Operation & maintenance

Crude oil refining

The purpose of crude oil refining is to convert crude and other feed stocks into saleable products. The desired products are mainly gasoline, kerosene, jet fuel, gas oil and diesel. In order to obtain these products, crude oil is first separated into fractions by distillation, and then the different fractions are further processed in order to obtain the desired characteristics and optimum yield.

Crude oil refiningThe different processes used in a modern refinery are summarized as:-1. Crude oil desalting2. Atmospheric distillation3. Vacuum distillation4. Catalytic cracking (e.g. fluidized bed catalytic cracking)5. Catalytic hydro cracking6.Visbreaking7. Coking (delayed coking, fluid coking, flexi coking)8. Catalytic reforming and Isomerization9. Alkylation10. Merox11.Hydrotreating12. Sulphur recovery

Crude oil refining 1. Crude oil desaltingWater and inorganic salts are removed in an electrostatic field. The main purpose of crude oil desalting is to protect the refining process units against corrosion.2. Atmospheric distillationCrude oil is a product with a very wide boiling range. In an atmospheric distillation column the fractions boiling below 360°C are distilled off under reflux, and, according to boiling range, recovered as naphtha, kerosene, and gas oil type stocks. Atmospheric distillation is limited to a maximum temperature of 360°C, because otherwise coking would start to occur, and this is not desirable at this stage of crude oil refining.3. Vacuum distillationIn order to distill off a heavier cut, without exceeding the 360°C temperature limit, a second distillation is done under reduced pressure: the vacuum distillation. The distillate fraction of the vacuum distillation unit is the feedstock for a catalytic cracking unit (see item 4).

Crude oil refining4. Catalytic cracking (e.g. fluidized bed catalytic cracking)The main feedstock for a catalytic cracker is vacuum gas oil. The cracking operation breaks large molecules into smaller, lighter molecules. The process runs at high temperatures, and in the presence of the appropriate catalyst (crystalline aluminum silicate). Atmospheric residue, with a low metal content, can also be used as catalytic cracker feed, necessitating an adjustment of the catalyst type The main purpose of a catalytic cracker is to produce light hydrocarbon fractions, which willincrease the refinery gasoline yield. Additional streams coming from the catalyticcracker are light cycle oil (increases the gasoil pool) and heavy cycle oil (base stock for carbon black manufacturing). Both streams are also used in heavy fuel oil blending.5. Catalytic hydro crackingSome refineries have catalytic hydrocracking as a supplementary operation to catalytic cracking. Catalytic hydrocracking further upgrades heavy aromatic stocks to gasoline, jet fuel and gasoil type material. The heaviest aromatic fractions of a catcracker are the normal feedstock for a hydrocracker. Hydrocracking requires a very high investment , but makes the refinery yield pattern nearly independent from the crude oil feed.

Crude oil refining6.VisbreakingThe feedstock of a visbreaker is the bottom product of the vacuum unit, which has an extremely high viscosity. In order to reduce that viscosity and to produce a marketable product, a relatively mild thermal cracking operation is performed. Theamount of cracking is limited by the overruling requirement to safeguard the heavy fuel stability. The light product yield of the visbreaker (around 20%) increases the blendstock pool for gasoil.7. Coking (delayed coking, fluid coking, flexicoking)Coking is a very severe thermal cracking process, and completely destroys the residual fuel fraction. The yield of a coker unit is lighter-range boiling material,which ultimately goes to the blending pool for the lighter products, and coke, which is essentially solid carbon with varying amounts of impurities. The heavier distillate fraction of a coker can be used as feedstock for a hydrocracker (see item 5).The above listed processes give an idea of the different type of operations done in a refinery, but many more process units are required before the end products leave a refinery. Most processes improve the characteristics of the different refinery streams to meet the technical requirements of the end products,others are needed to meet environmental limits (mainly sulphur reduction, both in the end-productsand in the refinery emissions).

Crude oil refining8. Catalytic reforming and IsomerizationBoth processes are in fact catalytic reforming, and are intended to upgrade low octane naphtha fractions of the crude distillation unit into high octane components for gasoline production. The type of catalyst and the operating conditions determine if the reforming is mainly to iso-paraffins, or to aromatics. The terminology “reforming” is generally used for the change to aromatics, while the change to iso-paraffins is referred to as “isomerization”. Isomerization is normally done on a lighter fraction (C5/C6), while reforming is done on the heavy naphtha fraction (C7 and heavier, up to 150°C).9. AlkylationThis is another process intended to increase the yield of valuable gasoline blend components. Alkylation is a catalyst steered combination reaction of low molecular weight olefins with an iso-paraffin to form higher molecular weight iso-paraffins. Thefeed to the alkylation unit is C3 and C4s from the catalytic cracker unit, and iso-butane.10. MeroxA merox unit is used on naphtha and kerosene streams. It is a catalytic process which is intended to remove mercaptan sulphur type molecules (corrosive, and with a very obnoxious smell) into disulphide type molecules.

Crude oil refining11.HydrotreatingA hydrotreating process is, as the name indicates, a process, which uses hydrogen to remove impurities from product streams, and replaces them with hydrogen. Hydrotreating is generally used to remove sulphur (re very low sulphur limits in thespecifications of gasoline and gasoil) and is then called hydro-desulphurization. It is a catalytic process. The process is generally used on kerosene and gasoil fractions. Residual hydro-desulphurization is an existing process, and is in theory feasible, but the economics are not favorable.12. Sulphur recoveryAs a result of the removal of sulphur from the refinery streams by hydrotreating, the generation of hydrogen sulphide during cracking and coking, refinery gases contain a very high concentration of hydrogen sulphide. The simple solution to eliminatethe hydrogen sulphide (extremely toxic) is to burn it, but this then generates SO2, which contributes to acidification problems. In order to safeguard the environment, the hydrogen sulphide is converted in refineries to elemental sulphur. This is typically accomplished by extracting the hydrogen sulphide from the refinery gas by a chemical solvent, e.g. an aqueous amine solution. The rich solution is then preheated and stripped by steam. The Claus process consists of the partial combustion of the hydrogen sulphide rich gas stream (sufficient air is introduced to combust 1/3 of the H2S to SO2). This SO2 then reacts (under influence of a catalyst) with H2S in the order of 1 SO2 for 2 H2S, and thus provides elemental sulphur. The tail gas of the Claus unit is still rich in SO2, and environmental legislation can require thefurther clean-up of this tail gas.

Crude oil refining• A complex refinery processing scheme

can be separated into two parts:• 1. Crude oil distillation (atmospheric and

vacuum distillation)• 2. Streams from the vacuum distillation

unit are converted through catalytic and thermal cracking processes.

The most important specifications to ensure reliable engine operation with fuel originating from complex refining are:

•Maximum density limit: Important for classical purifier operation and to ensure satisfactory ignition quality for low viscosity fuel grades

• Maximum Al+Si limit: In a complex refinery, HCO is used as a blending component. Mechanically damaged aluminum silicate catalyst particles ofthe catalytic cracker are not completely removed from the HCO stream, and are found back in mg/kg amounts in heavy fuel blended with HCO. In order toavoid abrasive damage in the fuel system onboard the vessel, it is necessary to limit the amount of Al+Si to a level, which can be adequately removed by the ship’s fuel cleaning system.

• Maximum total potential sediment limit: The stability of asphaltenes is deteriorated by the visbreaking process, and instability problems can cause fuel purification and filter-blocking problems, hence the need for a specification to ensure adequate fuel stability.

Combustion/ignition qualities

FUEL OIL STABILITY AND COMPATIBILITYTotal potential sediment is an important specification for heavy fuels. Currently, nearly all heavy fuel is marketed with the stability guarantee of total potential sediment (ISO 10307-2) 0.10% (m/m) max., and stratification in heavy fuel oil tanks due to stability problems of a heavy fuel delivery can be excluded when this specification is met. The reason for the specification requirement is the presence of asphaltenes in the heavy fuel. Asphaltenes are present in crude oil, and are defined as the fraction insoluble in n-heptane, but soluble in toluene. Their concentration in the crude oil is dependent on the crude oil origin itself. Asphaltenes are the highest molecular weight molecules in the crude, and contain all of the organically bound vanadium and most of the nickel found in the crude. They further contain a relatively high percentage of sulphur and nitrogen. Their hydrogen content (and hence combustion characteristics) can be quite different from one crude to another. Asphaltenes have a predominantly aromatic structure and the C and H atoms are combined in ring structures as shown

Asphaltenes are molecules, kept in colloidal suspension by their outer molecular structure. Thermally cracked asphaltene molecules have lost part of their outer structure (depending on the severity of the thermal cracking process) and even visbreaking, which is a relatively soft thermal cracking process, affects this outer molecular structure. If too much is removed, part of the asphaltenes can start clogging together, and will no longer be kept in suspension in the fuel matrix: sludge will be formed. Avoiding the formation of this sludge during the manufacturing of visbroken fuel is the responsibility of the refinery. A change in the fuel matrix composition by blending a stable visbroken fuel to a lower viscosity can also affect the stability of the asphaltenes. This means in practice that viscosity reduction of a visbroken fuel with a paraffinic type cutter stock can make the fuel unstable; when this would happen, the two fuels are said to be incompatible. When two fuels, which are mixed together, do not cause any asphaltene coagulation, they are compatible with each other. Test methods exist to predict the final stability of a fuel mixture, and hence the compatibility of the two components. In practice one chooses a cutter stock with a high enough aromaticity to keep the asphaltenes dispersed (e.g. by adding heavy and/or light cycle oil) and to provide an adequate stability reserve.Two heavy fuels with diverse compositions (e.g. one an atmospheric type heavy fuel from paraffinic type crude, and the other from a relatively severe visbreaker operation) can also be incompatible with each other. The potential occurrence of compatibility problems between two different heavy fuels has to be kept in mind for fuel storage, also by the end user.

Colloidal suspension: a suspension in which gravitational forces are negligible.

COMMINGLING OF FUELSFuel suppliers guarantee the stability of the fuel they deliver, but can not be held responsible for compatibility problems with another fuel.Rules, in descending order of safety:• Do not commingle fuels.• If commingling is considered unavoidable,check the compatibility of the fuels in advance, and make a final decision based on the test result.• If a compatibility check is impossible (one component unavailable at the moment the decision has to be made), reduce the amount of one fuel to a minimum before adding the second fuel.Note:Note: There is generally not a big risk of incompatibility between fuels of the There is generally not a big risk of incompatibility between fuels of the same viscosity grade when they have very similar densitiessame viscosity grade when they have very similar densities.

FUEL CONTAMINATION IN LUBRICANTSDuring the last decade, the demands on marine lubricants have changed considerably for medium speed engines: the engines are run at higher pressures and temperatures. This makes it more difficult to cope with the problems of borderline lubrication. The changes to engine construction and the deterioration of the heavy fuel quality in the late 1970s, early 1980s, have generated problems with conventional type marine lubricant formulations (high detergency, low dispersancy), such as liner lacquering,Under crown deposits, increased oil consumption, base number depletion, hot corrosion of the piston crown, oil scraper ring clogging, and increased piston deposits.Medium speed engine blackening due to HFO contamination of the lubricant, piston head corrosion and under crown deposits were the typical consequencesof increased fuel pump pressure and the change over from atmospheric type heavy fuel to visbroken type heavy fuel when conventional type marine lubricantwas being used. Oil cos. has developed high dispersancy lubricants to cope with the trunk piston engine lubrication problems and the asphaltene contamination of thelubricant. This oil was developed specifically to cope with the increasing mechanical and thermal stresses and changes in fuel quality. The following text and photos, taken from a lubrication brochure, further illustrate the problems encountered with the use of a conventional lubricant in medium speed engines, using visbroken heavy fuel.Figure clearly illustrates the camshaft box cleanliness obtained with the new technology, and contrasts dramatically with the blackening of the camshaft boxusing a conventional lubricant, as illustrated.

Camshaft box cleanliness obtained with new tech lubricant despite 4% heavyfuel contamination

Typical blackening of camshaft box asa result of heavy fuel contamination of conventional type lubricant

FO analysis• Carbon residue• Asphalteness• Diesel Index• Sulpher,• Vanadium and sodium• Water**• Ash,• Silica and Alumina• Viscosity **• Density **• Pour point **• Flash point **• Compatibility**

SIGNIFICANCE OF THE MARINE FUEL PROPERTIES LISTED IN ISO 8217:1996

Kinematic viscosity: Is a measure for the fluidity of the product at a certain temperature.Density:Density is used in the calculation of the quantity of fuel delivered. From a technical point of view, the density gives an indication of the ignition quality of the fuel within a certain product class, this is particularly the case for the low viscosity IFOs.Cetane number, Cetane index:Only applicable for gas oil and distillate fuels. It is a measure for the ignition quality of the fuel in a diesel engine. The higher the rpm of the engine, the higher the required cetane number. The cetane index — an approximate value of the cetane number based on the density and the distillation of the fuel — can be calculated and is usually used. Carbon residue:A carbon residue determination is a typical laboratory test performed under specified reduced air supply and does not represent combustion conditions in an engine. It gives an indication of the amount of hydrocarbons in the fuel which have difficult combustion characteristics, but there is no conclusive correlation between carbonresidue figures and actual field experience.Ash:The ash content is a measure of the metals present in the fuel, either as inherent to the fuel, or as contamination.Flash point:

SIGNIFICANCE OF THE MARINE FUEL PROPERTIES LISTED IN ISO 8217:1996

Sulphur:The sulphur content of a marine fuel depends on the crude oil origin and the refining process. Oxides reach the lubricating oil via the blow-by gas.These oxides are corrosive to engine piston linings and must be neutralized by the cylinder lubricant. Marine engine lubricants are developed to cope with this acidity (high BN). If the correct lubricant is used, the sulphur content of a marine fuel is technicallynot important (although it has environmental implications in sensitive areas such as the Baltic Sea).Water content:Water in fuel is a contamination and does not yield any energy & the steam takes away valuable heat. If water-contaminated fuel reaches the injectors, combustion can be erratic. Water in fuel which remains standing in lines for a longer period can cause corrosion.Pour point:With marine diesels with a high content of heavier n-paraffins vigilance is required if strong temperature changes are expected (wax settling can occur, even when the pour point specification is met).

SIGNIFICANCE OF THE MARINE FUEL PROPERTIES LISTED IN ISO 8217:1996

Elements:Vanadium and nickel are elements found in certain heavy fuel oil molecules (asphaltenes). Upon combustion Vanadium oxides are formed and some of them have critical melting temperatures. The most critical are the double oxides/ sulphates with sodium. Some countries have implemented maximum Ni concentrations forinland use of heavy fuel (emission regulations).Total sediment, potential:Inorganic material naturally occurring in crude oil is removed in the refineries prior to the atmospheric distillation. Some minor contamination (e.g. iron oxides) of a finished heavy fuel can not be excluded. The biggest risk for sediment formation in heavy fuel is due to potential coagulation of organic material inherent to the fuel itself: visbroken asphaltenes, if insufficiently stable, can form sediment (coagulation is influencedby time and temperature).. In cases of heavy fuel instability, it is only a relative small fraction of the asphaltenes which forms sediment, but this organic sediment includes in its mass some of thefuel itself, and water (onboard purifying problems), and the amount of generated sludge can become quite high. If the total potential sediment of the heavy fuel oil markedly exceeds the specification value (0.10 % max for all grades of IFOs and HFOs), problems with the fuel cleaning system can occur, fuel filters can get plugged and combustion become erratic.

SIGNIFICANCE OF THE MARINE FUEL PROPERTIES LISTED IN ISO 8217:1996

Catalytic fines:As described previously, HCO is used worldwide in complex refining as a blending component for heavy fuel. Mechanically damaged catalyst particles (aluminum silicate) cannot be removed completely in acost-effective way, and are found in blended heavy fuel. Fuel pre cleaning onboard ships has a removal efficiencyof approximately 80% for catalyst fines. In order to avoid abrasive wear of fuel pumps and injectors, a maximum limit of 80 mg/kg for Al+Si has been definedin ISO 8217.Calculated carbon aromaticity index, CCAI:CCAI is an indicator of the ignition delay of an IFO. CCAI is calculated from the density and the viscosity of the fuel oil. Although it is not an official specification, it has found its way in many users’ bunker fuel specification requirements. Somemanufacturers specify CCAI limits for their engines, depending on engine type and application.

Centrifuges are essential for the efficientcontrol and removal of solids and water fromfuel oils in fuel oil safety treatment systems; they will continue to be so in the future.Efficient treatment protects the diesel enginefrom wear to important components suchas cylinder liners, pistons, piston rings and the injection system.

CentrifugeCapacity of the purifier depends on various factors• Grade of oil• Purification temperature and viscosity • Type of impurities• Degree of purification required.• Through put• The centrifugal force exerted on the oil and other impurities directly relating to the speed and radius at which the fluid is made to rotate.

The following factors are important to understand the function of a purifier. Increasing the sg of the oil will tend to push the interface outward and cause overflow from the heavy phase outlet until the equilibrium is restored. Should the interface be moved so far as to breach the dam oil will be issued from the heavy phase outlet and an alarm will sound. The ideal position for the interface is to lie over the distribution holes. Reducing the sg of the oil will tend to bring the interface towards the axis, this reduces the force of separation on the oil mix and reduces the efficacy of the unit possibly leading to contaminants and water carryover with the light phase outlet. the "gravity" disc are changeable on virtually all purifiers. Their centre bore is governed by the sg of the oil being centrifuged. The largest bore should be used without risking overflow The flow rate of a purifier should be set to optimize removal of whole system impurities. The lower the oil feed the greater the time for impurity removal and the more efficient the purification. The higher the rate the greater the amount of system oil is treated per unit of time. For a system such as main engine oil where contaminants are continuously being added to the system. As a rule of thumb the total volume of the system should pass through the purifier three times every 24 hours, this rate may vary depending on operational parameters. A similar calculation has to be made with fuel oil to ensure removal of water and sludges which may accumulate over time.

Interface position sensitivity

Centrifuge

These produce huge centrifugal force when it rotates. Pay proper attn while operating, disassembly and reassembly, maintenance and check up.

Complete stopping of the machine shall precede any attempt of work on the equipment

Many points are of threaded connection. Make sure of secure tightening particularly of the

Bowl nutDisc nutCap nut of vertical shaftFrame cover boltPipe securing bolts. Make sure of tally marks and other so marked parts are aligned.

Periodically check for any corrosion or erosion.

Others

a) Before disassembling turn off starter

b) Before changing a treated liquid other than originally specified ; check up

c) As the bowl is factory balanced never interchange bowl parts (Discs) even of the same model

Routine ChecksFeed rate Normal

Feed temperature Normal

Frame vibration Normal

Operation noise Normal

Motor ampere Normal

Gear pump discharge Normal

Gear pump connections Normal

Pipes Normal

Leakage monitor Normal

Discharge monitor Normal

MaintenanceIt is advisable to replace the expendable parts of the bowl with new ones in keeping with the bowl opening intervals

Bearing inspection and replacement

For vertical and horizontal shafts use bearing as specified. Upper bearing of the vertical shaft is very important.

For safety reasons replace bearings of both shafts in 2 years weather machine was in use or not.

Bowl

Under stringent and severe operating conditions corrosion might occur in the form of pitting. This do happen at places that can be in contact with sludge.

Set proper discharge intervals and clean those portions as frequently as possible.

Maintenance

MaintenancePerform checks on parts where sludge deposit occurs

a) Seat portion of the sliding bowl

b) Sludge discharge ports

c) Inlet part

d) Thoroughly clean and remove deposits

e) Visual inspection

f) Any abnormality is observed then go for color test

g) Small pitting up to 0.5 mm can be smoothened by grinding and buffing

h) The part need replacement where pitting is more than 0.5mm or cracks are detected.

i) Disc need to be replaced when corrosion and cracks are evident

Remove the sludge deposited on the disc by the use of a cleaning oil and waste cloth or soft copper wire brush. Avoid using scraper or any other things that damage the surface

1) Check the portion above 1 for corrosion and cracks. And if dimension at A is less than mentioned in the manual replace the shaft

2) If a ring groove is observed at the bowl bush seating portion replace the shaft.

3) If too much damage at the threaded portion replace

4) If the inner race of bearing slips on shaft replace the shaft

5) If pinion is worn or damaged replace it

Performance

The optimum throughput rate of continuously bypassed LO is approximately one third of the rated capacity of the purifier. And at that rate generally can handle 2-3 times total system capacity in 24 hrs

Performance• In the Diesel engine LO, which get contaminated with Sulpher

combustion products, decomposed oil products like harmful acids etc can be washed with water flushing the oil at correct temperature.

• Excess flushing can wash away & deteriorate the detergent additives and its effect of oil

• For purification of the residual fuel oils a 2 stage clarification and purification is advised

• For LO the purifiers are normally arranged to operate on a continuous bypass system

• The system layout can vary. But for LO a good process would be to take the oil from a point where the oil has passed thro’ the engine, had time to settle, and hence at its dirtiest. This oil after the purifier is delivered close to the LO supply pump suction

Bathtub Curve

The life of an antifriction bearing is limited and failure will occur. Usually this is due to exfoliation (peeling of the surface layer). This type of failure can be catastrophic and it is normal to renew the bearing of these type as it approaches the probable fatigue limit.

Frequency failure of a random batch of working components

Running hrs

Failure rate

Mechanical failure

Fatigue failure

MICROBIAL DEGRADATION OF OIL.The hydro-carbons are an excellent energy source. Micro-organisms can thrive in any water phase present in a lubricating oil. If there is no water present, the organisms become inert but survive to germinate if a water presence does occur.They can, after germination, double in size and divide into two every 20 minutes, particularly if slightly alkaline conditions prevail. Only a very small amount of water is needed to initiate the germination. Such an amount could result from a condensation process. The growth products of the bacteria are corrosive They are organic acids such as Hydrogen Sulphide. Hence, pitting of bearing surfaces can result. The growth products can also stabilize a dispersed water phase into a corrosive stable ItP emulsion. Centrifuging is ineffective against an emulsion. The base hydro-carbon molecule of the oil can be attacked, with subsequent alteration in oil properties, particularly viscosity. This, in turn, affects lubricating value.

MICROBIAL DEGRADATION OF OIL.

Indications Of The Infestation:-Unusual smell, from the Hydrogen Sulphide. Fine golden brown film on non-bearing, oil wetted surfaces (slime). Increased oil acidity, or reduced oil alkalinity. Corrosion of bearing surfaces and possibly the centrifuge bowl. Water separation problems. Action Taken Renovating tanks, for medium infections, where the oil is just starting to cause water separation problems. The oil Should be heated to as high a temperature as possible(80’C to 90°C) for a two hour period. The jacket water can be replaced, with a biocide added. A biocide can be added to the oil, the type will depend on the oil characteristics. Typical is Benzyl Creosol. In extreme cases the oil is replaced and the whole system cleaned with steam lances.

Load tests

mark

mark

Drive

DriveConsiderable torque would be required to direct drive the bowl up to speed using an appropriately sized electric motor. In addition very high loading would occur on the gear train, to prevent scuffing due to oil film breakdown would require large mating areas there by large gear trains which would again increase the starting load.A centrifugal clutch arrangement is fitted which has between 2 and 6 ferrodo lined brake pads. These are designed to slip during the start up period and also to a much lesser extent during the speed up period after de-sludge. Purifier manufacturers will usually quote a maximum and MINIMUM start up time. As the pads wear it may be necessary to remove and restore the mating surface to keep the start up time correct. As a last measure the number of pads should be altered The electric motor may be of special design allowing for a long period of slight overload during the start up period.The gear train is generally a single stage worm and wheel arrangement with the wheel being made of a softer material. Lubrication is normally splash only, the viscosity of the oil is essential to prevent wear as the form of lubrication is mainly boundary therefore the wear is governed by the viscosity and additives contained within the oil.

DriveWhen wear occurs it will be scuffing and relative movement between the mating faces polishes out any pitting. As wear worsens galling occurs destroying the running surface. This damage is reflected in both elements therefore both should be changed. As well as overload other causes of premature failure are poor design poor material choice, poor lube oil choice, too long a de-sludge period relative to supplied oil quality, out of balance bowl, failing bearing set in particular the vertical shaft upper resilient bearing arrangement The use of planned maintenance is essential particularly with respect to bearing changes. It is strongly recommended to monitor condition using vibration analysis

Bowl Cleaning& Water washing

Cleaning Should be carried out at regular intervals not exceeding manufacturers recommendations. Every care should be taken not to score the surfaces of the bowl especially the sliding surfaces for de-sludging types. The disc stack is generally numbered and should be built up as per this system as the stack is a balanced unit.

Water washing was a techniques employed some time ago to improve purification of lube oil and to remove acids. It involved continuously adding a small quantity of water at oil temperature to the oil inlet which would pass through and overflow. This is much out of favour as it tends to remove the essential oil additives in particualr detergents. An alternative is to inject steam which improves the removal of colloidal carbon by causing it to coagulate

DamageShown is typical damage to the sealing face of a sliding bowl. This has been caused by either poor assembly or by hard material being trapped aft the bowl closes. Unfortunately it is more likely to be the former.Failure is detected by loss of sealing water as seen down the sludge chute. In addition there will be carryover to the heavy phase and loss of discharge pressure as the seal is exhausted.

Typical alarms and shut downs

The following gives a general list of alarms only some of which may be fitted.

Back Pressure shutdown- this measures the discharge oil pressure and alarms and initiates a shut down when below a set value Heavy phase overflow. Oil has a much higher viscosity than water. The heavy phase outlet is led to a small catchment tank containing a float. The outlet from the tank is restricted in such a way that water flows freely but oil tends to back up. This initiates an alarm and shut down Bowl not open- This may be dome in several ways, typically by a lever switch operated by the discharged sludge hitting a striker plate. Another method is by measuring the motor current, when the bowl opens the bowl speed is dragged down due to friction effects of the discharging sludge and water. The motor current rises until full speed is reestablished. This is detected by a current sensing relay Water in oil- This found on modern designs which have a detection probe mounted in the oil discharge High temperature alarm and shut down

Low control/seal water pressure. Where control water is supplied via a fixed small header tanks a float switch may be fitted

Sharples constant sludging

This consisted essentially of a standard non desludging bowl into which were drilled small holes on the circumference fitted with nozzles. Seal water was pumped continuously from a small catchment tank mounted adjacent to the purifier into the bowl where it passed though to be ejected through the jets. It then drained to the catchment tank. Dirty oil would float to the surface where it would overflow though a surface mounted skimmer to the sludge tank. Theoretically the bowl could run for considerable periods without cleaning. The reality was one to two weeks, bowl cleaning included patiently trying to clear the small bore nozzles.

Rudder• A rudder allows the ship to turn, simple plates have been

superseded by plates welded to cast or fabricated frame. Rudders are hollow and so provide for some buoyancy . In order to minimize the risk of corrosion, internal surfaces are provided with a protective coating and some are even filled with foam. A drain plug is provided to allow for the drainage of water , enable internal inspection to be made using fiber optic device and even allow for the limited application of a protective coating. Plates are welded to the frames internally in order to provide flush fitting , the final closing plate must be welded externally. A means of lifting is provided taking the form of a tube as close to the center of gravity as possible. Rudders are tested to a pressure head 2.4m above the top of the rudder.