Maximum gas velocity in steel pipe.
Maximum gas velocity in steel pipe.
(OP)
I am designing new piping header for a Natural gas piping. The pressures are around 50 psig and the flow is around 20 MMSCF/D of 0.6 SG NG. I know that 60fps is the rule of thumb for maximum velocity in a steel pipe. But my customer is questioning using smaller pipe (20 in pipe) which is already in place instead of the ideal designed pipe size (~24in). I know one of the major draw backs to high velocities is corrossion.
Aside from not being a recommended practice and the corrossion issue what are some other drawbacks to using smaller pipe and having gas velocities higher than 60 fps? and what would be an absolute maximum velocity in steel pipe to avoid excessive corrossion and other problems? 80fps? 100 fps? 120 fps....
Thanks.
Aside from not being a recommended practice and the corrossion issue what are some other drawbacks to using smaller pipe and having gas velocities higher than 60 fps? and what would be an absolute maximum velocity in steel pipe to avoid excessive corrossion and other problems? 80fps? 100 fps? 120 fps....
Thanks.





RE: Maximum gas velocity in steel pipe.
RE: Maximum gas velocity in steel pipe.
Other clients look at keeping dP/mile under 5 psi/mile in 20-inch (84 ft/sec at 50 psig) to minimize hp required to recover friction drop.
When I look at 20 MMCF/d in a 20-inch pipe with an exit pressure of 50 psig, I get a velocity of 25 ft/s using the AGA equation. That sounds perfect to me.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
RE: Maximum gas velocity in steel pipe.
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdo/
RE: Maximum gas velocity in steel pipe.
regards
Katmar
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Maximum gas velocity in steel pipe.
Regards,
athomas236
RE: Maximum gas velocity in steel pipe.
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
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
RE: Maximum gas velocity in steel pipe.
RE: Maximum gas velocity in steel pipe.
I do understand the example you described with the velocity difference at Standard conditions and at Actual Conditions. And I do convert to actual conditions to get and accurate velocity. But that is very good point to make.
Knowing that some customers use 100 ft/s and 40 m/s (131ft/s) as cut offs for maximum velocity makes me feel more comfortable about using over 60 ft/s if the customer requests it. After all it is their policy and if they want to exceed it, it is their decision.
RE: Maximum gas velocity in steel pipe.
RE: Maximum gas velocity in steel pipe.
David
RE: Maximum gas velocity in steel pipe.
Larry