Fuel Quality and Fuel Quantity Errors

7/30/2019 Fuel Quality and Fuel Quantity Errors

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Fuel Quality and Fuel Quantity errors:The marine industry is constantly expressing concern about fuel quality and fuelquantity problems yet until now very little has been done to address these problems.It isnt because nothing can be done, indeed, these are all problems solved decades

ago for the custody transfer of hydrocarbons, but simply because any initiative toaddress the problems will require investment. In fact, the custody transfermethodology represents a good model for bunker fuels, if the investment can bejustified.Background:Properly managed a fuel, when bunkered, will be in the amount invoiced and of thequality originally declared.However, is is common to discover that between the fuel being sampled and analysedprior to bunkering and bunkering the fuel, that the fuel quality will have beencompromised in some respect or other. There may also be errors in the amount

reported to have been bunkered due to tank dipping related errors or density errors.Some of these problems are inadvertent and simply a consequence of the methodsand equipment used and some will be deliberate frauds, frauds made more easy dueto the lack of effective fuel management and instrumentation. For examples, visittheBad Bunkerspage.Market Forces:Costs and LegislationMuch of the problem associated with bunker fuels; the disputes about both quality andquantity, reflect the existing fuel management practices which have evolved in amarket dominated by cost.Fuel is one of the single largest operating costs of a vessel. Indeed, some operators

report fuel as between 60% and 80% of the operating costs. Thus for many vesselsthe drive to obtain the cheapest fuel possible has created a situation where manysuppliers necessarily invest very little in fuel management or in instrumentation whichwould add to the cost of the fuel.It is hardly surprising that in so many cases the fuel quality and quantity is not asexpected because of the lack of required or voluntary investment nor that as a result,the opportunities exist not just for inadvertent compromises of the quality but also forfraudulent supply.Thus, in recent years, because fuel costs have risen dramatically and freight rateshave been poor, many operators have found profitability affected and attention hasfocused on quality and quantity issues.However transient (i.e. until profitability returns) this concern might be in isolation,there is another factor to consider: environmental protection legislation andspecifically, so far as fuel is concerned, MARPOL Annex VI the fuel sulphur limits thatimpact on fuel management.Financial Risk:Legislation creates financial risks for both suppliers and vessels.The financial penalties would appear to be limited to the possible fines that might belevied if a vessel is discovered to be using a fuel with excessive sulphur content. Thatrisk depends on how often such a situation occurs and the probability of it being

detected.
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It is the responsibility of Port State Authorities to ensure that the risk of discovery isvery high and because discovery depends on establishingdue cause (see Integrityfor Port State Authorities) the financial risks increase simply because the due causerisk is much higher than the risk of having a non-compliant fuel.Risks:

Risk of not achieving value for money bunkering

The risk of having to de-bunker a fuel

Detention by Port State Authorities for due cause

Prosecutions for violation of the treaty.

Digital Viscometers and fuel management::Under the influence of MARPOL it now becomes necessary to keep fuels fromdifferent batches segregated from each other and to maintain those fuelshomogenous in storage. Fuel samples must be representative and fuel quality mustbe accurately reported not just during bunkering but also on board the vessel whichmust log not only the sulphur content but also the density of the fuels used.The value of digital viscometers is that they not only can be used to validate the fuelcertificate but they can also reveal a great deal bout the fuel condition, whether it isaerated, if it is homogenous, if it has been adulterated (and can provide someindication as to possible cause) and generally demonstrate to all concerned partiesthat the fuel has been properly managed and in so doing it can also help limitfraudulent practices.The Integrity system also helps ensure value for money as well as avoiding the risksof de-bunkering, detention and prosecution.For examples of how Integrity helps suppliers and vessels avoid risk and achieve

value for money bunkering, please continue to the other INtegrity pages.
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Introduction:The examples shown here are the actual results of bunker operations which, throughINtegrity, reveal the nature of some of the problems with fuel quality. Undoubtedly someof these problems not show a failure to achieve Valuefor Money but they also illustratesome of the sources of quantity problems. Note that all the examples shown are frominline blending on a barge.Integrity and the digital viscometer can highlight these problems as they are occurringand because operators can tell at a glance that there is a problem and what that problemis, many of these problems will be resolved simply due to better operatorinformation,.allowing them better control of bunkering.Equipment problems or operational procedural problems are resolved by a one off actionto remedy the identified problem..Fraud becomes very visible and much more difficult to perpetrate.Fuel quality is assured and, in conjunction with tank dipping, tank gauging or flowmetering, most fuel quantity problems are managed or eliminated.To view the full size graphic, simply click on the graphic and a new window will open ontop.
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Expected:In any bunking operation there are three distinct elements in the log:

1. Start: The start may include fuel left in the pipes from a previous operation or distillate

used to flush the fixed pipework. For blended fuels it will also show where the fuel isinitially off-spec.

2. The main sequence: the main sequence should show the fuel quality as consistent

and homogenous throughout the bunkering operation.

3. The end sequence: once bunkering is complete, the flexible hoses are flushed

with air creating highly disturbed measurements.

This chart illustrates what may be expected when monitoring fuel properties as thefuel is flowing. The results are apparent in real time allowing real time decisions andactions.Initially INtegrity will respond to whatever was in the pipe work at connection. Once

flow starts the fuel will arrive at the digital viscometer and its properties will becomeapparent.If the fuel is a pre-blended and pre-tested fuel (green trace), then from the momentthe fuel first reaches the sensor and until bunkering is complete, the density andviscosity values will be constant and correspond to the values in the fuel analysisprovided that the fuel has been properly managed and that the original sample wasrepresentative.If the fuel is being blended as it is being bunkered then dependent on the method, theit will be apparent that the blender must first arrive at its control condition. In the caseof meter blending (red trace) the system should rapidly reach the set point values.Some mechanical blenders (blue trace) may require longer to reach the set points andmay show some slight drifting.A good bunker::More precisely, an acceptable bunker. Discounting the start and end sequences anddespite some evident instabilities in the middle of bunkering, the overall result, asshown by the mean and standard deviation results, is for a good blend. It is not asgood as it could be but far from being atypical.Non-homogenous:Any time the fuel is not maintained homogenous in storage or if it is crudelyadulterated then the density and viscosity will show variation during bunkering the

fuel.
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In this example, the cause f the problem is not clear and it is also relatively slight (notethe scales) and could simply indicate that the fuel, or one of its components, mostprobably the residual oil, has partially stratified in storage. It might also indicate someproblem on the barge. Note that because this is an inline blender, some of the effectshave been moderated by the response of the blender to changing viscosity.

Note that the graph is exaggerated for clarity.Consolidate batches:Common practice for some operators has been to follow the ISO in, ISO outprincipal that if all the fuels put into a common storage tank are of the same grade andare ISO 8217 compliant, then any fuel bunkered from that tank will also be ISO 8217compliant.Unfortunately, this is not good enough for MARPOL. The fuels may have differingsulphur contents and densities. Density is not only an important parameter for

accounting for the amount of fuel bunkered (which is the density that will be reportedhere?) it must also be accurately reported in the vessel logs.Here it is evident that a more dense fuel has been consolidated with a less dense fueland the more dens has been pumped from the bottom of the compartment (or aseparate compartment) first. What does the fuel calculation show?Air in the Residual fuel component:In this example there is a significant amount of air dispersed in the residual fuelcomponent on the barge. This may have been inadvertently introduced during loadingof the barge.In this case we not only see evidence of bubbles in the fuel but also of pockets of air.The effects are dramatic, instantly evident and recognisable. Samples taken andmeasured with offline instruments (e.g. Hydrometers and falling ball viscometers) willreport false low densities and false high viscosities. The digital viscometer also willshow false high viscosity and false low density but most importantly, the entrained airwill also result in false metered flows (especially pockets of air) and false tank dippingvalues on the barge or on the vessel.Blender Air:In this case the air is present just as a dispersion of bubbles and is consistent with air

being drawn into the blender as the fuels are being pumped. Even though this is a lowviscosity intermediate fuel, there is no evidence of the air being in the fuel in thecompartments. Evidently the problem has been detected and the cause found(possibly a partially open valve or a leaking gasket in the pump suction) and remedialaction taken with instant effects.Notice the quality of the density and viscosity signals after the air has been eliminated.These are very stable and consistent as the blend continues and these should matchthe fuel blend calculation if the component certificates are valid. Note also that theeffect of air on the values is to increase the viscosity and reduce the density.Summary:


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These are all real data from actual bunkering operations. In normal operations thedata is not processed nor evaluated in this manner, the viscometer is simply toprovide viscosity trim control. INtegrity allows automatic comparison of fuel densityand viscosity with the certificate values, air detection and some diagnostics to suggestpossible causes when the fuel quality is not consistent with the certificate or blend

calculation.

Any problem of quality or any operational problem such as entrained air is instantlydetected, is recognisable and allows immediate remedial actions. When first usedINtegrity problems associated with equipment or operational procedure can beidentified and remedied for once and for all.All parties to the bunkering operation have access to full and instant data on theproblem and its possible causes and appropriate action can then be taken, even if thataction is to reject the bunker. Rejecting a bunker can save the vessel having to de-bunker later on once the commercial sample analysis is recieved. .I

Fuels in storage and bunkering:Though many fuels are blended, only some fuels are blended during bunkering, therest will have been blended into storage and then sampled and analysed prior to

bunkering. Some will be still be supplied as straight run residual fuels.In this section the comments on fuel management mostly also apply to fuels that willbe used as components for blending and they will als apply to loading barges andbunkering vessels.In this section the concern is with fuels that are bunkered and a bunker fuel qualitycertificate supplied which has been produced from sample analysis. This analysismay have been produced by the refiner or it may be the result of samples taken bythe immediate supplier from the fuel in storage.Considering that as much as 65% of fuel supplied is imported or blended fromimported fuels, some care must be taken to ensure that the refiners certificate is still

valid or else a new sample must betaken and a fresh analysis provided; the longerthe supply chain and the more fuel transfers that have taken place from refinery tostorage, to tanker and transit etc., the more opportunities for the fuel quality to havebeen compromised.Today it makes more sense to monitor the fuel quality at each transfer using theIntegrity system to test the validity of the certificates and, if necessary, to take freshsamples for analysis.Fuel Management in storage is a key to the resolution of many qualityproblems:Fuel may be transferred from one tank to another, transferred to barges for delivery

or to mixing processes or simply retained in storage until required.It is in storage that many problems originate.


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In each of these cases the INtegrity solution is to use the digital viscometer tomonitor fuel conditions and to use the fuel fingerprint method to match the fuel toa fuel analysis.For each storage tank there will be an associated fuel analysis. That reports thedensity and viscosity as well as the other properties. Integrity tests the density and

viscosity values of the fuel against the certificate.Changes in practice:For some suppliers there will be no significant changes in how they handle fuels butfor other suppliers some changes are needed to stay competitive and supplydemonstrably compliant fuels.Fuels will stratify and they may tend to separate over time.One supplier using static storage and consolidating different batches togetherdiscovered that the 380cst fuel in storage had stratified into 450cst fuel at thebottom and 150cst fuel at the top.Hence the fuels drawn from the tank could not be guaranteed as ISO 8217compliant fuels.For MARPOL it is not possible with this system to provide an exact density or anexact sulphur content and especially if different batches of fuel have beenconsolidated together in the same tank.Fuels in storage should be maintained homogenous and, if consolidating fuelstocks, it is essential to ensure they are compatible and then homogenise the fuelstogether. The resultant fuel must then be sampled and analysed before transferringfuel out of the tank.In Tank Monitoring:It is not necessarily practicable to measure the quality or homogeneity of the fuelwithin the tank.A re-circulation system allows the fuel to be monitored entering or leaving the tankand while it is being homogenised using the external re-circulation circuit. The fuel ishomogenous if the measured values are constant and when homogenous, samplesmay be drawn from the circulation loop for analysis.The diagram (right) illustrates a simple arrangement of the INtegrity viscometer tocontinuously monitor fuel quality.The Fuel Fingerprint will match the fuel to its analysis and validate the fuel whensubsequently transferred or bunkered.Functions:

To validate that the fuel and the fuel analysis agree whether receiving fuels ortransferring them

To monitor the tank contents for homogeneity To check for entrained air To monitor consolidated fuels during mixing to determine when they are fully

mixed and ready for sampling To provide a fingerprint for identification of the fuel sample taken for

analysis and to match the analysis when returned. To monitor fuels when transferred and validate them against the analysis for

the tank contents.


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INtegrity software will manage a number of such tanks and collate the fuel analyseswith the appropriate tank contents.Most importantly, when any transfer occurs, the program will automatically identifythe fuel by selecting the appropriate certificate or comparing to the certified analysisfor the fuel held in that tank.All deficiencies will initiate an alarm. All data is logged.

Stratification in storage


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Using tank eductors to keep the fuel continuously agitated and homogenous.If the discharge pump is unsuitable for use with eductors, then a tank mixer may berequired.

One of the typical displays provided by Integrity


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Batch Blending is the method of blending fuels where the pre-calculated amounts ofeach component are batched into a mixing vessel sequentially and then mixed prior tosupply. The amounts are calculated typically based on the Wright equations for fuelblend viscosity as used in the ASTM D341 standard. Many of the other properties arefound from simple additive calculations.Batch bending is, perhaps, one of the earliest and simplest methods but also often themost suspect. In some regions it is not allowed, but INtegrity can help resolve theobjections by ensuring accurate quality determination and helps with some quantityproblems.INtegrity CMV not only helps solve the problems, it restores confidence in this method.Effective and accurate Batch Blending requires:

1. Knowing the properties of the components to be blended together.

2. Determining the volume or mass ratio of the components using a fuel blend

calculation.

3. To calculate the mass or volume of each component required to produce a

batch of the required total amount.

4. To accurately batch these amounts sequentially into the mixing tank (or barge

tanks if blended on the barge).

5. To thoroughly mix together

6. To sample, test and adjust as necessary.

In principal, this approach is as capable of producing as good a blend as any other

method; in practice, pretty much everything that can be wrong often is. INtegrityidentifies these problems and allows them to be solved. It is a valuable tool for suppliersand vessels.The problems with Batch Blending:Ratio Errors:Ratio errors result from:

a ratio calculation based on fuel properties which are in error, and results in incorrect

volumes being determined for each component;

ratio errors result from the incorrect batches of components being added (measurement

errors).Fuel Property Errors:Assumptions made about fuel quality often prove invalid, especially if the componentsare not properly managed in storage (seeFuel Transfer) and this leads to serious errorsin the blend ratio calculation.It is essential to homogenise and analyse the components prior to blending.Measurement errors:The volume / mass measurements made represent a significant source of error. Flowmeters are not always used, Tank level measurements are commonly used and aresometimes inappropriate for the task.Inadequate mixing:
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For any mixing system there is a minimum finite time required for satisfactory mixing.This time can be used as a measure of the mixing efficiency, and often the simplicity ofthe equipment means that this time can vary from very long to infinite.Problems occur when the time available is insufficient e.g. Voyage time betweenterminal and vessel.Sample, analyse and adjust:In refinery and some terminal operations this may happen but all too often this sampleand adjust step is omitted altogether, especially in blend-on-demand bunkering wherethe fuel is delivered before the sample results could be obtained.In some cases the resultant blend isnt a bad blend, it is just that the variousmeasurement errors mean that the actual quality of the resultant blend is not known.INtegrity:The diagram illustrates a batch mixing system where tank eductors have been used toprovide enhanced mixing in the tank but the principal addition is of the viscometer (7).It determines if:

The fuel (or component) is free of entrained air The fuel (or component) is homogenous The fuel is well mixed The fuel (or component) is a consolidation of different batches (commingled but

not mixed and re-analysed) The density value used in the delivery note, both for MARPOL and for calculating

the amount delivered, is correct The fuel is ISO 8217 Compliant.

There is no original fuel analysis against which to compare the fuel fingerprint; just a fuel

blend calculation. If the component analyses are correct (validated as the componentsare batched) and it is a two component system, then Integrity CMV has a facilitycalled Blend Ratio Error Recovery which will determine the final fuel properties.


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This diagram illustrates how a single viscometer can be used, with an appropriate pipework layout, to monitor the component qualities as they are batched into the tank (orcompartment on a barge), as it is re-circulated during mixing and as it is bunkered/.

Fuel barges where batch blending is performed usually have re-circulation pumps to mixthe blend. Where insufficient time is available using simple re-circulation to mix the fuels,tank eductors are an effective way to speed up and improve the mixing process.Tank eductors are commonly used aboard offshore support vessels to keep driling mudsfrom separating out.

Fuel Blend calculators rely on a set of equations to calculate the individual propertiesof a blend from the properties in the component fuels and the ratio in which thecomponents are blended.Most usually it is the viscosity which is the target parameter. The viscosity of the blendis calculated using the Wright Equation. Most of the other properties are additive.To assist suppliers blending fuels to the clients specification or to a fuel gradespecification, many of the Oil Majors make available free fuel blend calculators as dosome others including some test agencies.Shell Marine Fuels, Exxon Mobil Marine Fuels and DNV PS all supply free calculators.Integrity uses fuel blend calculations during blending to test if errors between themeasured density and viscosity and the calculated viscosity are due to inaccuracies inthe component specifications or due to ratio errors.


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On Demand Blending:Many fuels are supplied as on demand blends: the fuel is blended as and whenrequired and only as much as is required is blended. In some regions these methodsare not allowed for HFO (yet are successful with white products such as diesel, aviationfuel and gasoline).This may be in the terminal when loading fuel barges or on the Jetty loading vessels, orit may be performed on board the barge during bunkering.This means that the fuel blend properties cannot be supported by an analysis of thefinal blend but instead, a fuel blend calculation is supplied, which is, or should be,based on the fuel component analyses.Inline Blending:There are two principal methods of inline blending:

Mechanical blenders Meter blenders

In both systems the components are inline blended and the blend ratio is achieved bycontrolling the flow rate ratio of the components.Mechanical Blenders:Mechanical blenders use linked proportioning valves where as one valve opens theother closes. This action is controlled by balancing the pressure drops in the two flowstreams. The balance point is adjusted to produce a blend of the required properties.The valve position is affected by the pressure drop in each of the two component flow

streams; this will vary according to changes in flow rate and/or changes in the fluidviscosity.Accuracy depends on the efficiency of the mechanism and the original calibration of thevalve control.Meter Blenders:Meter blenders maintain the flow rate ratio based on the measured flow rates of each ofthe component streams.The properties of the final blend depend on the accuracy with which the blend ratio iscalculated and maintained and the known properties of the components.INtegrity:Both inline systems benefit from the use of the INtegrity digital viscometerfor viscositytrim control.Viscosity is not a simple additive property; it is very sensitive to small variations in ratio.Small errors in the blend ratio can result in significant viscosity errors. The immediateadvantage to using the digital viscometer is that blend can be very accurately controlledto the target viscosity and hence to the target ratio.This results in an almost complete elimination of the give-away, whichever system isused: the excess amount of the more expensive cutter stock often used to ensure asafe blend is reduced virtually to zero with significant cost savings.(This is illustrated most plainly in charts 2 and 4 on the Bad Bunkers Illustrated page.)Benefits: the INtegrity system determines if:

The fuel is free of entrained air
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The fuel is homogenous The fuel is well mixed The fuel (or fuel components) is a consolidation of different batches (commingled

but not mixed) The density value used in the delivery note, both for MARPOL and for calculating

the amount delivered, is correct The fuel is ISO 8217 Compliant.

The simplest implementation ofINtegrity with inline blenders is the addition ofthe INtegrity digital viscometerto monitor the line density, the base density and thebase viscosity of the final blend.This is illustrated in the diagram at right:Meter 8 is not an actual meter but represents the summated values of the componentstreams to yield the total amount delivered.In this system Mass flow rate and total depend on using the density value declared in

the fuel component analyses.

A variety of cross checks can identify if there is a mass flow measurement error and ifthis is due to component quality errors.As with batch blending, it also uses the Blend Ratio Error Recovery application tovalidate the blend ratio and or that the fuel component analyses are true.Component Quality Validation:In the Batch blending schemes, because each component flows past the digitalviscometer in turn, each component analysis can be verified and it can also beestablished that the component is free of entrained air, is homogenous and is not aconsolidation of two or more different batches itself.This means that component analysis error is not a source of the final blend propertieserror. All that remains is a simple error in achieving the target ratio which is simplycorrected.Blend ratio Error RecoveryThis mechanism, developed by Razaghi Meyer International, which can be used withtwo-component batch blends also works with two-component inline blends (using anadded algorithm to test if the source of the error is in the component properties) allowsthe final ratio to be found and the properties recalculated accordingly. This is effectivefor HFO blends with distillate.Blending one HFO with another (e.g. For sulphur content) reduces the effectiveness ofthis method.It is not valid for multi-component blends; it can inform if the results are as expected butif they are not there is no recovery available from Integrity; the final blend propertiesmust then be determined by analysis.Alternative Approaches for Multi-component blends:For multi-component blends there is another approach validating the componentproperties and which can also be applied to two component blends. Enhanced Two-Component blenders and Multiple-Component Meter Blenders:This is illustrated (below right) as a two component scheme.Each of the component flow streams has its own digital viscometer.Thus, for each component, we can verify if:

The component is free of entrained air


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The component is homogenous The component is well mixed The components is not a consolidation of different batches, commingled but not

mixed The component has the properties described in the component analysis (Fuel

Fingerprint match)In addition:

The density values used for the mass flow rate and total calculations are now livetrue values

Meters can also be density and viscosity corrected, if required, using live truevaluesDensity is measured inline and in each flow stream so the determination of the massflow rates is more precise and more dependable than simply using the base densityattributed to the component to calculate the density at the meter temperature.Because the density and viscosity are measured in each component stream flow metersmay be viscosity corrected for improved wide range accuracy and the mass flow ratecomputed live.INtegrity Diagnostics:The digital viscometer in the blend header provides the final quality check on the blendand it is used to trim the blend ratio to maintain the target viscosity.Because the component properties have been validated, various other factors can alsobe validated and the wealth of process data available to INtegrity means that a variety ofdifferent quality checks can be performed both instantaneously and cumulatively tomonitor the efficiency and accuracy of various measurements.INtegrity can be very simple or it can be highly sophisticated.With fuel costs such a significant issue; with the penalties for off spec fuels so damagingboth financially and (with the advent of stricter pollution legislation) because thepenalties can involve criminal sentences, suppliers and buyers of fuels are ever moresensitive to the question of fuel quality. This makes multiple viscometers a valuable aideven for two component blends.INtegrity is a vital tool not just to help achieve those quality targets but also todemonstrate that quality in a meaningful and documented form, with a variety of cross-checks, to provide confidence in many aspects of the fuel supply operation.To find out more, contactRazaghi Meyer International.
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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Single viscometer for two component blending systems

Three viscometers for mulit-component blending systems

Documents

Fuel Quality and Fuel Quantity Errors