MODULE 1 : ENGINE STRUCTURE (STATIC)
The engine structure is the foundation of the engine, and must provide:
1. Sufficient rigidity for:
a) Uniform loading on the main bearings, so that crankshaft stresses are kept within design limits by avoiding excess bending loads.
b) True alignment of piston and running gear, so that no uneven loads are imposed on the crosshead guides, diaphragms, and cylinder blocks.
c) Controlling the structure natural frequency away from continuous operating range of engine RPM. The natural frequency will be quite low for high mass objects such as the engine frame, thus increased stiffness is required to increase the primary critical speeds and its multiples (i.e. x2, x3, etc) away from `normal' speed ranges. Engine must be designed to run sub or over critical.
2. Sufficient strength to resist fatigue failure. Fatigue failure is a significant cause of structure defects, and thus the fatigue strength of the structure is more important than the actual UTS.
The engine must be constructed to resist:
A. Twisting, due to fluctuating torsion and guide forces and this occurs mainly in the transverse plane.
B. Bending, due to vessel movement, this occurs mainly in the longitudinal plane.
The strength transversely is required to ensure running gear alignment is maintained and comes from:
1. Strength of the transverse girder to absorb the inertia and combustion forces imposed on it from the main bearing.
2. Rigid attachment of the transverse girder to the longitudinal girders, and deep sided longitudinal girders. This box section which is well away from the neutral axis, provides high resistance to twisting.
3. Construction of `A' frame units to impart additional transverse strength, and to transmit guide forces into the bedplate without undue deformation. A single welded `A' frame unit stiffens the engine by 40%.
4. Cylinder block units resist transverse flexing, although production simplicity usually means that single units, bolted together are used for large engines.
5. Top bracings increase transverse strength; they dampen lateral structure vibration/fluctuations, rather than prevent them.
6. Tie bolts impart compressive stress to the structure this reduces the tendency for the structure to distort/separate, which would reduce the transverse/longitudinal strength.
The strength longitudinally is required to ensure crankshaft alignment is maintained and comes from:
1. Wide horizontal flanges attached to the top and bottom of longitudinal girders. These ensure that girder strength is increased, and also provide increased attachment strength to `A' frames and holding -down arrangements.
2. Single welded unit `A' frame unit will increase longitudinal strength, and reduce the chance of fretting occurring at bolted joints.
3. Cylinder block units also impart longitudinal strength, especially multi cylinder castings.
4. Rigid attachment to a highly stiffened tank top, with extended longitudinal girders. (Close spaced framing of 750mm is mandatory for double bottom construction)
Other points relating to structure rigidity:
A. Manufacture using jointed individual units are simpler for repair, production (easy tooling), and handling; but their bolted construction will increase alignment difficulties at initial erection, and later in life should fretting occur.
B. Rigid engine structure must be coupled to a rigid tanktop, otherwise shafting alignment will be affected. Also the reaction of the engine to the shaft torque must be resisted eventually by the tanktop itself. Class rules dictate the stiffening required in way of the engine seating. This stiffness is increased by making the double bottom as deep as possible, and the `stiffness' is gradually reduced away from the engine seating to avoid stress concentrations.
C. In the modern bedplate design, the main bearing is situated well below the top joint. This much deeper transverse girder increases both the transverse and the longitudinal strength, and hence stiffens the engine without increasing the engine dimensions or mass.
CH.1.1 ENGINE STRUCTURE DEFECTS
Regular inspections should be made of the engine crankcase, to ensure that the engine structure is free from defects and cracks. These defects can be found in the following areas:
1. At change of sections, where stress levels are concentrated, i.e. crosshead guides, & holding down sites.
2. At joints in the material, i.e. at bolt holes and welds, where shear stresses are concentrated.
3. At areas of high bending stress, such as beneath main bearing
4. At points where forces are transmitted or resisted from one component to another, such as the anchoring point for top bracings.
Engine builders construct the engine structure to absorb the high levels of forces imposed on it, thus once initial design defects have been modified, cracks will occur due to a fatigue mechanism caused by one or more of the following:
A. Poor level of manufacture, and inspection. This will affect the quality of the welds primarily, but can also account for stresses locked into the structure from new (residual), although post manufacturing heat treatment should reduce this.
B. Incorrect operation of engine. Imbalance of engine firing loads, or rotating masses (due to piston removal) will affect stress levels
C. Incorrect tension and maintenance of fastenings, such as holding down bolts, tie bolts, and top bracings.
D. Incorrect operation of vibration dampening units. Vibration occurs due to three main sources; hull structure vibration excited by axial vibration of the crankshaft, hull structure vibration excited by varying propeller thrust induced by varying torsion, and transverse vibration due to crosshead guide forces. Most detuners or axial dampeners are fixed units with little adjustment. However they can internally wear, and their efficiency should be checked by vibration measurements or regular internal inspection.
CH 1.2 : INSPECTION & REPAIR OF ENGINE STRUCTURE CRACKS
In order to determine if cracks are present, inspections should be concentrated on the areas where cracks/defects are most common (see earlier comment).
The initial search will probably be carried out visually, with cracks appearing as paint defects. The defects should be recorded as to: position, length, and orientation. If possible clean the surrounding area and use NDT to improve inspection. Dye penetrant is easy to use and interpret, but magnetic particle inspection (MPI) will show those cracks which are just beneath the surface. Note that fatigue cracks occur with very little plastic deformation, and the absence of any deformation makes detection more difficult.
The following course of action could be taken, but remember that the extent of each defect will determine the specific action.
1. Do nothing! Most cracks are dormant or will only grow slowly and not pose any problems. Always check tension of surrounding bolts.Thus a crack can only grow when the stress levels imposed on it are higher than the strain energy which it can dissipate through the parent material.
2. Should the crack be found to be growing, especially if accelerating then action should be taken. As the crack develops the area of metal resisting the stress reduces so the actual stress increases. Eventually the remaining cross-section is unable to resist the load = failure.
3. If shore facilities are not available, then try to bridge the crack, and place the crack affected area in compression. This will include drilling and tapping, but try to avoid hot work as this will probably increase stress levels not reduce them. Drilling the end of the crack can be beneficial BUT finding the crack tip will be difficult, as it is assumed to have a radius equal to the atomic spacing.
4. If shore facilities are extensive, then the crack should be removed and filled by metal similar or superior to the parent. This will include gouging, by grinding or arc, then welding with possible pre & post heat treatment. Obtain approval of class surveyor before and after such work, and closely monitor area following work. Try and obtain the welding procedure approval certificate to be used, so that checks can be made that welder is following laid down procedures.
Poorly carried out work may worsen the defect rather than improve it, as `Delayed Cold Cracking' may occur. This type of failure is the most common in higher tensile steel, heavier steel structures, and joints involving castings.
The cold crack results from a combination of four factors:
1. High residual stress in the joint
2. Small defect to trigger the crack
3. Hardening in the heat affected zone (HAZ)
4. Presence of dissolved hydrogen
If any two of the above can be eliminated then the crack is unlikely to occur.
1. When the bedplate is constructed stress relieving is carried out on the transverse girder (580-620oC and hold for 1 hour/25mm plate thickness), this should also be carried out following any weld repairs.
2. Defects are very difficult to avoid, and weld ripples or stop/starts can be sufficient.
3. To control the hardness, we must avoid rapid cooling from 800 to 300oC. This will prevent forming a martensite structure, which is hard, brittle, and slightly swelled, and encourages cracking usually just outside of the fusion line.
To slow down the cooling rate we can: Use a larger electrode and hence larger welding current; shorten the deposit length by weaving the electrode; or preheat the plate to 100-250oC. This heating must be controlled and applied continuously for 200mm either side of the joint. NB Heating by gas torch is useless and should never be used for pre or post heating.
4. Whilst the cooling rate of the joint determines the hardness of the HAZ and the weld metal itself, the hydrogen in the weld metal will determine the extra embrittling effect that will be added to the already stressed weld. Hydrogen is very soluble in liquid steel, and when the steel cools the dissolved hydrogen must be given time to dissipate or it will segregate to the grain boundaries where it can build up extremely high pressures (up to 7000 bar). The crack thus seldom occurs until the weld has completely cooled and the remaining hydrogen has come out of solution. Hence the term delayed cold cracking. Maintaining a post heat of 100oC for several hours will allow the hydrogen to escape from the weld area. However we can reduce the hydrogen in the weld by using low hydrogen rather than rutile coated rods, and ensuring the rods are dry. Metals with higher carbon contents suffer from increased hardening, and as such the Class rules dictate an upper limit of 0.23% carbon. (Note that other elements also have a hardening effect, and these elements are included in the Carbon Equivalent figure. CEV values above 0.41% are treated similar to steel of above 0.23%).
CH.1.3 : TIE BOLTS
These are provided to keep the whole engine structure in compression (clamping force), which:
1. Increases the fatigue strength of the engine structure.
2. Maintains the running gear in alignment, as fretting is prevented.
3. Reduces the bending stress imposed on the transverse girder. The gas force acting on the cylinder head is transmitted via the tie bolts, whereas the firing pressure on the piston is directly transmitted to the main bearing, which is then spread to the engine frame via the tie bolt support.
Tightening method (MAN B&W MC Engine)
1. Slacken all pinching screws,
2. Mount hydraulic jacks, starting at mid-engine,
3. Pressurise up to 700bar, and tighten nut with tommy bar,
4. Work out from starting point with alternate sides,
5. Check that pinching screw is free to move, and tighten up so that tie bolt is nipped only.
Tightening method (Sulzer RTA Engine)
1. If tensioning bolt from new, slacken off main bearing jack bolts,2. Ensure clamping (or pinching) screws are slack,3. Mount the hydraulic jacks, starting at mid-engine,4. Pressurise to 350 bar, and tighten nut with tommy bar,5. Work away from starting point, using alternate sides,6. Tighten all bolts to 350 bar, then using the same procedure/sequence as before, tighten to 600 bar,7. The elongation of the bolt may be checked, and these should be the same as the book reference value, and the other bolts,8. If just checking tension, then increase pressure to 600 bar and check with feeler gauge for any gap between nut and washer,9. Retighten clamping bolts.
If the makers tightening procedure is not followed correctly then non-uniform tightening may result. This will produce uneven loading on the transverse girders causing possible crankshaft misalignment, and reduced tension/possible fretting or over tension/possible yielding in some of the tie bolts.
If a tie bolt is operated with low tension then the fretting which occurs may permanently misalign the affected components. So following fretting, placing the correct tension on the component will probably cause misalignment, with corrective machining being an expensive remedy.
Although checking the tension of the bolt will indicate any slack bolts, visual inspection of the joints between cylinder block/`A' frame/bedplate will show some movement if the bolts are incorrectly tightened. If fretting wear is seen (fine rust particles like cocoa powder) then the bolt tension should be checked, as well as the joint sealing bolts, at the earliest opportunity.
For engines that are constructed with only one main component, tie bolts are not required. Thus the firing force will stretch or place the single component in tension. These engines will have underslung crankshafts, with the main bearing caps fastened both vertically and horizontally to the engine block.
One of the exam question deals with the operation of an engine with a broken tie rod. Obviously the compression of the engine structure in the location of that defective tie rod will be reduced and this will:
• Reduce the fatigue strength of that area,
• Reduce the clamping force between components allowing fretting/misalignment and possible fluid leakage.
The normal procedure that should be carried out when any component is damaged is to reduce engine power, especially in that affected area. This would be the prudent course of action until detailed advice from the engine builder can be received. It would probably be advisable to reduce the engine load uniformly rather than just one cylinder, as this would increase all the associated problems from a poorly power balanced engine.
However MAN B&W have stated that a single failure will not result in as much damage as long as the engine components are not moving excessively (see class overhead). Whereas Pielstick advise that only the fuel rack setting on the affected cylinder be reduced to avoid leaks at the cylinder head/liner gasket.
CH.1.4 : HOLDING DOWN ARRANGEMENTS
The engine is attached to the ships hull via the holding down arrangement. The holding down bolts and chocks will:
1. Fix the engine position within the ship's structure, so that good alignment of the engine and the transmission shafting is achieved. The main purpose of correct alignment is to ensure an even and acceptable load on all the bearings within the engine and transmission shaft, including and often especially the stern tube.
2. Provide the clamping force, so that the friction between bedplate, chock, and ship’s structure resists the propeller thrust.
3. Strengthen the stiffness of the engine, as the stiffness of the double bottom structure is coupled to the engine (2S).
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| MAIN ENGINE HOLDING DOWN BOLTS |
For steel chocked engines, the use of longer, more resilient bolts is quite widespread. The increased length produces increased extension under tension; thus the bolt will retain a higher proportion of tension should wear/fretting occur at the bolt or fastened components. Coupled with spherical washers, so that the bolts only have tensile loads applied, this design of bolt is less likely to fail or become slack in service.
Should an engine be operated with slack holding down bolts, then:
1. Fretting will occur between chock /bedplate/tank top. This will permanently misalign the bedplate should the bolts be re-tightened; and reduce the stiffness of the holding down arrangement. Vibration of the engine and ship’s structure will probably occur.2. Increased load will be applied to the friction grip of the remaining chocks, which may induce fretting here. Thus in extreme cases the bolts will be subjected to shear which they are not designed to withstand. Premature failure would probably result. Note end chocks are fitted to prevent shear on holding down bolts.3. Following re-tensioning of a slack bolt, the possibility of the bolt becoming slack once more is increased. This is because for the same bolt tension, the chock/bedplate surfaces may not be pressed together sufficiently for fretting to be prevented.4. The fretting will cause mis-alignment of the engine with the transmission shafting. This will increase the torsional stresses on the shaft with possible premature fatigue failure, plus increased imbalance of bearing loads on the engine and transmission bearings.
To rechock an engine on steel chocks is a major, hence expensive operation; thus resin chocks would probably be used.
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| HOLDING DOWN BOLTS ARRANGEMENT |
CH.1.4 : RESIN CHOCKS
The popularity of resin chocks has increased with experience of their use. They were primarily used for repairs, but now they are increasingly used for installations. These chocks are the only viable method for re-chocking repairs, especially on fretted/uneven foundation plates.
They offer the following advantages:
1. 100% contact on uneven surfaces,
2. Cheaper to install, as no hand fitting required,
3. Do not corrode and resist chemical attack,
4. As they achieve good contact there is little chance of fretting, thus the expensive resilient bolts are no longer required,
5. Noise and vibration damping.
Disadvantages:
1. Overstressing bolts can damage/shatter chocks,
2. Max temp limit of 80oC,
3. Incorrect fitting/pouring may drastically reduce chock life, and this could require complete resin chock renewal.
Method of application:
1. Obtain calculations from contractor for chock area and bolt tensions. Ensure Class approve these calculations before work begins,
2. Clean and degrease the area engine frame and tank top,
3. Build dams from metal sheet and putty sealant to contain the chocking liquid,
4. Ensure that all hull renewals in the area are complete, and that final engine re-alignment is satisfactory,
5. Ensure that no heavy work is carried out during the cure period,
6. Cure time is dependent upon ambient temp, which MUST BE above 20oC, cure time approx. 36 hours. Black heaters can be used if required,
7. Thickness must be no greater than 25mm, otherwise 2 or more pours required,
8. Torque down bolts after cure time, and hardness check of chocks.
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| RESIN CHOCKS |
The drawing illustrates a large 800mm bore slow speed engine mounted on resin chocks. Only the main support chocks are fitted with epoxy, the side and two end chocks are still produced from steel, which are tapered to allow easier fitting in the yard. As with all fastening devices, the bolts must be checked regularly to prevent chock wear.
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| 800 mm BORE, SLOW SPEED ENGINE ON RESIN CHOCKS |
As normal the main support chocks are fitted beneath the longitudinal frame, whilst the side chocks are fitted in-line with the main bearings. The two end chocks are fitted at the aft end of the engine, and provided with through-bolts to ensure that forward engine motion is also limited by these end chocks.
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| SIDE CHOCKS ARRANGEMENT |
Recent introductions by MAN B&W have changed the standard specification for new buildings to using epoxy side chocks spaced every other crossgirder together with standard “thinner” bolts rather than the original long “resilient” type bolt with spherical washers.
CH.1.5 : RESILIENT CHOCKING
For medium speed engines, mounting the engine on resilient chocks is an option. This mounting system will significantly dampen the level of vibrations transmitted from the engine to the tank top, with Wartsila quoting only 10-20% transmission.
The natural frequency of hull (2-5 Hz), stern (4-7 Hz), and decks/bulkheads (10-15 Hz) can not be easily changed due to the costs involved. Thus to avoid resonance, the level of engine vibrations must be reduced.
For 2/S engines the high rotational and static masses of the engine, will produce higher out-of-balance forces and preclude the use of resilient mounts which also need to support the weight of the engine. (Weight of Wärtsilä Vasa 12V46 10860kW = 155 tonnes, weight of MAN B&W 5L60MC 9600kW = 305 tonnes).
Thus 4/S engines are lighter, have lower out-of-balance forces, are smaller (so easier to produce more rigid engine seating), and most of the hull natural frequencies are away from the operating range of the engine (engine operation 400-1500 rpm transmits frequencies in 6.7 - 25Hz range depending upon number of cylinders).
Another vibration output of the engine is the frequency of ignition which is 27-450 Hz for 4/S and 4.7-25 for 2/S. Thus we can mount the 4/S engine on resilient mounts more easily, and obtain better results as there are less frequency ranges, and amplitudes to subdue.
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| RESILIENT CHOCKS ARRANGEMENT |
The number and location of the flexible mounts are given by the engine manufacturer, and Wartsila state that conical mounts are usually used. These vertical mounts are easier to install than the inclined mounts, but do not minimise the engine movement as well. The rubber element of the mount is designed to withstand both compression and shear loads, but the mounts also have built-in buffers to limit excessive movement of the engine during heavy sea conditions and during starting/stopping. As well as the main mounts or chocks, side and end buffers can also be fitted to limit excess engine movement. Typical figures given by Wartsila state that the engine crankshaft centre will move +/- 1mm and the top of the engine +/- 5mm during starting. Further movements of similar dimensions will also occur due to the torque reaction when the engine is fully loaded. Hence these movements, plus the effects of creep and thermal expansion must be accounted for when the engine is installed.
The engine is aligned to the gearbox (or the generator) before the mounts are installed, taking into consideration the mis-aligning factors stated above. The load or compression of each mount should be similar, and there is a tolerance of 2mm when using the conical mounts. Adjustments to the height of the chocks are made by a single steel shim, which are individually fitted beneath the mount.
As the mounts are made from natural rubber, (due to their inherent properties of vibration dampening), care must be taken to avoid contamination with oil, or even oily water. Hence the covers must always be in place, and regular checks carried out to ensure contamination is not occurring.
Note that for flexible mounted engines, all the rigidity for crank alignment must come from the single cast engine block, as the flexible mounting dictates that the stiff engine seating will not aid engine stiffness. Hence it is only under-slung engines that are chosen for resilient mounting.
The other factor with resilient mounting is the connection to the engine of all the associated pipework. All connections must be flexible, and this even includes access ladders and cabling. The flexible connections must be adequately secured at both ends, and the engine must not be allowed to impose high tensile stresses on the connections, otherwise premature failures will result.
CH.1.6: TOP BRACING OF MAIN ENGINES
These braces are fitted at the top of the engine in order to stiffen the upper part of the engine, and to resist the twisting on the engine imposed by the crosshead guide forces. The braces are intended to be fitted in pairs (or three for the larger engines) to one side of the engine only, usually the exhaust side, but the camshaft side is an acceptable alternative.
By introducing this bracing we increase the stiffness of the engine to ship attachment, thus increasing the natural frequency of the engine and the ship structure. Hence resonance of the engine structure will not occur within the normal operating range.
The bracing can be either the standard mechanical friction or hydraulic bracing where larger hull deflections can occur.
MECHANICAL FRICTION BRACING
This consists of friction shims clamped between two steel plates by a hydraulically fastened bolt. The pressure on the bolt is nominally set at 60 bar. However if movement occurs which exceeds +/- 0.02mm, then the bolt can be tensioned up to 120 bar maximum, or until the engine structure vibrations or brace movement have disappeared.
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| MECHANICAL FRICTION BRACING |
HYDRAULIC BRACING
This unit was introduced for those vessels which suffer from high engine/hull deflections (usually vessels made with a high proportion of high tensile steel). The spherical bearings absorb the relative vertical and longitudinal movements between the hull and engine.
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| HYDRAULIC TOP BRACING |
In this unit, a hydraulic force controls the bracing force via the setting of the main “high” relief valve. To initially pre-tension the device, an oil accumulator is used and should any movement in the brace occur due to engine vibration or thermal expansion, then the oil pressure will increase, but only up to the settings of the “high” relief valve.
The solenoid bypass valve is activated when the main engine is stopped and allows the hydraulic unit to be partly released when high hull movements are expected (i.e. during vessel load/discharge). The solenoid valve places the “low” relief valve in parallel to the main “high” relief valve, which releases at a lower pressure. During adverse weather the unit would release or slip, just as the mechanical unit would do.
This unit is attached to the engine and hull via bolts, rather than welds, as defective weld problems were also present in some engines with the mechanical brace.
Manufacturers recommend that the bracing should be checked at least once a year for security of the brace, and for possible cracking of the attachments to the hull on the mechanical units.
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ReplyDeleteCastingMFG
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