motors % SMCR Sea chests must be arranged so that Engine shaft power blocking with ice is avoided. Fig. 12: Expected steam production by exhaust gas boiler at winter ambient conditions (0 C air) for main engine 6S60MC-C7/ME-C7 with/without a load-dependent low air exhaust gas bypass system 16 Influence of Ambient Temperature Conditions

17 decrease by about 1.6ºC for each 1.0ºC reduction of the intake air. The load-dependent exhaust gas bypass system will ensure that the exhaust gas after the turbochargers will fall by only about 0.3ºC per 1.0ºC drop in the intake air, thus enabling the exhaust gas boiler to produce more steam under cold ambient conditions. Irrespective of whether a bypass system is installed or not, the exhaust gas boiler steam production at ISO ambient conditions (25ºC air and 25ºC C.W.) or higher ambient conditions, will be the same, whereas in wintertime the steam production may be relatively increased, as the scavenge air pressure is controlled by the bypass valve. As an example, Fig. 12 shows the influence of the load-dependent exhaust gas bypass system on the steam production when the engine is operated in wintertime, with an ambient air of 0ºC and a scavenge air cooling water of 10ºC. The calculations have been made for a 6S60MC-C7/ME-C7 engine equipped with a high-efficiency turbocharger, i.e. having an exhaust gas of 245ºC at SMCR and ISO ambient conditions. Fig. 12 shows that in wintertime, it is questionable whether an engine without a bypass will meet the ship's steam demand for heating purposes (indicated for bulk carrier or tanker), whereas with a load-dependent exhaust gas bypass system, the engine can meet the steam demand. Closing Remarks Diesel engines installed in ocean going ships are often exposed to different climatic conditions because of the ship s trading pattern, but as the variations on the sea surface are normally relatively limited, the engines will normally be able to operate worldwide in unrestricted service without any precautions being taken. Even if the ship has to sail in very cold areas, the MAN B&W two-stroke engines can, as this paper illustrates, also operate under such conditions without any problems as long as special low precautions are taken. The use of the standard load dependent low ambient air exhaust gas bypass system may as an additional benefit also improve the exhaust gas heat utilisation when running at low ambient air s. Furthermore, at the other end of the scale, if the ship should need to sail in unrestricted service in areas with very high ambient air s, higher than 45 C, this will also be possible provided a high matching of the engine is applied. Even when sailing should be needed at very high seawater s, this will be possible provided a specially designed scavenge air cooler is installed on the diesel engine. Influence of Ambient Temperature Conditions 17

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20 All data provided in this document is non-binding. This data serves informational purposes only and is especially not guaranteed in any way. Depending on the subsequent specific individual projects, the relevant data may be subject to changes and will be assessed and determined individually for each project. This will depend on the particular characteristics of each individual project, especially specific site and operational conditions. Copyright MAN Diesel & Turbo ppr Sep 2014 Printed in Denmark MAN Diesel & Turbo Teglholmsgade Copenhagen SV, Denmark Phone Fax MAN Diesel & Turbo a member of the MAN Group