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MAN B&W sli de type typ e fuel valves.
The fuel valve design was changed a few years ago from the conventional type to the slide type.
Conventional fuel valve with a sac volume of 1690 mm3 For this model, fuel from the sac enters into the combustion chamber at low pressure and contributes to the emission of smoke and unburned hydrocarbons. Carbon deposit occurs.
The relatively large sac volume in a standard design fuel nozzle thus has a negative influence on the formation of soot particles and hydrocarbons. A new type of fuel valve, essentially eliminating the sac volume, was subsequently developed and introduced by MAN B&W as standard to its larger low speed engines.
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The slide type fuel valve is designed for eliminating the sac volume to prevent any fuel oil from entering in combustion chamber without being injected. The combustion is improved. However this function is assured by the “cut off shaft“ which is a fragile piece requiring a special attention.
In pressure testing conventional fuel valves, the testing device is only capable of supplying 1 to 2 per cent of the normal fuel flow rate on the engine, which is not sufficient to ensure proper atomization. As the atomization test can be omitted, it is not described in new testing procedures, so verification of a humming sound is neither longer possible nor necessary. Pressure testing procedures for slide fuel valves are quite different from those for conventional valves. Slide fuel valves must be dissambled and cleaned before pressure testing and an atomization test must not be performed. Cleaning is necessary because the cold and sticky heavy fuel oil, in combination with the very small clearance between the “cut off shaft” and the fuel valve nozzle, would significantly restrict movement of the spindle. An atomization test is not acceptable because the very small needle lift obtained during such a test would result in an unequal pressure distribution on the “cut off shaft”, resulting in a relatively hard contact in a small area. This, together with the high frequency oscillations during an atomization test, and the low lubricity of the test oil, would significantly increase the risk of seizure. The full lift of the needle and the good lubricity of the fuel oil ( attention with the low sulfur HFO ) completely eliminates this risk during normal engine operation.
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MAN BW 6 S MC-C 90.
FUEL VALVE.
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The fuel valve consists of a valve h ead 197, union nu t 148 and valve body 124. Fitted within the valve body are non return valve 220 with a combined slide/valve, thrust spindle 232 with spring 256, thrust foot 268, and spindle guide complete 053. When the fuel valve is fitted in cylinder cover, the valve parts are tightened together by the pressure from the nuts being transmitted through valve head, non return valve, thrust spindle, and spindle guide complete to the valve body, which is pressed into the tapered bore in the cylinder cover. The union nut keeps valve head and valve body together during dismantling of the fuel valve. The spindle guide complete 053 consists of spindle guide 077, nozzle 090, thrust piece 016 and cut off shaft 028. The spindle guide is assembled with a press fit. The cut off shaft 028 is pressed against the tapered seat of spindle guide 077 by the action of the spring 256, the spring pressure being transmitted through the slotted thrust foot 268. The thrust spring determines the opening pressure of the valve. Optionally, an extra disc can be inserted to raise the opening pressure by 20 bar. The non return valve 220 consists of housing, thrust piece, slide and spring. The non return valve is assembled with a press lift. The slide of the non return valve is pressed by the spring against the tapered valve seat inside the non return valve. In this position the head of the slide uncovers a small bore arranged for circulation purposes in the thrust piece.
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The function ing of t he Fuel Valve. Position 1. The electrical fuel oil circulating pump circulates preheated oil through the fuel pump and fuel valve. In the fuel valve the oil passes through the central bore of the valve head and continues to the thrust piece in the non return valve, leaving through the circulation bore of the latter. Thence the oil is passed through the interior of the valve body to an outlet pipe on the side of the valve head.
The space round the tapered valve seat of the spindle of the non return valve is also filled with oil, but the circulating pump pressure is insufficient to overcome the force of the spring and the lift spindle.
If for some reason, cut off shaft 028 should not close during engine standstill, then the closed spindle of the non return valve will prevent the circulating pump from pressing oil through the nozzle, and thus obviate the risk of the engine cylinder being filled with oil.
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Position 2. When, at the beginning of the delivery stroke, the pressure has risen to about 10 bar, the force of the spring of the non return valve will be overcome and the spindle pressed back against the shoulder of the thrust piece. Position 3. When the spindle of the non return valve is pressed upwards, the circulation bore of the thrust piece is closed, and the oil passes the seat of the spindle and enters the space round cut off shaft 028 in the spindle guide. When the pressure has risen to the preset opening value of the fuel valve, the spindle is lifted, and oil is forced through the nozzle into the engine cylinder.
At the termination of the delivery stroke first cut off shaft 028 and then the spindle of the non return valve will be pressed against their respective seats, the injection of fuel stops, and oil is again circulated through the valve (position).
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Fuel Valve checking. The fuel valves must be given the utmost attention and care, as the greater part of irregularities that may occur during the running of the engine can be attributed to defects in these valves. If the engine gives normal performance in accordance with diagrams and exhaust temperatures, it is only necessary to inspect the fuel valves after the service period stated in the Checking and Maintenance Programme. In order to obtain reliable results during testing of the fuel valves, all the fuel valves that are dismantled from the engine must be disassembled, cleaned, inspected and re-assembled before testing. In the event that the slide type fuel valve is pressure tested without being cleaned between the fuel nozzle and the cut off slide, the opening pressure value measured might be considerably low er than specified.
1. Pressure testing pump. Working air of 7 bar could be used with hydraulic oil ( rust preventing) with a viscosity of between 7 and 10 cst at 50°c.
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2. Setting up the fuel valve.
Place the fuel v alve in the test rig and secure it with the spring housings and nuts. Tighten the nuts until the top face of the pressure disc is flush with the top face of the spring housings. Mount the oil pipe between the pressure testing pump and the fuel valve.
To ensure that overtightening has not taken place, check that the locking/indicating pin has not been bent or broken off. In the event of overtightening, replace the spring housing by a new one.
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3. Flushin g and jet cont rol. To remove air in the system and check the fuel jet.
The control handle must be in the open position, slowly increase the working pressure until straight jets of oil are ejected from the nozzle holes without atomization. There is to be a continuous jet of oil through at least one of the nozzle holes. Owing to the geometry of the internal part of the nozzle, and because the height to which the spindle is lifted during pressure testing is lower than the height it is lifted during normal engine operation, the fuel oil will not necessarily flow from all of the nozzle holes.
Cause of fault. If the jets do not fulfil the above point C, the cause may be : Dirt in the nozzle holes. The nozzle is not mounted correct.
Do not attempt to carry out an atomization test on slide type fuel valves, as this m ay damage the cut o ff s lide and nozzle. The reason is that the atomization test may damage the valve because it makes the needle oscillate, with a small lift at a very high frequency. The high pressure drop across the cut off edge and the high contact pressure between slide and fuel nozzle, in combination with poor lubricity of the test oil, increase the risk of seizures between cut off slide and nozzle. All these conditions involve the risk of seizure between the cut off slide and the nozzle.
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4. Opening pressure. To check the opening pressure. The control handle is to be in open position. Increase the oil pressure until the oil is admitted through the nozzle holes, check the opening pressure on the pressure gauge. Cause of fault.
If the opening pressure is higher than specified, the cause may be that wrong type of spring is used. Replace the spring on the thrust spindle, if necessary, replace the complete thrust spindle. If the opening pressure is lower than specified, the cause may be that the spring has sagged. Replace the spring or add a special thin disc. If a spring or a disc has been changed, the pressure testing procedure of the fuel valve must repeated .
5. Sealing t est and sliding funct ion. To check the needle valve seat for tightness and the slide for correct closing. The control handle must be in open position. Slowly increase the oil pressure to about 50 bar below the opening pressure. Maintain the built up pressure by moving the control handle into closed position. Repeat the procedure two or three times. Oil must not flow from the nozzle holes. The pressure drops relatively slowly to about 15 bar, after which it drops quickly to 0, the slide is pressed against the conical seat and opens for circulation oil. Oil flows out of the leak oil outlet when the fuel valve is full of oil. Cause of fault : If oil flows out of nozzle holes, the cause is either 1. Defective spindle guide at needle seat or a sticking spindle. 2. Too quick pressure drop – The clearance of the movable parts both of spindle and of thrust piece / valve slide in the non return valve are too large.
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3. The seats between the thrust piece / spindle in the spindle guide or thrust piece / valve slide in the non return valve are damaged. 4. Sliding function. If a quick pressure drop from 15 bar to 0 can not be registered , the valve slide is sticking or the vent hole in the thrust piece is blocked. 6. Pressure test, O ring sealings. To ensure that the leak oil ( circulation oil) remains in the closed system.
The control handle is in the open position, build up a working pressure of about max. 10 bar until oil flows out of the leak oil outlet.
Close the leak oil outlet with a gasket and a plug screw. Increase the working pressure to about 100 bar. Move the control handle to closed position. The build up pressure of 100 bar should be maintained.
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If the oil leaks out at the union nut, the O ring inside the fuel valve head is defective, and must be replaced.
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Fuel valve dismantlin g and overhauling . Safety precautions. Stopped engine Block the starting mechanism Shut off starting air supply Shut off fuel oil Drain t he high pr essure pipe and the fuel valve.
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Close the fuel oil inlet and outlet valves, and drain the high pressure pipe and the fuel valve. Dismantle and remove the fuel oil high pressure pipe.
