Flowmeter

Not certain what you mean, but you certainly can't measure fluid velocity if the fluid isn't there! So by definition the line needs to be full.
Flow meters are usually positioned so that air can't into the line and the flow is relatively laminar, e.g a straight pipe run for (say) a length of at least 5 times the pipe diameter, either side of the meter.
Sorry if I'm off target here...(now where's my banjo and that cow?)

Del

You're not off target but spot on.

A magnetic flowmeter operates by using the fluid passing through a magnetic field to generate an AC or DC voltage. If the fluid is not there, there will be no generated voltage. If there are bubbles in the fluid, you'll get a very noisy signal.

The banjo and the cow are at the town hall performing at a concert.

regards,

Vulcan

The velocity in a pipe is not uniform it will be faster at the centre and slower against the wand. The uneven distribution will have an effect on the result.

The friction in a pipe will be less if the pipe is not flowing full (the ratio of wet perimeter vs area changes fastest just below full) Therefore a pipe almost flowing full may deliver more water for the same measured velocity.

At any flow not fill the reduced wet area need to be considered.

Hendrik

No, what you explained is not the reason.

What Andrew Ward explains is the real and only reason for requirement of full flow for flowmeters.

Also What Andrew tells about Krohne meters is also true.

(I am in design and production of Magnetic Flowmeters)

My GA to Andrew Ward

Hi gsuhas

My last sentence is about the same as his. Therefore not false.

The problem with flowmeters is that it measures avarage velocities. If not full the level may be anywhere but I think it will mostly be close to full because what is after the meter..

With almost full the area stays almost at 100% and can therefore not have a big influence but the changing velocity distribution pattern will have an influeance on the avarage and therefore on the calculated flow.

I havent tested this on meters but on tunnels and I can assure you that the flow on a 95% full tunnel is more than on a full (submerged) tunnel with the same slope.

As you say, magnetic flow meter measures average velocity. This is not a problem, but it is advantage. As it measures average velocity, to find out flow rate we can use the multiplication of meter measured velcity and the cross sectional area.

Beside, there is no assurance that the pipe is full if the installation is not proper. In that case, if line is not full the meter with built in multiplier assumes that the line is full and give multiplication of average velocity and the full cross sectioanl area of the line (which is not really full of liquid. This gives higher reading than actual flow rate as Andrew say.

Besides, when we are talking of accuracies of the order of 0.1 to 0.3%, 95% filled line will give big error, which is not acceptable by industries.

If flow meter which measures local velocity instead of average velocity (type insertion flowmeter), we can not infer what will be the flow rate unless it is mounted at a point where velocity is average velocity.

GA. You figured out the simplest and most important reason to keep the pipe full all the time you want to measure flow.

Dear Andrew,

There is a situation in our plant where there is gravity flow throuth a line of 4''. and the process does not allow to provide any bend on this line as the process may get checked (in this condition mechanical will have to cut the line as steam fushing doesnt work in this process). I have enquired krohne for the partial flow mag flow meters for 4'' line but they said that krohne manufacture partial flow type magflowmeters on 8'' and larger dia flowmeters. do you any suggestion on this?

Raj,

One suggestion I have is to fabricate a Palmer-Bowles flume (some people spell them Palmer-Bowlus) into your pipe. It may be difficult to do depending on if your pipe can be open to atmosphere or not. If not, you would probably need a tee flange above the measurement point to mount an ultrasonic level sensor. Flumes generate a velocity head into the flow that is proportional to flow. I've used these flumes in wastewater applications where solids plugging the pipe would be an issue. Something that small might be a problem to calibrate accurately, but it may be good enough if you have nothing now and the flow stays relatively constant.

I'm not a mechanical guy and can't really suggest where to get a flume that will fit, but I hope this is enough information to get you started.

A flowmeter on a rising main should always be full. Air trapped in there will cause problems. All needed to do is to get rid of air some distance before the meter.

You are right, if the flow meter is in rising mains.

But, it is not true if flowmeter is in horizontal line. If it is in horizontal line, it is recommended that line be raised by 1 D after flowmeter to assure full line condition.

If the two electro-devices (two electrodes) aren't wetted all the time, the magnetic-devices won't operate correctly, and therefore the electro-magnetic flowmeter won't work.

If all else fails, read the manufacturer's installation instructions supplied with the equipment.