The present invention relates to a pipe unit for a fuel system of a piston engine in accordance with claim 1 .

Background of the invention

In large piston engines, such as ship or power plant engines, which can be operated using a gaseous fuel, the gaseous fuel is typically conveyed from a gas tank to the cylinders of the engine using double-wall pipes. The pipes comprise an inner pipe for conveying the fuel and an outer pipe that is arranged around the inner pipe to form a barrier. The purpose of the outer pipe is to increase safety in case there is a leakage in the inner pipe. Typically an air space is formed between the inner pipe and the outer pipe. Through the air space, leaking fuel can be vented to a safe place. The air space can also be provided with means for detecting fuel leakages. From the gas tank, the gaseous fuel is supplied into a gas manifold. The gas manifold is an elongated receiver, from which the fuel is fed to the cylinders of the engine. A V-engine can be provided with two gas manifolds.

In large piston engines, each cylinder of the engine is provided with an own cylinder head. The gas manifold is connected to each cylinder head. When the engine is running, the cylinder heads move relative to each other due to the forces caused by the combustion. To allow the mutual movements of the cylinder heads, the gas manifold needs to be resilient. The gas manifold can be made resilient by providing both the inner pipe and the outer pipe with bellow- type structures between each two consecutive cylinder heads of the engine. A drawback of the bellow-structures is that they are expensive. The bellow- structures also complicate the maintenance of the fuel system. Summary of the invention

An object of the present invention is to provide a double-wall pipe unit for a fuel system of a piston engine. The characterizing features of the pipe unit according to the invention are given in claim 1 . The pipe unit according to the invention has a first end and a second end and comprises an inner pipe forming a flow path for gaseous fuel, an outer pipe surrounding the inner pipe, an annular space formed between the inner pipe and the outer pipe, a first flange that is connected to the inner pipe and to the outer pipe at the first end of the pipe unit for attaching the pipe unit to an adja- cent part of a fuel system, and a second flange that is connected to the inner pipe and to the outer pipe at the second end of the pipe unit for attaching the pipe unit to an adjacent part of the fuel system. The outer pipe is provided with a corrugated portion for allowing yielding of the outer pipe both in the axial and radial direction of the outer pipe, and the inner pipe is a smooth-wall pipe that is connected to at least one of the flanges of the pipe unit so that mutual movement of the inner pipe and the flange in the axial direction of the inner pipe is allowed.

The pipe unit according to the invention is flexible. It can thus be used for example as a part of a gas manifold, which is attached to several cylinder heads of an engine. The flexible pipe unit allows movements of the cylinder heads relative to each other. Because of the smooth inner pipe, the manufacturing costs of the pipe unit are low compared to pipe units, where both the inner pipe and the outer pipe are corrugated. In addition, the smooth inner pipe is easier to clean. Furthermore, due to the smooth inner pipe, the diameter of the pipe unit can be smaller.

According to an embodiment of the invention, the inner pipe is connected to both the first flange and the second flange so that mutual movement of the inner pipe and the flange in the axial direction of the inner pipe is allowed. Slida- ble connection at both ends ensures that a smaller moving range can provide the required flexibility.

According to an embodiment of the invention, the inner pipe is connected to the first flange and/or to the second flange so that tilting of the inner pipe relative to the flange is allowed. This allows mutual movement of the flanges also in the radial direction of the pipe unit. According to an embodiment of the invention, the length of the inner pipe is less than the distance between the outer end surfaces of the first flange and the second flange. This ensures that the inner pipe does not protrude out of the flanges as the length of outer pipe changes. According to an embodiment of the invention, the inner pipe is supported against the inner perimeter of the first flange and/or the second flange.

According to an embodiment of the invention, the diameter of the inner perimeter of the first flange and/or the second flange decreases from the end surfaces of the flange towards the middle of the flange. This allows tilting of the inner pipe relative to the flanges.

According to an embodiment of the invention, the pipe unit defined above is used as a part of a gas manifold of a fuel system of a piston engine. The gas manifold can comprise a header for supplying gaseous fuel to a plurality of cylinders of the engine and a plurality of feed pipes, each feed pipe being config- ured to connect the header to one cylinder head of the engine.

According to an embodiment of the invention, the pipe unit forms at least part of a feed pipe of the gas manifold.

According to an embodiment of the invention, the pipe unit connects two feed pipes of the gas manifold to each other. A piston engine according to an embodiment of the invention comprises a gas manifold defined above.

Brief description of the drawings

Embodiments of the invention are described below in more detail with refer- ence to the accompanying drawings, in which

Fig. 1 shows a pipe unit according to an embodiment of the invention,

Fig. 2 shows a cross-sectional view of the pipe unit of figure 1 ,

Fig. 3 shows an enlarged view of a detail of figure 2, Fig. 4 shows behavior of the pipe unit as the flanges of the pipe unit move relative to each other,

Fig. 5 shows a gas manifold according to an embodiment of the invention, and Fig. 6 shows a gas manifold according to another embodiment of the invention.

Description of embodiments of the invention

Figures 1 to 4 show different views of a pipe unit 1 that can be used as a part of a fuel system of a piston engine. The pipe unit 1 is suitable in particular for a gas feed system of a large piston engine, such as a main or an auxiliary en- gine of a ship or an engine that is used at a power plant for producing electricity. The expression "large piston engine" refers here to engines having a cylinder bore of at least 150 mm. In such engines, each cylinder of the engine is provided with an own cylinder head. Due to the forces created by the combustion in the cylinders, the cylinder heads can move relative to each other. There- fore, the fuel system needs to allow the movements of the cylinder heads. The gas feed system where the pipe unit 1 can be used supplies gaseous fuel, such as natural gas, into the cylinders of the engine. The engine can be a gas engine, which is operated on a gaseous fuel that can be ignited, for instance, by means of spark plugs or by using liquid pilot fuel. The engine could also be a dual-fuel or multi-fuel engine, which can be operated using either a gaseous fuel or a liquid fuel.

The pipe unit 1 comprises an inner pipe 2. The inner pipe 2 forms a flow path for gaseous fuel. An outer pipe 3 is arranged around the inner pipe 2. The outer pipe 3 forms a barrier around the inner pipe 2. The outer pipe 3 is coaxial with the inner pipe 2. The inner diameter of the outer pipe 3 is greater than the outer diameter of the inner pipe 2. An annular space 6 is thus formed between the inner pipe 2 and the outer pipe 3. Fuel possibly leaking from the inner pipe 2 enters the space 6 between the inner pipe 2 and the outer pipe 3. The space 6 can be vented outside an engine room for reducing the risk of fires. The space 6 could also be constantly flushed with air for effectively venting the space 6. The space 6 could also be filled with inert gas. The pipe unit 1 has a first end, which is on the left in figures 1 and 2, and a second end. The first end of the pipe unit 1 is provided with a first flange 4 and the second end of the pipe unit 1 is provided with a second flange 5. In the embodiment of the figures, the flanges 4, 5 are circular. At the first end of the pipe unit 1 , both the inner pipe 2 and the outer pipe 3 are connected to the first flange 4. At the second end, both the inner pipe 2 and the outer pipe 3 are connected to the second flange 5. Each of the first flange 4 and the second flange 5 can be used for attaching the pipe unit 1 to an adjacent part of a fuel system. The adjacent part could be, for instance, a gas manifold or a cylinder head. Two pipe units 1 could also be attached to each other.

Each of the flanges 4, 5 is provided with bolt holes 7 for allowing the pipe unit 1 to be attached to the adjacent parts of the fuel system. Each of the first flange 4 and the second flange 5 comprises at least one aperture 8, which allows outflow from the space 6 between the inner pipe 2 and the outer pipe 3. In the embodiment of the figures, each flange 4, 5 is provided with a plurality of apertures 8. In the embodiment of the figures, the apertures 8 are slots, but the ap- ertures could also have some other shape. For instance, the apertures 8 could be round holes. The apertures 8 allow the space 6 between the inner pipe 2 and the outer pipe 3 to form part of a leakage channel of the fuel system.

The outer pipe 3 is provided with a corrugated portion 3a. The corrugated portion 3a allows yielding of the outer pipe 3 both in the axial and radial direction of the outer pipe 3. The outer pipe 3 is thus flexible. In the embodiment of the figures, the corrugated portion 3a forms a major part of the outer pipe 3. This increases the flexibility of the outer pipe 3. However, also a shorter corrugated portion 3a could be sufficient for providing the required resilience. The outer pipe 3 also allows movements of the first flange 4 and the second flange 5 in relation to each other. Because of the flexible corrugated portion 3a, the outer pipe 3 can be rigidly connected to the first flange 4 and the second flange 5. Each of the first flange 4 and the second flange 5 has an outer end surface 4a, 5a and an inner end surface 4b, 5b. The inner end surfaces 4b, 5b of the flanges 4, 5 face each other. The outer end surfaces 4a, 5a point outwards in the axial direction of the pipe unit 1 . The outer pipe 3 is attached to the inner end surfaces 4b, 5b of the flanges 4, 5. The inner end surface 4b, 5b of each flange 4, 5 is provided with a groove 15 for receiving the outer pipe 3. The connections between the outer pipe 3 and the flanges 4, 5 are gas-tight. The outer pipe 3 can be attached to the flanges 4, 5 for example by welding. The outer pipe 3 can be made of steel or some other suitable alloy. Also the flanges 4, 5 can be made of steel or another alloy. The inner pipe 2 is connected to both the first flange 4 and the second flange 5 so that mutual movements of the inner pipe 2 and the flanges 4, 5 in the axial direction of the inner pipe 2 are allowed. The connections between the inner pipe 2 and the flanges 4, 5 also allow tilting of the inner pipe 2 relative to the flanges 4, 5. In other words, each of the flanges 4, 5 can rotate relative to the inner pipe 2 about an axis that is perpendicular to the axial direction of the inner pipe 2. The allowed range of movement can be limited. The inner pipe 2 can be rigid. The inner pipe 2 is thus a smooth-wall pipe, i.e. without corrugations. The inner pipe 2 can be made of steel or some other suitable alloy. Be- cause of the connections allowing mutual movements of the inner pipe 2 and the flanges 4, 5, the pipe unit 1 is resilient despite the rigid inner pipe 2. The flanges 4, 5 can thus move and rotate relative to each other. The allowed movement range, however, is limited. According to an embodiment of the invention, the corrugated portion 3a of the outer pipe 3 and the connections be- tween the inner pipe 2 and the flanges 4, 5 are configured so that the second flange 5 is allowed to move at least +/- 1 mm relative to the first flange 4 both in the axial direction of the pipe unit 1 and in any direction that is perpendicular to the axial direction of the pipe unit 1 .

The length of the inner pipe 2 is less than the distance between the outer end surfaces 4a, 5a of the first flange 4 and the second flange 5. This ensures that the inner pipe 2 does not protrude out of the flanges 4, 5 in case the flanges 4, 5 move slightly towards each other. The flanges 4, 5 can extend for instance 0.5 to 5 mm beyond the ends of the inner pipe 2.

In the embodiment of the figures, the inner pipe 2 is supported against the inner perimeters 4c, 5c of the first flange 4 and the second flange 5. The inner perimeter 4c, 5c of each flange 4, 5 is provided with a groove 9. The groove 9 is configured to accommodate a seal 1 1 . The seal 1 1 can be an O-ring. The groove 9 is arranged in the middle of the flange 4, 5 in the axial direction of the pipe unit 1 . The seal 1 1 seals the inner pipe 2 against the flange 4, 5 for making the space 6 between the inner pipe 2 and the outer pipe 3 gas-tight. The diameter of the inner perimeter 4c, 5c of the first flange 4 and the second flange 5 decreases from the end surfaces 4a, 5a, 4b, 5b of the flange 4, 5 towards the middle of the flange 4, 5. The inner pipe 2 is thus supported against a middle portion of the flange 4, 5. This allows the inner pipe 2 to rotate relative to an axis lying in the plane of the first flange 4 or the second flange 5. Figure 4 shows how the longitudinal axis 10 of the inner pipe 2 is tilted relative to a situation where the inner pipe 2 is coaxial with the flanges 4, 5.

The connection between the inner pipe 2 and one of the flanges 4, 5 could be rigid. However, if a slidable connection at both ends of the inner pipe 2 is used, a smaller moving range between the inner pipe 2 and the flanges 4, 5 is sufficient for providing the required flexibility.

Figures 5 and 6 show two examples of how the pipe unit 1 can be used. In both examples, several pipe units 1 are used as part of a gas manifold 12. The gas manifolds 12 are used for introducing gaseous fuel into cylinders of an en- gine. The gas manifold 12 comprises a header 13 and feed pipes 14. The header 13 receives fuel from a gas tank and supplies it further to several cylinders of the engine. An in-line engine can be provided with one gas manifold 12. A V-engine can be provided with either one or two gas manifolds 12. The feed pipes 14 are branches of the header 13. Each feed pipe 14 is connected to one cylinder head of the engine. The number of the feed pipes 14 therefore corresponds to the number of cylinders to which the gas manifold 12 is connected. Because of the movements of the cylinder heads in relation to each other, also the feed pipes 14 move in relation to each other.

In the embodiment of figure 5, pipe units 1 according to an embodiment of the invention are used as the feed pipes 14 of the gas manifold 12. The second flanges 5 of the pipe units 1 are attached to the header 13. The first flanges 4 can be attached to the cylinder heads of the engine. The gas manifold 12 is thus resiliently connected to the cylinder heads. In the embodiment of figure 6, the feed pipes 14 are part of T-shaped pipes. The T-shaped pipes are rigidly connected to the cylinder heads of the engine. The T-shaped pipes are attached to each other by means of pipe units 1 according to an embodiment of the invention. There is thus a resilient part between each two consecutive cylinder heads. This allows mutual movements of the cylinder heads.

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims.