13 replies to this topic

[panoska]

Hello everyone,

 

I would like to ask your view about what is the usefulness of using standard conditions to express volumetric flows?

 

 

For example, when we prepare PDS for compressors, we use standard volumetric flow, what is the real benefit of it?

 

The standard volumetric flow is dependent on the actual volumetric flow, which means that the standard volumetric flow at the inlet and at the outlet of the compressor is different.

 

 

 

[Bobby Strain]

Specifying flow at standard conditions does nothing but confuse. The datasheet you have came from some other company, where it is also confusing. Best not put it on a datasheet, it's just a redundant item that will probably be in error, and waste a lot of time. Best to give the flow in mass units along with molecular weight. The compressor vendor will be OK with this.

 

There are probably other datasheets with useless or redundant information. Cast them out!

 

Bobby

[breizh]

Hi,

Agree with Bobby , use Mass flow rate and specify the inlet and outlet conditions (P,T) .

 

Breizh 

[katmar]

My experience has been that compressor vendors get very confused when you specify capacities in mass flow terms.  Although a standardized volumetric flow could well be said to be a mass flow rate.  Somehow people don't see it that way.

 

I have to disagree with Bobby and Breizh. The concept of standard volumetric flow is so deeply entrenched in process engineering that you are best served by getting to understand it properly and to be comfortable with it.

 

Your comment "The standard volumetric flow is dependent on the actual volumetric flow, which means that the standard volumetric flow at the inlet and at the outlet of the compressor is different." is wrong and shows that you have not understood the concept.  The advantage of using standard volumetric flow is that it is the same at the inlet and outlet of the compressor (because it is really a mass flow expression).

 

There are plenty of resources on the internet to learn how to use standard conditions so I won't try to teach it to you here.  But what I will say is that there is nothing standard about standard conditions.  Have a look at the Wikipedia entry for standard temperature and pressure and you will see that there are more standards than you could shake a stick at. The lesson to learn from this is to remember that whenever you quote flow rates or properties at standard conditions you must specify which standard you are using.

 

 

 

[SilverShaded]

I use standard flowing conditions all the time

 

Hello everyone,

 

I would like to ask your view about what is the usefulness of using standard conditions to express volumetric flows?

 

 

For example, when we prepare PDS for compressors, we use standard volumetric flow, what is the real benefit of it?

 

The standard volumetric flow is dependent on the actual volumetric flow, which means that the standard volumetric flow at the inlet and at the outlet of the compressor is different.

I think you misunderstand what standard flow is, the standard flow in and out of the compressor is the same.

Flows are allmost allways measured in volume as flowing conditions.  To convert to mass you normally have to have the density at standard conditions (which depends on the particular industry and country you are in, in refining it's 15.5C EU / 20C Russia etc).  So:

 

Mass Flow = Standard volume flow * Standard Density.

Thats one reason why standard volumetric flows are important.

 

Standard condtions typically apply only to liquid flows, gas flows are typically reported at Normal Condtions e.g. Nm3/hr.
 

Edited by SilverShaded, 07 March 2022 - 07:18 AM.

[latexman]

When specifying a gas/vapor flow one must be thoroughly explicit and document the temperature and pressure, even if units are Standard or Normal conditions.  Effective communication is the solution to confusion on gas/vapor flows.

[Art Montemayor]

Approaching the same number of years as the number I wore on my high school football jersey, I still am impressed by the present state of many engineers struggling to cope with the term “Standard Conditions” for gases.

 

The best explanation – and the one I never forgot – was the one given by Brother Frederick, my high school chemistry teacher in 1955: “All gases are composed of molecules.  Molecules are composed of mass.  Since this mass exists in the gaseous state, it is very difficult to describe it in mass units – since the gas volume will vary depending on its temperature and pressure.  Therefore, we will use a standard gas condition of 22.4 liters of gas measured at 0 ºC and 1 atmosphere pressure being the volume of one mole.”

 

I had no problems with that explanation.  However, in college I got into other temperature and pressure definitions for Standard Conditions (S.C.).  And when I started my engineering career in the compressed gas industry, S.C. got even more confusing and varied!  But I soon learned what was causing all the fuss and confusion: “Different strokes for different folks”. 

 

S.C. is as varied as the world’s languages and religions.  There is NO universal standard – and there probably never will be.  The realization of this situation is discovered by those of us who have had to enter into legal contractual documents regarding the purchase-sale of compressed gases – such as pipeline natural gas.  The lawyers – not engineers – dominate the definition of S.C.

 

But the necessity of defining gaseous conditions still haunts us engineers – choke flow, compressible flow in pipes and its pressure drop, volumetric flow in machinery, etc., etc.

 

As long as we openly and explicitly state the pressure and temperature defining a gaseous state, we should never have any problems understanding each other.  Latex says it all.

 

To give you an idea of how long this “problem” has been discussed, refer to the attached documents.  I hope you enjoy the comments given.

 Standard conditions.docx   30.85KB   31 downloads

 Standard Gas Conditions for Temperature and Pressure.docx   53.8KB   27 downloads

[panoska]

Thank you all for your replies.
I would like to answer to Katmar.
My wrong statement before can be explained with the below example:
Assuming we have a compressor with actual suction conditions Q1, P1,T1 and actual discharge conditions Q2, P2, T2.
The normal conditions are Tnorm and Pnorm.
If I convert my actual conditions to standard then,
NQ1=Q1*(P1*Tnormal)/(T1*Pnormal)
NQ2=Q2* (P2*Tnormal)/(T2*Pnormal)
Then I cannot get how the normal volumetric flow remains the same. Since NQ1 is different than NQ2.
I TRY to figure out what is going wrong with my understanding about this concept.

[panoska]

Art we commented at the same time.
I don't delete my last comment. But, I will read your document and your comment carefully.

[SilverShaded]

PV = nRT

 

or

 

P1 . V1 / T1 = P2 . V2 /T2

[katmar]

If you take your 2 equations

1.   NQ1 = Q1 * (P1 * TN) / (T1 * PN)
2.   NQ2 = Q2 * (P2 * TN) / (T2 * PN)

And note that Q2 = Q1 * (T2 * P1) / (P2 * T1)

substitute this expression for Q2 into your second equation and you get

2'.   NQ2 = [ Q1 * (T2 * P1) / (P2 * T1) ] * (P2 * TN) / (T2 * PN)

          =  Q1 * (P1 * TN) / (T1 * PN)

which is exactly your first equation so it is true that NQ2 = NQ1

[panoska]

Katmar thanks a million!
You clarify me everything.
Thanks for your time!

[Molgeshen]

Using standard conditions to express volumetric flows has several benefits in engineering and industrial applications. One major advantage is that it allows for accurate and consistent comparisons between different systems or components. Standard conditions provide a common baseline for measurements, ensuring that data from various sources can be reliably compared and analyzed.In the case of compressors, using standard volumetric flow helps in assessing their performance across different operating conditions. It allows engineers to evaluate how efficiently a compressor is working by comparing its actual volumetric flow to the standard conditions. This comparison helps in identifying any deviations or inefficiencies in the system.Additionally, standard conditions provide a basis for designing and specifying equipment. Engineers can use standard volumetric flow rates to determine the capacity and sizing requirements of compressors or other devices, ensuring that they meet the desired performance criteria under specified conditions.Overall, standard conditions play a crucial role in ensuring consistency, accuracy, and reliability in engineering calculations, measurements, and assessments.For more information on engineering and technical topics, you can visit https://www.vpesport...gends-bang-bang.

Edited by Molgeshen, 24 March 2024 - 03:46 PM.

[shvet1]

For example, when we prepare PDS for compressors, we use standard volumetric flow, what is the real benefit of it?

 

Std vol flow is much easier to measure and to specify than mass flow. Because of this fact for most kinds of 'simple' design&conditions of a compressor it is more credible to specify and test machines in std vol for a manufacturer and a purchaser both. Especially taking into account that for most cases actual gas composition/conditions can be far away from a design point.

 

When talking about compressors one should bear in mind that gases are most hard part to specify, test, design, and control precisely. In the real cases mass flowrate of a gas is some kind of a miracle - every one have seen that but no one can ensure how exactly.

Edited by shvet1, 18 March 2024 - 08:32 AM.