natural gas compressibility factor question
natural gas compressibility factor question
(OP)
Hello all, I am sizing a small natural gas line with a starting pressure of 40psi for an industrial application. It will need to flow 8300cfh, the pipe run is 3500'. I was going to use the Renouard equation, then I read in a piping design handbook that the compressibility factor "Z" doesn't really need to be accounted for on this short of a run, and with pressures this low. Is this correct? Is there a more appropriate formula for this situation?
Best regards.
Best regards.





RE: natural gas compressibility factor question
Or, break it up into smaller sections (say 10 sections of 350 feet) and apply Darcy-Weisbach successively with different States 1 and 2 for each section, and add dP1 + dP2 + ... dP10.
Regards,
SNORGY.
RE: natural gas compressibility factor question
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: natural gas compressibility factor question
There are two considerations regarding Z. The first is that it determines the actual density of the gas at the start of the pipeline. The second is that Z may change over the length of the pipeline, thus complicating the prediction of the density at each point along the pipeline.
The first point applies to all pipelines - whether they are short, long or intermediate. In all cases you need to know the actual density at the start of the run (either as a density or as a compressibility, depending on the pressure drop equation you use). This knowledge may be as simple as in this case where you know Z=1, or it may be very complex.
The second point (Z changing along the pipeline) is more a function of the physical changes in the gas (pressure and temperature) than in the actual length of the pipe. A very short pipe going from very high pressure to a vent or flare may well still require consideration of the compressibility changes. On the other hand a very long line at relatively low pressure may not need compressibility to be considered.
So overall I would say the first part of that quotation (regarding pipeline length) is misleading, but the second part (regarding low pressures) is correct.
Just an aside - many experienced pipeline engineers will never have heard of the Renouard equation. I had to Google it myself, and as usual found the answer right here on Eng-Tips. It is apparently an empirical equation in the same mould as Panhandle or Weymouth and used mainly in Spanish and Portuguese speaking countries. These empirical equations are great for specialists in a particular industry who come to learn their strengths and weaknesses, and to know where and when they can be applied. For non-specialists it is better to use Darcy Weisbach which has a much more general applicability, and requires less industry knowledge in order to get to the "right" answer.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: natural gas compressibility factor question
Regards.
RE: natural gas compressibility factor question
At high pressures (I defing "high pressure" as above 150 psia) I find that it doesn't matter so much which equation you use, they all give acceptable results consisten with their underlying assumptions. Below 50 psig, the "incompressible" assumption in all the equations becomes a big problem. The 10% pressure drop mentioned above is a response to people trying to ensure that they satisfy that assumption. I've found that if I ignore compressibility, breaking up a long pipe into sections does not change the answer and is a waste of time. If I recalculate compressibility (based on average pressure) for each section then I get very different answers for the sum of short sections. I find that ignoring compressibility creates a very large error at low pressures. Even though the change from 40 psia to 36 psia seems to be really small, the cumulative effects are large.
For example, let's assume that I start at 40 psia and predict that I'll end at 20 psia (50% pressure drop) using a single step. This violates the incompressible flow assumption so I break the line up so that no segment has more than 10% pressure drop. For one set of conditions I found that the pressure at the tail really worked out to 10 psia instead of 20 psia which is a huge operational difference (vacuum operations vs. positive pressure).
If you are talking about a flow line going from 2,000 psia to 1,850 psia (7.5% drop), then calculating a compressibility for the average pressure gives you an excellent match to field data. Going from 175 psia to 25 psia (still 150 psi drop, but now 86% drop) gives you a meaningless answer in one step.
We need to pay attention to the underlying assumptions of the arithmetic we use, and a low pressure it is disturbingly easy to violate the incompressible flow assumption inherent in all of the mainstream flow equations, including especially Darcy Weisbach.
I never ignore compressibility, but at low pressures I pay very special attention to it.
David
RE: natural gas compressibility factor question
The isothermal and adiabatic equations are simply the DW equation integrated for the change of gas density using either an isothermal or adiabatic model.
Another very loose use of language in my post above was the failure to distinguish properly between "compressible flow" and the "compressibility factor - Z". I agree with you absolutely that in gas flow you should always include the compressibility (but not necessarily the compressibility factor).
Because I have the compressible versions of DW built into a computer program it is no extra work for me to always use the compressible equations when dealing with gases, so I never treat a gas as incompressible unless it is for some specific purpose. On the other hand it is relatively rare for me to have to worry about the compressibility factor Z in the industries where I usually work.
Thank you very much for prodding me into this realization. I hope mod231 is still following the thread and re-reads my earlier post in this (hopefully) clearer light.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: natural gas compressibility factor question
Again, language can be tough. All of the equations mentioned above are incompressible. The math for compressible flow is a LOT tougher. Most liquids are inherently incompressible to a large extent (nothing is truly incompressible, but most liquids are close enough) and if you drop pressure a significant percentage, density will change very slightly--generally the density change in a liquid is far outside the accuracy of the equations used and can be safely ignored.
Incompressible flow of an intrinsically compressible flow is a different kettle of fish. When you say "use the compressible equations" I think you mean "use the incompressible equations that have been tailored for compressible fluids". The compressibility factor goes into the calculation of density, Reynolds Number, friction factor, and it is explicit in several of the incompressible flow equations.
I work with compressible flows in ejectors pretty often and I wouldn't wish those calculations on anyone.
David
RE: natural gas compressibility factor question
Thanks
RE: natural gas compressibility factor question
David
RE: natural gas compressibility factor question
Equation 1-6 assumes the perfect gas law, which allows for the gas density changing along the length of the pipe as the pressure drops (temperature assumed constant). Thus it definitely takes into account the compressible nature of the gas (i.e. density changes significantly with a change in pressure).
In Eq 1-6 one of the input values is the specific volume of the gas at upstream conditions. This value could (actually should) be calculated using the compressibility factor Z and as long as Z does not change along the pipeline then it will give the correct result. Just to make this absolutely clear I will state explicitly - it allows the density of the gas to change along the length of the pipe (i.e. compressible flow) but does not allow the compressibility factor Z to change.
The difference in complexity between Crane Equations 1-6 and 1-7 is so small that there is no point in making the additional simplifying assumption required (i.e. neglect acceleration) for 1-7 if you are using a spreadsheet or a program.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: natural gas compressibility factor question
For NG the compressibility factor Z can be neglected below 250 psi because for temperatures between about -20 deg C and 400 deg C the compressibility factor Z does not vary from 1.00 by more than 1%.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: natural gas compressibility factor question
I misunderstood what people meant when they said compressibility is not an issue.
Thank you
RE: natural gas compressibility factor question
Equation 1.7a in Crane is almost the SCF/hour version of the GPSA equation 17.15 (which is in SCF/day). The three differences are (1) the GPSA equation includes an "effeciency" term; (2) GPSA has "compressibility based on average pressure"; and (3) GPSA includes a T(b)/P(b) to allow for base conditions other than 60F and 14.696 psia. The effeciency term magically reappears in Crane's version of Panhandle A (eq 1.9), but not in Weymouth (eq 1.8). In GPSA it is in both Weymouth and Panhandle A.
The assumptions for eq 1.6 in Crane include GPSA doesn't have that assumption.
The tenth section of Chapter One of Crane is titled "Principles of Compressible Flow in Pipe". That is a source of incredible confusion in the industry. They further confuse the issue by saying that equation 1.6 is appropriate for pressure drops greater than 40% of inlet pressure, but the assumptions for eq 1.6 includes The only way the friction factor can be constant is if the Reynolds Number is constant which is only true if the density is constant. If I halve the pressure, I approximately halve the density, which just about halves the Reynolds Number, so if I have an ε/D of 10^-8 (smooth pipe) and a Reynolds Number of 10^6 at the head of the pipe, a 50% reduction in pressure would change the friction factor by 13% which violates the assumptions of eq 1.7 (and by extension 1.7 and 1.7a).
The equations for incompressible flow of compressible fluids all came from the Bernoulli Equation. They are very useful and allow us to do many things that we couldn't have done without them, but they need to stay withing their limitations.
I'm starting to feel like I've moved a useful conversation way too far into the weeds and I apologise. In my defence, I work with a lot of engineers in a lot of companies who feel that it is reasonable to use Panhandle A in 2-inch pipe or pipe with a Reynolds Number < 5X10^6 or use Weymouth for a line with pressure dropping from 900 psig to 500 psig (Weymouth was developed for pressures under 130 psig, and none of the equations work for a dP much greater than 10%). I keep trying to get people to acknowledge the underlying assumptions in the math they use.
David
RE: natural gas compressibility factor question
I must admit that when I wrote in my first post in this thread that Panhandle etc are best left to the experts in the field it was people like you and BigInch that I had in mind. You know what efficiencies to use and when to use which equation. But for generalists like me it is best to stick to Darcy Weisbach (or at least my expanded definition of DW, which includes the compressible versions).
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: natural gas compressibility factor question
RE: natural gas compressibility factor question
When I said I was just plain wrong.
Reynolds number is usually stated as:
Re= Dvρ/μ
ρ does change as I said, but velocity changes in the other direction by the same amount. Reynolds number is constant down a pipe. That is easy to see if you rearrange the Reynolds Number equation as:
Re=k*W/(d*μ) (where "k" is a constant for unit conversion and to include π)
Since "W" (mass flow rate) is constant everywhere in the pipe as long as flow is not added or removed, then except for minor changes in viscosity with pressure changes Reynolds Number is constant.
I really hate the taste of crow, but I dislike having wrong information associated with my handle even more.
David
RE: natural gas compressibility factor question
http:
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: natural gas compressibility factor question
mod231,
Give a shot to the calculator at the link below (it relies on the isothermal gas flow assumption).
http://www.pipeflowcalculations.com/gasflow/
I would also give a read to this article, to better undestand limitations of the assumptions on which eqautions are based.
www.aft.com/news/pdf/CE%20Gasflow%20Reprint.pdf
RE: natural gas compressibility factor question
I'm a bit puzzled by your statement "For NG the compressibility factor Z can be neglected below 250 psi because for temperatures between about -20 deg C and 400 deg C the compressibility factor Z does not vary from 1.00 by more than 1%" from your post (4 May 11 10:04).
I've run your Uconeer to calculate compressibility factor for methane and played with pressures and temperatures in the ranges you've mentioned and got results which exceeds the 1% threshold.
Just to quote an example:
T1 = 0 °C
P1 = 200 psia
Z1 = 0.9598
Could it be that this difference is due the fact I used methane instead of NG?
RE: natural gas compressibility factor question
This GPSA NG chart was for a MW of 18.85, so I think the value for pure methane should be very close. If anyone wants access to the original GPSA book the single page I have is labelled FIG 23-6 and page 23-14.
Thanks for pointing this out - it certainly underlines the need to double check anything you read on the internet! My apologies to anyone else who was confused by my mistake.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: natural gas compressibility factor question
The methane line is self-explanatory, CBM is 8% CO2, 92% methane. "Sweet Gas" is a 1173 BTU/SCF mix of hydrocarbons. Sour Gas has 3% H3S and 7% CO2 (963 BTU/SCF).
I'm not sure what EOS REFPROP uses but it has consistently given me good results. UCONEER uses Peng-Robinson which is also very good For the conditions the Ione entered, REFPROP yields 0.96752 instead of UCONEER's 0.9598 so the two methods are within 0.7% of each other. I find the attached graph useful for getting orders of magnitude quickly, but an EOS or something like Hall-Yarborough are a lot better for the heavy lifting.
David
RE: natural gas compressibility factor question