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(This is closely linked with measuring the Mach number inside a nozzle but it is not regarding the supersonic flow)

Friction and heat transfer have effects on the Mach number of the compressible flow (Fanno and Rayleigh flow). As it is extremely important to have tight control over the flow properties, here are my questions:-

  1. how does one know the Mach number of the flow inside long pipelines carrying some sort of gas (e.g. WEPP)?

  2. What Mach number is maintained through these pipelines?

  3. Considering the cyclic temperature changes and friction inside the pipes, how is the Mach number maintained constant / within a range?

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  • $\begingroup$ I don't feel knowledgeable enough to submit an answer, but I did intern for EQT corp in 2008, and it was my understanding that (at the time) most of their gas transmission lines are only monitored at pumping stations. Lots of meters are used, including orifice plate flow meters, turbine meters, and ultrasonic meters. But again, they mostly only measure at the pumping stations (though I'm sure there's exceptions to this). $\endgroup$
    – Rick
    Jan 28, 2015 at 14:08

2 Answers 2

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It's important to remember that these pipelines don't exist as a single isolated line and will have a number of branches that tie into the main pipeline and split off for gas to be sold to different locations. This touches on a very broad subject of flow assurance and pipeline network modeling.

The pipeline will also be broken into smaller sections by compressor stations because, as you mentioned, friction losses will require the gas to be re-compressed as it travels along the pipeline.

To answer your question, the gas flow rate will be regularly measured using a variety of flow meters such as an orifice meter, ultrasonic meter, Coriolis meter, turbine meter etc. This will occur at any point were gas flows into the main pipeline or is branched off the be sold. It also may occur at regular intervals (i.e. the compressor stations) along the pipeline. If you want to determine the mach number you could simply calculate it from your measured velocity.

I am not aware of any target mach number or velocity in pipelines, it will certainly be subsonic, the rest will depend on the specifics of the section of pipe. For example, the elevation profile, the presence of liquids, corrosion, and the required pressure at the destination all will have an effect on the velocity you can transport the gas.

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For question 1, as far as I recall, fiscal metering of long hydrocarbon gas pipelines is done with coriolis effect mass flow meters. To calculate Mach number, all that would be required is the mass flow and the temperature from a thermocouple (along with some estimate of the mixture's heat capacity ratio.)

This answer needs checking against reliable references. (CW as I didn't check any books.)

Questions 2 and 3 are more difficult, and again may need to be separated.

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