Marine Oily Handling Devices and Pollution Prevention Chapter 2 Lesson 3 Pre-treatment of Marine Fuels

Marine Oily Handling Devices and Pollution Prevention

Chapter 2 Lesson 3

Pre-treatment of Marine Fuels

2.3 Pre-treatment of marine fuels

Learning Objectives The main objectives of this section are to make

all engineers familiar with: Pre-treatment of marine fuels. Design criteria, or problems related to settling

and service tanks. Draining of settling tanks. Preheating. Separation, purifier and clarifier. Sludge and sludge handling.


Design criteria, or problems related to settling and service tanks. Draining and cleaning:

It is most important to examine and clean settling and service tanks regularly for sludge.

If satisfactory fuel is transferred into a tank containing sludge, waxy sludge may be thrown down from it as a result of a seeding action by the micro-crystals already present.


Draining and ventilation: Tanks containing heated fuel oil should be vented to a

safe position outside the engine room and as in the case of all fuel oil tanks, the end of the vent pipes should be fitted with wire gauze diaphragms.

Openings in the drainage systems of tanks containing heated fuel oil, should be located in spaces where no accumulation of fuel vapors at temperatures close to the flash point can occur.

There should be no source of ignition in the vicinity of these vent pipes or near the openings in the drainage systems


Settling tanks: Normally, two settling tanks should be fitted, each

having sufficient capacity of 24 hours normal service consumption.

The 24 hours capacity is provided to enable some natural gravitational separation of free water to take place, and to enable the temperature of the fuel to stabilize prior to pre-combustion treatment.

Both settling and service tanks should have sloping bottoms, to enable easy draining from the tanks. In addition it is preferable with perforated plates fitted parallel to the tank bottoms to keep possible sediment in place.


Remember: It is of great importance to establish routines for draining the

settling tanks to prevent build up of water and sludge, which may cause problems due to the separation process.

Service tank: Design features of the service tanks should be similar to those

of settling tanks. Overflow from the service tanks should be led to the bottom

of the settling tanks, or preferable, 300mm above the bottom and away from the outlets.

High and low suction pipes in service tanks should be located above water and sludge drains.

Figure 2.1

Preheating


Service systems: Service systems provide the final supply to the

engine and should be designed to give a flow rate proportionate to the maximum fuel consumption.

Pumps for purification and engine fuel oil service systems must be capable of operating with the highest temperature of the fuel to be handled.

Pressurized fuel systems are preferable when operating diesel engines on high viscosity fuels.

When using high viscosity fuels requiring a preheat temperature far above 100℃, there is a risk of boiling or foaming if an open return pipe is used , and especially if there is water present in the fuel.


Pressurized fuel system: In a pressurized fuel system, the fuel oil is gravity fed,

or drawn by a feed pump, from the daily service tank, through a course strainer and a flowmeter to a mixing or return tank or pipe.

From this tank a booster pump is delivering the fuel through preheater, viscosity controller and a fine filter to the engine fuel racks.

About 30% of the fuel supplied by the booster pumps are consumed by the engine, while the rest is circulated back to the return tank or pipe, through a return line. Normally there are two of each pump types and duplex strainers.


Return tank/pipe: Has a dual purpose: First, to ventilate vapors from the

oil. Second, to serve as a reservoir when or if, switching from operation on heavy fuel oil to operate on diesel oil.

Should have a capacity of 10 to 15 minutes of normal fuel consumption.

Normal temperature range would be 110 to 140℃ Pressure in order of 3 to 4 bars.


Fuel oil preheaters: Each preheater should be capable of heating the

fuel oil from the service tank temperature to 150℃ at full flow rate.

This is considered sufficient for fuels with viscosities up to 700 cst at 50℃. Steam preheaters are preferred to electrical, which have a tendency to give carbonation.


Heating elements and their thermostats must never become uncovered under normal operation and should be situated below the tank outlets.

Heating arrangements demand careful attention to avoid operational problems, because failure to provide heating surfaces of correct capacity can result in inadequate fuel temperatures and inability to pump the fuel.

The line heater output should not exceed 1.5 Watt/cm, but to avoid local cracking of the fuel, metal surfaces should not exceed 220℃.

These requirements clearly stress the need for large heating surfaces.

Figure 2.2 Separation


The fuel to be separated in a separator is a mixture of two or more phases. The dispersed phase may be solids or a liquid.

To enable separation, the phases must not be completely soluble in each other.

The phases must be of different density to enable separation.

Choice of equipment depends on the concentration of solids and particle size of the feed.

Table 2.3 Component affection by separation:

Components

Not affected:

Components

Slightly affected:

Components

Strongly reduced:

Density Nickel

Viscosity CCAI

Flash point Pour point

Micro carbon

Residue Sulphur

Vanadium

Asphaltenes

Total sediment

Ash

Calcium

Water

Sodium

Aluminum

Silicon

Iron

Magnesium


Purifiers: Multiple disc type of purifiers, as made by Alfa Laval,

Westfalia is the most common on vessels of today. It is self cleaning and fitted with 50 to 150 thin cone shaped, pressed steel discs, carried on a central conical distributor.

At least two purifiers should be installed in the fuel oil pre-treatment system.

Due to the recent tendency to increase the density of fuel oils, it is advisable to install purifiers able to cope with fuels having a density above 1000kg/m2 at 15℃.

Figure 2.2


Traditional purifier: Traditional purifiers are designed for fuel oil densities

of up to 991 kg/m at 15℃, with fuel oil temperature at inlet of purifier at 98℃.

These types of purifiers have a gravity disc for the important purpose of removing possible water from the fuel.

A number of gravity discs are supplied with the purifiers.

Each of these discs have different internal diameters, all correctly identified by the relative density clearly marked on the disc.


The density of the fuels decides which gravity disc should be used.

Generally the gravity disc with the largest internal diameter, which does not cause a broken water seal, is the most suitable one.

The importance of fitting the right gravity disc can not be overstressed.

This has become increasingly important, with the higher fuel densities now a reality.


• Latest generation purifier: • The new type of purifiers are specifically designed to

deal with fuel oil densities of up to 1010 kg/m2 at 15 and viscosities up to 700 cst at 50 .℃ ℃

• Instead of using the gravity disc, the new purifiers use a water transducer which responds to changes in the water content of the cleaned oil.

• This causes stop in the fuel supply to the purifiers and the water to be discharged from the purifier bowl.

2.3 Pre-treatment of marine fuels Purifier/clarifier:

The main difference between purifiers and clarifiers is that clarifiers do not have the water seal.

If traditional purifiers are used, they should be designed so that they can be operated independently of each other as purifier and clarifier.

All purifiers should be of the same size and be capable of individually handling the maximum quantity of fuel oil.

The clarifier following the purifier in series operation, will give an additional improvement to the separation result and also act as a safety net.

If the interface of the preceding purifier, for any reason, has moved into the disc stack.

It will not act as a safety net or any water present.

Figure 2.4

Figure 2.5


Mspx303 features: Separation of oil, water and solids simultaneously

from oil sludges of varying composition and density without adjustment.

Integral paring disc pumps, for continuous discharge of separated oil and water.

Separate outlets for oil, water and sludge.

Figure 2.6


MSPX303 benefits: Substantial savings in total sludge

handling costs. Maximum separation of water prior to

discharge to the collecting tank. Maximum recovery of oil for direct use in

boiler or for incinerator disposal. Automatic start and stop sequence.

Documents

Marine Oily Handling Devices and Pollution Prevention Chapter 2 Lesson 3 Pre-treatment of Marine Fuels