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In my job as a field service engineer at Bronkhorst, I have seen a lot installations in the field and I often get questions regarding the influence of the pipe length on the performance of a mass flow meter.
In today’s blog I will explain why the correct choice of piping is essential for an optimal performance of your installation using thermal mass flow meters or controllers and why this has an influence on:
Deviation in the measurement data can be caused by using a too short pipe length, because the pipe length is a parameter for the gas temperature. For an optimal performance we advise to avoid excessive temperature fluctuations during commissioning and process operation as much as possible, especially in a process with thermal mass flow meters and controllers. If you use mass flow meters based on the Coriolis principle, temperature fluctuations have no influence on the measurement data, as this measurement principle has been based on measurement of real mass.
In case of a high velocity of the gas flow, the temperature of the gas can change really quickly. In general it can be said, that the higher the flow rate, the more fluctuatuions you can get in gas temperatures. This can interfere with the temperature of your instruments, as the temperature of a gas will lower much faster than the temperature of the instrument itself. This can cause a deviation in your measurement data.
Therefore, for optimal performance of a thermal mass flow meter the gas temperature should be equal to the instrument temperature. Choosing the appropriate length of piping can help you here. If the piping is long enough, the gas has the ability to cool down gradually, more at the same pace as the instrument. This will help you minimize the temperature deviation.
Another effect which I encounter in the field is frozen pipes. How do frozen pipes occur?
When a cooled gas flows with a high velocity through the piping, the temperature of the piping will lower, especially when restrictions in the piping are used, such as narrowing of pipe diameter or the use of (shut-off) valves in the piping. As a result, the piping will attract moisture. If the ambient temperature lowers beneath zero degrees, the moisture will freeze.
Not only can your pipes freeze on the outside, it can also happen within the pipe when the medium (gas) contains moisture. In this case, you can avoid freezing by using a refrigeration dryer to dry the gas in the pipes. Solution to make sure the gas which is used in the process is dry, to avoid freezing as well.
I talk about “too short” and “long enough”. But what is long enough? Generally, we advise to use a minimum pipe length of:
The two effects discussed here are very common in all kinds of processes with high gas flow rates (>500 l/min), such as:
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