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

 

[enrjdean]

Hi, the flow rate was specified at 40psig

Jim

 

[Latexman]

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)
L_m = 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

 

[Latexman]

enrjdean,

What reference book are you using?

Good luck,
Latexman

 

[enrjdean]

Hi, I've been using the equations from this link:

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

I've also had a look at to see if I was along the right lines

http://www.pipeflowcalculations.com/pressuredrop/index.htm

Jim

 

[Latexman]

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

 

[JohnGP]

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 (?).

 

[enrjdean]

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

 

[Bribyk]

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

 

[Bribyk]

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

 

[BigInch]

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/

 

[JohnGP]

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

John

 

[enrjdean]

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

 

[BigInch]

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/

 

[Latexman]

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

 

[katmar]

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

 

[BigInch]

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/

 

[enrjdean]

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!

 

[quark]

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.

 

[JohnGP]

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

 

[BigInch]

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/