from the techno stuff department...
Normal vs. Standard Flow
Application Questions and Problems topic
Posted by Nick on 21 July, 2003 - 7:47 pm
I have a customer who wants his gas regulator (that we make) to flow with "normal" litres per minute. So I need to calculate his Cv - but the flow calcs we use in-house are based on 'standard' litres per minute and I cannot find anywhere that explains the difference between 'normal' and 'standard' flow. Does it mean that a normal flow rate is bigger or smaller than a standard one?

I have searched via the net but cannot find any reference to 'normal' litres per minute of gas.


Many thanks - any help is appreciated - Nick

Posted by Jeff LeBlanc on 22 July, 2003 - 5:25 pm

Could he mean standard flow at normal conditions? Sea level, standard temp and pressure?


Posted by Anonymous on 23 July, 2003 - 12:03 am
Standard flow rate is a correction applied to an Actual (or Normal) flow measurement; based on measured temperature and measured pressure of the flow system. The correction is applied using the ideal gas law. Standard conditions, 1 atm at sea level and 77 deg F, are used for comparison.

Again, P1V1=n1RT1 (T in absolute, R or K) for standard conditions. P2V2=n2RT2 is the second equation at Normal or Actual conditions. Let n1 = n2 and solve for V1 = (T1/T2*P2/P1)*V2. Most useful are standard flows, which can be tabled across the plant for direct comparison of various temperature, pressure streams.

Posted by Herman Vogel on 12 November, 2013 - 10:55 am
Standard flow rate is a correction applied to an Actual flow measurement at a given temperature and pressure. The correction is applied using the ideal gas law. We need standard conditions because they actually simplify flows and are used to compare different flows existing at different flowing conditions of pressure and temperature. The analogy is with our monetary standard based on the dollar over other denominations where we compare absolute costs.

However, the problem with Standard conditions is that they have several different definitions depending upon whom you talk to (as seen by the above example). The most commonly accepted definition value is 1 atm at sea level and 59 deg F (used mostly by engineers). Another reference flow condition is called Normal flow, and this is more commonly used by engineers using the Standard International (SI) metric system of measurement. Normal is defined as 1 Bar pressure (which is a virtual metric pressure that represents 0.98692 atmospheres) and 0 C or 32 deg F.

So that ideally: P1V1=n1RT1 (T in absolute, R or K; P in absolute not gage) for Standard conditions of pressure and temperature. P2V2=n2RT2 is the second equation applied at Actual measured conditions. Since n1 = n2 we can solve for Standard volumteric flow or basically volume V1 = (T1/T2*P2/P1)*V2. Having Standardized our flow, it can now be usefully compared against various other flows existing at different temperatures and pressures.

Posted by Gray on 23 July, 2003 - 12:15 am

Good question, but after talking with a colleague, it looks like you've opened a can of worms, and we can agree to disagree on the following standards;

There's the Standard Temperature & Pressure (STP), where calculations are with Temperature at 0 deg C at 1 Atmosphere.

There's also the Normal Temperature & Pressure (NTP), where calculations are with Temperature at 70 deg F (21 deg C room temperature) at 1 bar absolute.

But as far as Normal Flow is concerned, eg Nm3/ h, calculations are with Temperature at 0 deg C at 1 Bar absolute (1 Mol=22.4 Litres).

Confused? I am. But hope someone else on the forum can elaborate. Please can you forwards a answer if you get one. Thanks in advance.


Posted by Bruce Durdle on 23 July, 2003 - 12:24 am
In some cases, "normal" refers to gas volumes converted to 0 deg C and 1.013 barA - "standard" may refer to 15 deg C or 15.5 deg C or 60 deg F.

The only safe way to guarantee that you get it right is to ask your customer what conditions he uses for "normal" and "standard" volumes.

But it's not going to make a lot of difference - with the definition above, 1 Normal litre will be equivalent to 288/273 Standard litres - abut 5%.


Posted by Anonymous on 23 July, 2003 - 10:52 am
My guess is:
Standard is at atmospheric pressure.
Normal is at the actual pressure.

Just a guess.

Posted by Dave W on 23 July, 2003 - 10:21 pm
The difference between normal and standard conditions is the temperature. Here is an explanation that I found.

The most common units of industrial gas measurement are; SCFH and Nm3/hr. Note, that there is a difference between Normal cubic meters and Standard cubic meters. The difference lies in the reference temperature that each specific unit is measured with respect to. Normal meters are measured with respect to the Metric system (0C, 1 atm), and standard meters are measured with respect to the imperial system of standard conditions (70F, 1atm).

1 Normal Cubic Meter equals 38.042 SCF

There are also other defined "standard" conditions at other temperatures (32F, 60F), but normal is always 0C.

Posted by Standard Plan on 23 July, 2003 - 11:06 pm
The values used for "Normal" & "Standard" conditions vary by country and industry. Some places they are the same others quite different. In this country 60, 68, 70, and 75 F are used depending on the industry as are 14.696, 14.7, 14.73 psiA. The most common that I see are;

Standard: 70 F & 14.696 psiA (21 C & 1 Atm)
Normal: 32 F & 14.5038 psiA (0 C & 1 Bar) or, 32 F & 14.696 psiA (0 C & 1 Atm)

Its always best to confirm with the customer; assuming they know!

John Catch

Posted by Dave W on 24 July, 2003 - 2:47 pm
If you do a Google search on normal cubic meters, you should find articles that go into more detail. Gas flow in metric units is expressed as nm3/hr for flow at conditions of 1 atm. and 0 degrees C. (1 m3 = 1000 liters). Imperial flows are expressed as scfm. Here standard conditions are 1 atmosphere and, usually, 70 degrees F. Some standard conditions (e.g. natural gas flow) are taken at 60 degrees F.

Posted by Farhad on 2 April, 2012 - 10:27 am
> I have searched via the net but cannot find any reference to 'normal' litres per minute of gas.

you can find your answer in the below link:

normal liter (NL or Nl or Ndm3)
a unit of mass for gases equal to the mass of 1 liter (0.035 3147 ft3) at a pressure of 1 atmosphere and at a standard temperature, often 0 C (32 F) or 20 C (68 F). Air flow is often stated in normal liters per minute (Nl/min).

I hope you get my answer.

Be success
CSMS Company
Changsha , Hunan, China

Posted by ravichandra on 16 December, 2013 - 9:42 am
normal flow means flow of gas at atmospheric temperature. and standard flow means flow of a gas at specified temperature

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