World Bunkering > News > Winter 2010 > Coriolis: the new black?

The bunker industry has been ‘instrument free’ for far too long. Signs of the inevitable change are evident in the announcement of one shipping company taking unilateral action on quantity issues by fitting Coriolis meters on its vessels.

The news was not well received by some suppliers. There are always dangers from unilateral action; one concern was that vessels would challenge the supplier’s figures only when it was in their interest to do so, perpetuating the mutual mistrust between suppliers and vessels.

What is needed is an industry standard that delivers an equitable system, properly administered, that favours neither supplier nor shipowner, nor any one technology or manufacturer, and that deals with issues of different accounting methods reasonably.

Considering the alternatives

In its February article, ‘Entering a new Era’, World Bunkering reported on a welcome SPRING proposal to address quantity accounting. But the article implies that SPRING is likely to deliver an “any colour you like so long as it is black” solution: Coriolis meters. Sensitive regulation depends on supporting industry initiatives and in this case the supplier initiative is based on the Coriolis meter, but are other technologies to be evaluated, are there other initiatives pending?

Any new standard ought to be such that the industry is able to choose the optimum solution for a particular application from a range of viable alternatives. It should not exclude other choices, especially if they may be better solutions in some cases.

“70% of installed flow meters are either the wrong technology or the wrong size,” says an article in Control Engineering titled ‘Flow meter selection: Right size, right design’. It isn’t saying the installed meter won’t work, except in rare cases; it is saying that it may not be the best technology for the application or that it may be wrongly sized. In an individual application this may be survivable, but if it affects an entire industry, the consequences could be serious.

It is therefore necessary to ask: “Are Coriolis meters the best choice?” Coriolis meters may be every bit as good as people say, but there is a tendency to wrongly treat them as a universal solution. It is this presumption that can lead to the neglect of the proper selection process and the failure to properly evaluate and compare other technologies. Coriolis may be a solution, but it is not the only solution and perhaps not even the best solution.

Choosing a technology

The way to choose a technology is to identify the key factors and evaluate each technology against them.

Fluid Viscosity

Fluid viscosity is an obvious and defining factor. We quickly reduce the range of possible technologies to just a few that are able to handle bunker fuel viscosities. This includes Coriolis meters, positive displacement (PD) meters and ultrasonic meters.

Accuracy

The industry needs an equitable standard, ie one that is fair to both supplier and vessel. There are many examples of cross-boundary metering where different standards apply, but which is appropriate as a model?

• Domestic water meters deliver equitable metering. The accuracy requirement of 2% is consistent with a low value product and it is not based on a single transaction but on a sequence of transactions. It factors in the lifetime performance of the meters.
• Forecourt metering (of petrol and diesel) measurement error must always be in the purchaser’s favour and thus a very tight accuracy requirement is essential to minimise the give-away of high value product.
• Fiscal metering is intended to deliver performance suitable for high value, duty payable products with optimum single transaction accuracy. But which concept best fits bunkering?

Conventionally, mass is determined from the volume measured by tank dipping, the density reported in the BDN, and the temperature at which the fuel is delivered. In the marine industry density readings are often rounded up to the nearest 0.5kg/ m3. Tank dipping is wide open to both inadvertent error and deliberate fraud but obviously, where well conducted, it has delivered an acceptable accuracy.

Taken together, the existing volume and density measurements suggest that the industry doesn’t need a full blown fiscal standard. Bunker fuel is not petrol. It isn’t taxed and it may not be desirable to be ‘taxation ready’, so what accuracy is required? The industry might consider that a good accuracy could lie somewhere between 2% and 0.1%. SPRING is apparently suggesting 0.5% accuracy. Is this based on the industry’s needs or the capability of the chosen technology?

In fiscal applications we are not limited to a single technology: our possible technologies of Coriolis, ultrasonic and PD are all capable of delivering 0.15% accuracy but the accuracy the industry chooses need not be the best of which the best meters are capable. It may choose to permit the use of less expensive meters designed for lower accuracy. So is 0.5% too low or too high?

Operational factors

Operational factors may well be behind most wrong choices, where the meter manufacturer does not fully appreciate the application and the purchaser does not fully understand the technologies. In bunkering we might isolate two key factors: pressure drop and air.

Pressure drop is already an issue for bunkering but consider where the industry is going: bigger vessels with larger stems and higher lifts, faster turnarounds and higher viscosities All these trends have one common consequence: they each introduce higher pressure drops.

If we rank our choices according to pressure drop, ultrasonic devices are the best because they have no more pressure drop than the pipe they replace; PD are next, but already sometimes a problem; and Coriolis have the least favourable pressure drop. Note that I am not excluding any technology, but simply recognising that pressure drop is a factor that has to be managed.

One way to manage pressure drop is to find some suitable combination of pump size and meter size or to elevate the fuel temperatures. Already, some operators are over-sizing Coriolis meters, but does this involve unnecessary compromises or simply higher costs? And what happens if both the vessel and the barge have Coriolis meters? We still have our three technologies as options, but cost is now likely to be a more significant factor in determining the optimum solution.

Dealing with the ‘cappuccino effect’

The cappuccino effect (the entrainment of lots of bubbles) and air pockets both ‘inflate’ the measured volume. Handling air pockets is a problem for all technologies and all generally have the same range of solutions. So is the cappuccino effect the real differentiator? And how can we deal with it? Remove the air? Meter the fluid with entrained air?

Getting air out of a heavy fuel oil is far from easy due to the high viscosity. This suggests that the only option is to meter the fuels with the air in them. This would eliminate ultrasonic meters, but it would still leave PD meters as an option, if there is a suitable accuracy budget.

But we should ask if this is the only solution. It is essential to understand the nature of the problem. Entrained air is not an intrinsic property of fuels. Air is introduced either inadvertently or deliberately just prior to bunkering. (It was reported that in trials in Singapore, air had to be deliberately introduced.)

Thus, there is a third option, to prevent air being introduced. In an ‘instrument free’ industry the cappuccino effect is a serious problem because it is very difficult to detect. However, if it is detectable, then it can be prevented.

Air is very easily detected by most vibrating element sensors, including (conventional) Coriolis meters (see data plot). The plot shown is the real time data from a digital viscometer collected as the bunker progresses. It shows both the density at 15ºC and kinematic viscosity at 50ºC and it clearly shows the difference between air entrained fuels and air free fuels. (The final section shows the effect of air blowing or pockets of air passing the sensor.)

Instantly recognisable and easily detected, it can now be dealt with by better fuel management. New operating procedures will solve the problem of poor operating practise. If the cause is poor equipment, eg fuel delivered by sparging, rather than stilling, then the solution is relatively easy and comparatively cheap. In either case, once fixed, always fixed. Possibly a significant cause of the cappuccino effect is fraud, which depends on being undetected and profitable; once entrained air is ‘visible’, it is neither. It is probable that if entrained air Coriolis meters are used, the problem will rarely again be presented simply because the profit element has been removed.

An effective approach would be to reject any bunker where entrained air is detected. A simple alternative, again intended to defeat fraud and encourage better fuel management, is to calculate mass based not on the BDN density value but on the online mean density measured using an EG (Entrained Gas) density meter. Either way, air due to fraud will be detected. Note that just as the online sensor cannot report the true density and accuracy when there is air, offline measurement of samples is equally vulnerable unless samples are specially handled, eg centrifuged to remove air then remixed. Offline measurements will give false low density and false high viscosity values, and the calculated ignition index (now a feature of the new ISO 8217 standard) will be suspect.

As quantity accounting is coming under scrutiny, is quality measurement far behind? Maybe entrained air shouldn’t be tolerated, even if we can measure mass accurately. It would be ironic to adopt a meter standard simply because it can handle entrained air if it is later decided that entrained air should no longer be tolerated for quality assurance reasons. However, if we are going to meter the fluid with entrained air, there is more than one technology we can use. The solution on offer at the moment is special versions of the Coriolis meters that can measure the mass of the fuel, even if at reduced accuracy.

An alternative is to use PD meters with EGA (Entrained Gas Amplifier) Density meters. The PD meter may accurately record 100m3 delivered, but the fuel volume fraction is unknown. If there is a 50:50 ratio of fuel to air and the fuel density is 950kg/m3, then this is going to contain only 475kg of fuel and not 950kg.

But if we don’t know the volume fraction or the fuel density, what then? An EGA (Entrained Gas Amplifier) densitometer will report the density as 475kg/m3 and the mass is thus calculated as 475kg (the mass of air is negligible, so in effect we have the mass of fuel) and we need to know neither the fuel density nor the volume fraction. In either case, Coriolis or PD meter, there is a necessary accuracy penalty.

It is inferred from the proposed standard that for Coriolis meters this is 0.5%. For PD meters the accuracy is the combined volume accuracy and density meter accuracy, which for the EGA densitometer is 0.5%, ie perhaps 0.6-0.7% overall. Is 0.5% too tight a cut-off for accuracy, even if we accept metering as the necessary solution? Should we expect an accuracy that allows a lower pressure drop solution? Or if we want better accuracy, and meter choices, then entrained air cannot be acceptable.

Decision time

This is the point at which the industry needs to decide what accuracy it wants or can afford; to decide if air is a real problem or not; to decide if headloss is a serious enough issue to worry about and to start working out the costs associated with each technology choice.

The cappuccino effect is not an insoluble problem and thus it is not a unique differentiator between meter technologies. Some may prefer to prevent air and others to manage it. If the cappuccino effect can be prevented then greater accuracy is possible, if it is needed. If it cannot, then is 0.5% a suitable compromise?

Perhaps the most important factor will be installation cost. Prevention may require some capital investment. This has to be balanced against the costs of metering air entrained fuels and those costs must include the issue of pressure drop and the future direction of the industry. If headloss is to be minimised using larger pumps, meters, pipes or even high fuel temperatures, this will be expensive. The optimum answers will likely be different for different operators with different bunker supply options.

The industry needs to be able to choose the best solution for the individual application. More colours than ‘black’ are needed, and if it is to be just one colour, maybe it shouldn’t be black.

The Spring 2011 Issue of World bunkering will include a feature on quantity measurement. Readers’ views would be very welcome.