Square Root Parameter in Flowmeter

So that we can have a measured flow indication/process variable that is proportional to the actual flow rate.

As you probably know, fluids will either conform to linear or turbulent flow. Turbulent flow is difficult to model, and flow meters are not suitable for turbulent flow measurement. Most systems will only ever see linear flow fluids. In this manner, the fluid molecules in contact with the pipe wall are stationary, while the center of pipe molecules are at maximum flow. There is a perfectly linear change from static to maximum flow between these points. Hope this helps.

Please disregard comment #2

I dont know what planet the commenter stepped off of but not from the Oil & Gas Industry. In my 20 yrs the only time I have not had turbulent flow was the 18 months I spent making treacle / syrup.

Turbulent flow is difficult to model on a molecular level but as far as the bulk properties go - which to mind mind is engineering - the complex flow balances out to an almost even flow velocity across the pipe diameter making flow measurements quite simple.

There are a huge range as has been mentioned by many. I concur that a square root function is only applied to a pressure differential measurement to convert that reading to a flowrate by as mentioned elsewhere the worjk of Bernouilli.

As far as i am aware other flowmeters tend to use velocity / mass flow principles to measure flow.

The one query I have is that you call your meters electrmagnetic meters - I just want to make sure that you mean that a mag field is applied to the fluid and an induced voltage generated which is proportional to the flow (ie current - the fluid must be electrically active) which has been generally assumed.

If not could you clarify the meter.

WJMFIRE sorry if you think this is hostile and perhaps in your field linear flow is the norm if so could you identify your field.

I can see that the finite element analysis (or similar that you seem to be describing)would be easier in linear flow than turbulent but my post was a bit long and the issue in yours that I had real issues with were the two comments

flow meters are not suitable for turbulent flow

Most systems will only see linear flow fluids

These two statements are in my opinion (hence the reason I gave my background)wrong. I have specified 100's (at least) of flowmeters over the years for use in turbulent flow regimes. Now they may have inaccuracies and limitations and built in assumptions but plants work safely based on these flow measurements, customers are charged based on custody transfer meters operating in the turbulent regime.

Perhaps what I should say is that I have experience in one area where your statement is wrong and so although that may be the only case I think your comment to the OP is potentially confusing.

And honestly no line I have worked on in an oil and gas plant was streamline flow. I am very surprised that you have streamline flow in firewater circuits (except maybe when just the jockey is running) and again I would be surprised if my case were the only exception but it is dangerous to say but equally I cannot hazard a guess at the range of other industries so I should hold off saying most is wrong but as O & G is such a big part of the industries dealing with fluid flow I think I am correct (ish)

"... why we use square root parameter in flowmeters ...?"

Because the flow rate is proportional to the square root of the differential pressure.

"... why we use linearization in flow value ...?"

Because the physical limitations of the system (variation in flow type depending on actual flow rate, for example) mean that the relationship described above is not exact.

Flow meter can't measure flow directly, they calculate the flow wrt Differential pressure across the line.

It depends on what kind of flowmeter.

yes, this is for pressure based measurement where we require sqare root function.

AA

Please Google "Bernoulli principle" and view the level you are comfortable with. (Kids , example . . . )

Dear All,

Thanx for giving valuable information. We are using electromagnetic flowmeters and we have used square root function in them. How can we use electromagnetic flowmeters to measure pressure difference and then flow rate?

Best Regards,

Please tell us the manufacturer and type number of the devices (or better, post a link to the datasheet).

Electromagnetic flowmeters usually have an analog signal output directly proportional to the flow as advised in the manufacturers' selection criteria. So it is not possible to determine why any end-user would want to use a square root function with them as such an application is practically unheard of until now.

>We are using electromagnetic flowmeters and we have used square root function in them.

If this was done, the result is the square root of the flow rate, not the flow rate.

If this was done, the square root function had to be applied outside of the magmeter (in the controller, indicator or recorder) because no commercial magmeter has ever had a square root function.

Magmeters are inherently velocity meters, they do not measure differential pressure and have no use for a square root function. It is common for 'electronic differential pressure transmitters' to have a square root extractor function, but never a magmeter.

>How can we use electromagnetic flowmeters to measure pressure difference and then flow rate?

If you've already used a magmeter with a square root function to calculate flow, why would you want to bother with measuring a pressure difference. You already have a flow reading.

Hold your hand out the window driving 10 mph. The wind drag will apply a force to your hand. At 20 mph, the force will be 4 times as much. You are stopping twice as much air going twice as fast. The force or pressure is proportional to the square of the velocity. Therefore, to measure the velocity from the pressure, you need to take the square root of the pressure.

The square root function in a flow meter is used only for applications employing inline restrictions such as orifice plates, venturies etc (which create a pressure drop) and differential pressure sensors. The square root extraction is required since the relationship between linear flow and differential pressure is a square root relationship i.e. flow = the square root of the dp.

Square root function is not required for magnetic flow meters since this type of flow meter measure actual flow velocity and gives a calculated volumetric flow rate.

That's right. Electromagnetic flow meters measure the velocity of the fluid whereas turbine flow meters measure the angular velocity of the vaned rotor in the pipe.

You can check our website for making out more difference on flow meters at the following weblink:

http://scharfautomation.com/what_is_a_flowmeter.html

http://scharfautomation.com/electro-magnetic-flow-meter.html

In case you have any queries regarding flow meters, please feel free to email us.

Thanks.