Questions about MMSCFD 1

[combatlan]

Hello all, I'm pretty new in oil and gas I have few questions any advice is much appreciated:

Question 1
Information of natural gas (assumes methane) provided by operator i.e. 5 mmscfd, 50 bar, 80 C am I right to say that it is referring to standard condition at 1 atm, 60 F not the specified conditions?

Question 2
So if I want to convert to mmcfd I need to do (PiVi)/Ti = (PfVf)/Tf
With P = Pressure, V = Volume, T = Temperature, i being initial and f being final,
which give me 0.52 mmcfd.
Is this the actual flow rate at the specified temperature and pressure, need confirmation please?

Question 3
What about converting to kg/s using gas constant?

Thank you

 

[LittleInch]

1), That's what the s in mmscfd stands for.(Standard)

2) This is for an ideal gas and gets you close, but not close enough for real gases and proper calculations. You need to look this up and do it properly. I think you've got a decimal point wrong and it's circa .0.01 mmcfd.
Remember temp is in degrees K and pressure in bara. Try not to mix units. It just confuses everyone.

3) Depends on the density of your gas.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

On question 1. "Standard" is anything but standard. For gas measurement in the U.S. you use 14.73 psia and 60°F. For gas measurement in the EU you use 100 kPa and 15C (14.5 psia and 59°F). In New Mexico in the U.S. you report standard volumes at 15.025 psia. And so forth. Standard conditions need to be written into every contract and into every regulation. The first step is always to determine the "standard" conditions. Then you can determine a density at standard conditions and can convert to actual conditions (only useful for calculating velocity) or mass flow rates.

On question 2, what the hell are you converting from? I always calculate density at standard conditions and at flowing conditions, then if I have a standard flow rate, I can get an actual flow rate by q_std*ρ_std/ρ_actual

Density = [P*SG]/[R_air*T*Z] Don't leave out the compressibility, even at very low pressures, you can't afford the 2-30% error that ignoring it will incur. Once you know density (either actual or standard) then mass flow rate is simply q*ρ. Work with density, not pressures/temperatures/compressibilities/gas constents--I can't tell you the number of times I've fixed stupid errors that come from not being able to do simple algebra.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[katmar]

LittleInch and zdas04 have nailed the explanation. Let me add an estimate of 0.12 MMCFD at actual conditions to make this a multiple choice question. Methane is a gas that is highly non-ideal and you should always take the compressibility into account, particularly at or below ambient temperature.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[combatlan]

Thank you all for the reply and helping me understand.

 

[MortenA]

As a matter of fact: gas volumes at "standard" condition is another word for a number of moles of gas. The Danish enegy agency btw. uses the Nm3 definition at 1.01325 bar (a) and 0ºC - equivalent to 0.0446158 kmol gas. A SCF is 0.0011981 kmol. ISO for some strange reason chose to use 1 bara as the reference pressure - this is not commonly adopted. In general its rather country specific what reference you prefer to use (Sm3/Nm3/SCF).

 

[zdas04]

Morten,
It is way worse than that. Queensland has a different number than NSW which is different from SA. This means that a field like the Cooper Basin that has production in all three must store data in three different standards (four if the sales contract specifies its own). Most people solve this by storing "standard" volumes at known pressure and temperature (usually one of the standard variations) and then convert on output. I am always amazed when companies get the conversion wrong, but it happens all the time.

As to "number of moles", standard "volumes" are a good surrogate for both moles and mass. I did an exercise this morning where I got a change in mass flow of 21% and a change in "volume" flow in SCF of 26%, red flags went off everywhere and I quickly found my bonehead error since a 21% change in mass must be a 21% change in moles and a 21% change in SCF. That is anything but true in actual volume flow rates (which changed by nearly 50%)

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[MortenA]

David,

I agree (and i think we have been there before ;-) ) and i general when i was a consultant i always added a statement of the actual "standard" reference conditions. So that the client could object if he preferred a different reference. WRT mol/mass - i think that IF you change composition (significantly) then mass is no longer the same as standard volume unit (it derives directly from the ideal gas law - and thats mol not mass)

 

[zdas04]

R(gas)=R(universal)/MW(gas)=R(air)/(MW(air) / MW(gas) = R(air) / SG

so

P*V=n*R(universal)*T*Z ==> P*V = m*R(gas)*T*Z ==> P*V*SG = m * R(air) * T * Z

Yep, looks like the same for moles or mass.



David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[MortenA]

Under the assumption that Z is the same for air and the gas that you are considering. So if you know your MW then yes, its the same, if you dont then its always mol (i think we digressed a bit now). Not thats a proof of my point in any way - but in HYSYS you can key in the molar flow in e.g. kmol/hr, mmscfd, Sm3/hr etc - but not the mass flow...

 

[MortenA]

a bit too trigger happy there - The first sentence doesnt account for much the rest from So if yopu know your MW... is what i intended to post.

 

[zdas04]

My point was that I almost always know SG, and from that I can get MW or moles, but it is an extra step since I almost never know MW or moles. The "Z" in the equations above are intended to be compressibility for the gas in question, not for air (which is close enough to 1.0 to ignore).

It has been a really long time since I had any degree of mastery of Hysys (think mainframes, think CICS) so I can't really speak to anything that the current owners (Aspen Technologies) do. My dim memory is that back in the dark ages the primary input for flow was mass/unit time. Moles were a secondary option and volume flow rates (even at standard conditions) were not allowed.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[TD2K]

Some day I hope to see Dave's slide where he has the various "standard" pressures and temperatures he has come across. I think he mentioned he's up to two slides worth now.

 

[zdas04]

Ask and yee shall receive. I pared it back to one table because by the time you go through a half dozen versions you are beating a dead horse. There are hundreds of variations of standard. My favorite is still gas measurement in ISO--1 bar (100 kPa, 14.5 psia) and 15C (59.000 F) makes so much more sense for an arbitrary value than 14.73 psia (101.56 kPa) and 60F (15.56 C).

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[TD2K]

Thanks Dave.

 

[25362]

To zdas04, just for information, in your example on nitrogen, using densities tabulated by NIST, the result I found differs by ~2%: ~641 vs the reported 654 cubic feet...

 

[zdas04]

I'll take a look at that before the next class (I presented that slide on Monday this week)

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[katmar]

David has stated that he is assuming Z=1.0 and that he can use the ideal gas law. The tabulated value I have for Z is 1.021 so that would explain the discrepancy. But for an illustrative example where the assumptions are stated I wouldn't worry about that small deviation from real gas behavior.

While we are knocking David ;-) - The gas constant for nitrogen (53.355) seems wrong. It has no impact on this calculation because it appears in both density calculations and would be "cancelled out" in the final ratio calculation. The value I have for Nitrogen is 55.165 ft.lbf/lbm.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[zdas04]

That is kind of what I was thinking, the magnitude of the error was the right magnitude for the difference between ideal and real.

Katmar, you have to read closer. It wasn't 53.335 it was 53.335/SG

R(gas)=R(universal)/MW(gas)

SG = MW(gas)/MW(air) --> MW(air) = MW(gas)/SG

R(air)= R(universal)/MW(air) --> R(universal) = R(air)*MW(air)

Substituting:

R(gas)=R(air)*MW(air)/MW(gas) = R(air)/SG

R(air) = 53.355 ft*lbf/(lbm*R)

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[katmar]

Sorry, my bad - I should have seen that.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"