14 Feb 06 9:32
Katmar's point is far too important to skip over--it is totally meaningless to calculate bulk velocity directly from a volume flow rate at standard conditions unless your gas is actually flowing at 14.73 psia (or 14.696, or 15.025, whatever "standard" means to you) and 60F (if that is your "standard" temperature).
This mistake is so ubiquitous that I made it in my masters theses and turned an important study into a pointless acedemic exercise--and no one on my theses committee caught it. Three years after I defended it, I got a call from someone who had gotten the document out of the library and said that at his conditions the equations didn't track reality. Once I got over the "denial" stage of conflict I checked the math and found my mistake. There was no way to salvage the relationships that had seemed valid in my data and I've been embarassed by it ever since.
While "volume flow rate at standard conditions" is a wonderful commercial concept and a pretty good surrogate for mass flow rate in many calculations, it has zero direct physical significance at virtually all flowing conditions. You have to convert it to "volume flow rate at actual conditions" before you can calculate velocity. That is true if you are calculating average (or bulk) velocity, if you are calculating a velocity profile, or if you need a maximum velocity--none of these concepts can reasonably be calculated using standard conditions.
In the example you gave, if the downstream pressure is exactly 14.73 psia at 60F then the bulk velocity is 109 ft/s, if the downstream pressure is 50 psig your bulk velocity is 25 ft/sec, at 100 psig it is 14 ft/sec, and at 1,000 psig it is 1.5 ft/sec (all these numbers are based on SG=0.6 and elevation=5535 so Atmospheric pressure is 12.0 psia, other elevations would result in slightly different velocities). So, depending on pressure your velocity is somewhere between way too slow and maybe too fast.
David Simpson, PE
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