Buffer tank sizing

[Jezovuk]

Hallo!

I have a task to determine dimensions of a buffer tank for nitrogen.
Tank must ensure enough quantity of nitrogen to supply working equipment for 10 [min], with pressure 7 [barg] and with flow rate of 4 [Nm3/ min].
Working temperature is -10 to +40 [°C].
Tank is filled up to 15 [bar] and during the working period of 10 [min] it can drop to minimum 10 [bar].

How big tank (buffer) [m3] do i need?


Thank You!

 

[LittleInch]

How do you think it should go?

You know the outlet flow at standard conditions - (flow times time) convert this to an actual volume at a mid point between min pressure and max pressure = Vout

Then add this to the volume left in the tank at the end of your 10 minutes = Vtx10bar = V finish

Volume of gas at start = Vt x 15= Vstart

V start= Vfinish + Vout

All pretty rough and you will need to check with compressibility factors, but will give you a start point.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EmmanuelTop]

See page 8 of the document

http://www.chagalesh.com/snportal/uploads/chagalesh/karafarinan farda/jozveh/process/8.pdf

The calculation is for Instrument Air but there is absolutely no difference if applied for Nitrogen.

Dejan IVANOVIC
Process Engineer, MSChE

 

[Jezovuk]

Thank you LittleInch, i had a similar approximation method on mind.

Thank you EmmanuelTop, this helped a lot, that is excelant literature.

(Hvala zemljace, ovo je bas pomoglo, odlicna je literatura.)

 

[EmmanuelTop]

Cool, so we both come from the same geographical area. I think there aren't many Balkan specimens around here, but I may be wrong.

Dejan IVANOVIC
Process Engineer, MSChE

 

[Jezovuk]

I think there are many of us here.

I have one more question, when i calculate the volume of the tank i get 8,1 m3. Is this volume expressed in Nm3 or in m3 (working conditions)?

In other word, do i have to divide 8,1 m3 with 10 bar and get volume of 0,8 m3 (800 lit)?

 

[EmmanuelTop]

It is 8.1 m[sup]3[/sup]. The terms "actual" and "standard" apply for volumetric flow rates, they do not have much meaning for physical volume itself.

Dejan IVANOVIC
Process Engineer, MSChE

 

[Jezovuk]

But my flow is not massflow, it is volumetric flow- 4 [Nm3/min].
And C is the book is also in Sm3/ hr.

 

[EmmanuelTop]

Yes, for flow units. If you say I have a vessel with standard 8m[sup]3[/sup] or I have a vessel with actual 8m[sup]3[/sup] of Nitrogen, what is the difference? It is the same physical volume, equal to 8m[sup]3[/sup].

Dejan IVANOVIC
Process Engineer, MSChE

 

[Jezovuk]

Yes, ok, i agree. So i must use 8000 [L] tank?
Diameter for example 1,8 m and length 3 m.

 

[EmmanuelTop]

That's about it.

Dejan IVANOVIC
Process Engineer, MSChE

 

[LittleInch]

I agree with the 8m³, but this is actual. The volume of gas at standard conditions, whatever they are, is dependent on the pressure, but actual is fixed.

What is the definition of Nm³ ?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EmmanuelTop]

How would you define actual m[sup]3[/sup] (in terms of volume, not flow)?

Dejan IVANOVIC
Process Engineer, MSChE

 

[LittleInch]

Physiczl size. The actual volume of a tank can't change. The standard volume of the gas within it can depending on pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EmmanuelTop]

I see your point. It makes sense to differentiate between STD and ACT if we are interested in how much is the equivalent mass of fluid inside the same volume for each case. But speaking strictly about volume itself, there is no difference between Am[sup]3[/sup] and Sm[sup]3[/sup]. Cubic meter is always cubic meter, regardless of pressure and temperature.

Dejan IVANOVIC
Process Engineer, MSChE

 

[Jezovuk]

Maybe this can clear some doubts?

p1=15 bar
p2=10 bar

p1xV1=p2xV2

V2=1,5xV1
V2-V1=4Nm3/minx10min=40 Nm3
1,5xV1-V1=40 Nm3
0,5xV1=40 Nm3
V1=80 Nm3
V2=1,5xV1=120 Nm3
V2r= 120 x 1,01325/16,01325 = 7,59 m3

 

[LittleInch]

Ypou need to use absolute figues in the first part as well as the second. Gets you much closer to 8m3

My point was how is Nm3 defined?? What does N stand for - Nominal??, Natural?? Nitrogen?? N in a unit is normally used for Newtons. Sm3 is a much more commonly used term.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jezovuk]

N stands for normal conditions.

http://www.eng-tips.com/viewthread.cfm?qid=97506

 

[LittleInch]

Ok, I kind of got the drift, but isn't one I often use or see.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[FredRosse]

"But my flow is not massflow, it is volumetric flow- 4 [Nm3/min]."

Actually, NormalMetersCubed per Minute is a MASS Flow Rate, although it appears to be a volumetric flow rate, examination of the fundamental definitions will show that it is indeed a mass flow rate. The volume concept was developed so lay people would have some grasp of the quantity, but the real definition is indeed mass flow rate. Same for SCFM (Standard Cubic Feet per Minute, also a mass flow, no doubt about that.