Selection of flow meter

[Abrabaracurxis]

Hi all,

I got the problem to measure a air flow to a burner. I have work it out up to 2 choises. I hope you can give me some advice, I have a thermal flow meter and a annubar to choose.
The process has the following characteristics
Pipe 18"
Fluid clean air
Pressur 250mBar
Fluid temperature 90°C
Environmet temperature 35°C
Type of measurement Volumetric
Accuracy up 2% of span
Environmet Lots of dust

The annubar is about 2 times more expensive than the thermal. What the sales annubar sales man says about the thermal instrument is that it requires frecuent calibration. Is this true?? I got no experience with this kind of measurement.

thanks

 

[danw2]

Cool, clean gas is the ideal measurement for thermal mass flow meters.

It might be a careful phrasing by the salesman, to make you believe that 'annubars' don't need frequent calibration, when they don't, it's the instrument attached to the annubar that needs calibration.

Frequent calibration is a policy, usually required for regulatory purposes. In former years when instruments drifted substantially, calibration was done for performance reasons, to make sure you could trust the numbers. My experience with thermal flow meters is that drift is negligible so there is no reason to calibrate for performance reasons, only for regulatory reasons (modern electronics are so much more stable than yesteryear's).

If, by policy, you were required to calibrate the flow meter at some periodic interval, like quarterly, you'd have to calibrate the instrument, the DP transmitter, on the averaging pitot tube (Annubar is a brand name) just like you'd calibrate a thermal mass flow meter.

Depending on the calibration protocol, you might not have to include calibrating the actual primary flow element, the pitot tube, given its construction and the service it is in. If the service conditions are such that it doesn't corrode or erode, what's to change?

But if you need to calibrate a flow meter, then

Assuming your environmental conditions, "lots of dust", is the surrounding area, and not the quality of the combustion air itself, a pitot tube is usually assumed to not have changed its characteristics so the tube itself isn't calibrated, but the instrument attached to it is.

So you need to find out if there's a policy you need to meet for calibration. If not,

I'm surprised that a pitot tube and DP cell is so much more expensive than a thermal flow meter. Is this thermal flow meter the kind you fill out an application sheet and they size a meter for you and its output is scaled for your flow rate? Not one of those cheap, screw- into-a-pipe-tee and get 5 LEDs showing relative flow (low, medium, high)?

 

[Ashereng]

I would have thought air to a burner would be relatively clean, and not "full of dust".

Thermal mass flow meter work best in gases that stay consistent, like good air. Annubar work well also. They both also have low pressure losses.

I would think that they are both probably equally good in the service of air to a burner, and the maintenance would be about the same.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[Abrabaracurxis]

Thank you,

well you are rigth by policy we have to calibrate all our instruments, this kind is only once a year... or if the production people thinks something is wrong with our readings... when the sales man said "frecuent calibration" i understood every 15 days or so...

Anyway I think I am not going to be able to use thermal flow meters beacuse i got only 6 m of straight pipe and according to the manual i need 9m (20 diameters)...
have you used them with less than 20 diametres upsetream??

and the annubar takes to much time to deliver...

looks like i am going to have to install DP transmiters in the piezometric ring of the blowers...

 

[Ashereng]

For our thermal mass flow meters, we usually allow for a minimum of 10D upstream and 3-5D downstream.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[dcasto]

Just about any meter will need either a long upstream straight run or some sort of flow conditioner, so do not let that be a factor in this choice. All the air must be moving exactly in parallel, no swirling, no eddys, straight flight for either meter to be considered accurate.

What will happen if you use a 5 meters upstream and 1 downstream is typically this: If the flow rate only changes +/- 10%, the meter will have a bias offset from actual. so the meter will appear to be say 3% high as the flow changes +/- 10%.

If the rate changes +/-90%, the error will appear to be random through the range. If you tested it enough, you could find a trend line. For example the meter may read 5% low a the x flowrate and 3% low at 2x flowrate, and 7% low at 3X flowrate.

 

[Cat3347]

FYI, many thermal mass meters have flow conditioners as options. Does your current vendor offer this?

 

[JimCasey]

You have not been really specific about what thermal meter you are considering, but one particular consideration is that it probably only measures one point in the pipe cross-section. That's why you need so many diameters upstream for the velocity profile to be fully-developed and stable.

Annubars read 4 points across the duct and average the pressures. So even if there is some distortion of the velocity profile you can get a reliable reading.

I am surprised you state that their delivery is too long. Annubar is one specific vendor and there are competitors who also make averaging pitot meters. I Googled that and got a bunch of hits.

http://www.google.com/search?hl=en&q=Averaging+pitot+flowmeter&btnG=Google+Search

SOMEBODY will be able to get you a probe in the timeframe you need.

 

[Abrabaracurxis]

Thank you for all your responses.

I decided for the DP transmitters in the piezo electric ring that came with the blowers...

Some of the answers of your questions...

- I located in Mexico and thats right there are many manufactures of "annubars" but overhere the shortest delivery time of all of them was 4 weeks...

- Installing a couple DP transmitter was easier, and cheaper than geting a conditioning section plus the thermal flowmeter.