IMO symbols

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

RT -FLEX ENGINES

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.


The Wärtsilä RT-flex50 low-speed two-stroke marine diesel engine:
 common-rail system for fuel injection.
 common rail system for exhaust valve actuation, and
 full electronic control of these engine functions
   instead of the traditional mechanical camshaft system.

Benefits:

Smokeless operation at all operating speeds

Lower steady running speeds, in the range of 10–15 per cent nominal speed, obtained smokelessly through sequential shut-off of injectors while continuing to run on all cylinders.

Reduced running costs through reduced part-load fuel consumption and longer times between overhauls
Simpler setting of the engine. The ‘as-new’ running settings are automatically maintained.

Reduced maintenance costs through precise volumetric fuel injection control leading to extendable times between overhauls.

The common-rail system with its volumetric control gives excellent balance in engine power developed between cylinders and between cycles, with precise injection timing and equalised thermal loads
Reliability is given by long-term testing of common-rail hardware in component test rigs.

Higher availability owing to the integrated monitoring functions.

High availability also given by the built-in redundancy, provided by the ample capacity and duplication in the supply pumps, main delivery pipes, crank-angle sensors, electronic control units and other key elements.
The common rail for fuel injection is a single-piece pipe running the length of the engine at just below the cylinder cover level.

 The common rail and other related pipe work are arranged beneath the top engine platform and readily accessible from above.

The common rail is fed with heated fuel oil at the usual high pressure (nominally 1000 bar) ready for injection.
The supply unit for the fuel has a number of high-pressure pumps actuated by cams driven through gearing from the crankshaft.

Fuel is delivered from this common rail through a
separate injection control unit (ICU), mounted directly on the rail, for each engine cylinder to the standard fuel injection valves which are operated in the usual way by the high-pressure fuel oil.

Using quick-acting Wärtsilä rail valves, they regulate the timing of fuel injection, control the
volume of fuel injected, and set the shape of the injection pattern.

Each ICU serves the two fuel injection valves in its corresponding cylinder cover.
Each injection valve is separately controlled so that, although they normally act in unison, they can also be programmed to operate
separately as necessary.

The exhaust valves are operated in the same way as in RTA engines by a hydraulic pushrod but actuating energy now comes from a servo oil rail at 200 bar pressure.

The servo oil is supplied by high-pressure hydraulic pumps incorporated in the supply unit with the fuel supply pumps. The electronically-controlled actuating unit for each cylinder gives full flexibility for setting the timing of valve opening and closing.

All functions in the RT-flex system are controlled and monitored through the integrated Wärtsilä WECS-9520 electronic control system.

This is a modular system with a separate FCM-20 microprocessor control unit for each cylinder.

An additional FCM-20 unit provides all connections to other systems such as the remote control and alarm systems.

Lower turbocharger efficiencies at part load normally result in low excess combustion air with fixed valve timing.
Another important contribution to fuel economy of the RT-flex50 engines is the capability to adapt easily the injection timing to various fuel properties having a poor combustion behaviour.
Exhaust gas emissions have become an important aspect of marine diesel engines. All Wärtsilä RTA and RT-flex engines as standard comply with the NOX emissions limit set by IMO in Annex VI of the MARPOL 73/78 convention.
   RT-flex engines, however, come comfortably below this NOX limit by virtue of their extremely wide flexibility in optimising the fuel injection and exhaust
   valve processes.
 
A visible benefit of RT-flex engines is their smokeless operation at all ship speeds.
The superior combustion with the common-rail system is largely because the fuel injection pressure is maintained at the optimum level irrespective of engine speed.
At very low speeds, individual fuel injectors are selectively shut off and the exhaust valve timing adapted to help to keep smoke emissions below the visible limit.


 

VIT AND SUPER VIT A DISCUSSION

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

Modern large slow speed MAN B&W 2-stroke engines have Super VIT mechanism fitted to advance fuel injection at lower loads for increasing maximum firing pressure resulting better fuel economy. Details of Super VIT are available in the engine manuals.

Why use the term "Super"? was there an "ordinary" VIT earlier, which has been changed to Super VIT?
If yes, can I have a brief description of how the ordinary (non-Super) VIT worked?

Before the Super VIT was introduced variable injection timing was obtained by means of a special profile on the fuel pump plunger. Hence there was a fixed relationship between the injection timing and the fuel index. Thus it was not possible to adjust the fuel index of the individual pumps without also altering the injection timing. For this reason the Super VIT was introduced, where it is possible to adjust the fuel index and the injection timing independently.

Is it possible to achieve Super VIT action by means of mechanical linkage or can it be only done if we have electronic (microprocessor) controlled fuel injection system. Or is it right to say that most efficient Super VIT can only be achieved by electronic fuel injection?

The Super VIT is available in both a mechanical and an electronic version. In the electronic version an I/P converter supplies the pilot air pressure to the individual servo cylinders, instead of the pilot valve activated by the fuel rack used in the mechanical version. The I/P converter receives its pilot signal from the governor system.

The advantage of the electronic version is that the break-point is calculated from the actual conditions, why the ambient conditions are taken into account. The engine load is calculated from the engine speed and the fuel index, while the compression pressure is calculated from the scavenging air pressure. Based on these calculations the governor calculates the output to the I/P converter

Am I right in assuming break point is where max cylinder pressure has been reached before MCR which is about 85% MCR when using VIT, after which injection timing is retarded back to its original setting at 100% MCR

Also, does the MAN B&W engine have a fuel quality setting lever on the VIT control similar to that used on the Sulzer RTA, or must the fuel pumps be adjusted individually if a fuel of differing ignition quality is bunkered.

The breakpoint is the point where the maximum cylinder pressure has been reached and the injection timing is advanced the most. Above the breakpoint the injection timing is gradually retarded back until it reaches its original setting at 100% MCR load. The position of the breakpoint is determined by the layout of the engine. Formerly it was generally considered to be at approximately 85% MCR load, but it also has to be ensured that the maximum pressure rise from compression to maximum cylinder pressure is 35 bar or less (recommended by MAN B&W Diesel A/S). For this reason the breakpoint has tended to be somewhat higher on the latest engines (approximately 90% MCR load).

In order to compensate for fuel related differences in the maximum cylinder pressure it is possible to adjust the VIT according to the experience with the different fuels.
In case of the mechanical VIT an offset is introduced by moving the pilot valve bracket horizontally towards or away from the lever, by means of the adjusting screws.
In case of the electronic VIT it is possible to adjust an offset value on the governor panel.

What the term fuel index and how is it adjusted?

The fuel index is an indication of the active stroke of the fuel pump. This is controlled by push rods on the individual fuel pumps connected to the fuel rack, which in turn is controlled by the engine governor.

When the engine is given a speed command the governor increases the fuel index, subject to certain limitations, untill the requested engine speed is reached

if i am not wrong, the vit pilot line pressure increases to approx 3 kg at break point and then reduces till the mcr keeping the pmax const.
i would like to draw your attention to the inlet pressure of this vit pilot valve which is 7 kg (control line pr).
i had experienced sticking of this pilot valve and full 7 kg outlet pressure was fed to the servo positioners. luckily it was noticed immediately and corrective actions were taken.
suppose this had happened during ums period in an engine room, would any enine alarm sound?
what would the peak pressure be during that limited time with pilot line pressure at 7 kg and an engine operating at 90 % mcr?

If the servo signal increases way beyond the normal operating range, e.g. as you state the worst case is equal to the control system pressure of 7 bar, this will naturally result in an increased pressure rise from the compression pressure to the maximum pressure.
No alarm is provided for this malfunction, as it does not directly influence the operation of the engine. The increased pressure rise will actually improve the performance of the engine, but will over time overstress the piston rings. If such a condition is allowed to exist for a long time, cylinder condition problems are thus likely to occur. However, an abnormal servo signal ought to be discovered by the engine crew during their routine inspection rounds - even with the engine room under UMS conditions.

But if we observe the VIT index at break point, this would be around 6 notch and the peak pressure developed at this point would be the design 140 bar in the cylinder. If 7 bar is admitted to the servo the VIT index becomes around 14 notch. and as each notch gives an increase in peak pressure of 1 bar, the peak pressure may exceed by 8 bars. this difference may increase more if the engine is operated above the break point. Wouldn’t this effect the load on the bearings and other running gear if the engine is operated in this condition for more than 8 hrs.
My doubt is , why isn’t a reducer fitted before the vit pilot valve, when the max pressure needed is 3.5 bars, which is given during engine starting and astern movements.
the data above are speculated and may not be fully correct, please correct if wrong.

The actual VIT index at the break point depends on a variety of factors, e.g. fuel properties and ambient conditions. The latter is taken into account in the calculations of the electronic VIT version, as the scavenging pressure is used in the calculations. Consequently, it is not possible to state default values for the VIT index break point setting or rate of change.

In any case, the mechanical limits of the VIT system also have to be considered. The travel of the servo actuators is designed to be going from a minimum at a servo signal of 0.5 bar to a maximum at a servo signal of 5.0 bar, why the mechanical limit is reached earlier than a servo signal of 7 bar would otherwise indicate. In case of the mechanical VIT system, the break point is determined by the point where the lever rests on both pivot points. In this case it is not possible to depress the pressure adjusting valve further, thus preventing a too high servo signal as the control air inlet will always press the piston of the pressure adjusting valve against the lever.

Even if the maximum pressure is over the specified maximum, the engine is not overloaded due to this. The engine load is determined by the power required to turn the propeller at the requested revolutions and a malfunction of the VIT system does not change this and e.g. the auxiliary systems are not affected by this either. Consequently, the only real impact on the engine is the increased pressure differential over the piston rings, due to the too high pressure rise from the compression pressure to the maximum pressure, and a very limited increase to the bearing load.

As stated earlier, the over stressing of the piston rings may possibly require maintenance, if the malfunction is allowed to persist, but does not affect the possibility to safely manoeuvre the engine. Hence, it has not been deemed necessary to introduce an alarm or other measures to prevent or limit the duration of such a malfunction.

 

ENERGY AUDIT

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

What is an Energy Audit ? An energy audit is a preliminary activity towards instituting energy efficiency programs in an establishment. It consists of activities that seek to identify conservation opportunities preliminary to the development of an energy savings program. The Role of an Energy Audit To institute the correct energy efficiency programs, you have to know first which areas in your establishment unnecessarily consume too much energy, e.g. which is the most cost-effective to improve. An energy audit identifies where energy is being consumed and assesses energy saving opportunities - so you get to save money where it counts the most. In the factory, doing an energy audit increases awareness of energy issues among plant personnel, making them more knowledgeable about proper practices that will make them more productive. An energy audit in effect gauges the energy efficiency of your plant against “best practices”. When used as a “baseline” for tracking yearly progress against targets, an energy audit becomes the best first step towards saving money in the production plant. Contents of an Audit An energy audit seeks to document things that are sometimes ignored in the plant, such as the energy being used on site per year, which processes use the energy, and the opportunities for savings. In so doing, it assesses the effectiveness of management structure for controlling energy use and implementing changes. The energy audit report establishes the needs for plant metering and monitoring, enabling the plant manager to institutionalize the practice and hence, save money for the years to come. The energy audit action plan lists the steps and sets the preliminary budget for the energy management program. 1. Analysis of energy use Identifying where energy is used is useful because it identifies which areas the audit should focus on and raises awareness of energy use and cost. The results of the analysis can be used in the review of management structures and procedures for controlling energy use. Analysis of energy use can be done by installing submeters in different plant locations to pinpoint actual energy usage per area. This is a good source data for allocating energy use. The plant manager can also list all equipment used and the corresponding operating hours. With this information, he can create spreadsheet information and generate charts useful for analysis. Important Points to Consider When Collecting Site Load Data a.     Operating hours - This can be gathered from plant personnel. It is important to ensure the accuracy of this data because much of the potential for energy savings lies on correct estimation of the equipment’s operating hours. b.     Duty cycle - Machines such as large electric motors have varying loads and hence, different power requirements. c.     Actual power consumed - For electric power users, this is based on either 3-phase current/voltage readings or power analyzer measurements (e.g., direct kW which incorporates power factor). For fuel users, tank readings of monthly consumption estimates and flow meters with totalization can be sources of measurement. 2. Identification of energy projects Opportunities for energy savings can range from the simplest, such as lighting retrofits, to the most complex such as the installation of a cogeneration plant. The important thing to remember is to focus on major energy users and areas. Always apply the 80/20 rule, focus on opportunities that provide 80% of the saving but require 20% input. After the preliminary identification of opportunities, spend more time on those which have shorter payback periods. 3. Cost benefit analysis The identified energy conservation opportunities should be analyzed in terms of the costs of implementing the project versus the benefits that can be gained. If you want to, say, install a heat plate exchanger to recover waste heat, you need to calculate the total cost of installation and compare that with the savings you will derive from recovering waste heat. It makes sense to go on with the project if there is a net positive benefit from the project. 4. Action plan to set implementation priority After passing the cost benefit test, an action plan should be developed to ensure that the opportunities identified are implemented. The action plan should include all the major steps for implementing the opportunity as well as the people responsible. Furthermore, there should be a plan for monitoring the results.

 

STABILITY DEFINITIONS

MARINESHELF publishes articles contributed by seafarers and other marine related sites solely for the benefit of seafarers .All copyright materials are owned by its respective authors or publishers.

Stability Definitions Centre of Gravity A point on the vessel through which all forces of gravity act vertical downwards Forces of Graphic All forces of gravity acting vertically downwards Centre of Buoyancy A point on the vessel through which all forces of buoyancy act vertically upwards equal to the water displaced Forces of Buoyancy A floating body experiences an upward force equal to the water it displaces Metacentre A point on the centre-line of a vessel through which all the forces of buoyancy pass when the vessel is heeled Righting Lever When the vessel is heeled by an external force, the centre of buoyancy/centre of gravity are not in the same line, now a horizontal distance exists, the buoyancy pushing the vessel upright (the righting lever Gz) Metacentric Height The distance from the Centre of Gravity to the Metacentre (G.M.) Height of the Metacentre The distance from the Keel to the Metacentre (K.M.) Displacement Is the total weight of the vessel equal to the water it displaces (Displacement = Lightship + deadweight Draught The vertical distance from the Keel to the waterline Freeboard The vertical distance from the waterline to the lowest deck-edge Under keel allowance The distance from the keel to the seabed Trim This is the difference between the fore and aft draughts Mean Draft This is the forward and aft draft added together and divided by the number 2 Stable Equilibrium This is when a vessel has a positive righting lever (G below M) Neutral Equilibrium This is when the vessel has no righting lever (G & M together) (Danger of Capsize) Unstable Equilibrium This is when the vessel has a negative righting lever (G above M) (Capsizing lever) Stiff Vessel This is a vessel with a very large righting lever (G near the Keel) Tender Vessel This is a vessel with a vessel small righting lever (G very near M) Angle of Loll This is a vessel that is initial unstable but when heeled has a vessel small righting lever (Very dangerous condition, get rid of any weights on deck either by putting it overboard or down into the hold) (Caution watch an angle of loll through ice accretion, always take the ice off all rigging first the from the high side and push it towards the low side giving you a bigger list but your forces of buoyancy work harder to keep your vessel upright) List A list is caused by you moving anything on the vessel to one side Curve of Statical Stability this is a curve that shows the following : (1.) angle of maximum stability (2.) maximum g.z. (3.) the righting lever at any angle (4.) angle of vanishing stability (5.) the range of stability (6.) angle where deck-edge immersion begins (7.) the amount of dynamic stability a vessel has (8.) the point of contra flexure (9.) the angle of inclination (10.) the initial g.m. (11.) the radians for that vessel Stability This is an act of keeping the vessel stable Transverse or Statical Stability The vessels ability to return to the upright position Reserve Buoyancy This is the volume of air trapped in a watertight space above the waterline Centre of Floatation This is the centre of the water-plane area of a vessel at any draught Deadweight This is the cargo, stores water, fuel that you've taken aboard Light Displacement The total weight of the vessel, machinery etc that stays on the vessel and cannot be moved, (stores, fuel water etc not included) Lightship The total weight of the vessel, machinery etc that stays on the vessel and cannot be moved, (stores, fuel water etc not included) A righting moment or a moment of statical stability The total weight X the righting lever (Gz) A moment A moment = weight x distance Loaded weight regarding the centre of gravity When a weight is loaded onto a vessel the centre of gravity moves towards it Discharged weight regarding the centre of gravity When a weight is discharged from a vessel the centre of gravity goes back to where it was before the weight came on board (Opposite direction from where the weight was placed at on the vessel) Shifted weight regarding the centre of gravity When a weight is shifted on a vessel the centre of gravity moves from where the weight was to the weights new position Dynamic stability The amount of work taken to bring a vessel back to its upright position Range of positive stability This is on a curve of statical stability , where the curve starts on the angle of inclination to where the curve stops at the point of vanishing stability Angle of vanishing stability This is on the curve of statical stability and where the curve comes down and has no (g.z.) ( + or - ) then this is where stability vanishes Initial GM This is on the curve of statical stability, on the angle of inclination at 57.3 degrees there is a radian line , and a tangent line which starts from 0 degrees and leaves the first arc of the curve of statical stability and where the tangent line and the radian line at 57.3 degrees meet then this is the initial g.m. Angle of Maximum stability This is on the curve of statical stability, on the curve itself at the top of the curve down to the angle of inclination and this is the angle of maximum stability Maximum GZ (on curve of static stability) This is on the curve of statical stability, at the top of the curve look at the distance on the scale (metres) and this is the maximum g.z. Importance of adequate freeboard With freeboard raised then this will give you (1.) a greater range of stability (2.) a greater range of vanishing stability (3.) a greater maximum g.z. (4.) the maximum g.z. occurs at a greater angle (5.) greater dynamic stability Density The mass of any object expressed in cubic metres (i.e.) a dice is length x breadth x width = Volume of displacement This is where the vessel is equal to the water displaced and expressed in cubic metres