Slide Type Fuel Valve

8/16/2019 Slide Type Fuel Valve

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MAN B&W slide type fuel valves.

The fuel valve design was changed a few years ago from the conventional type to theslide type.

The relatively large sac volume in a standard design fuel nozzle thus has a negativeinfluence on the formation of soot particles and hydrocarbons. A new type of fuelvalve, essentially eliminating the sac volume, was subsequently developed andintroduced by MAN B&W as standard to its larger low speed engines.

Conventional fuelvalve with a sacvolume of 1690

mm3

For this model, fuelfrom the sac entersinto the combustion

chamber at lowpressure and

contributes to theemission of smoke

and unburnedhydrocarbons.Carbon deposit

occurs.


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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 notsufficient to ensure proper atomization. As the atomization test can be omitted, it isnot described in new testing procedures, so verification of a humming sound isneither longer possible nor necessary.Pressure testing procedures for slide fuel valves are quite different from those forconventional valves. Slide fuel valves must be dissambled and cleaned beforepressure testing and an atomization test must not be performed. Cleaning isnecessary because the cold and sticky heavy fuel oil, in combination with the verysmall clearance between the “cut off shaft” and the fuel valve nozzle, wouldsignificantly restrict movement of the spindle. An atomization test is not acceptable because the very small needle lift obtainedduring 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 thehigh frequency oscillations during an atomization test, and the low lubricity of the testoil, would significantly increase the risk of seizure.The full lift of the needle and the good lubricity of the fuel oil ( attention with the lowsulfur HFO ) completely eliminates this risk during normal engine operation.

The slide type fuel valve is

designed for eliminatingthe sac volume to preventany fuel oil from enteringin combustion chamberwithout being injected.

The combustion isimproved. However this

function is assured by the“cut off shaft“ which is afragile piece requiring a

special attention.


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MAN BW 6 S MC-C 90.

FUEL VALVE.


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The fuel valve consists of a valve head 197, union nut 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 complete053. When the fuel valve is fitted in cylinder cover, the valve parts are tightenedtogether by the pressure from the nuts being transmitted through valve head, nonreturn valve, thrust spindle, and spindle guide complete to the valve body, which ispressed into the tapered bore in the cylinder cover. The union nut keeps valve headand 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 theaction of the spring 256, the spring pressure being transmitted through the slottedthrust foot 268.

The thrust spring determines the opening pressure of the valve. Optionally, an extradisc 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 valveseat inside the non return valve. In this position the head of the slide uncovers asmall bore arranged for circulation purposes in the thrust piece.


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The functioning of the Fuel Valve.

Position 1.The electrical fuel oil circulating pump circulates preheated oil through the fuel pumpand 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 thecirculation bore of the latter. Thence the oil is passed through the interior of the valvebody to an outlet pipe on the side of the valve head.

If for some reason, cut off shaft 028 should not close during engine standstill, thenthe closed spindle of the non return valve will prevent the circulating pump frompressing oil through the nozzle, and thus obviate the risk of the engine cylinder beingfilled with oil.

The space round the tapered valve seat of the spindle ofthe non return valve is also filled with oil, but thecirculating pump pressure is insufficient to overcome theforce of the spring and the lift spindle.


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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 spindlepressed back against the shoulder of the thrust piece.

Position 3.When the spindle of the non return valve is pressed upwards, the circulation bore ofthe thrust piece is closed, and the oil passes the seat of the spindle and enters thespace round cut off shaft 028 in the spindle guide. When the pressure has risen tothe preset opening value of the fuel valve, the spindle is lifted, and oil is forcedthrough the nozzle into the engine cylinder.

At the termination of the delivery stroke first cut off shaft 028 and then the spindle ofthe 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 ofirregularities that may occur during the running of the engine can be attributed todefects in these valves.

If the engine gives normal performance in accordance with diagrams and exhausttemperatures, it is only necessary to inspect the fuel valves after the service periodstated in the Checking and Maintenance Programme.In order to obtain reliable results during testing of the fuel valves, all the fuel valvesthat are dismantled from the engine must be disassembled, cleaned, inspected andre-assembled before testing.

In the event that the slide type fuel valve is pressure tested without beingcleaned between the fuel nozzle and the cut off slide, the opening pressurevalue measured might be considerably lower than specified.

1. Pressure testing pump.Working air of 7 bar could be used with hydraulic oil ( rust preventing) with a viscosityof between 7 and 10 cst at 50°c.


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2. Setting up the fuel valve.

Place the fuel valve in the testrig and secure it with thespring housings and nuts.Tighten the nuts until the topface of the pressure disc isflush with the top face of thespring housings. Mount theoil pipe between the pressuretesting pump and the fuel

valve.

To ensure that overtightening has not takenplace, check that the locking/indicating pin hasnot been bent or broken off.

In the event of overtightening, replace the springhousing by a new one.


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3. Flushing and jet control.To remove air in the system and check the fuel jet.

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 fuelvalves, as this may damage the cut off slide and nozzle.

The reason is that the atomization test may damage the valve because it makes theneedle oscillate, with a small lift at a very high frequency. The high pressure dropacross 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 betweencut off slide and nozzle.

All these conditions involve the risk of seizure between the cut off slide and thenozzle.

The control handle must be in the

open position, slowly increase theworking pressure until straight jetsof oil are ejected from the nozzleholes without atomization.There is to be a continuous jet of oilthrough at least one of the nozzleholes.

Owing to the geometry of theinternal part of the nozzle, andbecause the height to which thespindle is lifted during pressure

testing is lower than the height it islifted during normal engineoperation, the fuel oil will notnecessarily flow from all of thenozzle holes.


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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.

5. Sealing test 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 toabout 50 bar below the opening pressure. Maintain the built up pressure by movingthe 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.

If the opening pressure is higher than specified, the causemay be that wrong type of spring is used. Replace thespring on the thrust spindle, if necessary, replace thecomplete thrust spindle.

If the opening pressure is lower than specified, the causemay be that the spring has sagged. Replace the spring oradd a special thin disc.

If a spring or a disc has been changed, the pressure testingprocedure of the fuel valve must repeated .


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3. The seats between the thrust piece / spindle in the spindle guide orthrust 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 issticking 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.

Close the leak oil outlet with a gasket and a plug screw. Increase the workingpressure to about 100 bar. Move the control handle to closed position. The build uppressure of 100 bar should be maintained.

The control handle is in the openposition, build up a workingpressure of about max. 10 bar untiloil flows out of the leak oil outlet.


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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 dismantling and overhauling.

Safety precautions.

Stopped engine

Block the starting mechanism

Shut off starting air supply

Shut off fuel oil

Drain the high pressure 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 fuelvalve. Dismantle and remove the fuel oil high pressure pipe.

Documents

Slide Type Fuel Valve