1 Arrangement and installation of pressurized oil fuel systems
1.1 Major factors which can lead to failures of oil fuel system components are:
- poor installation, especially insufficient care being taken to provide adequate support (pipe brackets, etc.) and lack of attention to thermal expansion and possible machinery movement due to flexible mountings;
- the frequent partial dismantling and reassembly of the system for maintenance purposes;
- the effects of high frequency, short duration pressure pulses generated by the action of the fuel injection pumps, which are transmitted back into the oil fuel supply and spill rails; and
1.2 The causes of high pressure pulses in the oil fuel supply and spill systems
1.2.1 The most common fuel injection pumps (monobloc or “jerk” pumps) are comprised of a plunger moving up and down in a barrel which contains ports for fuel to enter and leave. The pump is designed to provide the variable fuel flow required for the engine to operate under fluctuating load or rpm, by adjustment of the plunger delivery stroke. At a point determined by the engine’s fuel requirement, the plunger will uncover the ports and the internal pressures between 80 N/mm2 and 150 N/mm2 will be spilled back into the fuel supply and spill piping.
1.2.2 Each injection pump action generates high magnitude spill pressures followed by periods of reduced pressure. The pressure differences accelerate columns of fuel within the piping system and, when combined with the action of the circulating pump relief valve, cavitation and reflected pressure waves can be caused. Cavitation implosions occur quickly, and can induce very short duration pressure pulses in excess of 10 N/mm2.
1.2.3 Tests have determined that the magnitude of pressure pulses in the fuel system of a typical medium speed diesel engine installation are greatest at 40% to 60% engine load, and will reach 6 N/mm2 to 8 N/mm2. The pulses are approximately eight times the nominal pressure of the system. High-speed engines, such as those installed on high-speed craft, generate higher injection pressures and it is likely that the fuel system of these engines will experience correspondingly higher pressure pulses.
1.2.4 High pressure pulses lead to vibration and fatigue and are responsible for many failures of equipment such as thermostats, pressostats and mechanical dampers. The failure of fuel lines and their components will invariably involve fatigue and the initiation of fractures due to tensile stress.
1.3 Design consideration
1.3.1 It is essential that the fuel system is designed to accommodate the high pressure pulses which will be generated by the injection pumps. The engine manufacturer and/or the fuel installation manufacturer and the piping installer, etc., should be consulted for an explicit statement of the fuel system parameters including the maximum pressures which will be generated. Many engine manufacturers, aware of the potential risks due to high pressure pulses within the fuel system, now aim to limit the magnitude of the pulses to 1.6 N/mm2 at the engine fuel rail outlets.
1.3.2 The alternative approaches which may be considered by the designer are:
- to design the fuel system such that it is able to contend with the magnitude of pressure pulses generated. Piping systems should be designed and installed to an appropriate classification society or ISO specification;
- to install pressure damping devices; or
- to specify injection pumps which are designed to eliminate or reduce high pressure pulses.
1.3.3 The fuel line between the fuel tank and the engine is made up of several parts often from different suppliers. The fact that these suppliers may be unaware of, and therefore do not take into account, the pressures that may be placed on their equipment by the other components of the system, is often the reason for the system’s failure. The specification, design and installation of all of the components of the fuel system should be carefully coordinated to ensure that they are all suitable individually, and in combination with the other components, for the anticipated high pressure pulses.
1.3.4 There are a number of pressure damping devices which have been fitted within fuel systems. Mechanical pressure accumulators and gas filled bellows have both been used however, in some cases, problems of slow response and failure due to fatigue and vibration have been reported.
1.3.5 Fuel pipes should be of steel and supports should be adequate to prevent fatigue due to vibration through the structure from the engines and propellers. The support arrangements should also protect the system from vibration caused by high pressure pulses. Copper and aluminium-brass pipes should not be used as their inherent work hardening characteristics make them prone to failure when subjected to vibration.
1.3.6 Experience indicates that compression couplings require careful attention to tightening procedures and torques to avoid leaks or damage to the pipe when subjected to over-tightening. They should not be used in the fuel supply line of the injection pumps and spill system. Flanged connections should be used in place of compression couplings.
1.3.7 In multi-engine installations supplied from the same fuel source, means of isolating the fuel supply to and spill from individual engines should be provided. The means of isolation should be operable from the control position. Without the ability to isolate the fuel supply and spill lines on each engine a single leak could necessitate the need to stop all engines, thus putting the manoeuvrability of the vessel at risk.
1.4.1 One designated person should be responsible for coordinating the initial onboard installation of the complete fuel system.
1.4.2 The coordinator should be able to understand the overall design criteria and ensure that the design intent is fully implemented at the time of installation.
1.5 Inspection and maintenance
1.5.1 The ship safety management system should contain procedures to identify vibration, fatigue, defects, poor components and poor fitting of the fuel system and ensure that proper attention to protecting hot surfaces is maintained. Means, such as check lists should be prepared to ensure that all procedures are followed at major overhauls and that all components, supports, restraints, etc., are refitted on completion of such work. The installed system should be routinely inspected for:
- verification of the adequacy of its supports and the condition of its fittings;
- evidence of fatigue stresses to welded or brazed pipes and connections;
- assessment of the level of vibration present; and
- condition of the lagging or shielding of hot surfaces.
1.5.2 Components of the fuel system should be comprehensively examined, particularly threaded connections, at each dismantling.
.5.3 Injection pump holding-down bolts should be proved tight by testing with a torque spanner at frequent intervals (not to exceed 3 months).
1.5.4 The supports and retaining devices of the low pressure fuel system should be checked at regular intervals (not to exceed 6 months), to be proved tight and to provide adequate restraint. The lining of such devices should be examined for wear and renewed if they provide insufficient support.
2 Spray shields for joints of pressurized flammable oil piping systems
Spray shields should be fitted around flanged joints, flanged bonnets and any other flanged or threaded connections of oil fuel and lubricating oil systems having an internal pressure exceeding 0.18 N/mm2 which have the possibility of being in contact with potential ignition sources by direct spray or by reflection. The purpose of spray shields is to prevent the impingement of sprayed flammable oils onto a high temperature surface or other source of ignition.
2.2Design and installation
2.2.1 Many types of spray shields are possible to avoid spray at flanged connections. For example, the following may be treated as spray shield:
- thermal insulation having sufficient thickness;
- anti-splashing tape made of approved materials. Caution should be taken to avoid using the anti-splashing tape in areas of high temperature so as to maintain its adhesive characteristics. In case of rewrapping of the new tape, the surface area of the tape should be clean and dry; and
Figure 3-1 – Example of correct taping method
- where an anti-spray cover is wrapped around the side of flange, it is not necessary to wrap tightening bolts completely.
Figure 3-2 – Recommended arrangement of anti-spray cover for flange joint
2.2.2 Anti-splashing tape or other equivalent method may be treated as spray shield on threaded connections. Additionally, the use of sealing tape at thread of union joint is strongly recommended to prevent spray.
2.2.3 Spray shields should be applied not only to a piping system but also to pressurized equipment and/or fittings on oil fuel systems, such as heat exchanger, tube plate and filter or strainer body joints.
2.3 Inspection and maintenance
Spray shields should be inspected regularly for their integrity and any which have been removed for maintenance purposes should be refitted on completion of the task according to the manufacturer’s instructions.
3 Jacketed high pressure fuel lines of internal combustion engines
3.1.1 All external high pressure fuel delivery lines between the high pressure fuel pumps and fuel injectors are required to be protected with a jacketed piping system capable of containing fuel from a high pressure line failure.
3.1.2 The requirements are applicable to internal combustion engines installed in any area on board ships irrespective of service and location.
3.1.3 Single cylinder and multi-cylinder engines having separate fuel pumps and those having multiple fuel injection pump units are included.
3.1.4 For the purpose of these Guidelines, lifeboat engines and diesel fire pumps are excluded.
Figure 3-3 – Sample of jacketed pipe
3.2.1 For engines of less than 375 kW where an enclosure is fitted, the enclosure is to have a similar function to jacketed pipes, i.e. prevent spray from a damaged injector pipe impinging on a hot surface.
3.2.2 The enclosure should completely surround the injection pipes except that existing “cold” engine surfaces may be considered as part of the enclosure.
3.2.3 The enclosure should have sufficient strength and cover area to resist the effects of high pressure spray from a failed fuel pipe in service, prevent hot parts from being sprayed and to restrict the area that can be reached by leaked fuel. Where the enclosure is not of metallic construction, it should be made of non-combustible, non oil-absorbing material.
3.2.4 Screening by the use of reinforced tapes is not acceptable as a suitable enclosure.
3.2.5 Where leaked oil can reach hot surfaces, suitable drainage arrangements should be fitted to enable rapid passage of leaked oil to a safe location which may be a drain tank. Leaked fuel flow onto “cold” engine surfaces can be accepted, provided that it is prevented from leaking onto hot surfaces by means of screens or other arrangements.
3.2.6 Where the enclosure has penetrations to accommodate high pressure fittings, the penetrations should be a close fit to prevent leakage.
Two systems have been successfully used in meeting this requirement, namely, rigid sheathed fuel pipe and flexible sheathed fuel pipe. In both systems the sheathing is to fully enclose the pipe and is to resist penetration by a fine spray or jet of oil from a failure in the pipe during service. Also the annular space and drainage arrangements should be sufficient to ensure that in the event of complete fracture of the internal pipe, an excessive build up of pressure cannot occur and cause rupture of the sheath. The suitability of such pipes should be demonstrated by prototype testing. The drainage arrangement should prevent contamination of lubricating oil by oil fuel, and should include an alarm to indicate leakage has occurred.
3.4 Inspection and maintenance
Regardless of the system selected, little additional maintenance or periodic inspection is required to keep the jacketed fuel lines in proper working order. However, jacketed pipes should be inspected regularly and any drainage arrangement which may have been disconnected for maintenance purposes should be refitted on completion of the task.