The coriolis effect on massflow instruments

[Cyanate]

Hi Guys,

 

I am diving into the working principle of a Coriolis mass flow meter.

I do understand how the mass flow is calculated and how the Coriolis principle works in theory based on rotating systems.

The problem I encounter is that I can't match the theory with the practical application.

 

The theory describes the force on an object in a rotating system that makes its path bend.

But what is the initial cause for the bending of the tubing in the image underneath? (green arrow force)

Is it the medium that shifts direction into the tube?

 

Thanks

[exchemist]

2 hours ago, Cyanate said:

Hi Guys,

 

I am diving into the working principle of a Coriolis mass flow meter.

I do understand how the mass flow is calculated and how the Coriolis principle works in theory based on rotating systems.

The problem I encounter is that I can't match the theory with the practical application.

 

The theory describes the force on an object in a rotating system that makes its path bend.

But what is the initial cause for the bending of the tubing in the image underneath? (green arrow force)

Is it the medium that shifts direction into the tube?

 

Thanks

This is interesting. I had vaguely heard of Coriolis flow meters from my time in the oil industry, but never looked into the principle of operation. So thanks for posting this query. I have learnt something.

I think I've got it. It's the inertia of the flowing fluid inside the tube. The arrows show what takes place during the "upward" phase of oscillation of the tube. Fluid entering from the left resists being made to flow slightly upward due to its inertia, creating a downward force on that part of the tube. Conversely, the fluid leaving, back to the left again, has by now been made to flow slightly upward and therefore resists being made to flow horizontally once more, creating an upward force on that part of the tube. So, seen end-on from the right, the tube will have a slight twist as shown. When the tube is in the "downward" phase of its oscillation, the converse happens. When it is in the centre, there is no twist. So, again as seen end-on from the right, there will be a rocking or twisting motion superimposed on the up-down oscillation. In effect, on the side the fluid leaves, the phase is slightly advanced relative to the oscillation with no flow, while on the side the fluid enters the phase is slightly retarded.  The downstream side leads the upstream side.  

The magnitude of the force will depend on the rate of flow of mass, because when it flows faster more mass has to be made to change direction in unit time, i.e. the rate of change of momentum, d(mv)/dt is greater.

d(mv)/dt = ma = F.  

And then of course there a load of fancy stuff about detecting this distortion of the resonant frequency via phase shift etc.

 

 

 

Edited yesterday at 10:36 AM by exchemist

[studiot]

Just now, Cyanate said:

But what is the initial cause for the bending of the tubing in the image underneath?

You have missed out the external oscillation dive.

 

Quote

https://www.emerson.com/en-gb/automation/measurement-instrumentation/flow-measurement/coriolis-flow-meters

A Coriolis meter is based on the principles of motion mechanics. When the process fluid enters the sensor, it is split. During operation, a drive coil stimulates the tubes to oscillate in opposition at the natural resonant frequency. As the tubes oscillate, the voltage generated from each pickoff creates a sine wave. This indicates the motion of one tube relative to the other. The time delay between the two sine waves is called Delta-T, which is directly proportional to the mass flow rate.

Watch the animations on the website.

[Cyanate]

51 minutes ago, studiot said:

You have missed out the external oscillation dive.

 

Watch the animations on the website.

Hi Studiot,

Is it possible to explain the "external oscillation dive"?

I already watched the videos of all manufacturers, but like this one, all explain the consequence and not the cause of the bending.

2 hours ago, exchemist said:

This is interesting. I had vaguely heard of Coriolis flow meters from my time in the oil industry, but never looked into the principle of operation. So thanks for posting this query. I have learnt something.

I think I've got it. It's the inertia of the flowing fluid inside the tube. The arrows show what takes place during the "upward" phase of oscillation of the tube. Fluid entering from the left resists being made to flow slightly upward due to its inertia, creating a downward force on that part of the tube. Conversely, the fluid leaving, back to the left again, has by now been made to flow slightly upward and therefore resists being made to flow horizontally once more, creating an upward force on that part of the tube. So, seen end-on from the right, the tube will have a slight twist as shown. When the tube is in the "downward" phase of its oscillation, the converse happens. When it is in the centre, there is no twist. So, again as seen end-on from the right, there will be a rocking or twisting motion superimposed on the up-down oscillation. In effect, on the side the fluid leaves, the phase is slightly advanced relative to the oscillation with no flow, while on the side the fluid enters the phase is slightly retarded.  The downstream side leads the upstream side.  

The magnitude of the force will depend on the rate of flow of mass, because when it flows faster more mass has to be made to change direction in unit time, i.e. the rate of change of momentum, d(mv)/dt is greater.

d(mv)/dt = ma = F.  

And then of course there a load of fancy stuff about detecting this distortion of the resonant frequency via phase shift etc.

 

 

 

This makes sense to me, thanks for the explanation.

I think I've found a graph that describes what you've explained.

 

[studiot]

Just now, Cyanate said:

Is it possible to explain the "external oscillation dive"?

My fault, I really should get a better spelling manager.

It really should be external oscillation drive.

As in the words from the article I linked to

 

Quote

a drive coil stimulates the tubes to oscillate

What they are saying is that it not not just the interaction between the piped fluid and the pipe that needs to be considered, but also some external ( to the fluid filled pipes ) drive physically moving the pipes as well.

Edited yesterday at 01:52 PM by studiot

[exchemist]

30 minutes ago, studiot said:

My fault, I really should get a better spelling manager.

It really should be external oscillation drive.

As in the words from the article I linked to

 

What they are saying is that it not not just the interaction between the piped fluid and the pipe that needs to be considered, but also some external ( to the fluid filled pipes ) drive physically moving the pipes as well.

You’re not the only one: the diagram in the OP refers to a “twist angel” [sic]. 😁

 

[Cyanate]

Alright, so let me recapitulate:

 

The tube is moving up and down due to the external oscillator.

When the angle of the tube is positive, relative to its starting position, the medium puts a downward force on the tube due to inertia on the incoming side, an upward force on the outgoing side. Because of the oscillator, the forces switch direction twice a period.

[exchemist]

3 minutes ago, Cyanate said:

Alright, so let me recapitulate:

 

The tube is moving up and down due to the external oscillator.

When the angle of the tube is positive, relative to its starting position, the medium puts a downward force on the tube due to inertia on the incoming side, an upward force on the outgoing side. Because of the oscillator, the forces switch direction twice a period.

Yes I think that’s right. What I don’t follow yet, not having read it up, is how this phase shift thingie gets detected and converted to a mass flow readout. Some explanations show a pair of tubes in parallel. Not sure what that does.

[Cyanate]

20 minutes ago, exchemist said:

Yes I think that’s right. What I don’t follow yet, not having read it up, is how this phase shift thingie gets detected and converted to a mass flow readout. Some explanations show a pair of tubes in parallel. Not sure what that does.

The pair is made to exclude the external vibrations from the receiver. Since both tubes are vibrating oppositely, external factors are canceled out this way.

The phase shift is detected by two hall-principle sensors for every tube, depending on the manufacturer. 

 

Please let me know if you stumble upon extra information.

[exchemist]

2 minutes ago, Cyanate said:

The pair is made to exclude the external vibrations from the receiver. Since both tubes are vibrating oppositely, external factors are canceled out this way.

The phase shift is detected by two hall-principle sensors for every tube, depending on the manufacturer. 

 

Please let me know if you stumble upon extra information.

Hmm, I see. But it must be hard to tune them so they have identical resonant frequency. Or do they have some ingenious way round that?

[Cyanate]

15 minutes ago, exchemist said:

Hmm, I see. But it must be hard to tune them so they have identical resonant frequency. Or do they have some ingenious way round that?

I suppose that both tubes are brought in resonancy by the same oscillator.

[exchemist]

1 hour ago, Cyanate said:

I suppose that both tubes are brought in resonancy by the same oscillator.

I don’t think so. To me, resonance implies it is the natural frequency of the pipe, not a forced oscillation. I might do a bit more reading about it tomorrow (right nowI’m suffering from a bad cold and have little mental energy in the evenings🙂).