Deviation in measurement of the thermal mass flow meter
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 fluctuations 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.
Frozen pipes
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.
The right pipe length for flow instruments
I talk about “too short” and “long enough”. But what is long enough? Generally, we advise to use a minimum pipe length of:
- 10x the pipe diameter, at the inlet of the instrument
- 4x the pipe diameter, at the outlet of the instrument
For gas flow rates between 100-1500 l/min it is common to use a 12mm or ½” pipe, and we advise a larger pipe diameter for gas flows > 1500 l/min.
Where can you encounter deviation in measurement and frozen pipes?
The two effects discussed here are very common in all kinds of processes with high gas flow rates (>500 l/min), such as:
- Plasma vapor deposition technique; used to provide rotor blades with a coating to make them suitable for high temperatures
- Blast furnaces; to make stainless steel out of conventional steel
Need more information?
If you need any advice about choice of piping for flow meters or applications, our team will be more then happy to help you.
Related blog articles
Managing vibrations in Coriolis flow meters
In-line filters for gas flow instruments
Miniaturization in flow solutions