Loadcell assemblies overcome cool metering problem

Marco Weighing is working with Chive Fuels in the setting up of a nationwide network of liquid natural gas (LNG) refuelling stations.

Marco Weighing is working with Chive Fuels in the setting up of a nationwide network of liquid natural gas (LNG) refuelling stations to supply this alternative fuel to the steadily growing number of operators of gas powered commercial vehicles.

Chive Fuels believes it can make a worthwhile contribution to the improvement of air quality by providing the means to displace traditionally fuelled vehicles with those powered by natural gas.

However the company realises that the rate of growth of LNG powered vehicles is directly proportional to the availability of the gas, and the lack of suitable refuelling sites has, until now, limited its use.

As a result, it is installing a number of LNG dispensing stations conveniently located to motorway stations for vehicle operators around the UK.

Commercialisation of natural gas vehicles is underway.

After five years of trials Government backing is providing real incentives for hauliers to convert.

Converting vehicles to run on gas has been shown to cut pollution by up to 80% and the annual cost savings to vehicle operators are significant, especially when grants of up to 75% of the conversion cost are available.

At today's UK prices, LNG is up to 30-35% cheaper than diesel, and the road tax for gas powered vehicles is in the region of GBP 500 lower.

LPG (liquified petroleum gas) has been used as a fuel for light duty vehicles for many years and a growing number of petrol stations in Europe have LPG pumps.

However LPG is not seen as a practical fuel for diesel powered vehicles over 3.5t GVW, due in part to the space and weight limitations and part to the disappointing power output.

LNG is often confused with LPG but there are significant differences in its composition and method of storage.

LPG is composed primarily of propane (upwards of 95%) and smaller quantities of butane.

LPG can be stored as liquid in tanks by applying pressure alone.

In comparison, natural gas is composed primarily of methane (at least 90%).

'Pipeline' natural gas also contains small quantities of ethane, propane, oxygen, carbon dioxide and sulphur compounds as well as water.

When the gas is cooled to approximately -160C at atmospheric pressure it condenses to a liquefied natural gas, LNG.

The liquefication process removes the oxygen, carbon dioxide and water and can also be designed to purify the LNG to almost 100% methane, making it theoretically the cleanest of the 'alternative' fuels.

LNG has a high energy density because it occupies 1/600th the volume of natural gas under ambient conditions, making it ideal for energy storage in double-walled tanks.

The LNG fuel tank is a cryogenic container that is designed as two separate pressure vessels, one inside the other.

The insulated inner vessel stores the cold LNG in its liquid form and the space between the inner and outer vessel is then evacuated to produce a superior insulation system.

The typical weight of a 95gal tank when full is approximately 365kg.

On a dedicated gas engine operating at 80% efficiency, 1gal of diesel is equivalent to 3.92kg of natural gas.

Therefore for large vehicles, especially fleet vehicles, LNG becomes attractive due to the reduced on board weight and space requirements, excellent power output combined with lower system and running costs.

The refuelling facilities are paid for by Chive, and customers simply pay for the fuel they use.

The LNG dispensing system is designed by Triscan Systems which already supplies vehicle refuelling systems for conventional fuels and LPG.

To measure the amount of LNG dispensed from the storage tanks, the company initially looked at using a standard flow meter method.

Although this works well for LPG, it found that it was far from satisfactory for LNG.

Because LNG is stored at -160C, the change in temperature seen by the meter during an individual fill operation is significant, and this introduces large errors.

It is not until the meter reaches a steady temperature that reasonable accuracy is achieved and discrepancies of up to 40% were seen in complete fills - a situation which is clearly not acceptable.

The only solution using this method would be to place the meter inside the tank, which was not seen as practical.

The only alternative was to weigh out the gas, and at this stage Chive turned to Sussex based Marco Weighing Systems for advice.

LNG is stored in super insulated vessels using proven and safe cryogenic techniques.

Each tank weighs 11,000kg when empty and holds 6000kg of gas.

The solution offered by Marco involves mounting the complete storage tanks on four special self-aligning high-accuracy 5000kg load cell mounting assemblies.

The fully welded, IP66/68, stainless steel ring type load cells are ATEX certified for Zone 1 operation and connected through the latest MTL intrinsic safety barriers to a Lucid weight indicator in the safe area.

The special mounting assemblies, developed by Marco for aerospace weighing, ensure optimum load introduction even under adverse conditions and thus provide exceptional weighing accuracy and repeatability.

When a vehicle is refuelled, the system recognises the vehicle and zeros out the weigh system.

The amount of gas dispensed is then measured as a weight loss from the tank.

This information is transmitted via the Triscan system to the Chive offices in Reading, and the operator is then invoiced for the amount of fuel dispensed.

Marco has already installed systems on three such tanks and accuracies of better than 1kg are being achieved.

Roy James, MD of Chive, is delighted with the system: "Marco's enthusiasm has matched our own in this project and they have been willing to be extremely flexible.

Initially we thought that we would need to weigh the gas in a smaller intermediary vessel but the results we are getting mean we do not have to go to this extra cost.

The weighing system not only tells us how much gas is being dispensed to individual operators but also gives us an accurate indication of the contents of each tank at any one time.

This information allows us to forward plan our gas deliveries in the most expedient way".

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