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Build in 1978, the Fluxys LNG Peak Shaving site in the back port of Zeebrugge, Belgium, stores liquid natural gas. When the gas demand is higher than the primary supply the reserve from the LNG tanks is heatedby gas burners and sent into the network. Magnetrol is providing the needed up-to-date flow measurement for the burners' air supply.
The heating of LNG or Liquefied Natural Gas from -162°C to gaseous form at 2°C, takes place in a regasification installation for which the energy is supplied by gas burners. To keep the exhaust gases of this installation as clean as possible, controls are used on the burners, whereby the amount of air supply to the burner basins is a key factor. Tobetter control the supplied amount, the pressure measurement controlon this air supply was replaced by a mass flow meter. This more accurate measurement method increased the efficiency of the burners and ensured compliance with the local exhaust regulations for CO and NOx.
In Belgium, Fluxys is responsible for the transport of natural gas.The LNG Peak Shaving installation was started in 1978. In those days,the installation was at the end of the gas line and its single purpose was to gasify stored LNG in periods of high demand (winter), and send it into the network to maintain a constant pressure. In periods of low consumption (summer) natural gas was taken from the pipelines and liquefied for storage by means of a liquefier installation. Today the natural gas is supplied in liquefied form by trucks from the LNG-terminal in Zeebrugge.
Zeebrugge is an important transnational centrefor the distribution of natural gas. It has the capacity to handle 15% of the natural gas consumption of continental Western Europe. Norwegian and British natural gas is supplied through pipelines, and LNG-tankers deliver gas via the front port of Zeebrugge.
Today, the Peak Shaving installation consists of three parts. The first part is theintake, where trucks deliver 41 Nm³ of LNG at a time from the terminal. The second part is the LNG storage, consisting of two tanks above ground with a capacity of 57.250 Nm³ each, where the gas is preserved in liquid condition at -162°C. The cryogen boil-off gas is diverted bycompressors and pumped into the gas network.
The third part isthe evaporator, which will be activated when demand rises. If the installation is in standby, the evaporators can deliver natural gas withintwelve hours; otherwise it will take twenty-four hours to deliver gas. During this time the gas distribution network with 3.800 km of conduit-pipes acts as a buffer, providing enough time to start up the evaporators on the installation, without causing a spectacular pressure drop.
There are five LNG evaporators in the LNG Peak Shaving installation. They consist of a heated 40.000l water basin and a gas spiralrunning through it. The spiral is the heat exchanger in which the transition of LNG to gas takes place. Each unit can evaporate more than 80.000 Nm³/h of LNG at -162°C to gas of 2°C. This natural gas is then pumped into the conduit pipes. The energy needed is delivered by two 9MW burners for each evaporator. Their exhaust gases bubble through thewater basin, heating up the water and improving the thermal dispersionbetween the heated water and the gas spiral.
The burners are controlled by the air flow of the ventilators. The correct amount of air delivers a higher combustion return, but the air control also has its effect on the environment. VLAREM (Flemish Environmental Administration) regulations makes it necessary to minimize the CO and NOx, requiring a correct calibration of the combustion air supply. In general this means that too little air supply creates CO, too much air results inNOx.
The air flow was previously measured by a differential pressure system based on the classic pitot’s tube. It only offered a small measurement window, making it difficult to measure or control the air flow accurately, but it was the only measurement principle available at the time. Tests had been conducted with positive displacement meters, working very well in the summer, but in the winter it had a tendency to freeze up.
In 2002 Fluxys tested the Thermatel TA2 thermal mass flow transmitter, provided by Magnetrol. The unit passed thetest and devices were installed on all burners. The choice of Magnetrol’s Thermatel TA2 is based on good experiences with the material and the supplier - Magnetrols's Echotel ultrasonic level switches were already used on the oil reservoirs of the boil off compressors - and on the technical features such as the wide application range of the instruments. The transmitters are calibrated for a range between 12.000 and 19.000 Nm³ and have a turndown ratio of 100:1.
In this application, two RTDs (resistance temperature devices) were placed in the tubethrough which the gas flows at a fixed distance from each other. One RTD measures the gas temperature; 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 amount of energy, necessary to keep this delta of 20°C, is used to calculate the gas flow. 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 as well as to pressure and temperature because it is a comparative measurement. A limitation for this type of meter is that the instrument does not react to sudden flow changes, but this was not important for this application. As the meters require energy, and as they are installed in a hazardous environment zone 2 it needed to be equipped with an Exd housing.
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