Measurement method ups efficiency

01 June 2007

The Fluxys LNG (Liquefied Natural Gas) Peak Shaving in the back port of Zeebrugge, Belgium dates from 1978. Here, liquid natural gas, supplied by trucks from the LNG terminal in the front port of Zeebrugge, is stored. When the gas demand is higher than the primary supply, through pipelines from Norway and Great Britain and LNG tankers, the ‘supply’ in the LNG Peak Shaving is ‘heated’ to 2°C and sent into the network.

The Fluxys LNG Peak Shaving installation in the port of Zeebrugge (Belgium), dates from 1978
The Fluxys LNG Peak Shaving installation in the port of Zeebrugge (Belgium), dates from 1978

The transition of LNG / gas takes place in one of the five evaporators at the installation. These consist of a heated 40 l water basin and a gas spiral, which runs through it. This spiral is the heat exchanger. Each evaporator can evaporate more than 80 Nm³/h of LNG at - 162°C to gas of 2°C. This natural gas is then pumped into the conduit pipes. The energy (per evaporator) to evaporate gases is delivered by two burners with a capacity of 9MW each. Their exhaust gases bubble through the water basin and heat up the water while it improves the thermal dispersion between the heated water and the gas spiral.

The burners are controlled by the air flow of the ventilators. Too little or too much air can create CO or NOx, substances that, according to VLAREM (the Flemish Environmental Administration), must be minimised.

Previously a pressure measurement was used to control the air flow. This system was based on the classic pitot’s tube over which differential pressure was measured. The maximum air velocity is about 60 m/sec in a 400mm diameter tube. The created overpressure is limited to 235 mbar and the differential pressure, used for control, is not higher than 6,5 mbar. Based on such a small measurement window it is difficult to measure or control accurately.

When it was decided that the measurement system needed to be replaced tests were conducted with positive displacement meters, a measurement system based on a little paddle rotated by the flow. These systems had a tendency to freeze up in the winter.

Finally Fluxys tested the Thermatel TA2, provided by Magnetrol. The unit was approved and TA2’s were installed on all burners.

With this type of measurement system, two RTD’s (resistance temperature devices) at a fixed distance from each other, are placed in a tube through which the gas flows. One RTD measures the ambient temperature (gas temperature), and the other one is heated to exactly 20°C higher than the process temperature.

The gas flow cools down the heated RTD and makes it necessary to continuously add heat. The energy, necessary to keep this delta of 20°C, is the measure for the gas flow, which is the mass flow that passes by the sensor. The microprocessor in the meter compares the consumed energy with a calibrated curve and translates the energy consumption to mass flow. This measurement is insensitive to vibration and therefore very useful. It is insensitive to pressure and temperature because it is a comparative measurement. The TA2 sensor measures the process temperature and compensates the mass flow in function of this temperature because the heat transfer at various temperatures can be different.

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