Converting Density from kg/Nm^3 - Pipelines, Piping and Fluid Mechanics engineering (2024)

To convert kg/m3 to lbm/ft3 is straight forward. Convert kg to lbm, then m3 to ft3.

The normal part you leave as is. So, you now should have the kg/Nm3 --> lbm/Nft3.

If you want to change normal condition to your actual process condition, you can do so using any of the equation of states, graphs, correlations, etc. for dry air.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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kg/Nm³ is already a density. One should specify the pressure and temperature, which in this case would probably be 0^oC and 1 atm. Conversion to lb/cf at the same normal (or standard) conditions involves applying factors such as 1 ft³ = 0.02832 m³, and 1 lb = 0.4536 kg.

For changing the selected standard conditions, refer to thread798-106556 and the links therein.

In order to understand how to do the conversions go to Milton Beychoks site at
http://www.air-dispersion.com/formulas.html

To do the actual conversions you can download a free calculator that does it all automatically.Click on the link in my signature below and follow the links to Uconeer. Once you have installed and run Uconeer click on the fan icon in the toolbar and the conversion calculator will open up. But make sure you understand what you are doing before you use the calculator. GIGO.

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

Assuming that ideal gas behavior applies, and that the Normal cubic meter was defined as being at 0 °C and 1 atmosphere of pressure, you can divide kg/Nm³ by 16.928 and you will obtain the density as lb/SCF where the SCF is defined as being at 60 °F and 1 atmosphere:

lb/scf = (kg/Nm³) ÷ 16.928

If you want your density at some other temperature and pressure:

lb/cf @ T and P = (lb/scf) × (520 ÷ T) × (P ÷ 14.696)

where:
T is the temperature in °R = 460 + °F
P is the absolute pressure in psia (pounds per square inch absolute)

If you want to leave your density in the metric units of kg/Nm³ but want to convert it to anothertemperature and pressure:

kg/Nm³ @ T and P = (kg/Nm³) × (273.15 ÷ T) × (P ÷ 101.325)

where:
T is the temperature in °K = 273.15 + °C
P is the absolute pressure in kPa (kiloPascals absolute)

Are you guy's sure about this?

Nm3 is not a unit of volume, it's 1/22.4 of a kmol of gas (which just happens to occupy 1 m3 at ntp). Hence 1.3138 kg/Nm3 is equivalent to saying Molecular Weight = 29.43. Isn't it?

Nm3 was defined to be Normal meter cubed in the OP. Hence, it is a unit of volume.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

So what's the volume of 1 Nm3? We don't know - we need more information.

But we do know how many molecules are there.

Beg to differ Ashereng, the units are kmols.

I agree with sethoflagos that the units kg/Nm³ are equivalent to Molecular Weight. Nm³ defines an amount of matter, which is the number of kmols. If you divide the number of kmols into the total mass you get MW.

One small disagreement. Since the definition of Normal conditions was redefined by IUPAC to be based on 100 kPa and not 1 atm, a Nm³ is now 1/22.681 of a kmol of gas.

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

1.3138kg/m^3 is 0.0820 lbm/ft^3.

Sloth4Z, be careful with your units."N" in metric means newtons which is a measure of force.Saying "normal meters" is meaningless in the metric system; there is no abnormal meter, for example.

Also, the correct measurement for mass in the imperial system is "slugs".Since "kg" is correct for the metric mass measure, I think it proper that you should list the density as "slugs/ft^3".This is not to say that "lbm/ft^3" is wrong, just be aware that the computations involving mass would be out by a factor of gravity acceleration.I prefer the metric system for this reason, then convert the answer to imperial if the need be.

Trick question!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

The conversion by co*ckroach from kg/m³ to lb/ft³ is correct

if the P,T conditions (normal or standard) are the same. Mass wouldn't change but volume might. Refer to thread798-106556, and to the above post by mbeychok.

Huh.... there is something

Probably a mix-up of units

Specific mass (density):kg/m³ should become lbm/ft³

Specific weight: N/m³ should become lbf/ft³

N stands for Newton, metrics

Regards

The use of the capital "N" to represent Normal and Newton is confusing. Strictly we should put a point (full stop) between the units if there were such a thing as a Newton metre cubed. It would be written as N.m³

On the other hand, when we talk Normal metre cubed there would be no point involved and it would be written as Nm³.But this convention is so sloppily applied that you can't really rely on it and you have to take the context into account to determine whether the N stands for Normal or Newton.

I'm 100% with co*ckroach in being against any system of units that involves the dreaded g_c. It is purely by coincidence that g_c has the same numerical value as the acceleration of earth's gravity in the US customary system of units. This coincidence leads to untold confusion. But being realistic, we engineers will have to deal with g_c for a while yet.

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

Just a thought: it would be easier to understand if mass would be expressed in kg, and weights in N.

A man with a mass of 70 kg on Earth would still have the same mass on the Moon.

His weight (a force=m.g) on the surface of the Earth would be
9.8 m/s²×70 kg = 686 N.
On the Moon his weight would be 1.6 m/s²×70 kg = 112 N, and on Mars it would be 3.74 m/s²×70 kg = 262 N.

Thus a rope supporting a maximum weight of 350 N couldn't support that man on Earth, but it could on the Moon and on Mars.

Good point 25362.

But the sitution is actually worse than having to worry which planet you're on. The gravitational 'constant' gc is tied to sea level at latitude 45 or thereabouts. Now I live within a stones-throw of the equator where gravity's a little (not much, but a little) stronger.

Is my psi the same as your psi?

You know where you stand with N/m2 !

Katmar, thanks for the info on ntp, though I know nothing of this IUPAC of which you speak. Which pub do they hold their meetings in?

Sethoflagos,

The gravity at the equator is a little, not much, weaker -not stronger- because of the slight increase in distance from Earth's center.

From the CRC Handbook of Chemistry and Physics: the mean acceleration of gravity is 9.78036 m/s² at equator vs 9.83208 m/s[sup]2]/sup] at poles.

Quote (sloth4z):

I know that Nm^3 stands for "normal cubic meter" which means it is at standard temperature and pressure,

Guys, sloth4z clarified that the N stands for normal.

I still think he is confused about what normal is, and how to get it to his working conditions, or to standard.

He is converting kg/m3 to lbm/ft3, and then try to figure out how to deal with the difference in temp and press.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

Quote:

The use of the capital "N" to represent Normal and Newton is confusing.

The metric system does not use Normal in its units.

Quote:

it would be easier to understand if mass would be expressed in kg, and weights in N.

it is mandatory in the metric system.

Quote:

Sethoflagos

Now I live a stone-throw of the equator where gravity's a little (not much, but a little) stronger.

The point is that gravity is weaker (not stronger) at the equator.

The number was listed as "density" with the units "kg/Nm3".After doing some research on the internet, I discovered that Nm3 can mean "normal cubic meter".Apparently the "normal" denotes that the density is the density of the gas at standard temperature and pressures (which weren't listed in the original list).The english equivelant is scf (standard cubic foot).Most of the website were more concerned with flowrates.

My problem wasn't with converting from SI to English units.I am comfortable with my abilities in this area.My concern was calculating the density at other temperature and pressures.

As an engineer you should not take every thing for granted, especially from the internet

Our academic books are far more reliable resources, because they have been written and revised by technical knowledgable people.

Every lunatic is free to write some rubbish on the internet and claim it is the truth.

This normal cubic meter is most probably the translation of some student who don't know the difference between an inch and a decimeter. I could do a guess the nationality of this person and within 4 attempts I would have the right one.

Normal cubic meter has been discussed and clarified numerous times and is NOT a mistranslation:
http://www.air-dispersion.com/formulas.html#standard

I came across NM³ so many times, particularly dealing with pneumatic accessories (manufacturers from all parts of the world), that I don't mistake them to be alphabets in wrong order even if the exponential is missing.

The simple conversion given by Milton is what the manufacturers consider. It is volume of the gas at normal conditions.

Quote (Sethoflagos):

Are you guy's sure about this?

Nm3 is not a unit of volume, it's 1/22.4 of a kmol of gas (which just happens to occupy 1 m3 at ntp).

I don't see any striking coincidence in it. As per Avagadro's law, one gram mole of any gas occupies 22.41 liters volume at 0⁰C and 1.013 bar. As most of the members agreed NTP as 0⁰C and 1.013 bar (in SI units, in many previous threads), 1 NM³ can be, without doubt, considered as the volume occupied by 1/22.4 kmol gas and not the other way round.

I am very surprised at the number of people reponding in this thread who seem to think that a Normal cubic meter is some new exotic expression, or somehow a misinterpretation of Newtons, or that it is somehow related to molecular weight.

Normal cubic meters as a volumetric measure of gases have been in worldwide usage for many, many years.All that has changed is that the set of reference temperature and pressure that applies to a Normal cubic meter has changed ... and there is no longer any universally accepted set of Standard or Normal reference conditions of temperature and pressure for gases.

In a Wikipedia article at http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure there is a tabulation of the standard reference temperature and pressure conditions used by 15 different organizations including the ISO, NIST, ISA, OPEC and the IUPAC (both the IUPAC's old values of 0 °C and 1 atmosphere as well as the newer values of 0 °C and 1 bar) among others. A careful reading of that article will make it clear that there just is no universally accepted references conditions of temperature and pressure any longer.

It is not often that a poster is able to prove his point as clearly and as unequivocally as svanels has done here. A new bench mark has been set.

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

I support Katmar on this one and his earlier point regarding "Nm" would most likely be confused for "newton (N) and meter (m)"

Correctly "Nm" would be the units associated with torsion, that is, a moment.So I disagree with earlier comments that "Nm" is typical in the metric system.Quite simply, kg/Nm^3 would not balance out dimensionally consistent to density measurement.But this point has already been raised and correctly so by Katmar.

You cannot bastardize the metric system."Normal" is not a proper SI related term under the present convention.Also, "N" does not stand for any prefex associated with a linear measure, milli, mega, hexa, nano for example.Trying to rationalize it as such is simply wrong!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

For what it's worth, and in support of Milton Beychok's point, I have used kg/Nm3 since 1961 as a measure of the standard volumetric density for gas, referred to a base temperature of 0 C and a base pressure of 1 standard atmosphere.At BIPM (Shell), usage of Nm3 for volume measurements goes back to the earliest days after the adoption of the SI system.

It appears from reading this thread that those unfamiliar with traditional usage of the Nm3 terminology (for the volume corrected to standard conditions, especially in the petroleum and gas industries) find it very confusing, as it does appear to violate the strictest nomenclature rules for the SI system.This is a pity, but this is a highly entrenched convention going back at least 45 years, I believe, so it's unlikely to go away even in places deeply committed to the SI system.

Svanels:

Quote:

This normal cubic meter is most probably the translation of some student who don't know the difference between an inch and a decimeter.

As Umesh said in his posting on this thread, the Nm³ has been in world-wide usage as a measure of gas volume for over 40 years and still is in wide usage.The world existed for centuries before the SI system of units was in being. Newton and Einstein came up with their outstanding and innovative scientific contributions before the SI sytem existed.We designed and/or invented bridges, jet aircraft, nuclear power plants, radio, electric lights, telephones and etc. before the current set of SI unitsexisted.

Just because one is unaware of a particular usage of measurement units (or too young to be aware of that usage) does not mean that one should deride or make fun of those who are aware the usage.

I just did a search on Google for the keyword "Nm3" and got 586,000 hits ... a great many of which deal with gas volume amounts. So it is clearly obvious that a great many people are aware that usage.

The SI metric system provides a very useful, consistent set of units. But rules such as the SI system should not be endowed with the fixity of rock-ribbed law. We should not be obsessive about such rules and we should have enough elasticity to tolerate other usages that have been in widespread use for decades.

Milton you didn't get my point
Normal is not part of the units like kg, Watt, Newton, Ampere and many others.
If you feel offended about my statement about the Internet, I feel sorry for you.
Actually I have a copy of your post about the balance of cooling tower pinned on my board behind my desk.

Not because Nm is on the Internet, and have 30 millions hits, it will invalidate the ISO rules which had the participation of a lot knowledgable and respected members of the engineering community and also have contributed to the ASME, ASRAE and API standards.

Websites and calculators are made by programmers, if they do not have guidance of engineers like us, they will make a mess of it.

Too many times I have seen formulas in an excel spreadsheet with some "magical conversion factor" wich are an afront to all laws of physics.

About the term!! normal is widely used in the catalogues of Atlas Copco, Ingersoll Rand and many others. In fact not being aware of the term "normal" makes it almost impossible to size an aircompressor, but that does not promote normal to unit for calculation.

Maybe I am pissed off because just this week I had to explain to some s why the receiver of an instrument-air watercooled compressor was filling with water. The humidity of surrounding air is almost 82% and the trap is undersized. The argument that it have been working for 1 year, thus it is not undersized I call BS. The iceberg was growing during 1 year and just surfaced.

Correction

Quote (svanels):

Not because Nm is on the Internet, and have 30 millions hits, it will invalidate the ISO rules which had the participation of a lot knowledgable and respected members of the engineering community and also have contributed to the ASME, ASRAE and API standards

Should read

Quote:

knowledgable and respected members of the engineering community and who also have contributed

to avoid chauvinistic reactions

No one has, near as I can tell, promoted "normal" as a unit.

Rather, it's simply a shorthand adjective for STP

Standard conditions of Temperature and Pressure

Regard

See thread378-97454 in particular Montemayor's atticism of April 22, 2004.

Quote (Ashereng):

Nm3 was defined to be Normal meter cubed in the OP. Hence, it is a unit of volume.

There are the "right way" and the way the person is using.

The OP stated that Nm3 was to mean "normal m3". He defined it as volume.

So, I took it as a volume. Is it right? Does it matter? If people have been using Nm3 as a volume for 45 years, then they believe it is useful. Right or wrong, SI or not, they use it.

We as engineers also need to be aware that not everyone, every industry, every location, uses the same "conventions" of units.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

Irrespective of the professed irritation of some about the ubiquitousness of the term "Nm3", the fact remains that there are innumerable existing, immutable, and legally binding gas suppy contracts all over the world that specify use of gas volumes in Nm3.

Unless these contracts are abrogated unilaterally and replaced by something more aesthetically pleasing to the purists, the rest of us will simply have to continue to muddle through and continue using that unit of volumetric measure, at least in the gas industry.

Only a fool will defend use of deliberately confusing terminology; however, usage of this term precedes the careers of the vast majority of chemical engineers.When the term was first coined, it made a lot of sense, firstly because it was logically equivalent to specifying flows in molar units, and secondly because it replaced other less "standardized" usages.I used it in process calculations for over 25 years as it made a lot of physical sense, at least to me.

Finally, since this term was not coined merely to add to the ranks of the perplexed, I humbly suggest that we close out this thread.