How to Calculate Nitrogen Purge Velocity in Fixed Volume Tanks?

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Discussion Overview

The discussion revolves around calculating the nitrogen purge velocity in fixed volume tanks, specifically addressing the complexities involved in determining the velocity and volumetric flow rate of nitrogen gas being purged through a pipe. The context includes considerations of fluid dynamics and the behavior of gases under varying pressures.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant presents a practical problem involving the purging of nitrogen gas from tanks and seeks guidance on calculating the velocity and flow rate.
  • Another participant notes that the problem is more complex than it appears, referencing a related discussion on pressure vessel depressurization through an orifice.
  • It is suggested that the velocity of the gas is influenced not only by the fluid properties (pressure, viscosity, density) and the pipe diameter but also by factors such as the length of the pipe, its roughness, and the geometry of the entry point.
  • A participant questions whether major and minor head losses in fluid mechanics apply similarly to compressible fluids, indicating a background in fluid mechanics focused on incompressible fluids.
  • There is mention of upcoming data from flow meters that may affect the theoretical calculations, particularly regarding temperature changes in the pipe.

Areas of Agreement / Disagreement

Participants express varying views on the factors influencing the purge velocity, indicating that multiple competing perspectives exist regarding the complexities of the calculation. The discussion remains unresolved with no consensus reached on the best approach to the problem.

Contextual Notes

Limitations include the potential impact of compressibility on flow calculations, the need for specific data from flow meters, and the influence of temperature changes on the theoretical models being considered.

bluesman4509
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This is not homework, but a problem from work.

If 4 tanks with fixed 90m^3 volume contained Nitrogen gas at 701.3 kPag were purged to 131.3 kPag through a 100mm pipe how would I work out the velocity/volumetric flow rate of the output?

Thanks
 
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The velocity isn't dependent only upon the fluid (pressure, viscosity, density), and not just on the diameter of the 100mm pipe, either, but also the length of the tube, it's roughness, and the entry geometry (exit, not so much provided its flush or free air). Is the tube's entrance flush with the interior of the tank, or in free air?
 
Thanks all for the pointers,

mugsliens I've completed a subject in fluid mechanics that only looked at incompressible fluids so would I be correct in saying that major (friction) and minor (inlet/outlet exspansions) head losses relate smilarly in compressible fluids?

I will receive data from some flow meters soon, since I read in another post that I would need to consider the change in temp. through the pipe I guess introducing measured data would make the theoretical calc useless anyways.
 

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