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Conversion between Nm3/hr to Sm3/hr and vice versaHelpful Member! 

chemprocess (Chemical) (OP)
28 Oct 04 0:16
Hi all,

I am just curious when do you use Nm3/hr and Sm3/hr. How do you convert between the two units?

Please help.

Thank you.

tickle (Chemical)
28 Oct 04 0:46
See previous posts on this topic on Eng Tips
Helpful Member!  katmar (Chemical)
28 Oct 04 7:42
Unfortunately neither Nm3 or Sm3 (or Standard cubic feet for that matter) are complete definitions in themselves. It is essential to know the temperature and pressure conditions that the author believed were "Normal" or "Standard".

Once you have the temperature and pressure references it is a simple matter to do the conversion.
25362 (Chemical)
28 Oct 04 9:57

Messrs tickle and katmar are both right.

Some experts consider NTP and STP -reference conditions- as equivalent terms. Notwithstanding their expertise, they should clearly indicate T and P to enable making conversions to differing T or P values.
Razif (Chemical)
30 Oct 04 15:07

The importance of the defining T & P values are very important. I had one experience where there was a dispute on the amount of hydrogen exported to another plant from one plant in an integrated petrochemical complex. It turns out that one plant was designed by a European company and the other was designed by a Japanese company. They had their definitions of  Normal (NTP) and Standard (STP) conditions in reverse (Don't ask me who got it right!). This led to inconsistent basis for calculating the mass of hydrogen transferred from one plant to the other.

It is easy to assume that there is an internationally accepted 'standard' for STP or NTP, but this is'nt so. Looking back at your university text books may not help either in this case. You must refer to the design basis of your respective plants.


kingsss (Chemical)
1 Nov 04 9:30
What I have experienced so far Nm3/hr is the volume flow considered at 0°C and 1 bar (absolute).
Sm3/hr is the flow taken at 15°C and 1 atm (1,013 barA).
However, the 15°C is sometimes subjected to dicussion: 20°C is taken as well.

Conversion can easily be made by using (P*V)/T = C

Regards, Kingsss
Guidoo (Chemical)
1 Nov 04 9:51
Just to make your life easier: an extensive thread on this topic can be found on Thread378-97454
Homayun (Chemical)
3 Nov 04 5:37
The standard conditions are as follows:

Temp.: 15 degr C, Press.: 1.01325 bara

THe normal conditions are as follows:

Temp.: 0 degr C, Press.: 1.01325 bara

How to convert between the two in obvious.

katmar (Chemical)
3 Nov 04 13:29

Those may be your definitions of Standard and Normal but there are many others.
Homayun (Chemical)
4 Nov 04 4:00
No katmar,

These are to my knowledge the widely used conditions, I haven't invented them! So I don't know what you are talking about.
25362 (Chemical)
4 Nov 04 5:02

Homayun, sorry to tell you that standard and normal conditions have various definitions as katmar says. I suggest you visit the following threads to get an idea of the situation:


katmar (Chemical)
4 Nov 04 7:17

I must admit that it was only relatively recently that I discovered that Normal is not a universal definition. The IUPAC (International Union of Pure and Applied Chemists), which is a well respected organisation, defines it as 100 kPa. Most older books use 101.325 kPa.

Standard conditions are most frequently used in the USA. {/Bait on} I don't want to be rude, but until a few years ago most engineers there didn't know what degrees C were, so they would not have made their definition in those units. I won't tell you what most American engineers think a bar is, but as a hint, the Absolut part is associated with Vodka.{/Bait off}

The bottom line is - the gas conditions need to be defined for every calculation, contract etc where they are used.

quark (Mechanical)
4 Nov 04 8:06
Katmar is right. I know atleast three temperature conditions for STP with 0(no..not NTP), 15 and 160C, in use.

25362 (Chemical)
4 Nov 04 8:38
Adding to quark's list, at least until a few years ago:

The American Gas Association: 60oF (15 5/9oC) and 30 inches (762 mm) of mercury.

The Compressed Gas Institute: 20oC (68oF) and 1 atmosphere.
Homayun (Chemical)
4 Nov 04 10:19
But this is not good to have different conditions with same name: standard  / normal conditions ! This makes things complicated I would say at least, and in some cases even dangerous.
25362 (Chemical)
4 Nov 04 10:45
Corect !! That's the reason for the obligation to clearly state what are the P, T conditions every time to avoid confusion.  

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