Legal Pneumatic Fittings - Nipples & Elbows?

[nuclearnerd]

Thanks for the input. I'm glad to get some backing on my interpretation.

Personally I'm not worried about a "pipe" being mistaken for "tubing". In this area, "tubing" usually means flexible (or at least, bendable). I'm guessing the rule is there to minimize damage if the vulnerable plastic tubing is cut or disengaged, which isn't a risk with hard pipe. I agree the question is probably worth a Q&A (I don't know how to do that though - I'm a first year mentor).

[FrankJ]

Personally I'm not worried about a "pipe" being mistaken for "tubing".

In that case "pipe" is not a fitting & not on the list of approved items. You probably OK with short nipples, without a Q&A only your inspector knows for sure.

[Al Skierkiewicz]

The rule was simplified this year as 0.160 ID tubing has the same flow restrictions as saying Cv of 0.32 or about the same flow as 1/8" NPT port diameter.

[lcoreyl]

Originally Posted by Al Skierkiewicz

The rule was simplified this year as 0.160 ID tubing has the same flow restrictions as saying Cv of 0.32 or about the same flow as 1/8" NPT port diameter.

If 1/8" NPT port diameter has a Cv of 0.32, then why would a solenoid with 1/8" NPT output advertise a Cv of 0.75? (mcmaster 6124K511)

[IndySam]

Originally Posted by lcoreyl

If 1/8" NPT port diameter has a Cv of 0.32, then why would a solenoid with 1/8" NPT output advertise a Cv of 0.75? (mcmaster 6124K511)

The CV of a valve is a calculated number that includes the fitting size and the internal restriction of the valve and is basically just used for valves.

I think what Al was saying that using port size and hose ID is just a way of restricting flow like quoting a CV value.

Using CV can be pretty confusing because some valves give flow rate and some give CV which is not the same number. Eliminating the CV restriction and just using port size and hose ID is much easier to understand and inspect.

[lcoreyl]

Originally Posted by IndySam

The CV of a valve is a calculated number that includes the fitting size and the internal restriction of the valve and is basically just used for valves.

I think what Al was saying that using port size and hose ID is just a way of restricting flow like quoting a CV value.

Using CV can be pretty confusing because some valves give flow rate and some give CV which is not the same number. Eliminating the CV restriction and just using port size and hose ID is much easier to understand and inspect.

(emphasis mine)

right, so if a solenoid has a 1/8 NPT port, and the Cv of the 1/8 NPT is 0.32, then how can the Cv of the solenoid which includes said port and internal restriction be 0.75?

[FrankJ]

Cv is the conductance of flow. (inverse of resistance.) It is a handy way of knowing how much a particular valve or fitting flows. The easiest example is with liquid. A valve with a Cv of 1 will flow 1 Gal/Min with a pressure drop of 1 PSI. Note the unit dependance. Cv with pneumatics is more difficult because the flow is referenced to standard conditions. (Cubic feet per min at 14.47 PSI at 32 F, or 60F or 68F or 70F). Lets not get started with metric definitions. Anyway Cvs determined with consistent standards with let you know the relative size without worrying about all the illogic & math behind it. Which is what standards are all about.

Any way no matter how free flowing a valve is, the restrictions in the 1/4 OD tube restrict the flow to something acceptable to the GDC. I think that was what Al was saying. You would have to know the length of tube to calculate the exact Cv for it.

[Al Skierkiewicz]

I read the flow specification on the valve as without fittings for the exhaust ports. So a 1/8" port with no fittings ought to have a higher flow rate than the same port with fittings inserted.

[lcoreyl]

Originally Posted by Al Skierkiewicz

I read the flow specification on the valve as without fittings for the exhaust ports. So a 1/8" port with no fittings ought to have a higher flow rate than the same port with fittings inserted.

well, this still strikes me as a "1080p" TV sold with only an s-video input.

thanks for the info on the Cv of tubing and fittings, based on reading CD threads about flow restriction, I had assumed past KOP solenoids were significantly* worse than tubing and fittings, but I guess not...

*maybe a Cv difference of at least 0.1, as opposed to 0.03

[Levansic]

Going back to the OP's question, I would suggest testing your setup with a nipple, and with the tubing. See if there is enough difference, or even good difference that would be worth risking the creation of a new rule.

One of our mentors had the same thought, and attached one of our higher Cv solenoids to an accumulator with a short brass nipple. Imagine our surprise, when the increased "performance" turned out to be detrimental to the actual performance of our catapult. Everything was promptly changed back.

Test your theory, and find out if you are just tilting at windmills with trying something that will attract attention from inspectors. You will at least satisfy your curiosity, and you may just find, as we did, that it isn't worth it.