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Pressure drop of gas in a pipe line
9

Pressure drop of gas in a pipe line

Pressure drop of gas in a pipe line

(OP)
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

RE: Pressure drop of gas in a pipe line

Do you know the density of your exhaust gas (kg/m^3 or lb/ft^3)?

If you can find/figure that your approximate pressure drop will be approximately:

P(inH20) = 21850 X Density (lb/ft^3) [engine manufacturer's reference]

I'm guessing about 400 degF for your (constant) temperature so your density works out to about 0.17 lb/ft^3.

Therefore, your pressure drop should be about 925 kPa (3714 inH2O).  

Are you sure on your units?  It looks like your 4036 estimate is about right if your results are supposed to be in inches of water column, not Pa.
 

RE: Pressure drop of gas in a pipe line

(OP)
Hi, I've used SI units throughout, so the pressure drop should be in Pa?

To calculate the density, I've used rho=Pabs/R*T which gives: 376kPa / ((287J/kg*K)* (333K)) = 3.94kg/m^3

The temp is actually 140F, so 333K and Ive used the universal gas constant for air. I've then had a look at the Reynolds number so I can calculate the friction coefficient which I calculate to be 4.8*10^-5.

Jim

RE: Pressure drop of gas in a pipe line

Is this a (natural?) gas being vented/flared or the products of combustion (exhaust gases)?  

We need some more info on the gas/application before we can help.

 

RE: Pressure drop of gas in a pipe line

2
A flowrate of 6.55 m3/s in a 12" pipe gives a velocity of around 90 m/s. This velocity over 1609 m is definitely going to give you a pressure drop of more than 4 kPa. Your friction factor seems much too low to me.  How did you calculate it?

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Pressure drop of gas in a pipe line

(OP)
Hi

I don't have any other information yet on the gas other than it is a natrual gas being flared off from an oil rig.

To calculate the friction factor I used: 64/Re

I've been through my calcs over and over to make sure I'm making an error with the units.

I agree that the pressure drop should be higher through gut feeling, but ovbiously I dont have as much experinence as you guys.

Jim

RE: Pressure drop of gas in a pipe line

Using the simplified natural gas formula I get a pressure drop of about 13.3 psi (40 psig at inlet, 26.7 psig at outlet).

RE: Pressure drop of gas in a pipe line

64/Re is for laminar flow!  Your friction factor is too low.  Use a friction factor for turbulent flow and be VERY accurate on pipe roughness.  Using a 0.0018 inch roughness for new commercial steel, I got ~ 35 psi pressure drop.  when I used a 0.004 inch roughness for "design", the flow choked.

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

Assuming your gas is pure methane with a MW of 16 gives a density of 2.2 kg/m3 at the upsteam condition. If we treat this flow as incompressible (i.e. like a liquid) we get a pressure drop in a 12" line of 650 kPa. Your inlet pressure of 40 psig (375 kPa abs) is less than this so it is an impossible situation. Taking the gas compressibility into account makes the pressure drop higher and the situation worse.

Under the incompressible assumption the Reybolds number is 5x106 and the friction factor is 0.0135 (Moody/Darcy) or 0.0034 (Fanning) with a pipe roughness of 0.05 mm. Sorry for the mixed units - you say you are working in SI but you give your data as psig and Fahrenheit?

If you have not made an error in your data then it looks to me that you need at least an 18" line. In this case Re= 3.6x106 and friction factor = 0.0128 (Moody).

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Pressure drop of gas in a pipe line

BTW, the 35 psi drop was for adiabatic compressible flow with MW = 19.5 and cp/cv = 1.27.

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

I have got yet another value but mine is close to Bribyk. At 60C and 19.5MW, pressure drop is about 0.83 bar, Reynolds Number is 4780461 and f is 0.0133. I am just following Crane's method. I considered 0.05mm roughness.

 

RE: Pressure drop of gas in a pipe line

Sorry, I forgot to convert kg to lb for my software.  Now I'm aligned with katmar.  You either have an error in your data or you have/need an 18 inch line.  Sorry quark, I think you have an error too.

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

(OP)
Hi Guys

Thanks for the replies, I really appreciate it! However, I'm still not able to get the values you have quoted. I've attached a spreadsheet with four different cases with different bore sizes. Maybe you might be able to point out where I'm going wrong?

Jim

RE: Pressure drop of gas in a pipe line

Using your numbers from Case 3.0 and row 18, I get the same answer, 15.7 bar pressure drop using incompressible methods.  However, this is impossible because the pressure drop exceeds (significantly) the starting pressure!

In gas (compressible) service if the pressure drop divided by the inlet absolute pressure is < 10%, you can use incompressible methods using either the upstream or downstream conditions.  If the pressure drop divided by the inlet absolute pressure is > 10% and < 40%, you can use incompressible methods using the average of upstream and downstream conditions.  This is iterative.  If the pressure drop divided by the inlet absolute pressure is > 40%, you need to use compressible flow methods.





 

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

At those low pressures, I wouldn't bother with compressibility factors.

**********************
"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/

RE: Pressure drop of gas in a pipe line

Latexman,

So far, it didn't come to my observation that either you are katmar missed a point grossly, in any previous posts. So, I give both of you benefit of doubt.

However, there are couple of issues that may require some understanding. First, why the assumption of either incompressible flow or adiabatic flow in this case?

Secondly, if the flow is choked, how 6.55 cu.mtr/s flow rate is maintained?

Panhandle's formula gives me much higher flow rate at 0.83 bar pressure drop.

Am I missing a point here?

 

RE: Pressure drop of gas in a pipe line

quark,

All good questions!  I bet the inconsistancies in the units of this post have contributed to these differing opinions.  I still have some doubts about that.  The spreadsheet says 20 MCFD.  In my industry and location, which uses King George's units, that's 20,000 CFD.  In the mixed-Metric world I've seen it mean 20,000,000 CFD.  I hope the OP can shed more light on this.

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

(OP)
Hi

I've taken 20MCFD to be: Million cubic foot per day? Which converts to 6.55m^3/s.

I must appolgise for the inconsistancies in units, I've had these past onto me, then have had to convert them into something I recognise, such as SI units.

Jim

RE: Pressure drop of gas in a pipe line

For natural gas work M is 1000, MM = 1E6
and flow should be discussed Standard units, unless pressure & temperature are also stated.  It doesn't hurt to also specifically state the standard conditions in any case, just to be sure.

kPag is a more conventional international gas pressure unit.
Standard flowrates are most often mentioned in m3/h or m3/day.

So was that original 6.55 m3/s a flowrate at 40 psi or at STP?
 

**********************
"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/

RE: Pressure drop of gas in a pipe line

(OP)
Hi, the flow rate was specified at 40psig

Jim

RE: Pressure drop of gas in a pipe line

quark,

I used Panhandle's from Crane's TP 410 (Equation 3-13).

E = 0.92
d = 12 inch
P'1 = 54.7 psia
P'2 = 42.7 psia (dp = 0.83 bar = 12 psi)
Lm = 1 mile

q'h = 1,023,000 scfd

1,023,000 x (14.7 psia/54.7 psia) x (600 R/520 R) = 317,000 acfd

317,000 acfd = 0.104 m3/sec

The OP said 6.55 m3/sec.  Where did I go wrong?


 

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

enrjdean,

What reference book are you using?

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

BigInch and I have a concern on the interpretation of 20 MCFD.  Are you absolutely certain it's 20,000,000 CFD.  Your words "I've taken 20MCFD to be: Million cubic foot per day" do not sound like that has been verified 100%.  Just reading between the lines!  That's a huge difference of 1,000 compared to 20,000 CFD and it has to be chased down or we are wasting our time.

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

When I was working in Oil & Gas some years ago, mmcfd represented "million cubic foot per day", so that would make mcfd = thousand cu ft per day (?).

RE: Pressure drop of gas in a pipe line

(OP)
Hi, throughly understand your concern, I'm really grateful you guys are discussing this - its a big help! I'm positive its 20 million cubic feet per day.

Jim

RE: Pressure drop of gas in a pipe line

Yep, around these parts "1 MCF" or "1 mcf" is 1000 cubic feet.

RE: Pressure drop of gas in a pipe line

The "M" being the Roman numeral for 1000...

RE: Pressure drop of gas in a pipe line

So, now its 20E6 ft^3/day @ 40 psig, since that's still not a flowrate specified at STP conditions, technically one would need to know the temperature and the altitude of the gage.  

And now its not exhaust gas, its natural gas, or is it really well head gas containing a mixture of various well stream gases, in which case you need the mole% of each component?  

So can we also specify some pipeline hydraulic analysis boundary conditions, such as inlet pressure and outlet pressure?  Is the inlet pressure 40 psig and the outlet pressure is what, 0 psig?  And the temperature was what?

If the pressure drop is greater than that 40 psig, or whatever is available as determined from the boundary conditions, then the flowrate must be adjusted to match whatever inlet and outlet pressures you can hold.  

So, what's really going on here?

**********************
"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/

RE: Pressure drop of gas in a pipe line

Quantity of gas should be verified - 6.55m3/s corresponds to 1.86 x 106 cu ft per day. (Or am I being dumb?)

John

RE: Pressure drop of gas in a pipe line

(OP)
Guys, I really appreciate your help on this, I can only give you what I have been given though which is on the spreadsheet.

40Psig at 140/90F, 20/50/70MCFD at 1 and two miles from the platform. I've been told that it is an exhaust gas being flared off. I really don't have any more info than that other than bore sizes and the calcs I've done. I'll go back in tomorrow and see what else I can dig up.

Again, I appreciate everyones input, thanks

Jim

RE: Pressure drop of gas in a pipe line

6.55 m3/second @ 40 psig is just under 20E6 ft3/day @ 40 psig.  Converting to STP (or standard atmospheric pressure, I don't know what the temperature is) gives appx 20E6 *(40+15)/15, running the actual numbers yields approximately 86.5 MMCFD.

Using natural gas density of 0.6 in relation to air at STP(at 0.0745 lbs/ft3) and a 1.6 Km 12 inch line, I get a pressure drop of 42 psi, at a rather high velocity, so I would recommend an 18 inch line giving a pressure drop of only 5 psi (the area of an 18" pipe is quite a lot more than a 12) giving a velocity of around 140-something feet per second.  Still a velocity a bit on the high side, but if you can tolerate that, go for it.

**********************
"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/

RE: Pressure drop of gas in a pipe line

Jim,

You are using the density of air in your calculations.  You need to use the physical properties of the flowing gas.

The inlet pressure is 54.7 psia (40 psig) or 3.77 bara (bar absolute).  You can use your method as long as the pressure drop or delta P (dP) < 0.38 bar.

If the dP > 0.38 bar and < 1.5 bar, you need to use the density of the gas at the average of the inlet and outlet conditions.  At the bottom of your first reference speaks to this.  This will be iterative.  I'd use Solver in Excel.  (I think this method is where you'll end up.)

If the dP > 1.5 bar, you need to come over to the dark side and use compressible flow methods.  The references you showed are not going to be much help.  Google "compressible flow", open up your old college thermo or fluid flow book, or buy a good flow reference that covers compressible flow.

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

Jim, it sounds as though whoever sent you the spreadsheet also passed the monkey on to you, and you have accepted that it now belongs on your shoulders. Do not accept this. You are fully entitled to go back to the originator of the data with your queries.

They must specify the molecular weight of the gas. They must use proper nomenclature rather than MCFD (which I would take as a thousand rather than a million). They must specify the temperature and pressure at which this volumetric flow is measured. If possible get them to specify the flow in mass units - a kg is always a kg, even on the moon.

I am impressed that you already have sufficient engineering judgement to pick up that the calculation was suspect and you have done the correct thing to ask for help. You have seen how even experienced engineers such as those who have commented above can be confused by sloppy specifications. Ask questions and eliminate the confusion. When the design does not work you cannot say "I thought it was.....". Your reputation is on the line and you are fully within your rights to demand the necessary information.

BigInch - I agree that the compressibility factor (i.e. Z in PV= nZRT) can probably be ignored, but he should not ignore the compressibility of the gas and therefore should not use the standard Darcy-Weisbach equation. It is necessary to treat it as a gas (i.e. compressible).

Quark and Latexman - as far as I understand Panhandle is an empirically derived equation that is accurate over a narrow range.  With velocities of 90 m/s it is probably not the right tool to use. I suspect that the flowrate is off by a factor of 1000, and those m3's are actually Nm3 or maybe SCF. Once these corrections have been applied Panhandle may be OK.

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Pressure drop of gas in a pipe line

You're right, I used standard pressure density.

For an 18" I now get 20 psi pressure drop.  

**********************
"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/

RE: Pressure drop of gas in a pipe line

(OP)
Guys, I can't thankyou enough for looking at this with me, its been a big help to have the help of experienced Engineers!

The calculations on the spreadsheet are my own, I'm not trying to pass off somone else's work as mine, so I have to except I've made a balls up treating the flow as incompressible - looks like the thermo books are definitely coming out again!

Thanks guys!
 

RE: Pressure drop of gas in a pipe line

I would like to go with reverse approach here, presuming the flow rate (what ever it may be) is a strict requirement of the user, by first limiting the pressure of the gas just before exit to be less than the pressure that can cause choked flow conditions. To be on safe side, I consider it to be 25 psia or 10.3 psig. So, inlet pressure psig - 10.3 psig should be the minimum frictional resistance required.

Latexman and Katmar,

I am using the simplified compressible flow equation given in Crane. For Re calculation, I used the density values corresponding to pressure drop to inlet pressure ratio (same as you guys did) and this gave me good results in the past with compressed air and nitrogen piping.

Crane says Panhandle's can be used for flows beyond 5x10^6 Re. I will recheck my calculation again.

RE: Pressure drop of gas in a pipe line

This looks like study work, so I don't think the flow will be a strict requirement beyond ensuring the system can handle a set of design flows.

I haven't got rid of doubts about the actual flow conditions - I know it's been stated that the 20 mmcfd was at 40psig (so not 20 mmscfd), but that seems a lot of gas to be flaring (even 20 mmscfd seems a lot) - are the cases for upset conditions? But if you've confirmed the actual flow conditions, then that's Ok, and I'll stop worrying about it.  

John

RE: Pressure drop of gas in a pipe line

Hopefully its a rate with a way-out-of-scale time unit and it really wouldn't be flowing for a whole day, or anywhere near it, but depending on where he is, you never know.  Well gas streams in some parts of the world are still being flared off 24/7 !!!

**********************
"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/

RE: Pressure drop of gas in a pipe line

Jim,

Please give us some feedback when you can; we've got a fair amount invested in this problem.

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

(OP)
Hi Guys

I've managed to get hold of the composition of the gas, so I've uploaded an excel sheet, my calculation for density is in red.

What is a good text/reference book that would guide me through such compressible problems and help me understand the theory a bit better. I've trawled through some of my fluid mechanics/thermo books and tried to apply examples from there to this problem, but I'm getting even more rubbish out.

Jim



 

RE: Pressure drop of gas in a pipe line

(OP)
Thanks BigInch, exactly the type of literature I was looking for! Clear, concise explanations followed by examples and no mumbo jumbo, thanks!

Jim

RE: Pressure drop of gas in a pipe line

The procedure you have used in this spreadsheet will give you the density at 0 deg C and 101.325 kPa (the "old" normal conditions).  For flow calculations you need the density at the upstream conditions.  It seems to me that you are applying recipes for which you perhaps do not know the theory that underpins them.  If you do know this theory then forgive me for rehashing the basics below.

It would be better to use the procedure that you used in your second post (13 Apr 09 15:02).  This is based on the ideal gas law, which is a good approximation of the truth under your conditions.  Note that it is wrong to call 287 J/kg*K the "universal gas constant for air".  It is more accurately called the "gas constant for air".  The universal gas constant has a value of 8.31447 kJ/(kgmol.K) and calculations using it must be done on a molar basis, but it applies to all (ideal) gases.

The ideal gas law is

PV = nRT  where P is in kPa absolute, V is in m3, n is the number of kg moles, R is as above and T is in Kelvin

Using n = 1.0 kgmol and re-arranging we get V = RT/P (which is the basic form you used before, but now in molar terms)

I am confused as to what your actual flow temperature and pressure are, but we can pick any numbers to illustrate the method.  Using "old" normal conditions gives

V = 8.31447 x 273.15 / 101.325 = 22.414 m3/kgmol

This is the well known value that you have used in cell E22.  The SUM(E7:E21) gives the average molecular weight of your gas as 19.86 kg/kgmol and dividing it by the volume calculated above gives the density as 19.86/22.414 = 0.886 kg/m3.  You got this answer correctly in your spreadsheet, but seemingly without knowing why - as I said above, forgive me if I am wrong on this.

We can now use this same procedure to give the density at 65 psig and 89 F (549.4 kPa abs and 304.8 Kelvin). Now

V = 8.31447 x 304.8 / 549.4 = 4.613 m3/kgmol and applying the average molecular weight we get the density as 19.86/4.613 = 4.31 kg/m3.

Once you have understood this procedure you can download a free units conversion program called Uconeer from my web site (see signature below) which has a calculator for converting between mass flow and volumetric flow for gases.  This calculator will calculate the gas density for you, based on the molecular weight, temperature and pressure.

For calculating your pressure drop you should look up isothermal, compressible, turbulent flow.  Perry's "Chemical Engineers Handbook" is a good starting point, or the Crane 410 "Flow of Fluids through valves, fittings and pipe". In a long pipe isothermal conditions are most likely to apply, but if your flow is adiabatic then using the isothermal assumption will most likely slightly over estimate the pressure drop, which is usually the conservative scenario.  

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Pressure drop of gas in a pipe line

(OP)
Thanks Katmar, much appreciated!

Jim

RE: Pressure drop of gas in a pipe line

I agree with Isothermal, compressible and turbulent air flow model. Adiabatic conditions are least likely to prevail in this case.

My final remark is that I would be careful about being conservative in pressure drop calculation if choked flow is the least thing what I want.

RE: Pressure drop of gas in a pipe line

FOURe,

That calculator is for incompressible flow.  While the flow rate in this post is somewhat in question, the velocity is probably high enough for the diameter and length of line that good engineering judgement calls for compressible flow methods.  That could be accomplished several different ways.  One way is by using the density of the gas at the average of the inlet and outlet conditions.  This would be iterative, but one could use the calculator you posted to crunch the numbers.  Another way is by using compressible flow methods like the closed form, isothermal compressible flow equation.  Yet another way is use a numerically integrated form of the differential isothermal compressible flow equation in a spreadsheet.  Of course at the end of the day, the final solution and the method chosen must be evaluated for appropriateness.

Good luck,
Latexman

RE: Pressure drop of gas in a pipe line

Latexman:

I agree with you more than 100 [%].

I do know that the stated problem has been going on for quite some time.

My idea was just to provide another quick way of coming up with some output values ...

I do know that the provided source is for incompressible flow.  In this case, we are dealing with compressible flow.  Therefore, two different cases.  However, if the compressible flow gas velocity is "small", the provided eFunda.com calculator could be used to get some preliminary output values.

In the end, no matter what gets used for the purpose of engineering calculations, "the final solution and the method chosen must be evaluated for appropriateness".

In my opinion, there is no doubt about your words and piece of advice -- you and other participants in this problem are "good" ...   

http://www.engineering-4e.com

RE: Pressure drop of gas in a pipe line

2

At the risk of being reprimanded for adding yet another post to this lengthy (but interesting) thread, I will just add one of my favorite, old spreadsheets for calculating compressible flow problems.  It has served me well and it responds to what Jim Dean requested.  I hope it helps complement what Katmar and Latex have so ably explained.

RE: Pressure drop of gas in a pipe line

Mr. Montemayor:

It is always good to hear from you and to get your input on the subject matter.

http://www.engineering-4e.com

RE: Pressure drop of gas in a pipe line

Montemayor,

Your advice is always appreciated, as is the spreadsheet.
Thank You.

**********************
"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/

RE: Pressure drop of gas in a pipe line

(OP)
Mr. Montemayor

Thanks for the spreadsheet, much appreciated! Rather than blindly plug my values into it, I'm trying to work through the sheet so I can gain a better understanding of whats going on with the calculations. However, theres a few constants croping up, that I can only guess at what they are.

CELL G25 =C7+((C17^2)/C24*(2*C22*C11/(C9/12)+LN((C26+14.7)/(C7+14.7))))/144/32.2

In this equation for finding P2, I'm assuming that the value of 14.7 is in psig i.e 1atm? However what are the values of 144 & 32.2 refering to, are these simply for converting the units?

Jim

RE: Pressure drop of gas in a pipe line

Montemayor will be able to give you a more detailed explanation of the maths, but very quickly, they're conversion factors for those nasty non-SI units - 144 converts from in2 to ft2, because there are "feet" in the flow, density, and length units and "inches" in the diameter and pressure units. The number 32.2 is the gravitational constant (I think it's called that, because I haven't used it for more years than I can remember) to handle the mix of "pounds mass" in the flow and density values, and "pounds force" in the pressure numbers.

The pressures are given in "psig", so to convert to "psia" 14.7 is added (which is I atm as you have noted).
 

RE: Pressure drop of gas in a pipe line


On the roughness factor (which is important), you might want to consider the fact that the surface smoothness could degrade over time.

RE: Pressure drop of gas in a pipe line

But, would surface roughness inside a flare line be increasing that much over time?

I'd expect much worse for, say, seawater or condensate water in a pipe in continuous service, but is nat. gas in a flare that corrosive?   

Or is it much more corrosive than "simple" raw water?

RE: Pressure drop of gas in a pipe line

Natural gas by itself is not corrosive, but when combined with trace amounts of water, CO2, H2S, fostering chemical reactions that yield carbonic and sulfuric acids, which as you probably already know or guessed, can make short work of a piece of pipe.

As for friction factors changing during the life of a pipeline, I've actually seen one gas gathering systems with rather high gas velocities where the pipe in some areas actually appeared to decrease in roughness over the years.  As sand production was also increasing in conjunction with the wells in that same area, I supposed at the time it was due to a sand blasting effect on the pipe walls.

**********************
"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/

RE: Pressure drop of gas in a pipe line

(OP)
BigInch

I've been reading through the PDF that you posted which shows different examples and equations that are used for calculating the pressure drops/flow rates ect of a compressible fluid. These examples use a effiency factor of a pipe (E) and also a compressibility factor (Z). The spreadsheet Mr. Montemayor posted uses a roughness factor. Which is the more accurate of the two for predicting the pressure drop?

Have I missed something in the math or are the two, two different ways of calculating the pressure drop/flow rate which achieve the same goal?

I know that this thread has turned into a bit of an animal, but I would just like to understand how the two different equations differ - if in fact they do?

Jim

RE: Pressure drop of gas in a pipe line

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/

RE: Pressure drop of gas in a pipe line

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: Calculating specific gravity

Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com

RE: Pressure drop of gas in a pipe line

(OP)
Thanks Katmar/BigInch

Jim

RE: Pressure drop of gas in a pipe line

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
 

RE: Pressure drop of gas in a pipe line

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.

fs = fD/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.

RE: Pressure drop of gas in a pipe line

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

Lij
 

RE: Pressure drop of gas in a pipe line

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

RE: Pressure drop of gas in a pipe line

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

RE: Pressure drop of gas in a pipe line

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.

RE: Pressure drop of gas in a pipe line

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

RE: Pressure drop of gas in a pipe line

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

Regards,

Lij

RE: Pressure drop of gas in a pipe line

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.

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