Gas Flow Meters and Flow Rates

You can check your flowmeteres by very simple way just replacing their. If the result will be same ( wrong) so you have chimical process on the distance between the flowmeters location. Most probably that with the temperature dropping you have the carbon dust production and second flowmeter will indicate less consumption.

Are your flow meters the same brand, same model, same style? That's a good idea, to swap them and see if the readings stay the same.

A couple issues I've encountered with thermal dispersion flow meters I've installed:

1) The insertion depth of an insertion style probe (a long probe secured into a weldolet or with a compression fitting) is critical. The ones I've used insert until the probe hits the opposite pipe wall and then are retracted X distance in order to put the sensing element in the middle of the pipe. X depends on model and line size. Deviation from the retraction distance definitely affects the reading.

2) The single point sensing style wants to be upstream of a valve, rather than downstream, to minimize flow profile changes with flow changes. Do you valving in between the two meters?

3) Thermal dispersion in general works better at low flow velocity rates than high flow rates. You have to make sure that your meter will work at the actual flow rates.

I once put a meter in a liquid line that flat lined at about 2/3 of the max flow. The calibration curve indicated that, but I didn't examine it before installation.

Hi Sat,

Is each of the meters calibrated for the correct flowrate conditions, what is this?

What is your overall flowrate (is the variance 50 to 100 Nm3/hr small or large)?

Is gas moisture/dewpoint having an effect?

How accurate are they located for flow direction?

<...Can anyone suggest suitable methods or technique to get same flow in both the meters....>

One of the ways is to get rid of one of them.

Happy to comment but need more info: flow meter types; pipe diameters same?; why 30C drop in as many meters?; any turbulence or venturi effect?. Typically ultrasonics are affected by these factors as well as other types of FM's. RDGRNR

Thermal dispersion type flowmeter works based on temperature variation.When the flow passes through the flowmeter, since your process incudes two difference temperature at the installation points, sensnor senses the difference of temperature and gives you the flowrate.

So pls check up with ur process that why you have temperature drop after 30 m distance. pls check with process team.

remember this kind of flowmeter is good for clean gas and steam.( cheak out your process fulid whether is is having dirts,suspened particles)

for wide temperature ranges you have to provide compensation.If it is immerson type your performance may affected if it is not installed properly.

sudip

Hi friends, I thank all of you who replied for my question. Before I inform the test results, I want to inform this. Both the flowmeters are of same brand, same style. Also both were calibrated for 1200 Nm3/hr flow rate and installed in 4 inch pipe line with proper upstream and downstream considerations. As suggested we swapped the flowmeters, more than this we replaced one meter with a spare new one of same type.. But the result is same. One thing we noticed is that the gas is having considerable amount of moisture in it, which we observed while we drained the pipe lines using the drain traps provided in it. Whether providing thermal insulation for the pipe between the two meters and some meters after the down stream flow meter will solve the problem?

Your flow medium drops 1°C per meter between the flow meters.

I suspect that the flowing wet gas temperature drops below dewpoint and water condenses out of the flow stream. The thermal dispersion flow meter gets coated with condensed water, which alters the coefficient of heat transfer for which it was calibrated. So the measurement value changes.

There are lots of claims that thermal dispersion technology works in wet gases. My experience does not back that up (but who am I? just some guy on a forum . . .)

According to the link below, Endress & Hauser, a major flow meter manufacturer, does not recommend its thermal dispersion meters for use with wet gases.

http://www.engineerlive.com/Oil-and-Gas-Engineer/Production_Processing/Measuring_the_flow_of_gas__accurately_and_reliably/18344/

Thermal Mass Flow Meters: <snip> The use of these devices is not recommended in applications with strong pressure swings (eg from 2 to 10bar), wet gas applications, gases with changing composition and SIL applications according to IEC61508.

2) EPI (manufacturer of thermal dispersion meters) acknowledges the difficulties involved in condensed water in a power point on the web (my italics)

Equally important is the monitoring of Digester/Bio Gas (typically 65% CH4/35% CO2) downstream of the aeration basins to achieve an overall system balance. A secondary objective is to monitor the excess gas (Waste Gas) used as a fuel to power onsite generators and pumps, or to create energy for a more widespread power grid. These gases are often wet or saturated. Although water droplets will cause problems for thermal devices, EPI has strategies for reducing the likelihood of these problems.

but does not reveal their 'strategies' for dealing with such in the web presentation.

http://www.epiflow.com/EPI%202009%20PowerPoint.pdf

If condensing water is causing this variation in flow readings, then delaying the point at which the condensation occurs should bring the readings closer to one another. If the temperature is falling due to radiation from uninsulated piping, then insulating the piping will decrease the heat loss.

Be aware that dewpoint changes far more radically with pressure changes than with temperature, and it isn't clear what pressure changes are occuring in this process.