Mass Flow Meters

Mass Flow Meters

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When asked to do a thought piece on the reasons for using Mass Flow Meters, for custody transfer of marine fuels, on our bunker tankers, operating in South Africa, I wondered, why me? I’m not a flow meter expert, instrument technician, or an authority on fluid dynamics. What can I offer the audience prepared to listen to my opinion on the subject? My views are purely based on my personal experience with different types of flow meters, on bunker tankers, over the last 30 years.

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As a qualified marine engineer, I’ve spent many years on the receiving end of bunker hoses, then as bunker barge master, I spent more or less the same time on the supply end of the bunker hose. As an engineer it was frustrating to deal with significant quantity discrepancies nearly every time a bunkering operation was completed. Flow meters on bunker tankers were almost unheard of back then and most deliveries were done on physical soundings. Shortened sounding tapes, false sounding pipes and two, or more different sets of sounding tables were not uncommon. As barge master it was frustrating to deal with receiving ship’s Chief Engineers, not trusting barge figures. This even though the barge was equipped with the latest flow meter technology available on the market, at the time, and valid calibration certificates available for all to see. One could hardly blame them, considering the bunkering industry’s notorious reputation.

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All South African bunker tankers were contracted to the big oil majors, who would under no circumstances, compromise on quality of service, with big emphasis on on-time, safe delivery, of correct grades and quantities. There was no option of returning back to the terminal and getting rewarded for ROB’s. If anything, we would have been penalized for such. I have to believe, that this was quite a unique model, resulting in regular callers to SA ports, very seldom querying delivered quantities. ?

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I later got the operations manager’s job, by which time I fully understood the importance of proper stock management and the critical importance of getting it right every time. With all the variables at play, temperature, density, viscosity and flow rate, to name but a few, stock calculations can become confusing, for the untrained. Temperature compensation of flow meters, automatically compensating from actual volume at actual temperature, to a corrected volume at a specific temperature, 20degC in our case, in the beginning was difficult to explain to a number of receiving ship’s chief engineers.

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It was only in 2006 when I was tasked, with the help of some very clever people, to design and equip our current fleet of 3 ships, that the choice of flow meter, make and type, prompted me going into the subject in more detail. By this time, I have had experience in and an above average understanding, of the various types of PD meters, as well as early generation, mass flow meters. We tried to narrow the options down to one, based on the following:

1.????Accuracy range. This is the flow rate range at which the meter operates at optimum accuracy and some meters have a much bigger range than others. The ultrasonic option had a range between 80 and 1000mt/hour, which covered all the rates that we would generally deliver at. PD meters had a much narrower range which was seen as a limitation.

2.????Maintenance Requirements. PD meters, whether screw/gear type, or rotary vane type, have moving mechanical parts that wear over time, could affect accuracy levels and require routine maintenance and regular calibration. Turbine meters have similar negatives. Mass flow meters and ultrasonic meters have no moving parts, resulting in very limited maintenance requirements and increased calibration intervals.

3.????Service back up in South Africa. Traditionally, we’ve had good service back up from different local PD meter agents. Mass flow and Ultrasonic meter back up was new territory to us, which had to be explored first.

4.????Reputability. This speaks for itself.

5.????Temperature range. HFO flow curves change dramatically with actual temperature dropping, resulting in higher actual viscosity (refer to Reynolds number for more detail). This doesn’t affect PD meters, apart from maybe slowing down the pumping rate. Mass flow meters not affected. With ultrasonic meters the accuracy level will be determined by the number of ultrasonic beams, the more beams, the more accurate.

6.????Size. Ultrasonic meters with a high flow range, was at the time of going to market, substantially smaller than any of the other meters. This has subsequently changed with mass flow meters.

7.????Cost. We found ultrasonic meters to be somewhere between mass flow meters, at the higher end and PD meters at the lower end. Costly flow computers with remote readouts also had to be fitted with ultrasonic and mass flow meters, whereas PD meters could be fitted with mechanical, local counters.

In the end we opted for temperature compensated, 3 beam Krohne ultrasonic meters with software upgrades, to compensate for changing flow curves. It also made sense to go with the Kronhe brand, not only because of superior quality and serviceability, but also because we fitted Krohne Cargomaster radar tank level gauging systems to back up meters in case of catastrophic failure. Barge tanks were laser calibrated and Cargomasters programmed, using this data. This resulted in very close correlation between the two, totally independent stock measuring systems, and superior accuracy levels.

Mass flow meters, at the time, were big, bulky and expensive, but my biggest concern was that it measured mass. On the face of it, that’s absolutely fantastic, as it eliminates the human factor and possible calculation errors. However, for the meter to calculate mass, it had to also measure density and that’s where my concern was. Which density is used for custody transfer in the end, is it the lab density on the product spec sheet, or density measured by the meter? Which is the more accurate of the two can be debated, but with our oil major clients’ contractual obligations, we were obliged to use lab densities. Using lab densities when loading and meter densities when discharging, however small the difference, on the volumes we moved, would have made stock reconciliation rather difficult.

Times change, technology changes and market demand and expectations change, so if one wants to be at the forefront, you have to embrace change. About 4 years ago, we decided to change all our HFO ultrasonic flow meters out for the latest Krohne mass flow meters, now smaller, smarter, more affordable, able to plug into existing flow computers and last, but certainly not the least, now able to measure volume and not just mass. Did we make the change because of accuracy concerns? Certainly not, the ultrasonic flow meters lived up to and exceeded all expectations and one needs no more proof than looking at the less than 1% of deliveries resulting in quantity disputes. More than 20,000 deliveries executed.

Modern mass flow meters certainly tick more boxes than it did 15 years ago and the accuracy levels across most parameters are certainly impressive. Interesting to keep in mind that the 3rd party master meter, used for calibration/verification of all our meters, is nothing other than a certified Krohne 5 beam ultrasonic flow meter.

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In closing, if less than 1% of deliveries result in quantity disputes, I think it would be fair to assume that 99% of chief engineers have come to trust our integrity. Keep in mind that we have constantly maintained the 1% for over 14 years now.