Pressure drop of gas in a pipe line 9

[enrjdean]

Hi Guys

I'm currently looking at sizing a pipe to carry 6.55m^3/s of exhaust gas over 1609m @ 40 psig in a 12" pipe. I've gone through these set of equations to look at the pressure drop over the line:

http://www.engineersedge.com/fluid_flow/pressure_drop/pressure_drop.htm

But the answer I'm getting back (4036Pa) seems extremely low? I'm recently graduated, so perhaps its my lack of feel for this sort of problem.

Would this be a reasonable figure, are the equations I'm using appropriate or not?

I'd appreciate any help or comments you guys have

Jim

 

[BigInch]

Efficiency factors are usually calculated from hydraulically similar pipelines nearby in the same system.
Some equations use the efficiency factor in place of both entering an actual pipe roughness value and also account for losses due to the expansion of the gas when reaching areas of lower pressures as it travels down the line.

I haven't checked MonteM's spreadsheet, so I'm not sure what's going on there yet, but I'd say that using a value for roughness is by far the better way to approach a problem when one does not have an idea of what eff value one should be using. Efficiency factors can vary with gas density, pressure, temperature, diameter, length AND roughness.

If an equation uses standard cubic feet per day, you won't need to use the compressibility factors, but if it uses actual volumetric flow (ACF) in the pipe at the pipe temperature and pipe pressure, you will need to use the compressibility factor whenever you need to convert between ACF and Standard Cubic Feet (SCF).

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[katmar]

enjrdean, it is only the old empirical equations that use the efficiency factor E. If you use equation 2.7 from BigInch's document then there is no mention of E. Leave the old Weymouth and Panhandle equations to history and to those who know how to use them. As BigInch said, these old equations are used to fit experimental data by adjusting E to suit the data. This is useful to the specialists, but if this is not your core business rather stick to equation 2.7 and friction factors.

The Z that appears in equation 2.7 is just another way of getting to the gas density. I said in an earlier post that in order to calculate the pressure drop you need to know the upstream density. In Montemayor's method he enters the density directly. In equation 2.7 a suitable constant enables the equation to "know" the density of air at the reference (base) temperature and pressure. You enter G, which is called the gas specific gravity but which is really just the ratio of the molecular weight of the gas to that of air (See equation 1.6). This converts the built-in air density to the density of the actual gas at the base temperature and pressure. By including the flowing temperature and pressure with G we are able to get to the density of the gas under the assumption of ideal behavior at the upstream conditions. Adding Z into the mix gives us the actual density. There is nothing wrong with this method, but many people get confused over the definition of gas specific gravity. For an example of this confusion browse through thread798-233225

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[enrjdean]

Thanks Katmar/BigInch

Jim

 

[Lijantropo]

Good Morning,

I have been watching the spreadsheet of Mr Montemayor. I have a question about the friction factor that he uses:

f = ((8/Re)¹² + 1/(A+B)^1.5)^1/12
A = (2.457ln(1/((7/Re)^0.9 + 0.27e/D)))¹⁶
B = (37530/Re)¹⁶

In the FAQ 378-1236, Mr Quark presents other equations for Churchill Equation:

f = 8((8/Re)¹² + 1/(A+B)^1.5)1/12
A = (-2.457ln((7/Re)^0.9 + 0.27e/D))¹⁶
B = (37530/Re)¹⁶

The answer that I get using each set of equations are different -mainly by the "8" in the expression used to determine "f".

Is the first set used for gas pipes? (I was not able to find any reference to this in the Churchill's article attached to the spreadsheet)

Regards,

Lij

 

[quark]

Neither of the equations is wrong. What Mr Montemayor mentions is the friction factor defined in terms of shear stress on the pipe wall and the equation in my FAQ is Darcy's friction factor, which directly can be used in D-W equation.

f_s = f_D/8

If you have access to Chemical Engineering Journal, refer Friction-factor equation spans all fluid-flow regimes by Stuart Churchill appeared in November, 1977 edition.

Friction factor is independent of fluid flowing in the pipe.

 

[Lijantropo]

Thank you Mr.Quark for clarify this point.
Regards,

Lij

 

[Lijantropo]

Mr Montemayor:

In the spreadsheet you use the friction factor defined in terms of shear stress on the pipe wall, but I think that you should use the Fanning's friction factor in the equation 2.11-10.

Please let me know if I am missing something here.

Regards,
Lij

 

[katmar]

Lijantropo, I believe you are correct about the confusion between the versions of the friction factor used in Montemayor's spreadsheet. In the three equations in cells C25, G25 and K25 where ever the friction factor (i.e. C22, G22 or K22) appears it was multiplied by 2. I have changed each of these constants from 2 to 4 and then the spreadsheet gives good agreement with a variety of examples from the literature.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Lijantropo]

Good morning,

Thank you Mr. Katmar for review this point. I used Mr Montemayor's spreadsheet to compare my calculations and I noticed this difference.

I am very interested in this expressions -using friction factor- because I actually use AGA equation but it needs to adjust the efficiency factor "E", and I am not a specialist in compressible flow.

Regards,

Lij.

 

[katmar]

Lij, an article that discusses the comparison between using the Darcy friction factor approach and the classical efficiency approach is available at

http://www.psig.org/papers/2000/0112.pdf

Unfortunately it does not specifically discuss the AGA equation but the principles remain the same.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Lijantropo]

wow!
Thank you Mr. Katmar.
The article is very useful for me.

Regards,

Lij

 

[vzeos]

Attached is an excellent review of the pipe flow equations in use today.

http://www.scielo.br/pdf/jbsmse/v29n3/a05v29n3.pdf

P. S.
In a previous post I uploaded a spread sheet which incorrectly calculated the transmission factor, thus the results were incorrect. I have corrected the problem in the spread sheet below. Please note that this calculation uses the correct equation and methodology for incompressible flow.