During the last EXXETA training on fundamentals of the gas business a participant asked how fast gas travels through pipelines. As the training deals with commercial contracts and processes the question was deemed quite relevant. Imagine the following situation: A German station employee notices a sinking pressure on the manometer, hurries to the telephone and askes the Norwegian colleague to turn on the tap of the gas supply system. How long will it take until this event will show its effects in Germany?
We look at a distance of about 3,500 km, from Kjøllefjord (Norway) to Oberstdorf (Germany). For purposes of illustration, we compare the transmission of a telephone call with the transport of gas and petroleum.
The electrical part was sufficiently explained by James Clerk Maxwell. 1 Electric current is transported within an electric field. In this case, a distinction is to be made between the electrons in the telephone cable and the electromagnetic waves in the radio link or the mobile radio network. While the electrons only transmit the pulse and the transmission takes place in the electric field, the electromagnetic wave is already such a field. Over the distance of 3,500 km at the speed of light of 300,000 km/s the telephone conversation is therefore transmitted within fractions second of 12 milliseconds.
The gas, on the other hand, has its pitfalls: it is gaseous, under pressure, and flows in a pipe. Near the pipe walls the speed is 0 km/h, because the pipe material should remain where it is. In the middle the gas flows much faster, which complicates the calculation. For this reason, we have to distinguish between volume flow and pulse transmission. If you colour a gas molecule red, feed it into the pipeline in Norway and wait for it to arrive in Germany with a microscope, we would have to wait a long time for the red coloured molecule. For example, at a pressure of 80 bar, a pipe diameter of 1.2 m and a gas throughput of 26 billion m³ per year there is a flow velocity of the gas molecules of 40 km/h. 2
When a gas molecule is fed into the pipeline in Norway, a non-coloured one is displaced at the other end of the tube in Germany, because the pipe is evenly filled with gas molecules (the level of temperature and pressure is the same for all). The upper limit for the transport speed is the sound velocity. The lower limit depends on the type of gas, its temperature and pressure. For example, methane at a temperature of 25° C and a pressure of 80 bar, there is a given transport speed of 438 m/s. 3 4 After the tap is turned off in Norway the pressure change almost needs 8,000 seconds, which equals roundabout 130 minutes or a little over 2 hours.
Compared to gas, petroleum is slower and less compressible. This means petroleum does not change its volume under pressure at a constant temperature. Petroleum travels comfortably at 5 to 7 km/h and does not mix. If various different heavy petroleum products are fed into a pipeline one after another, they will come out at the other end with minimal mixing after roundabout 21 days. 5
To summarise, the telephone conversation has the fastest transmission, followed by gas and at some distance followed by petroleum.
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- 1 Srakar et al., “Who was James Clerk Maxwell and what was and is his electromagnetic theory?”, IEEE Antennas and Propagation Magazine, vol. 51, no. 4, pp. 97-116, 2009
- 2 Anna Sophie Müller, “Entwicklung des europäischen Erdgasmarktes bis 2030”, Universität Trier, 2010
- 3 https://lp.uni-goettingen.de/get/text/5138
- 4 TU-Dresden, “4. Gasversorgung – Antworten”, Fakultät Maschinenwesen
- 5 Mineralölwirtschaftsverband e.V., “Mineralölversorgung mit Pipelines”, 2006