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Flow Rate vs Area

04/29/2015 3:19 AM

Just want to ask the experts here. Let's say I have a electromagnetic flowmeter .The pipe size is DN 300 and the flowmeter size is DN250. Now,since the flowmeter size is reduced. Will the flow be the same.?What effect ,reduction in pipe size have on the flow. Since Flow is equal to Velocity times area and by reducing the area ,the velocity increases which leds me to believe the flow will be the same. Since Flowmeter measures velocity. Is this true.?

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#1

Re: Flow rate vs Area

04/29/2015 3:28 AM

Yes, but you still don't seem to understand it. Somewhere in the scheme of things, there should be a parameter for pipe size, and hence area. The flow meter will then compute Q = VA. (Or somewhat more complicated, because velocity varies with location in the pipe.)

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#2
In reply to #1

Re: Flow rate vs Area

04/29/2015 3:32 AM

Can you please explain me then

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#3
In reply to #2

Re: Flow rate vs Area

04/29/2015 5:08 AM

Perhaps not. I already tried.

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#4

Re: Flow Rate vs Area

04/29/2015 9:05 AM

I know that in the Waste water piping designs that I have worked with, the piping fore and aft of the flow meter was reduced in diameter. That was to get a more accurate number, or at least the engineers stated that is why it was designed and installed like that?

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#5

Re: Flow Rate vs Area

04/29/2015 9:34 AM

It depends on the source providing system pressure.

If supply pressure remains constant then no, the flow will not be the same and will decrease.

There will be a small loss in flow due to the pressure drop across the meter and at every restriction presented in the piping system including pipe diameter for a given pressure value applied to the fluid.

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#6

Re: Flow Rate vs Area

04/29/2015 10:52 AM

This depends on the pressure and velocity and viscosity, the smaller pipe may act as a restrictive device or, just as you say, increase the velocity...if there is any pressure difference upstream to downstream then the flowmeter may need to be swapped...

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#7

Re: Flow Rate vs Area

04/29/2015 11:00 AM

Nah, flow rate is the same through out the line it doesn't matter what size of reducer of expander you put on it. All through one line, the same flow is experience, the velocity will only vary depending on pipe size or diameter, dont fool yourself, study simply circuit either electrical or mechanical circuit. Chum..

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#8

Re: Flow Rate vs Area

04/29/2015 12:23 PM

The mass flow rate is the same; providing you are talking about liquids or slurries and not gases, the volumetric flow rate is also the same (apart from possible very minor expansion due to frictional heating). The only possible difference is if your meter acts as a restriction and reduces flow through the entire system.

Your flow meter should COMPUTE an accurate number based on measured velocity the meter, adjusted by whatever other factors it is capable of measuring and referenced to a preprogrammed constant for that particular size, model, colour and who knows what else of meter.

However, the size reduction may induce turbulence that may affect the accuracy of the meter. There are normally installation parameters that can be obtained from the manufacturer to avoid this problem; they will usually recommend certain L/D ratios both upstream and downstream from elbows, reducers etc. Call them.

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#9

Re: Flow Rate vs Area

04/29/2015 10:47 PM

For liquids, the flow rate would remain the same.

The following is from the "Engineering Toolbox":

The Law of Conservation of Mass states that mass can be neither created or destroyed. Using the Mass Conservation Law on a steady flow process - flow where the flow rate do not change over time - through a control volume where the stored mass in the control volume do not change - implements that

  • inflow equals outflow

This statement is called the Equation of Continuity. Common application where the Equation of Continuity are used are pipes, tubes and ducts with flowing fluids or gases, rivers, overall processes as power plants, diaries, logistics in general, roads, computer networks and semiconductor technology and more.

V1 x A1 = V2 x A2, in it's simplest form, since density is constant.

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#10
In reply to #9

Re: Flow Rate vs Area

04/29/2015 11:33 PM

In addition, for gases, the mass flow rate also remains constant, but the volumetric flow rate varies with density.

(If the pipe leaks outward, the mass flow rate decreases downstream, and conversely for inward leaks.)

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#11
In reply to #10

Re: Flow Rate vs Area

04/29/2015 11:34 PM

Did you just take a leak, by chance?

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#12
In reply to #11

Re: Flow Rate vs Area

04/29/2015 11:36 PM

Either that, or gave one. The effect is about the same.

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#13

Re: Flow Rate vs Area

04/30/2015 1:31 PM

Until 1970 or so, it was believed that the transition between laminar and turbulent flows is gradual, but increased understanding of turbulence through supercomputer modeling has shown that the onset of turbulence is abrupt.

When flow is turbulent, the pressure drop through a restriction is proportional to the square of the flowrate. Therefore, flow can be measured by taking the square root of a differential pressure cell output. When the flow is laminar, a linear relationship exists between flow and pressure drop. Laminar flowmeters are used at very low flowrates (capillary flowmeters) or when the viscosity of the process fluid is high.

In the case of some flowmeter technologies, more than a century elapsed between the discovery of a


Figure 1-4: Click on figure to enlarge.
scientific principle and its use in building a flowmeter. This is the case with both the Doppler ultrasonic and the Coriolis meter.

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#14

Re: Flow Rate vs Area

04/30/2015 8:56 PM

You might understand it better if i were to say that the sensor part is matched to the bore. If you took the sensor part and put it in a different size pipe the reading would be different. Is that what you want to do?

Jim

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#15
In reply to #14

Re: Flow Rate vs Area

05/07/2015 3:55 PM

GA JimRat! That appears to be the page OP is on - in other words a perfectly good insertion flow meter set up improperly can be made to output garbage data. GIGO.

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