Basic Theory of Fire

Fire Triangle

Fuel, heat, and oxygen are the three components of fire.

All three components are require to start a fire and absence of any one component will stop the spread of fire

Tetrahedron fire Triangle

The base of a fire tetrahedron represents the fourth component in a fire triangle. It is the ‘chain reaction’ due to the molecular combination of fuel, oxygen and heat energy. 

When you break the chain reaction, fire does not spread.

The four phases of fire

1. Ignition
2. Developing
3. Absolute Fire
4. Burning Out

• Ignition phase

When there is sufficient air-vapour mixture and the temperature of the substance exceeds its ignition point, the fire starts.

This fire can be due to an external heat source, auto ignition or spontaneous ignition. Spontaneous ignition is the one in which certain materials become warm during oxidation producing more heat and in turn accelerate the oxidation process.

In the ignition or ‘incipient’ phase, there is a heat flow back to the material due to radiation, which results in a chain reaction and the components will have the ability to sustain fire.

• Developing phase

During developing phase, the flame spreads on the surface of the material. After sufficient heating, it gives off vapours, which act as a source of heat. Solids and liquids convert to vapour state before combustion. In case of flammable gases, there is a high risk of explosion, because the combustion process takes place at an extremely fast rate.

The burning speed depends on the rate at which the heat source gives off the vapours and its combustibility. Petrol has a very high surface burning speed, while wood has a low surface burning speed.

• Absolute Fire phase

In this phase, the heat spread on the surface of the material penetrates into the depth of the solid or liquid. Intense flammable vapour will be generated at this stage.

• Burning Out phase

The fire continues to burn, until it consumes most of the vapour. In the burning out phase,most of the fuel gradually burns out and the fire ‘dies’ thereafter.

Sparks from main engine exhaust

Engineer on duty sometimes get dreaded calls from the bridge reporting sparks from the funnel which could lead to panic.

Reasons for sparks from funnel

1. Slow streaming and frequent maneuvering could have built unburnt soot in the Exhaust Gas economizer.
2. Lack of maintenance in Exhaust Gas Boiler-Infrequent/delayed water washing.
3. Fault fuel injector, improper combustion, or incorrect governor setting
4. Indication for the start of an economizer Fire or a scavenge fire.
5. Water in the service/ settling fuel  tank or maintaining the wrong viscosity as per FO lab analysis
6. Improper FO treatment onboard or wrong FO purifier setting
7. Dirty turbocharger air inlet/ fouled air cooler 
8. Improper cylinder oil lubrication feed rate.
9. Dirty scavenge space or choke drains

Prevention

1. Frequent soot blow and water washing of EGB exhaust gas boiler.
2. Check the integrity of funnel wire mesh with frequent inspection.
3. Maintain soot-blowing equipment in good working condition.
4. Carry out proper maintenance on the main engine as per PMS.
5. Fuel oil purification, storage, and transfer should be carried out as per the FO lab analysis report recommendation.

Pro Tips

• Start boiler circulation pump after EGB water washing. Collect/remove soot at the bottom of EGB , this may have to be done multiple times if time permits before departure. 
• At times sparks do come from the engine exhaust right after EGB water washing so avoid water washing at port.
• In oil tanker do not carry out soot blow during cargo operation, venting, and tank cleaning operations. Always inform the bridge prior to soot blowing.
• Remove any combustible material around the poop deck or around the vicinity of the funnel and follow good housekeeping practices. 
• If sparking occurs keep a crew on standby around the poop deck for fire watch and keep fire fighting equipment on satanby until the problem is resolved.

As always prevention is better than cure so carry out proper maintenance as per PMS, have good watch-keeping and housekeeping practices. Share and guide the crew with personal experiences, and good practices.

Steam hammering Vs Water Hammering

What is Steam hammering?

It happens when there is direct contact of steam with liquid, as hot steam comes in contact with cooler condensate or liquid, creating a low-pressure zone. The liquid then rushes in to fill the low-pressure zone creating a pressure spike. As the steam flow increases, steam carries the water with it a lot of momentum is created and hammers the line loops with tremendous forces causing a lot of stress.

Effects of steam hammering

• Cause valve failure.
• Break pipe welds and rupture piping.
• Failure in heat exchange or tubes.
• Crack steam trap and pressure gauges.

How to prevent Steam hammering?

• Have a checklist for the start and shutdown of various steam systems onboard.
• Correct usage of the warm-up valve.
• Drain the steam line and make it free of condensate
• check and repair insulation to reduce  heat wastage and accumulation of condensate
• Gradual opening and closing of the valve.
• Ensure steam traps are working satisfactorily.

What is water hammering?

Water hammering is the sudden pressure surge caused by rapid change in the velocity in the pipeline, which sounds like being pounded with a hammer

It is caused when a fluid in motion is forced to change direction or stop abruptly. When a pump is suddenly stopped, or when an open valve is closed suddenly.

How to prevent Water hammering?

• Reduce operating pressure.
• Open and close the valve slowly.
• Installing pressure reducer.
• Good pipeline control (start-up and shut-down procedure).

What does CCA and Ah in battery mean

Cold-cranking amps (CCA)

It is a battery rating system that defines the battery's ability to start an engine in cold conditions.

What does a 400 CCA battery mean?

It means a battery rated 400 cca will give 400 amps of power for 30 seconds at 0 deg cel maintaining a voltage of 1.2v per cell or 7.2v for a 12v battery

And  battery rated 200 CCA will give 200 amps of power for 30 sec at 0 deg celsius, maintaining a voltage of 1.2v per cell

How much CCA do I need?

The thumb rule is one amp per cubic inch of engine displacement.

Why do batteries deteriorate at cold temp?

At cold temperatures, the reaction in the battery slows down impacting its ability to start the I.C. engine.

LO viscosity increase at lower temp making it harder to pump oil and start the engine. This makes a situation where your battery has to work harder to start the engine.

It is always better to have a higher CCA-rating battery for marine applications.

Ah rating?

Ah typically measures batteries' total energy-storing capacity.

A 100 Ah battery is capable to deliver10 amp for 10 hrs before the voltage drops to a standard value of 1.67 volts per cell or 10.02 volts for a 12v battery.

Similarly, 50Ah would supply 5 amps for 10 hrs

Ah = Amps x hours

Amps hour(Ah)= Current(I) x Discharge time (T)

CCA VS Ah

CCA is the ability of the battery to deliver a burst of power over a short time like a sprinter

Application- ➡Starting lifeboat engine, Emergency generator, etc.

Ah is the ability to deliver power over a length of time like a marathon runner. 

Application➡GMDSS battery, general service battery, etc (deep cycle application)

Difference between purifiers and clarifier

What are centrifuges?

A centrifuge is a device that uses centrifugal force to separate component parts of a fluid. This is achieved by rotating the fluid at high speed within a container, thereby separating fluids of different densities (e.g. cream from milk) or liquids from solids.

• It works by causing denser substances and particles to move outward in the radial direction.
• At the same time, objects that are less dense are displaced and move to the center.
• In ships, we use two types of centrifuges ie. purifier and clarifiers 

Definition

• Purifier is a type of centrifugal separator that we can use to separate two liquids with different densities 
• Clarifier is a type of centrifugal separator that we can use to separate solid impurities from fuel.

The main difference between the two

Purifier

1. In a purifier there exists an inference or a line separation between the oil and water, this is achieved using a dam ring. the position and the diameter of the dam ring plays a major role in the creation and maintenance of interface line
2. Purifier needs to be filled completely with sealing water for the generation of a seal that prevents the oil to leave from the water outlet
3.  The conical disc has feed holes/distribution holes to help with the interface
4. It has two outlets clean oil outlet and water outlet
5. Generally used for lubricating oil

If the interface is broken or misplaced water can go into the clean oil outlet or oil can enter the water outlet, hence disturbing the purification process 

Clarifier

1. In a clarifier, there is no interface between oil and water, this is achieved by using a sealing ring, which seals the water outlet. this allows the water and impurities to stay within the bowl and is discharged when the bowl disludges
2. The clarifier doesn’t need to be filled up with sealing water.
3. The conical discs in a clarifier usually don’t have feed holes in them but if they do, then a disc without any holes is fitted at the bottom of the stack.
4. It has only one outlet clean oil outlet
5. Generally used for fuel oil depending on fuel density and design

Power factor-Basics

 In AC circuits, the power factor is the ratio of the Real/true power that is used to do work and the apparent power that is supplied to the circuit.

• The power factor can get values in the range from 0 to 1.
• When all the power is reactive power with no real power (usually inductive load) - the power factor is 0.
• When all the power is real power with no reactive power (resistive load) - the power factor is 1.

Power factor=True power/Apparent power

=watts/volts-amps

=KW/KVA

Real power is the actual power consumed or dissipated due to the resistance load and is 

• Measured in Watts
• Symbolized by the capital letter P, as always

Apparent power is a phantom power combination of reactive power and true power and it is the product of a circuit’s voltage and current, without reference to phase angle,

• Measured in the unit of Volt-Amps (VA) 
• Symbolized by the capital letter

Why should I improve the power factor

Improving the power factor can improve the current-carrying capacities

• Improve voltage drop in equipment or even cancel completely the (inductive) reactive current in upstream conductors
• Reduce power loss/heat generated

Losses in cables are proportional to the current squared. The reduction of the total current in a conductor by 10% for example, will reduce the losses by almost 20%.

• Reduction in cable size
• Lower electric bill and save money

Generator safety and protection

The protection device is built into the main alternator circuit breaker to safeguard both the individual alternator and distribution system

1)Overcurrent protection

The alternator has overcurrent protection but the main consideration is to maintain the supply of power for as long as possible. The breaker is designed in such a way that is trip instantaneously only in the event of high overcurrent due to a short circuit.

When an overcurrent is not so high a delay with inverse time characteristic allow an interval of time (or a time delay )before the breaker is opened during this time delay the overload may be cleared, hence allowing the supply of power for as long as possible giving rise to IDMT RELAY(inverse definite minimum time Relay)

REASONS FOR OVERLOAD

• Due to serious fault in the switchboard causing the high current fault
• Partial/Straight overload due to starting of big motors

2)Preferential trips

This trip is designed to disconnect non-essential load (like ventilation fan, HVAC, reefer system, etc) in the event of partial/straight overloading with the aim to prevent operation of main breaker trip an loss of total power failure.

• Non-essential load are grouped together and disconnected at timed intervals arranged at 5,10,15 seconds
• Operated at a relay set at about 110% of normal fault load

Das pot arrangement is an older design provided to give a time delay or time lag for the relay to operate, it consists of a piston moving in a cylinder with silicone fluid. The piston moves up or down as the silicone fluid is displaced from top to bottom through a small hole or by the way of clearance around it.time delay can be adjusted bu the size fo the hole or clearance

3)Reverse power trip

It is intended when the alternator is running in parallel operation, this trip will release the breaker and prevent the motoring of the alternator if a reversal of power occurs.

Such a device is used to prevent physical damages to the prime mover, which has shut down due to mechanical faults.

For a detailed explanation

https://marinehack.blogspot.com/2017/07/why-is-reverse-power-motoring-of.html

4)Under voltage trip

• It helps to prevent the closure of the circuit breaker by mistake when the alternator is dead.
• This is fitted in the alternator which is arranged for parallel operations, the instantaneous operation of this trip is necessary to prevent the closure of the breaker.
• It also gives protection against the loss of voltage while the machine is connected to the switchboard.

Electrical Shore supply

Shore supply is generally required in dry dock, to supply the vessel with power when the onboard generator and auxiliary system goes offline for maintenance. There must be a suitable connection box conveniently located at the entrance of accommodation or the emergency generator room to connect the shore supply.

The connectin box may have the following

1. Circuit breaker or a switch and fuse to protect the cable linking the connection box
2. Data plate depicting the ships electrical system(voltage and frequency)
3. Voltmeter to indicate the voltage
4. Frequency meter to indicate the frequency
5. Phase sequence indicator usually a lamp to indicate that the shore power is available for connection to the bus bar
6. An interlock with the MSB to ensure the breaker is not closed when the ship's generator is connected.

Problems with higher frequency and voltage

• A higher frequency will cause the motor to run faster and overheat.]
• A higher voltage may generally cause equipment to draw higher current and overheat and may cause a motor to accelerate more rapidly and this may over stress-driven load

If the shore supply frequency differs

If the shore supply frequency differs with the ship's normal frequency then ideally the shore supply voltage should be different in the same proportions.

If the ships normal supply is 3phase,440V,60Hz, then the available supply should be chosen as 380V and 50hz

Checks and Test

• Ensure the circuit breaker of ships alternator is kept open
• Interlock provided should not enable the shore circuit breaker to close if the ships alternator circuit breaker is kept close
• Ensure correct voltage and frequency are supplied (as asked by C/E)
• Ensure the phase sequence is OKAY
• Earth connection to the shore should be made before connecting the shore supply
• Insulation of incoming cables should be tested

IF the ship-shore connection box is not supplied with phase okay indicator

Check the physical rotation of the smallest motor onboard keeping all the other motor breaker open, if the rotation of the motor is the reverse direction, interchange any two leads of the shore supply cable at the connection box