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Muhmmad Basharat
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Join Date: Dec 2010
Posts: 14

Gas Flow

03/16/2011 4:25 AM

Dear friends

I am a consumer of Natural gas and my flow meter is giving flow in m3 and my soft ware is converting it ito nm3 but their is lot of difference between to figures.If I say , today I got reading on flow meter 11665 m3 and soft ware is showing it 18971 nm3.where as calculation says that m3 is greater then nm3.Flow pressure after regulator is 0.8 bar and temperature is 26 celcius and this temperature can very during the day and night.Pressure before regulator is 2 .00 bar.Can some one help me to calculate these figure and relation between above given figure is correct or not. If not then what it could be. I will great full.Thanks.

Basharat
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Mikerho
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#1

Re: Gas Flow

03/16/2011 8:32 AM

The meter shows the actual volume in cubic meters(m3) at its current temp. and pressure. The software adjusts to give the volume in normal cubic meters (nm3), meaning what the volume would be at 0ºC and 1.013 bar. Since the actual pressure is lower and the actual temperature is higher, the meter will be reading higher. Don't worry.

For more on the relationship between volume, pressure and temperature of gases, see here.

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Muhmmad Basharat
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#2
In reply to #1

Re: Gas Flow

03/16/2011 8:50 AM

Dear Mikeho

Thanks lot for the quick respose.You ended my worry.

Bashrat
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lyn
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#3
In reply to #2

Re: Gas Flow

03/16/2011 9:24 AM

When someone gives the correct answer to your question, it is customary to vote them a GA, good answer, using the rating box below their good answer.

Here, I'll show you.
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Doorman
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#4
In reply to #2

Re: Gas Flow

03/16/2011 9:44 AM

I will also do the customary thing and award a GA vote for a spot-on answer.

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lyn
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#9
In reply to #4

Re: Gas Flow

03/16/2011 12:13 PM

Does this mean that we have to take back our GA's?
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Anonymous Poster #1
#5
In reply to #1

Re: Gas Flow

03/16/2011 9:53 AM

Your answer to the procedure is correct, however there is a word of caution.

The temperature of 273 deg K and pressure of 1 atm is not universal.

Different international and industry standards define these two parameters differently for the standard temperature and pressure.

Eg http://www.puregasproducts.com/flowrate.htm says

Design calculations have been made useing the following unit convention factors to maintain design consistency

Normal flow, Normal Cubic Meters per Hour, (Nm3/hr)
Normal Conditions are defined as: Tn = 20.0 ºC, (68 ºF) Pn = 1.01 bara, (14.72 psia)
Note: the definition of Normal conditions are consistent with the calibration of the CME. Standard flow, Standard Cubic Feet per Minute, (SCFM)
Standard Conditions for the calculation of SCFM are defined as: Ts= 15.6 ºC, (61 ºF) Ps= 1.01 bara, (14.72 psia)
Note: the definition of Standard conditions are consistent with the calibration of the CME.

http://www.engineeringtoolbox.com/stp-standard-ntp-normal-air-d_772.html is also clear

Note! There is a variety of alternative definitions for the standard reference conditions of temperature and pressure. STP, NTP and other definitions should therefore be used with care. It is always important to know the reference temperature and reference pressure for the actual definition used.
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Mikerho
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#6

Re: Gas Flow

03/16/2011 9:54 AM

Thanks, Doorman and lyn. It was good to actually see the positive results of an answer in OPs reply.

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Codemaster
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#7

Re: Gas Flow

03/16/2011 10:48 AM

Is the pressure 0.8 bar gauge (= 1.8 bar absolute) after regulator? Likely to be as it would need to be an engine or booster to give 0.8 bar absolute, and anyway the gas supplier would object to you pulling a partial vacuum on his pipe. Is this where the flowmeter is installed?

If yes, then Nm3 is greater than actual m3. Formula is Vn = Va*P/1atm*273/(T + 273)

With your figures I make that 18920 Nm3, which I would say is near enough. If your "calculation says that m3 is greater then m3" you're calculation is different from the software, but the software is right!

In the general case, whether Nm3 is greater or less than actual m3 depends on the actual temperature and pressure.

Mikerho - I don't quite follow your post #1, except for the conclusion Don't worry . The actual pressure, assuming 1.8 bara, is higher than standard 1.013 bara, and the meter reading is lower than the calculated normal volume, from figures given by the OP.

Cheers.........Codey

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Mikerho
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#8
In reply to #7

Re: Gas Flow

03/16/2011 11:25 AM

Yeah, my bad. I was thinking 0.8 bara, not barg.

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Anonymous Poster (1); Codemaster (1); Doorman (1); lyn (2); Mikerho (3); Muhmmad Basharat (1)

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