Flow Rates of Nitrogen Through Piping

[RJB32482]

Hey,
I saw in the Crane Fluid Manual that there was a chart stating the mass and volumetric flow rate of steam through a known size of pipe at a known pressure. Is there anything like that for nitrogen or instrument air? If not, any quick way to calculate the flow rate knowing pressure and size of piping?

Thanks

 

[Latexman]

Try page B-15, Flow of Air Through Schedule 40 Steel Pipe.

Good luck,
Latexman

 

[RJB32482]

Thanks for the initial help.

How wold this calculation work if the air was not at 100 psig. So if I have nitrogen flowing through a Schedule 40 Pipe at 35 psig, how could I calculate the flow rate using this table?

 

[Latexman]

Hmmmmm. Did you read the text to the left of the table?

Let’s say you have air at 35 psig and 100 F flowing through 100 ft. of 2” schedule 40 pipe. The measured pressure drop is 0.656 psi.

Crane tells you that to determine the pressure drop for inlet or average pressures other than 100 psi and at temperature other than 60 F, multiply the values given in the table by the ratio:

[(100 + 14.7)/(P + 14.7) x (460 + t)/520]

or

dP_{100 psi & 60 F} x ratio = dP_{P & T}

This means dP_{100 psi & 60 F} = dP_{P & T} / ratio

So, 0.656 / [(100 + 14.7)/(35 + 14.7) x (460 + 100)/520] = 0.264 psi

This pressure drop through 2" pipe applies for 25.63 cfm of air at 100 psig and 60 F. 25.63 cfm of air at 100 psig and 60 F is equivalent to 200 cfm of air at 14.7 psia and 60 F.

They also tell you that to determine the cubic feet per minute of compressed air at any temperature and pressure other than standard conditions, multiply the value of cubic feet per minute of free air by the ratio:

[(14.7/(14.7 + P)) x ((460 + t)/520)]

So, 200 x [(14.7/(14.7 + 35)) x ((460 + 100)/520)] = 63.7 cfm.

The flow of air would be 63.7 cfm at 35 psig and 100 F. Since nitrogen and air properties are so similar, I’d call this good for nitrogen too.

Good luck,
Latexman

 

[RJB32482]

OK I Understand, but you need the measured pressure drop for the calculation correct? You can't do it if you just have line pressure and line size? Using the steam table in Crane, you just need the pressure of steam, temperature of steam, and the size of the line to achieve flow rate.

Thanks again.

 

[Latexman]

Which table are you referring to?

Good luck,
Latexman

 

[Montemayor]

RJB32482:

You say you've got the Crane TP #410. Well, allow me to quote the opening paragraph on page 1-6:

"Flow in pipe is alway accompanied by friction of fluid particles rubbing against one another, and consequently, by loss of energy available for work; in othe words, there must be a pressure drop in the direction of flow." (underlined emphasis by me)

Since you are a Chemical Engineer, I assume you have read all through Crane's TP #410. There can be no fluid flow without a driving force; that driving force is pressure drop. As a corollary to this fact, there can be no flow table if there is no noted and identified pressure drop.

You've received some top-notch, practical fluid flow consultation from Latexman and I don't pretend to help him out on his presentation. But I just have to emphasize the basic fact behind the phenomena of fluid flow: pressure drop.

 

[RJB32482]

Table on 3-17

 

[UmeshMathur]

In my opinion, it is best to use a first-principles approach when solving compressible flow problems.

Personally, I first decide whether the situation corresponds more closely to isothermal flow versus adiabatic flow. Then, I use the first-principles formulas for both these extremes. The final answer is likely somewhere in between, unless you are heating or cooling the pipe significantly. For compressible flow with heat transfer, e.g., in a furnace, you have to break up the pipe into segments and use computers as the iteration is simply too much to do by hand.

Even for "simple" isothermal and adiabatic flow, the calculations require trial and error, unfortunately. This is fairly erasily handled by the "solver" in Excel, however. See Streeter and Wylie, pages 284-289 (adiabatic) and pages 294-296 (isothermal) in "Fluid Mechanics", (1st SI Edition, McGraw-Hill, 1983).

Another beautiful summary of the basic methods for both adiabatic and isothermal flow in conduits is shown on pages 133-139 of McCabe, Smith, and Harriott "Unit Operations of Chemical Engineering" (5th ed., McGraw-Hill, 1993).

A much higher level of complexity arises in case the gas is far from ideal conditions (at a high pressure and low temperature, when the compressibility factor is far from unity). In that case, you will need to divide the pipe into many segments (I use at least ten) and do the calculation by trial and error. Also, you will have to call a thermodynamic routine to find the average compressibility factor of the gas at flowing conditions at each segment of the pipe to get the density required for velocity calculations.

The most important item, as pointed out by Montemayor, is that flow rate requires pressure drop so you need to be clear about which problem you are solving. You can specify three of the following four variables and solve the equations for the fourth: length, diameter, flow, pressure drop. Of course, the inlet stream pressure and temperature of the gas must be known.

 

[Latexman]

The nomograph on 3-17 in my Crane is "Velocity of Compressible Fluids in Pipe". It can be used to solve for one of the following four variables given the other three - average velocity, mass flow rate, specific volume (or density), and internal pipe diameter. On the left side it has a neat graph of temperature versus pressure isobars to find the specific volume of steam. You can use it for any fluid though, even air or nitrogen. You just have to know the specific volume of the fluid and use the line(s) just to the right of the steam graph.

Good luck,
Latexman