- #1

Dullard

- 511

- 414

- TL;DR Summary
- Is this a valid technique?

I am a poor, dumb EE often stuck with the odd plumbing calculation. I am often asked questions like: "what size tubing do I need to convey 10 SLPM of 20 PSIG oxygen 200' with no more than 2 PSI pressure drop?"

I generally treat the fluid as in-compressible and use Darcy-Weisbach (I like Bellos for friction factor). So long as the pressure drop is < 10% of the inlet absolute pressure, my calcs correspond pretty well with reality. I try to stay conservative (It's not usually obvious when a tube is a little bit too big).

I had a situation this week where my installers were certain that they knew what they needed (they didn't ask me anything). They screwed up - the tubing will have to be replaced. When I did my normal calculation on what they actually installed it was too small by my standards, but should have worked (just - if I ignored the 10% rule). This got me thinking that I'd like to be able to (semi-accurately) go a bit beyond my previous comfort zone. I created an excel workbook (with some automation) to break a tubing run into 'n' segments. The outlet conditions for one segment are the inlet conditions for the next. This (mathematically) appears to work pretty well: The calculated pressure drop increases with 'n' and converges (increases at a decreasing rate). I'd appreciate any comments on the accuracy/validity/limits of this approach. If I'm missing an alternative approach, I'd love to hear about that, too. Thanks.

I generally treat the fluid as in-compressible and use Darcy-Weisbach (I like Bellos for friction factor). So long as the pressure drop is < 10% of the inlet absolute pressure, my calcs correspond pretty well with reality. I try to stay conservative (It's not usually obvious when a tube is a little bit too big).

I had a situation this week where my installers were certain that they knew what they needed (they didn't ask me anything). They screwed up - the tubing will have to be replaced. When I did my normal calculation on what they actually installed it was too small by my standards, but should have worked (just - if I ignored the 10% rule). This got me thinking that I'd like to be able to (semi-accurately) go a bit beyond my previous comfort zone. I created an excel workbook (with some automation) to break a tubing run into 'n' segments. The outlet conditions for one segment are the inlet conditions for the next. This (mathematically) appears to work pretty well: The calculated pressure drop increases with 'n' and converges (increases at a decreasing rate). I'd appreciate any comments on the accuracy/validity/limits of this approach. If I'm missing an alternative approach, I'd love to hear about that, too. Thanks.