18 replies to this topic

[chemmu]

Dear All, Good Day to all .. Please tell me how will you do line sizing for compressible fluid? Flow rate shall be taken in Nm3/hr or in actual m3/hr? quantity wise Nm3/hr more than Am3/hr at same temperature?? Regards, Chem.M

[thorium90]

Typically, Nm3/hr is for gases and m3/hr is for liquids. You would need stuff like the flowrate, pressure, pressure drop, velocity... Think this files and links might be of interest to you

http://www.cheresour...rent-standards/

http://www.cheresour...velocity-check/

http://www.cheresour...-pipeline-flow/

Edited by thorium90, 21 February 2013 - 11:19 AM.

[Art Montemayor]

Although written in bad English, the OP intends to state:

 

How is a line sized for compressible fluid flow?

Does one use the units Nm³/hr or actual m³/hr?

 

One can use mass flow rate (kg/hr), “standard” or “Normal” volumetric flow rate, or the actual volumetric flow rate – it all depends on which flow equation one employs .  In the end, one is basing the calculations on the ACTUAL flow rate – whether mass or volumetric.

You can initiate the calculations in “standard” volumetric flow rate and convert this value to actual flow rate in order to finish the exercise.

 

Contrary to what thorium states, there is nothing “typical” about which units are used for compressible or  incompressible fluids.  The important point that all engineers should realize and accept is that in the real world, it is vitally important to state what PHASE you are dealing in.  That is what is important, not which units are employed. 

 

 

 

[ankur2061]

chemmu,

 

For any line sizing there are basically two criteria, one is pressure drop and other is velocity restrictions. Every fluid is compressible whether it be liquid or gas, only the degree of incompressibility differs. While liquids are less compressible, gases are more compressible.

 

Liquids being less incompressible, the velocity change across a given pipe cross-sectional area and length is much lesser whereas for gases which are more compressible the velocity change is more pronounced. The relationship is simple:

 

Q = A*v or v = Q / A

where:

 

Q = volumetric flow

A = C/S area of pipe

v = velocity

 

Any volumetric flow for line sizing has to be considered based on the actual volume flow which is nothing but the mass flow divided by the density at the actual temperature and pressure.

 

Regards,

Ankur.

[markymaark]

Agreed.  I believe Chemmu is talking about gases and Nm3/h (at 1atm 25C) vs. Am3/h where you simply convert the volumetric flow from standard T and P to the actual system T and P.

 

In that case, I believe I've usually spec'd air filters in SCFM.

 

Mark

[katmar]

While this may appear to be a simplistic question, it is actually very relevant to the way pressure drops are calculated for gases.  As Art Montemayor has pointed out, in the final analysis you have to use the actual conditions.  But if you ask 90% of the engineers involved in pipelines and reticulation systems for petroleum gases they will tell you exactly the opposite and they will insist that you have to use standard conditions.  My experience of working with these engineers is that they want to have nothing to do with mass flows or actual volumetric flows.

 

The reason for the gas engineers believing that they have to work in flows at standard conditions is that historically they used empirical formulas like Spitzglass, Weymouth and Panhandle - all of which are indeed based on flows in standard conditions.  But if you examine these equations in detail you will quickly see that you have to enter data like the actual temperature, the actual pressure, the actual compressibility factor and all the other data that is necessary for the equation to (internally) convert the flow in standard conditions to the flow at actual conditions.  (The standard conditions are also in there, but are all combined into one constant.)  So, while the gas engineers think they are working in standard conditions, their equations are doing all the conversions for them behind the scenes and they are in fact working in actual conditions.

 

Old habits die hard, and these equations are still used - even though there are better alternatives these days.  It seems that consistency is more important than accuracy.

 

You can prove to yourself that you have to work in actual conditions by examining the Darcy-Weisbach formula, i.e.

ΔP = ƒ(L/D )(ρv²/2)

 

This is applicable to all fluids - gases and liquids - provided that L is small enough that the density (ρ) and velocity (v) can be regarded as constant.  With gases, which are more compressible than liquids, the increments of L have to be taken much smaller than with liquids to meet this requirement.

 

The density (ρ) and velocity (v) in this equation have to be the actual values at the flowing conditions (this is basic physics for kinetic energy) and shows why I (and Art) have said that in the final analysis you are working with actual conditions.

[thorium90]

 if you ask 90% of the engineers involved in pipelines and reticulation systems for petroleum gases they will tell you exactly the opposite and they will insist that you have to use standard conditions.  My experience of working with these engineers is that they want to have nothing to do with mass flows or actual volumetric flows.

 

Exactly my thoughts! Thanks for explaining it so nicely.

 

Just read markymaark's post again and noticed;

Nm3/hr is not at 25C but at 0C.

Edited by thorium90, 22 February 2013 - 01:50 AM.

[chemmu]

Dear All,

 

Thanks for your response.

 

We assume that we have to size a line for instrument gas.

 

The given flow rate is 108.8439 Am3/hr at Operating pressure of 11.01325 bara and 50°C.

If I consider the velocity of 15 m/s, then the approximate line size is 2 "

 

When we consider the same flow rate in normal condition, the flow rate is 1000 Nm3/hr at 0°C and 1.01325 bara

If we consider the velocity of 15 m/s (to be checked), then the approximate line size is 6"

 

When we consider the same flow rate in standard condition, the flow rate is 1057.11 Sm3/hr at 15.6°C and 1.01325 bara

If we consider the velocity of 15 m/s (to be checked), then the approximate line size is 8" (it came 6.5")

 

I had the doubt because of the different line sizes.

 

should different velocity  be considerd based on the pressure ?  

 

Regards,

Chem.M

[latexman]

Don't be in the 90% chemmu!  Re-read Post #6 carefully!

 

First, what equation(s) are you using?  This is critical!  Does the equation(s) require actual, normal, or standard conditions as input?  Now, input only the conditions that are required.  If you input standard conditions into an equation that requires actual conditions, you get garbage!  GIGO!  Garbage in = garbage out!

[thorium90]

I assume instrument gas means air?

108Am3/hr and you get 2"? 1000Nm3/hr, 15m/s and you get 6"? Hmm... the numbers dont look right. Care to put up a spreadsheet showing your calculation steps?

[katmar]

chemmu, you should not think of Nm³ as a unit of volume.  When you say that you have 1000 Nm³ what it means is that you have an amount of gas that if it were at normal conditions it would take up a volume of 1000 m³.  But it is not at normal conditions so it does not have a volume of 1000 m³.  It is possible to work out the actual volume - as you have done in your example - but it is also possible to work out the mass of the gas if you have its molecular mass.  At normal conditions most gases have a compressibility of 1.0 and a simple application of the ideal gas law gives you the mass or the number of moles.

 

In fact, a quantity expressed in Nm³ is a lot more like a mass than it is like a volume.  A very useful property of quantities expressed in this way is that they are additive - just like the conservation of mass principle.  If you have one flow of 100 Nm³/h and another of 200 Nm³/h then irrespective of the temperatures or pressures of either of the sources, or of the final mix, you can be certain that when you combine them you have 300 Nm³/h. It is because of these very useful properties that the petroleum gas industry has stuck to using standard flows.  There is nothing wrong with expressing flows this way.  Perhaps I was a bit hard on my gas industry colleagues earlier.  All I was trying to say is that it is good to understand what you are doing, and to be a little bit flexible.

[Shivshankar]

Chemu,

 

Check below links.

 

http://www.epa.gov/e...2/rate/rate.htm

 

http://www.sensortec...low_E_11153.pdf

 

Regards

Shivshankar

Attached Files

•  11_gas_properties.ppt.zip   435.7KB   31 downloads

Edited by Shivshankar, 22 February 2013 - 03:16 PM.

[narendrasony]

Dear Chemu,
Katmar and other esteemed members have already explained it very well, perhaps you are still not getting at it.

1) Velocity is actual velocity , so it has to be at actual flow conditions only. It is as simple as that.

2) Nm3/Hr or any other standardized flow rates (SCFM, MMSCFD etc) are used for material balance since they are proportional to molar and mass flow rates. (Note: For systems not involving chemical reactions)

Pressure drop also needs to be checked. 11 Bara for instrument air seems little higher. As per my experience, 6.5-7 Bara at process plant battery limit is sufficient .
Regards
Narendra

Edited by narendrasony, 23 February 2013 - 01:32 AM.

[kkala]

Following note could be useful,  since elasticity of liquids was mentioned.  Elasticity is inverse to bulk modulus, the latter expressing increase in pressure (bar) needed to decrease liquid volume by 1%. More exact definition and values can be seen in <http://hydraulicspne...HydraulicFluids> and <http://www.engineeri...city-d_585.html>, e.g. it is 21500 bar for water and 9200 bar for acetone (assumed at ambient temperature).

Suppose that a flowing liquid undergoes a pressure reduction of ΔP=100 bar isothermically, without phase change.

In case of water, density decrease will be Δρ/ρ = 100/21500 =0.47%, so velocity will  increase by this percentage. In case of acetone Δρ/ρ = 100/9200 = 1.09%. Some additional (but lower) increase in velocity will result from (metallic) pipe section reduction (*), yet all these are insignificant before usual uncertainties of pressure drop calculation.

Even though there are simplifications (actually bulk modulus depends on pressure and affected by dissolved gases), above indicates practically constant density of liquids for engineering calculations of ΔP (under conditions of no phase / temperature change). Division of liquid pipe line into segments (of some length L) does not seem necessary, at least for Newtonian fluids. In some non Newtonian fluid cases, segmentation could have a meaning, although most important would be to simplify these cases. In a case of thixotropic slurry, this was stronlgy agitated for viscosity reduction, then pumped to disposal area.

This reflects cases faced so far. Comments would be welcomed.

 

(*) See <http://www.cheresour...er-from-pr-loss>

Edited by kkala, 23 February 2013 - 07:59 AM.

[Steve Hall]

I'd like to chime in. Post #8. 1.01325 bar? Just because your calculator has lots of digits there is no excuse for actually reporting them! For a rough calculation as described, two, or at most three, significant figures are all that are warranted.

[kkala]

Well, I think intention was to say that  concerned pressure = normal atmospheric. This was just expressed through 1.01325 bara.

Other times we write the figures with many significant digits, just to ease somebody willing to repeat calculations manually. Not all these digits are physically precise in our engineering calculations.

On the other hand long figures make reading a bit more difficult, conclusion is not so clear.  Difference after second significant digit (and conclusions based on it) can be hardly noticed without effort. But integers up to 4 digits are not conventionally rounded off.

I think above had better be considered according to intended purpose, when writing posts. In most posts short figures would be better. Not a serious problem, but facilitating reading.

Edited by kkala, 24 February 2013 - 08:03 AM.

[chemmu]

Dear All,

 

Thank you so much for your technical support on this.

 

As per the comments from the technical expert, I understood that we have to take the value in the actual conditions.

 

Does the velocity for line sizing of compressible fluid vary depends on the pressure ?

 

Just for example, i am having a pressure control valve to reduce the pressure from 10 barg to 1 barg.

 

What is the velocity to be considered in the upstream of PCV and downstream of the PCV ? ( mass flow is same but there will be change in vol metric flow rate)

 

Regards,

Chem.M

[chemmu]

Narendra

 

Dear Sir,

 

Thank you so much for your reply. Just for example only i mentioned as 11 bara.

 

Regards,

Chem.M

[chemmu]

Dear All,

 

Once again thank you so much for your valuable information

 

Regards,

Chem.M