5 replies to this topic

[Hossein Deldari]

dear all

I want to calculate the required nitrogen flow rate to pressurize a vessel from atmospheric to 20 barg during 30 min.

based on ideal gas rule, the used formula for this calculation is as folowing:

m=(VM/RT×(P2-P1))/t

that V:vessel volume

M:nitrogen molecular weight

R:gas constant

T:temperature

P2:ultimate pressure (20 barg)

P1:preliminary pressure (atmospheric)

t:time (30 min)

after calculation mass flow rate based on above formula, I will convert it to normal volume flow rate based on P=1 atm and T=0°C.

any commnet/advice about this method will be appreciated

[djack77494]

Hossein,
You will indeed calculate the average nitrogen flowrate to pressurize your vessel using the methodology you describe here. So, if that is what you want, then you are finished. If you want more, then please be aware that you have an unsteady state process with your pressure difference between the vessel and your nitrogen source varying moment by moment. Especially if your nitrogen source is close in pressure to the final vessel pressure, the changes in nitrogen flowrate may be very significant.

[latexman]

I agree with djack. Your basis was 30 minutes. If this is for an existing system, no tests or calculations have been presented that prove this is realistic for the nirtogen supply system or not. If this is for a new system, then the same basis must be, or be met by, the design criteria for the new nitrogen supply system.

With no data or calculations, your basis is just a guess. Remember, in God we trust, all others bring data!

[Hossein Deldari]

djack
i want to determine maximum flow rate of nitrogen for presurizing mentined vessel. nitrogen will be supplied from nitrogen bottles after regulating its pressure equal to 20 barg. after determination maximum required nitrogen flow rate, the nitrogen injection line will be sized. so I need to know maximum flow rate to size nitrogen lines and nitrogen package capacity

[latexman]

Hossein,

The cylinders will be the supply. The pressure vessel will provide the demand.

At time (t) = 0, the cylinders will be full and vessel will be empty, maybe somewhat empty if it starts at atmospheric pressure. When the last valve between the supply and demand is opened, t = 0+, the cylinders will start to empty and the vessel will start to fill. Mathmatically, the supply is the equal, but opposite, unsteady-state process as the demand. I recommend you consult your college textbook or search the web for unsteady-state material balances and fluid flow. More than likely, the regulator(s) on the supply will determine the duration of time it takes to fill the vessel, therefore a good reference on control valve sizing (i.e. C_v) will be handy. Just be sure the supply has enough capacity or volume, so the cylinders do not run out before the vessel is up to pressure. You can solve this problem with differential equations or numerical integration (a spreadsheet makes this easy). You can probably find the integrated differential equations for emptying and filling in the general references I mentioned or on the internet.

[djack77494]

djack
i want to determine maximum flow rate of nitrogen for presurizing mentined vessel. nitrogen will be supplied from nitrogen bottles after regulating its pressure equal to 20 barg. after determination maximum required nitrogen flow rate, the nitrogen injection line will be sized. so I need to know maximum flow rate to size nitrogen lines and nitrogen package capacity

Hossein,
You will greatly enhance your cause if you think the problem through in more detail and carefully formulate your question. Often, a sketch will allow readers a much better understanding of your system than can be obtained using just text.

If you draw nitrogen from a supply header that is maintained at 20 barg and if your objective is to pressurize your vessel to 20 barg, then it will take forever. I'm not trying to "be cute", I'm just responding to the strict meaning of the data you have presented. As your vessel pressure increases and approaches 20 barg, then your driving force (and flowrate) drop to near zero. I don't think that is the answer you seek, and, in fact, I don't think you really care to limit the pressure in what I've called the header.

The pressure in a nitrogen gas cylinder is likely to be quite high. 200 to 300 barg would seem to be quite typical to me. Let's think of it as 250 barg. So during the pressurization process, the differential pressure will vary from 250 - 0 = 250 bard initially, to 250 - 20 = 230 bard near the end. Not very much difference. If this addresses your problem, then I think you can consider the average flowrate to be very close to the maximum (and minimum) flowrate and therefore quickly obtain your answer. The approach outlined in your original posting looks just fine to me.