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Showing posts from May, 2020

Oil cooled piston of medium speed diesel engine

Piston above has an alloy steel crown containing chromium and molybdenum to give increased strength and resist corrosion at high temperatures. Suitable for burning residual fuels. Aluminium alloy skirt (subject to lower thermal and mechanical loads) is light and has low inertia, reducing stress reversal on bottom end bolts. Clearances are larger than for a cast iron skirt, which although heavier has a low coefficient of expansion and self lubricating properties giving a low coefficient of friction. Large diameter gudgeon (piston) pin and large bearing surface keeps bearing loads to acceptable limits, eliminating edge loading due to bending of pin

Crankcase relief door

The spring loaded valve is of aluminium alloy for low inertia. A non stick heat resisting rubber O ring ensures positive sealing. The valve is designed to be fully open at a crankcase pressure of 0.2bar.  A dome shaped flame trap of oil soaked wire gauze inside the crankcase with a free area equal or above that of the valve opening area is designed to dissipate the heat from the explosion. A deflector shield secures the valve spring and directs any gas emitted in a downward arc of 120° where the damage caused will be minimal. Fitted to engines with bore of 200mm and above with a crankcase volume of 0.6m3 and above (one door at either end of crankcase). Engines with bore of 300mm and above must have a door fitted to each crank throw compartment. Free area of valve a min of 45cm2 and 115cm2 per m3 of crankcase volume. When a crankcase explosion occurs, flame speeds may reach 300m/s with a substantial rise in crankcase pressure. If this pressure is not relieved it can reach

Oil mist detector

LIGHT SCATTERING METHOD (NEPHELOMETRY) In this form of crankcase oil mist detector individual sensors are placed at each monitoring point – each crank throw space and chain case (where applicable). A suction fan draws the sample through each detector. Light is transmitted at one end of the head where the sample flows through. Directly opposite the transmitter is a compensating receiver. This adjusts the light intensity by  feeding back a signal to the transmitter. A measurement sensor picks up the scattered light produced by the oil mist particles. The result is transmitted as an analogue signal back to the monitor twice per second. The monitor compares this signal against a set point, and an average of the other readings. When the scattered light picked up by the sensor reaches a pre determined point an alarm condition will be reached. Advantages claimed for this system are:     •    Sampling points fitted close to crankcase - no long runs of piping.     •    Continuous para

Device for continous monitoring of viscosity of heavy fuel to M/E

The sensor is suspended in the oil being measured by screwing the device into a tapped hole in a stub pipe. The system uses the principle that the damping of a vibration signal is proportional to the square root of the viscosity. Not affected by vibrations or pressure and flow fluctuations. No moving parts to wear out. The sensor comprises of a stainless steel pendulum attached to a base plate via a torsion tube. Two piezo elements are driven by an alternating signal causing the pendulum to vibrate. A second set of elements sense the torsional vibration via a feedback, and a processor measures the phase difference between the transmitted and received signals. This phase difference is processed which results in a value proportional to the square root of  the fuel oil's viscosity. The viscosity of a fuel is dependant on the temperature; The higher the temperature, the lower the viscosity. The output signal from the viscosity measuring device is an electrical signal between 4 an

Rotating Piston

The rotating piston has an alloy steel crown to maintain strength and resist corrosion at high temperatures and a cast iron skirt to guide the piston and transmit the side thrust to the liner. Three chrome plated ring grooves are machined into the crown and an oil control ring groove is machined into the bottom of the skirt Instead of a conventional piston pin arrangement the top end of the con rod is machined to a spherical shape. This locates in a spherical bearing, which comprises of an upper and lower half as shown. A spring loaded ratchet ring is bolted to the piston skirt. The ratchet has an odd number of teeth. Two spring loaded pawls located in the top of the con rod engage alternately with the ratchet ring as the con rod swings. This imparts a rotary motion to the piston (about 9rpm). Oil is supplied up the centre of the con rod to lubricate the spherical bearing and ratchet assembly, and provide cooling to the crown. Cooling is effected by the "cocktail shaker&quo

Top bracing

 Top bracing is fitted to some 2 stroke crosshead engines. Sideways vibration or rocking due to reaction forces at the crosshead and main bearings may cause damage to turbochargers and attached pipework as well as causing vibration in the engine room and through the ships structure. Two modes of vibration known as H or X modes can exist. Mode and amplitude of vibration will depend on number of cylinders and size and stiffness of engine structure. Fitted between the upper gallery of the main engine and a very stiff location on the ships side, the bracings act as detuners, increasing the natural frequency of the system, so that resonance occurs above the engine running speed. It relies on the friction between the pads to brace the engine at the top so that the resonances with critical orders are above the speed range of the engine. Because it relies on the frictional grip to work correctly, the tension on the hydraulic bolts must be regularly checked. Also inspect the structure for

The most critical time for any bearing with regards to lubrication

 Most of the bearings used on the engines work on the principle of hydrodynamic lubrication.  During hydrodynamic lubrication, there is no metal-to-metal contact between the bearing and the shaft. The bearing and shaft is separated by lubricating oil film.  This oil film is formed due to the rotation of the shaft inside the bearing. As the speed of the shaft increases, more oil is drawn between the shaft and the bearing and thicker lubricating oil film is formed.  During starting of the engine, the speed is very low. Hence it is difficult to form a hydrodynamic lubrication between the shaft and the bearing.  During this time, for a short period, there is metal-to-metal contact between the shaft and the bearing surface.  Hence this is the most crucial period as the characteristics of the lubricating oil (oiliness) comes into use and this is time when more wiping out of the bearing takes place.  Hence it is necessary to coat the bearings with anti-friction material to reduce the

Piston cooling fins

 The underside of the piston acts like a cooling area for the piston crown. In this area the cooling medium is circulated and the heat transfer takes place at the underside of the piston crown. If the underside of the piston crown is provided with the fins, as the fins have better cooling effect with sufficiently thickened crown, we can have better cooling of the crown the thickened crown have more strength.  At the same time the fin effect will be utilized for better cooling.  Hence both, more strength and better cooling effect, is achieved by having a thicker crown and fins.

How lubricating oil is supplied to crank pin bearing

The internal passages in the crankshaft are made in such a manner, in case of small diesel engines where crankshaft size is comparatively small, that oil is supplied to the main bearing. From there to the passage in the crankshaft it is feed to the crank pin bearing. Thus avoiding the complicated piping to supply oil to the crankpin bearing and gudgeon pin bearing.  This is applicable only in case of small crankshafts. Bigger crankshafts may fail under fatigue if the holes are drilled out for the passage of the oil. Hence in case of the bigger crankshafts, no drilling of the holes is done on the crankshaft and hence this method of lubricating the bottom end bearing and gudgeon pin bearing is not applicable.  In that case, there is a separate supply of lubricating oil to main bearing and as well as to the crosshead bearing, which will be further bifurcated into two: the crosshead bearing and piston cooling, and the bottom end bearing, for lubricating.

Effects of using fuel oil with high sodium content

In general, fuels leaving the refinery have sodium content well below 50mg/kg. If the sodium content increases, which is normally caused due to seawater contamination. A 1% seawater contamination represents potentially a 100mg/kg increase.  Vanadium is also present in the fuel oil, which combines with oxygen to form V2O5 (vanadium pentoxide), which combines with sodium to form sodium/vanadium complexes. It is well known that there are low melting temperatures of sodium/vanadium complexes of certain critical ratios.  The most critical sodium/vanadium ratio is about 1:3. This will form a sodium/vanadium complex with a low melting point which will flow with the exhaust gases.  It will get deposited as a hard and brittle layer on the cold surfaces such as exhaust valve spindles, turbocharger nozzles and turbine blades. This layer is highly corrosive and corrodes the metal. It is also brittle and breaks away exposing the metal for fresh attack especially when they get deposited on exha

How heat damage to the fuel injection nozzle can be prevented

 If excessive fuel oil temperature is permitted and if there is no separate cooling is given to the injector and the circulating fuel itself is part of cooling system, in such conditions the hot fuel oil will keep the nozzle at high temperature. This will cause damage to the fuel injector because at higher temperature the fuel injector nozzle and the valve seats will lose its strength and it will fail.  Secondly, if the engine is working at high load for long time, the fuel injector nozzle will be subjected to high temperature from the combustion space due to which the fuel injector nozzle may get damaged.  Thirdly, if the quality of the fuel is bad, there is a possibility that hard carbon deposits may be formed. Also the needle guide may get coated with hard varnish and because of this the fuel oil nozzle operation may get affected and it would ultimately cause to seep in place or have excessive weardown and it will get damaged.

Sulzer Reversing Servomotor

Sulzer Reversing valve

When cylinder oil is admitted to Main engine cylinder unit

Cylinder lubricating oil, for lubricating the piston rings and the liner, has to be admitted when the piston, piston rings and the liner are in cool condition and the piston is moving upward so that oil can be retained on the piston rings and sprayed by the piston rings on the liner walls.  This is only possible during the compression stroke. Otherwise, the piston is hot and if the lubricating oil is sprayed on it, it will evaporate very fast and will not carry out any work of lubrication.  At the same time, if lubricating oil is injected during the expansion stroke, i.e. when the piston is moving downwards, it will have a scrapping effect rather than lubrication.

How injection pressure is controlled in a comon rail fuel system

In common rail system of fuel injection, common piping supplies fuel to the individual injectors by a branch pipe which is the feature of the common rail system. This pipe and the full system is under high pressure i.e. the injection pressure.  To control this, a bypass valve is utilized which is fitted at the end of the common pipe. By closing or opening this valve, more or less, the fuel is bypassed from the system back to the suction of the pump, thus reducing the pressure.

Turbulence in diesel engine

The shape and geometry of the combustion chamber is very important from the point of view of creating turbulence in the combustion space. As the air enters through the air inlet valve, exhaust gas is blown out through the opened exhaust valve. During this time, the incoming air will always follow the geometry of the combustion space.  The geometry of the combustion space governs the movement of the incoming air and exhaust gas. This creates a large amount of turbulence, which will be useful in the proper combustion of the fuel, and thus increase the thermal efficiency of the engine.  Also, turbulence is created because of the shape of the crown of the piston which is part of combustion chamber geometry.  As the air enters the chamber, it will hit against the crown of the piston and create turbulence due to change in direction, leading to greater combustion of the fuel.

Why exhaust valve opens before inlet ports are uncovered

When the exhaust valve opens before the inlet ports open, the exhaust pressure is slightly higher than the atmospheric pressure so it automatically drives the exhaust gas out from the cylinder. This is called blowing down effect.  This is highly important because it is the time when maximum amount of the exhaust gas is removed from the liner. When the piston is moving upwards, it is necessary that the exhaust gas pressure should be dropped sufficiently down so that energy is not exerted by the piston to remove the exhaust gas. This helps to avoid the pumping loses and the backpressure.  If sufficient exhaust gas pressure is still existing when the piston is moving upwards, then this exhaust gas has to be driven by the upcoming piston which causes the pumping loses as power required for this is taken from the engine. At the same time, it reduces the backpressure.  If there is backpressure on the upward moving piston, it will have the same effect i.e. it will have more resistance fo

Interferrance angle in diesel engine exhaust valve

The interference angle is the angle between the valve seat and the mushroom head of the valve which sits on the valve seat i.e. the difference between the angle of contact of the valve and the valve seat. This angle is required to have the proper contact of the valve and the valve seat when the valve has attained the running temperature. This angle is about ½ degree.  It gives the inner contact to the valve so that when the valve attains the working temperature it expands and have a proper full contact with the valve seat thus preventing any valve leakage.  At the same time, it also helps in sitting the valve quickly. The main reason for the interference angle is to sit the valve and valve seat accurately when they have attained the working temperature The interference angle is provided for the better sitting of the valve and the valve seat. It has nothing to do with the rotation of the valve or the breakup of the seat deposits because the breakup of the seat deposits is carried o

Boiler automation failure

C/E to be informed and then inform bridge. If steaming then there is some buffer time to sort out the fault. Trace the alarm events in the data logger; this will give some lead to what might have been the initial problem. Man the E/R if UMS ship. Or Change over the boiler and see if the other is working fine on Auto. call for assistance and report matter to Electrical officer If total automation failure then run on Emergency Mode. Procedures will be laid down in the SMS Manual

Action to be taken in case of electrical failure during sailing

In the event of loss of main power, there would be an immediate shut down of main propulsion, which would lead to dangerous situation, if they were to be manoeuvring in narrow congested water or near coast line. Although the emergency generator would start and come on load it is not possible to restart the main engine till the main alternators are restarted and taken on load. Communicate with bridge and if vessel is under manoeuvring in high traffic zone then exhibit the "NUC" signal. Raise engineers call alarm. All engineers to proceed to E/R. If stand by generator has not started, start same and take on load. Make sure to keep enough reserve air in the Main Air Bottles to start the other Generators if required or to start the ME which might be required in heavy traffic areas. Run the Emergncy air compressor and fill up the Emcy Air Bottle. Keep an eye on Running of Emcy Equipment.   Confirm sequential start of all essential M/C or start same. Change over M/E control to ECR

Action to be taken in case steering failure

Engage alternate or emergency steering system. Advise Engine Room. Call Master. Check vessels in vicinity. Check navigational hazards in vicinity. Use Engines as required. Make appropriate sound signals as required. Exhibit shapes / lights as required. Use VHF Channel 16 / 70 (DSC) as required. Consider anchoring if necessary and suitable depth is available. Fix position of vessel. Record time of failure. Ascertain cause of failure. Ascertain time required to repair. Ascertain if shore assistance is required. Make entry of all facts in log book. Forward initial report to all concerned.

Action to be taken in case of Main engine failure during running

Inform bridge and take controls to ECR. Raise engineers alarm and inform C/E Start Aux engine which is in standby. Record time of failure / Maintain timings of events. Assess the situation and if the repair / restarts need considerable time then prepare for anchorage if depth permits. Ascertain cause of failure. Ascertain if Shore assistance is required. Ascertain time required to repair start repair and inform bridge of the progress and expected time of completion. Inform company’s technical department. After rectifying the fault, start M/E and try out in ahead and astern direction Make a report of the failure and damage.

How to find which valve is leaking in compressor

The H.P. Delivery Valve can be tried by taking out the H.P. suction valve then putting on the air back from the receiver to the compressor. If the H.P. delivery valve is tight, no air will leak out of the suction valve hole. The H.P. Suction Valve can be tried by first taking out the H.P. delivery valve, then the pipe between the cooler and the H.P. suction valve. Now open the valve from the receiver, and if the H.P. suction valve is tight, no air will pass back through the pipe hole in the compressor head. The other valves can be tested similarly, after first removing the H.P. suction and delivery valves to allow the air to pass through. When under running conditions, if the L.P. compressor gauge shows a tendency to drop, the L.P. valves may be out of order. On the other hand, if the L.P. gauge rises, the intermediate valves may be out of order. Leaky H.P. valves cause the pressure to rise in both of the lower stage

Air Compressor requirements

Two starting compressors must be fitted, of sufficient total capacity to meet the engine requirements. Each compressor must be able to press up Air receiver from 15 bar to 25 bar in thirty minutes. Two air receivers must to be provided. Total air receiver capacity is to be sufficient for Twelve (12) starts of Reversible engines and six (6) starts for non-reversible engines. Additional one Diesel driven or hand operated (or if possible, both) emergency air compressor must be fitted to start auxiliary engines of a “Dead Ship”. Safety valves or preferably bursting discs must be fitted on the cooling water casing to give ample relief of pressure, should an air-cooling tube burst. Each compressor must have a safety valve designed so that the accumulated pressure, with the outlet valve closed will not exceed 10% of the maximum working pressure. The air compressor cylinders, covers, intercoolers and after-coolers, are tested by hydraulic pressure to twice their working pressure. The casing of

Flashing up and Raising ste procedure on a shutdown boiler

Check that the boiler is properly closed-up after repairs (if any). Cheek that all boiler mountings and equipments are in place and properly connected to tile boiler. Check that all the appropriate valves are shut such as steam stop, blow down valve, chemical injection etc; Open air vent completely. Open auxiliary feed cheek valve and fill boiler with pure water up to 1/4 of the gauge glass level. Do not fill the boiler up to the normal operating level, since the water will expand when heated and so cause the level to rise. Start forced draft fan with dampers opened correctly to purge the furnace and combustion space off any foul gases. If such gases are present, a ‘blow back’ is possible. Some times a ‘blow back’ may occur due to very dirty uptakes. 'Blow back' is a very rapid combustion of inflammable gases and is, in reality, all explosions, which can cause serious damage to equipment and personnel. Check must he made to ensure that there is no deposited oil in the furnaces.

Regulations concerning boiler water level indicators

 Every boiler is to be fitted with at least two independent means of indicating the water level in it. One of which is to be a glass gauge. The other means is to be either an additional glass gauge or an approved equivalent device. (A set of not less than two test cocks will be accepted as the approved equivalent device mentioned above, for boilers having a design pressure less than 8 Bar or internal diameter less than 1.83m.) For water-tube boilers the approved equivalent device is to be other than the test cocks, but where a steam and water drum exceeding 3.96 m in length is fitted two glass gauges are to be fitted in suitable position. The water level gauges are to be readily accessible and placed so that the water level is clearly visible. The lowest visible part of the water level gauge and the lowest test cock (if fitted), are to be situated at the lowest safe w6rkirg water level. The cocks of all gauges are to be accessible from positions free from danger in the event of th

Advabtages of water tube boilers

High efficiency (generally greater than 85%) hence reduced fuel consumption. Flexibility of design -- important space consideration. Capable of high out put (i.e. high evaporation rate). High pressures and temperatures improve turbine plant efficiency. Flexible in operation to meet the fluctuating demands of the plant, - superheat         control rapidly responsive to changing demands. Generally all surfaces are circular hence no supporting stays are required. Steam can he raised rapidly from cold if the occasion demands, (3 to 4 hours compared to 24 hours for a Scotch boiler) because of the positive water circulation.  Compact and relatively light (water content up to 7.5 tons compared with 30 tons for a Scotch boiler). With double easing radiation loss can he cut to 1% or less.