Pressure Loss Calculation - Nitrogen. GPSA - Darcy Weisbach method. 6

[ABenggie]

Hello,

I need some help in determining why my calculated pressure drop is so high using the GPSA methodology vs results from a calculator such as free calc. In the attached I have a side by side comparison of the results between the 2 methodologies. It is peculiar because the calculated velocity using freecalc is higher than what I get yet their calculated pressure loss is orders of magnitude smaller. If anyone could advise on what the issue is that would be appreciated.

Thanks

 

[katmar]

The units for B77 should be Pascal and not kPa.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[pierreick]

Hi,
Katmar already mentioned issue with units (system ) . If you want to know if you are on the right track let you compare dimensionless numbers, Reynolds number ,Friction factor , either Fanning or Darcy should the same using SI or FPS system .


Good luck
Pierre

 

[Latexman]

Also, you used D = 0.5 inches in B55 and 0.37 inches in "free calc". This will make ~ a 4X factor in pressure drop.

Good Luck,
Latexman

 

[LittleInch]

That all checks out as Latexman and Katmar says.

Id is smaller in the free calc therefore velocity in your spreadsheet is 188 ft/sec, in the other calc it is 337ft/sec.

If you correct the KPa to Pa, your calc has a pressure drop of 5.8 psi vs 23 from the free calc so a factor of about 4.

Find out what the ID actually is, but 1/2" tubing looks really quite small to me. Also make sure that the ID of your bend fittings is the same as the pipe. Even a fem mm makes a big difference in calculations for small ID pipe





Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Kind of a high velocity, no.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[ABenggie]

Thank you guys very much for reviewing the calculation sheet. I did the unit check per your suggestions and the result is much better.

 

[1503-44]

And the velocity is OK?

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[LittleInch]

it's the inlet to a relief valve so only in use very occasionally

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

WHAT? I CAN'T HEAR YOU.
I suppose the relief valve will be louder.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

 

[ABenggie]

This is for relief case so the higher velocity can be tolerated.

 

[Latexman]

ABenggie,

Is this a conventional PSV? If so, [&Delta;]P[sub]INLET[/sub] <= 3% of Set Pressure, so there is no "chattering". API 520 Part II applies. Your free calc says 23.3/203 x 100 = 11.5% of Set Pressure for 0.37" ID. Is this the criteria you are designing for?

Good Luck,
Latexman

 

[ABenggie]

I am no longer using the free calc data. I updated the datasheet per the suggestions above (divided by 1000 for pa to kPa) the inlet loss is just a hair below the threshold when using 1/2" tubing.

 

[Latexman]

Okay. Another issue, I don't know if you caught it. No one mentioned it above specifically. Tubing goes by OD, not ID as your spreadsheet used.

Good Luck,
Latexman

 

[ABenggie]

Thanks for catching that latexman. I am going to adjust that on the calc sheet as that does have a major impact.

 

[Latexman]

Glad to help! Is your threshold [&le;] 3% of Set Pressure?

Good Luck,
Latexman

 

[ABenggie]

Yea we are going to comply with the API guideline. Taking into account the wall thickness I am most likely going to have to upsize the inlet piping.

 

[Latexman]

Good Luck,
Latexman

 

[katmar]

Something that has nothing to do with the hydraulics, but which might be the deciding factor on what size pipe to install, is the design pressure and code of the vessel itself. I have worked in companies where a 1/2 inch nozzle on a pressure vessel would be frowned upon because it is simply not mechanically strong enough. I would be tempted to install a 1" Schd 80 nozzle. The cost difference between this and the minimum sized nozzle that you could get away with from the hydraulics point of view would be negligible.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[Latexman]

Great reply, katmar. I had ASSuMEd the OP had a very small pressure vessel. My company’s minimum nozzle size on a pressure vessel under normal circumstances is 1.5”.

Good Luck,
Latexman