Marine Engineering: Bunkering Oil on the Ship

It is great to have huge ships and massive diesel engines on board the vessels, but then how do you cater to the fuel hunger of these monsters. Just learn the basics of bunkering in this article

Introduction

What do you do when your car or bike runs out of fuel? Well the solution to this problem is pretty simple; you go to the nearest petrol bunk/gas station and get your vehicle refueled. The amount of petrol or gasoline that you purchase depends on the type of your vehicle and its tank capacity, not to mention your pocket allowance as well.

The ship is also a vehicle which floats on water and requires fuel for its operation. She has massive engines, which requires fuel to burn and produce power. The point to be noted is that the gasoline/fuel of your car or bike is stored in tank which is part of the vehicle. Similarly the fuel for ships is stored in tanks which form a part of the ship, called “Bunker Tanks”.

While the term used to describe the process of getting your vehicle filled with petrol or gasoline from a petrol bunk is known as refueling; a similar operation performed for filling fuel oil into the ship tanks from an outside source is known as bunkering.

Bunkers are mostly supplied onboard the ship through a bunker barge, which is a relatively small boat or ship supplying fuel or other types of oils to relatively bigger ships.

Bunkering Oil in the Ship

Depending on the size of the ship and speed at which it is designed to cruise, the capacity & number of bunker tanks are decided. Typical capacities of bunker tank onboard ships are varying from 150 cubic meters to as big as 3500 cubic meters. One might wonder why the ship’s bunker tank capacity is big. The answer is that as the engine power increases, the fuel consumption also increases.


For example a normal 800 cc car, which has engine of 37 bhp power output, gives a mileage of 18 kilo meters / liter, but in a ship of engine output power around 10,000 bhp, the consumption is approximately 25 cubic meters to 30 cubic meters per day. It may go as high as 200 cubic meters of fuel/day. So to accommodate such huge amount of fuel for a long voyage of 45 days, bunker tanks are designed and located.


You give you an idea how massive this volume is, say if the normal heavy fuel oil is considered which has a density of roughly 930 kg/m³, 30 cubic meters would work to nearly 28 metric tons which is the daily fuel consumption. So you can imagine the amount of oil required to be present on board in case the ship is on a month-long voyage of so.

Obviously the only practical way to deliver such large quantities of oil on board the ship from the barge is to use the arrangement of pipelines and pumps. There is a network of pipelines on the ship as well as the barge which are connected to the actual tanks deep below. The valves which control and regulate the flow of fluid to these pipelines are located on the deck at convenient places. You can see the arrangement of valves and pipelines in the adjacent figure. They are distinguished from each other using colour coding which we will take up in a later article.

Towards the left-hand side of the picture you can also see the flexible connecting hose which is used to supply the oil to the ship after connecting it to the appropriate bunker manifold.

The other image on the right hand side shows the bunker barge approaching a huge oil tanker for the bunkering operation.

The chief engineer of the ship is the official in charge of the bunkering operations and the exact amount of oil to be bunker in each time is decided prior to bunkering. We will take a look at the bunkering operation in detail in a separate article and we will also learn about the types of fuels used on ships.

Marine Engineering: Air Starting Valves Used In Ships

Introduction

 

What do you do when you want to start your car or bike? Well you simply turn your ignition key and the battery of the vehicle turns the engine with the help of a starting motor and the firing process starts which continues till the engine is able to run on its own, of course this process takes very short time. But how do you turn the gigantic engine like that of a ship's main propulsion plant? The answer lies in the use of compressed air for starting the engine, hence it is also known as the starting air.

Starting air admittance valves or air starting valves are provided in the cylinder head mountings of marine diesel engines. Their main function is to admit the starting air into the cylinder for starting the engine by air and closes when the engine picks up speed and starts running on fuel oil. In this article we will take a look at the construction and working principle behind these air admittance valves. Just take a close look at the picture given below which shows the full constructional details of the valve before proceeding to study further.

Construction

 

The air starting valve is operated by pressurised air from the air bottle. Each air admittance valve is equipped with its own distributor and Ahead-Astern cam.

These cams have been adjusted for their opening times in such a manner, so as to ensure overlapping for consecutive cylinders for a certain time period, based on the firing order of the engine

Each cylinder is equipped with a starting valve fitted at the lateral side of the cylinder cover. The housing for the valve is integrally cast with the lower part of the two piece cylinder cover.

The valve chest is made out of a steel casing. The miter faced valve is carried at one end of the spindle, and the other end is provided with the guide piston.

The control piston is screwed with the extension of the valve spindle. The valve guide piston is fitted in a bush and is equipped with a number of seal rings.

A store spring keeps the valve firmly seated. A control piston with its cylinder is secured with the valve body by two studs, thus the body makes a gas tight joint with the cylinder cover.

Operational Details

The starting air valve is PNEUMATICALLY operated to admit starting air at the instant of starting.

Basically two types of air are used in the operation of the air admittance valve namely - pilot air and the actual starting air. The pilot air acts to control the entry of the main air which in turns actually starts off the engine and the procedure is described as follows.

The opening of the starting air valve depends upon the PILOT AIR which is send by the distributer; the pilot air is delivered from the control air which passes through the hydraulic interlocks and air starting handle.

The distributer valve is pressed down by the PILOT AIR when the roller stands against the inwardly depressed segments of the cam profile.

This is the starting phase of the cylinder, pilot air get passage and acts on the control piston and opens the starting air valve, thus the starting air is admitted into the cylinder.

In the up coming articles we can see the consequence of leakage of air starting valve and starting air line explosion
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Marine Engineering: Reversing Of Marine Engines

Introduction

The propeller thrust must be reversible in order to do manoeuvring of a ship. Usually manoeuvring is done while entering a port or leaving a port. In case of a controllable pitch propeller an unidirectional engine is sufficient. In case of limited power systems like medium speed engines of high speed engines, clutches and reverse gears may be used. But in large diesel engines, the main engines must be reversible and should be able to produce thrust efficiently in both the directions ( ahead and astern )

To reverse an engine the engine cycle may require re-timing.

Large diesel engines have scavenge ports which controls the scavenge timing. This remains unchanged when reversed.

Engines working with constant pressure turbocharge systems also have symmetrical exhaust valve timing and hence no change in timing is required. Only the fuel timing to be changed since it will be the opposite flank of the cam.

Reversing of old M.A.N engines:

In old M.A.N engines, the reversing procedure is carried out by means of shifting the camshaft axially. For this purpose a separate astern cam is fitted to the camshaft.

Each astern cam is fitted next to the corresponding ahead cam.

The whole camshaft can be shifted axially with the help of hydraulic cylinders fitted to the camshaft. The hydraulic oil gets pressurized and forces the piston inside the cylinder which moves the entire shaft from ahead to astern or from astern to ahead.

Locking devices and safety cutouts are fitted to that the correct positioning of the cam is achieved.

In some engines reversing is carried by the above same procedure. Only one change is that instead of hydraulic pressure, air pressure is used for shifting the camshaft axially.

Sulzer engines

Sulzer engines have lost-motion servomotors on the camshaft. These servomotors are useful to rotate the fuel pump cams to their astern positions. In RTA engines a servomotor is fitted in between a pair of adjacent cams.

Lost motion is a term which means the timing has been retarded by a certain angle with respect to the new direction of rotation.

M.A.N. B&W MC Engines

These engines have a separate method. In this method instead of shifting the entire camshaft, we are shifting the cam follower rollers to alter the timing. Compressed air from the starting system is used to actuate the pneumatic cylinder and piston which displaces each follower unit.

If the position is not properly locked in its place, the fuel delivery will be stopped. This is done by a sensor fitted to each pump.

These are some of the basic methods used for reversing of marine diesel engines which of course plays a vital role while manoeuvring.