calculating amount of flow in a nitrogen pipe 4

[Maintenancetech]

I'm sure this is a simple question, but how doI calculate the flow in CFM going through a 2in nitrogen line made of copper and at 60psi. I know there will be some pressure drop from the lenth of pipe and elbow, but I am just looking for a ballpark number.

 

[Cockroach]

Use the Bernoulli Equation and obtain velocity from the pressure drop. This number times the cross sectional area of the pipe using the inner diameter will give you volumetric flow rate. Simply modify your units for cubic feet per minute.

Since you are interested in an approximation only, don't get into frictional losses, accelerations around the bend and other related pheonema. Mass is conserved, so anything going into the pipe will come out of the pipe in the absence of leaks. Therefore a chunk of straight pipe would be sufficient for computation purposes.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[katmar]

Cockroach,

If the 2" dimension mentioned by the OP was the length of the copper pipe then you are justified in neglecting friction. I assumed the 2" was the diameter and the pipe had some significant length. If this is the case, ignoring friction and the effect of fittings will introduce significant errors.

I deliberately avoided answering this question because the OP has made no effort whatsoever to think the problem through and has apparently done no research of his own. By his own admission it is a simple problem. An hour or two with his books would give him the answer. Even a 10 second Google on "online flow calculator" would probably solve it for him.

I am very grateful for the help and advice I receive through Eng-Tips and I am happy to help where I can, but I do not expect anyone to do my work for me and I will not do another engineer's work if he has not made a concerted effort to help himself.

It's early in the morning and I will probably be in a friendlier mood later on.

TGIF
Katmar

 

[pmover]

a star for Katmar!

i wholehearedly agree!

after reading the post again, my instincts do tell me that perhaps the author is not an engineer or has knowledge of solving such a problem; hence, the question. pls do understand that prudent engr's are relunctant in furnishing resolutions without knowing or understanding the entire situation. vague or ambiguous inquiries are subject to much interpretation and assumptions; which may/may not be suitable to the problem.

like Katmar, be glad to help & good luck!
-pmover

 

[monkeydog]

Katmar and pmover,
you guys need to up your caffine rating in your coffee!
The original poster did not ask for an answer, the poster asked how to solve the problem for a ballpark figure.
Cockroach told the poster how to solve the problem, he did not solve it for the poster.
A star for the Cockroach!

 

[Cockroach]

Thank you, MonkeyDog.

Yes, exactly. MaintenanceTech wanted a generalized approach to solve his particular problem. I merely provided direction or at least a place to start.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[sailoday28]

Cockroach (Mechanical) IF 60 psi is the pressure at a point in the pipe and friction losses, compressibility and elevation are neglected-- how does one get a pressure drop using the Bernoulli equation?

 

[katmar]

Although Sailoday28 asked the question, I am sure he knows the answer and only asked the question to show that "apply Bernoulli" is not the most helpful advice.

If Bernoulli is applied to a system where friction, acceleration, expansion and mechanical work are all ignored you get a very easy equation to solve, but a very wrong answer. These assumptions mean that we are working with an incompressible fluid, and if we assume that all 60 PSI ends up as velocity head we get a velocity of around 3000 ft/second. This is supersonic and clearly impossible.

We must at least include the compressibility of the fluid in our analysis. This means that the gas expands as it flows down the pipe and the velocity increases. To solve Bernoulli you need a relationship between density and pressure - so now we have to decide on a thermodynamic model. If you look back in your books and see how Bernoulli is solved under the assumption of either an isothermal or adiabatic model you will see this is a non-trivial exercise unless you are a professor lecturing this on a daily basis. This is why you find Lapple charts and all sorts of nomograms in the piping books.

 

[sailoday28]

The Bernoulli equation does not include friction. We as engineers have modified it to include friction.
With 0 friction, there will be 0 pressure drop.
A reasonable knowledge of upstream flow and pressure drop must be available or estimated.
The following is for low Mach no. flow.
If the 60 psi is a pressure in the pipe, then using the Crane or other manual
1) get ID of piping.
2) with material of piping get epislon/ID
3)Assume turbulence controlling (ie. Very high Reynolds number)and
4)From Moody diag get friction factor.
5)Use equiv length of elbow and length of piping.
Assuming density at 60 psi is known,-- from estimated mass flow and pipe flow area obtain velocity.
6) Obtain pressure drop.
7) If pressure drop exceeds available dp, then lower expected flow and recalculate the pressure drop.
To refine, for other then estimate, the friction factor, based on Reynolds no. will have to be itterated.

 

[Onno]

See

http://www.pressure-drop.com/

for a students version of a flow-pressure calculator in free of charge version.

 

[Cockroach]

Sailoday...change in velocity.

Also, friction can be used in the Bernoulli Equation by acknowledging it as a loss in head, i.e. Darcy-Weisback equation.

As this debate heats up, may I add that the thermodynamic tables would probably yield reasonable results given an understanding of the upstream and downstream states. Probably the air tables, i.e. 78% nitrogen, would be sufficient for buddy's purpose.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[sailoday28]

With no friction, compressibility and elevation neglected, the Bernoulli equation will yield no change in pressure or velocity in the fixed flow area piping.
With friction and compressibility included pressure and velocity will change.