A cylinder relief valve is a protective device fitted on top of the cylinder head that protects the engine and its surroundings from unprecedented damage from overpressure. It is one of the several-cylinder head mountings installed for the safe operation of an engine.
Provided for each cylinder of a marine engine, it lifts up releasing excess combustion pressure. Which protects the cylinder, cylinder head, and engine itself from unwanted consequences of the resulting combustion force. The valve operates when the cylinder pressure exceeds its safe operating limit.
Under normal circumstances, it is locked in its position under the effect of strong spring force. Now when the combustion pressure becomes too high to lift the valve spindle; the valve opens allowing the excess pressure inside the combustion chamber to escape out.
The Relief valve is designed in such a way that the cylinder pressure never exceeds that of 10% above the normal working pressure.
A spring keeps the relief valve under closed condition while the lifting pressure is adjusted by an appropriate packer thickness.
Once operated it reliefs the pressure quickly out of the cylinder avoiding any damage. The valve and the spindle are separated in such a way that the valve can be positioned correctly after opening.
What Cause The Cylinder Relief Valve To Lift
A normal lift of safety valve can occur in the following situations:
- If the engineer supplies too much fuel during the start of the engine.
- If air is being used as a break when stopping an engine for reversal or during a “rash stop” high compression pressure about top dead center may cause a safety valve to lift momentarily.
- It can also occur when the engine is run on the full astern direction for a long period of time. under such conditions, the bridge must always be informed of the astern running at the limit.
- Over-loading of the engine.
However, some situations can cause the relief valve pressure to rise when the engine operates under normal load conditions.
- Badly leaking fuel injector, through the loose nozzle or injecting hoses.
- Fuel pump setting is low or high or incorrectly timed.
- Water leaking into the combustion chamber.
- Scavenge fire.
Difference Between Safety Valve And Relief Valve
> It is a type of protective device fitted to protect the system, device, or instrument.
> It is a type of safety device fitted to protect the life of the working crew or surroundings.
> It is made to lift briefly, release excess pressure and sit back as in normal condition.
> It is made to be lifted once triggered releasing all pressure within the system.
> It lifts in proportion with the increase in pressure. For example, if the pressure increase is gradual it too will lift gradually.
> It lifts suddenly once the set pressure limit is exceeded.
> Can not be lifted manually.
> Can be lifted manually.
> Set pressure is generally 10 to 15% higher than normal working pressure of the system.
> Set at 3 to 5% higher pressure than normal.
> Immune to the backpressure effects; that is why the releasing pressure can be sent back into the system.
> Subjected to backpressure effects that is why the releasing pressure is always released directly to the atmosphere.
Now that you have understood what is the difference between a relief valve and a safety valve based on its definition, function, operation and set pressure; you can easily understand how do they work. But where we get into that lets take a look on the construction parts of cylinder relief valve.
Cylinder Relief Valve Assembly
Helical Valve Spring
The spring is made-up of vanadium /silicon-chrome mostly, spring wire is being tempered and hardened in oil to the appropriate standards. It is then mounted inside the housing which is held by the upper and lower spring carrier.
The Valve and Stem
In a cylinder relief valve the valve and the spindle is made up of high-grade stainless steel. The valve then sits on the machined surface that is exposed to the combustion space. In operation the cylinder relief valve is exposed to opposing upstream combustion pressure and downward spring force.
In some case the valve steam of cylinder relief valve is not connected to the valve itself; but rather it sits on top of it. The spring is located between the bottom spring carrier and the distance desk. You can adjust the spring tension by adjusting the lock-nut position.
A relief valve can have a piston or diaphragm based sensing element. A piston based sensing element is used to meet the need for a simple and reliable system that works great under over pressure conditions. Have higher relief pressures and less tolerance.
On other hand a diaphragm based sensing element is used In low pressure applications where we generally need a more accurate and sensitive system to operate properly. Using diaphragm system we can reduce the size of the relief valve and can eliminate the friction inherent with piston mechanism.
Cylinder Relief Valve Working Principle
Lifting of the safety valve relieves the dangerous pressure in the cylinder and warns of the incorrect condition. The reason for this must be ascertained and corrective action must be taken. The high-temperature gas frame expelled may damage the valve seat.
Safety valve area is only designed to relieve excess gas pressure and may be sufficient to prevent damage in the event of cooling water or oil leaking into a cylinder before starting ON engine for the first time it should always be tunned slowly with indicator cocks open to expel any leakage.
The cylinder valve relief the pressure when the pressure exceeds 10% to 20% of the maximum mean pressure. A small amount of lifting is only permitted and the escaping gases are directed toward a safe outlet. The spindle and valve are two different pieces, so that valve can correctly sit on the seat.
The operation of the cylinder relief valve is the indication of a fault in the engine, which should be identified and corrected as soon as possible. The valve itself should be examined as earliest as possible.
Maintenance Procedures of Cylinder Relief Valve.
1) Remove the relief valve from the cylinder head and clean externally
2) Hold the valve at bench vice and dismantle the parts.
3) Clean the parts in diesel and dry with lint-free cloths.
4) Inspect valve and valve seat for pitting? burning, carryout lapping to remove these and clean with diesel to remove the lapping paste.
5) Inspect spring for crankcase or wrapping. Measure free length repeal if necessary.
6) Assemble in reverse order.
7) Replace copper gasket after annealing.
8) Apply molyKot (Anti-Cleasing parts) on the thread.
9) Mount the valve on the test-rig and set light pressure as per manual.
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