July 18, 2007

Suction Vs Discharge Gas Flow

Today I was reading a very interesting discussion on one of the famous forums at Cheresources.

This was really interesting. Newcomers should learn these useful things to avoid confusing states and hence it enables them for better decision making skills.


Why there is a difference in the measured suction flow & discharge flow of a blower (No Spill Back or antisurge or recycle is there). She got Suction flow as 62000 Nm3/Hr and discharge as 48000. How it is possible??????????


I would rather focus on other aspect of it - the practical one.


How the flowmeters work...................? The root cause of this problem lie in this question.


Flowmeters are actually measuring (DP type - Orifice etc) differential pressure across the tappings used for Orifice. Now in case of compressible fluid flow the most important thing is its density or in general terms its P, T, Mol Wt & Compressibility Z (for high pressure systems you can ignore Z for low pressure systems as it doesnt vary too much).


When we use these flow meters they usually experience different P & T and sometimes if gas is not fixed MW also vary. Therefore, at same mass flow rate also the DP across flowmeter will be different due to change in its density.


Now if we convert this measured DP to flow value based on its design DP & flow range which is usually considered (or fed in the system) by instt people, it will give a wrong indication.


Why??????????


Becasue generally spec sheet is mentioned in Nm3 flow not in terms of actual M3 flow. So if spec sheet is having actual M3 flow than above conversion will also give actual M3 flow and would be correct..........Here comes next practical aspect...............


But indication on DCS is provided in Nm3 generally without thinking & checking all the units.............


So we must be careful for our actions when we install any flowmeter............Instt should consult technical services deptt for parameters correction.


Further there should be an standard practice for providing online compensation of P & T in each compressible fluid flowmeter for better understanding...........because uncompensated flow lead to some wrong decisions in emergencies, startup etc when variations are large compared to design values.


In the above case, it seems that both the flows are Actual M3 flows, when converted to Nm3 they are closer than given figures...........Its just a possibility.


Hope it is useful for you?????????


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2 comments:

Max said...

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