Product category:
Energy management; Boiler control
News Release from: Magnetrol International | Subject: Thermatel TA2
Edited by the Processingtalk Editorial
Team on 31 May 2007
Thermal flowmeter improves LNG emissions
The Fluxys Peak Shaving plant in the back port of Zeebrugge has installed Magnetrol Thermatel mass flowmeters to monitor regasification burner air flows and reduce harmful exhaust emissions
The Fluxys Peak Shaving plant in the back port of Zeebrugge is now a reserve supply of gas for the network, normally supplied by the Primary Supply in the front port, which receives natural gas direct through pipelines from Norway and Great Britain, and also LNG tankers, such as the Methania This "reserve supply" in the LNG Peak Shaving plant is "heated" to 2C and sent into the network at times of peak demand
This article was originally published on Processingtalk on 22 May 2003 at 8.00am (UK)
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The heating of the stored LNG (held at -162C) to gaseous form (approximately 2C), takes place in a re-gasification installation, for which the energy is supplied by gas burners.
To keep the exhaust gases of this burner as pure as possible (free of CO and NOx), specific controls are used on the burners, whereby the amount of air supply to the burner is the key control factor.
To provide better control of the air supply, the Pitot tube measurement system used to monitor air flows was replaced by a Magnetrol Thermatel mass flowmeter.
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This more accurate control increased the efficiency of the burners and ensured compliance with the local VLAREM exhaust regulations for CO and NOx.
This article describes the new flow measurement systems installed, and is based on an interview by Alfons Calder of the Belgian magazine Industrie Technisch Management with Mr Jean Joris, technical manager responsible for the Fluxys Peak Shaving, and Mr Gustaaf Schelkens, responsible for sales in Magnetrol Belgium, the company that delivered the mass flowmeters for this application.
In Belgium, Fluxys is the company responsible for the transport of natural gas.
The LNG Peak Shaving installation dates from 1978, when the plant was at the end of the gas line (natural gas entered Belgium from the Netherlands) and its purpose was to gasify stored LNG in periods of high demand (winter), and send it into the network to maintain a constant pressure.
In a period 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 only supplied in liquefied form by trucks from the LNG-terminal in the front port of Zeebrugge, which is an important transnational centre for the distribution of natural gas.
It has the capacity to handle 15% of the natural gas consumption of the whole of continental Western Europe.
Today, the Peak Shaving installation consists of three parts: the first part is the "intake".
Trucks transport 41 Nm of LNG at a time from the LNG terminal at the front port of Zeebrugge to the "intake".
The second part is the LNG storage: this consists of two overground tanks, which each have a design capacity of 57.250 Nm.
The gas is preserved in liquid condition at -162C.
The cryogenic boil off gas (LNG which evaporates at the surface) is diverted by compressors and pumped into the gas network.
The third part is the evaporator, which is activated when users in the network ask for gas supplies to meet increased demand.
When the installation is in standby, the evaporators can deliver natural gas within twelve hours.
When the installation is not in standby, it takes twenty-four hours to deliver gas.
This may seem a long time, but it is more than adequate.
The gas distribution network with 3.800 km of conduit-pipes (600 or 1200 mm for 66 and 80 bar) is itself an important buffer.
When the users of the natural gas distribution network require additional capacity which makes it necessary to activate the LNG Peak Shaving installation, the buffer in the pipes provides 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 40.000 Litre water basin and a gas spiral, which runs through it.
This spiral is the heat exchanger in which the transition of LNG / gas takes place.
Each evaporator can evaporate more than 80.000 Nm/h of LNG at -162C to gas of 2C.
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.
The correct amount of air delivers a higher combustion return, but the air control also has its effect on the environment.
VLAREM (The Flemish Environmental Administration) makes it necessary to minimise the CO and NOx, which requires correct adjustment of the combustion air supply.
Too little air supply creates CO, and too much air supply results in NOx.
However, too much air, to a certain level, also cools down the flame and reduces the formation of NOx (NOx is formed at high temperature, starting at a flame temperature of 800C).
Previously, a differential pressure measurement from a Pitot tube was used to control the air flow.
The maximum air velocity is about 60m/sec in a 400mm diameter tube.
The created overpressure is limited to 235mbar and the differential pressure, used for flow measurement, is not higher than 6,5mbar, a small measurement to achieve accurately.
At Fluxys, evolutions in measurement are carefully followed up and, within budgetary agreements, the installations are kept up to date.
Tests had been conducted with positive displacement flowmeters, a measurement system based on a little paddle rotated by the flow.
This worked very well in the summer, but in the winter it had a tendency to freeze up.
In 2002 Fluxys tested the Thermatel TA2, provided by Magnetrol.
The unit passed the test and TA2 units were installed on all burners.
THERMATEL TA2 FLOWMETER.
The Thermatel TA2 is based on thermal mass flow measurement.
The choice of the Magnetrol Thermatel TA2 was based on good experiences with the material and the supplier (Echotel ultrasonic level switches are already used on the oil reservoirs of the boil off compressors), but also on technical features such as the wide application range of the instruments (they are calibrated for a range between 12.000 and 19.000 Nm and have a turndown ratio of 100:1).
With this type of measurement, two RTDs (resistance temperature devices) at a fixed distance from each other, are placed in the tube through which the gas flows.
One RTD measures the ambient temperature (the gas temperature), and the other one is heated to exactly 20C higher than the process temperature.
The gas flow cools down the heated RTD and makes it necessary to continuously add heat to maintain the differential.
The energy, necessary to keep this differential of 20C, is used to derive the gas mass flow that passes the sensor.
The microprocessor in the meter is used to compare the consumed energy with a calibration 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.
A limitation of this type of meter (independent of the brand) is that the instrument does not react to sudden flow changes (temperature stabilisation takes a little time), but this limitation is not important for this application.
This mass flow meter requires a power supply, and is installed in a hazardous environment, specified as zone 2 (presence of gas possible).
Therefore, this instrument is equipped with an Exd housing.
Exd means that gas may penetrate the housing, but an explosion must be contained within the housing and the flame path or the gases that leave the housing, must have lost energy to such degree that they cannot ignite possible gas in the environment.
(Exi can be another requirement within hazardous environments, whereby the internal power, which supplies the instruments within the housing or the energy accumulated via capacitators and condensers is too small to deliver enough energy to ignite gases in the environment.
Also known is EExp, whereby the housing is kept in overpressure so no gas can enter).
In general, the Fluxys experience with Exd housings is that they can give condensation problems (moisture seeps into the housing, and is difficult to evacuate), which causes the internal electronics to corrode and - sometimes due to short circuiting - fail.
However, experience with the Thermatel TA2 shows that this instrument does not suffer from this moisture problem. Request a free brochure from Magnetrol International ...
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