flow through bypass 1

[DYV1973]

LS,

Another piping problem I have is as follows:

We do have a large pipe (DN200) in which a scoop (DN100)is placed in the middle of the flow, that reduces conically to DN50. This pipe goes out of the pipe and is by-passed for about 0,8 m. In the by-pass a measuring section is placed (and thus creating additional resistance), and then it is connected to the main pipe again under a 90° angle (see sketch).

How can I setp up a calculation which part of the flow is going through the bypass section, depending on flow and viscosity in the main pipe.

Thanks in advance.

DYV

 

[zdas04]

This is an amazingly complex problem. The driving force for the flow is the difference in dynamic pressure between the pipe wall and the center of the pipe. The convergent nozzle (scoop) converts some of this pressure into velocity, but the magnitude of the dP is really dependent on how well developed the flow is (no swirl, symmetrical flow profile, flow profile approximately a 1/9 power). If you've done something to establish the flow profile (like a flow conditioner described in AGA 3) then you can approximate the dP across the pipe from the dv.

I'm pretty sure you can safely disregard the friction drop in the main pipe over 80 cm, but the protruding pipe would constitute a "bluff body" in the flow that will shed von Karmen Streets and will play havoc with pressure at the point that the flow re-enters the main pipe. These perturbations can be pretty long lived and will certainly prevent the flow from being fully developed at the entry point.

I don't even know how I would set this up in a CFD program. Good luck with it.

David

 

[BigInch]

=0, or almost.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[Latexman]

I don't understand why anyone would want to know the flow of this "centercut". Can you enligfhten us?

Good luck,
Latexman

 

[tr1ntx]

Is this a homework problem?

 

[DYV1973]

No, it's not a homework problem.......

It is an excisting real situation. Through the piping flows oil of which flow and viscosity can vary, and the bypass with the scoop in the center is used to have a continuous sample from the main flow. In the by-pass there is a measuring section which could not be placed in the main flow. In the moment there is discussion about the refreshing rate through the by-pass, and whether there will be sufficient flow through the by-pass. This is why I would like to do some calculations to verify.

DYV

 

[Latexman]

Is it to measure viscosity? Density? Flow?

Good luck,
Latexman

 

[DYV1973]

Correct: density / viscosity (and also temperature, but that could also be done in main stream).

Flow would not be possible as it is a by-pass. Flow is what I would like to estimate.

 

[zdas04]

If I had this problem, I would place an orifice in the main flow to force meaningful flow through the bypass. The way it is designed should give you creep flow which can be anything by meaningful.

David

 

[BigInch]

If I had this problem, and I have had many similar for density, viscosity and specific gravity measurements, I would install a small pump to take or move the exact sample volume out of the line that I needed.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[Artisi]

My thoughts are that flow would be unknown if at all due to the pressure drop across the "device", the losses could be more than the pressure needed for re-entery into the main flow again.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[tr1ntx]

DYV,

I've seen this arrangement before. I'm sure the math is worked out already and this arrangement is intended to capture a specific percentage of flow. Research "sampling systems" or maybe "diversion systems". I don't think I have any of that old stuff where I can find it. Check in the field of Instrumentation Engineering or with instrumentation suppliers.

 

[BigInch]

trouble with no pump, orifice etc. is when the product or viscosity changes significantly, so does the flow. If viscosity varies widely, you'll need an adjustable orifice plate.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus