Help Settle a Tank Filling Discussion

[TN-Steve]

I'm a little confused. If all supply cylinder valves are open at the same time, you don't actually have a cascade, do you? My little basement fill station has 7 supply tanks leading to a manifold that has a gauge and leads to a whip with a needle valve. On the far side of the needle valve is another gauge (digital) and the whip to the receiving tank. But my supply cylinders are only opened one at a time. That's what creates the cascade. The supply cylinder is opened all the way. The gauge on the manifold reads the supply tank pressure. The bottle being filled is opened all the way. The gauge on the whip at the receiving tank reads that pressure. There is a check valve at the manifold so gas can not flow back into the supply side. The needle valve controls the flow.

Have I been doing it wrong all these years?


iPhone. iTypo. iApologize.

Nope, you're right. The fill station at the shop doesn't have a needle valve and a check valve, but same idea. I use the bank tank valve to control the flow. And you're right, if you open them (the bank bottles) up at the same time, you don't have a cascade. (as I had to educate some of my co-workers).

Thanks

Steve

 

[rx7diver]

our setup was a bit different...

bank bottles all the way open
manifold all the way open
third stage set at whatever you want the cutoff to be
tank valves all the way open
flow rate controlled at the fill whip.

tbone1004,

Yours was a great idea for an additional level of safety.

My scuba professor/instructor/coach was extremely cautious. He often referred to a serious fill station accident that had happened at a university in Kansas (KU? or KSU?) which maimed/killed someone, a student I think (I've forgotten the details), and this shaped his approach:

At that time our cascade bottles each had a 3,500 psig service pressure, and the course used (mostly) 3AA and (a few) AA steel 72's all of which had a service pressure of 2,250 + 10%. All—cascade bottles and scuba cylinders—were hydro-ed every three (!!) years, iirc. The 72's were plus-hydro-ed.

The compressor (an Army-surplus 4-stage Worthington, having cold tapwater flowing in jackets surrounding the high-pressure lines) had its pop-off valve set to 2,475 psig (so the 72's could *never* be filled above their 2,475 psig service pressure).

For class, we filled the 72's (in a cold tapwater bath) to only ~2,100 psig most of the time. For pool skills which required students to be submerged for *long* periods of time without breaking the surface, the 72's were filled to (only) 2,250 psig.

When we trailered the 72's down to northwest Arkansas for open waters, the 72's would be filled to 2,250 psig before we left. (While there, the fill station at the closest marina would fill them to 2,475 psig, however.)

Even with this degree of safety built in, my instructor would allow only a very few TA's to operate the fill station.

His paranoia left quite an impression on me!

Safe Diving,

rx7diver

 

[jbomb001]

I'm a little confused. If all supply cylinder valves are open at the same time, you don't actually have a cascade, do you? My little basement fill station has 7 supply tanks leading to a manifold that has a gauge and leads to a whip with a needle valve. On the far side of the needle valve is another gauge (digital) and the whip to the receiving tank. But my supply cylinders are only opened one at a time. That's what creates the cascade. The supply cylinder is opened all the way. The gauge on the manifold reads the supply tank pressure. The bottle being filled is opened all the way. The gauge on the whip at the receiving tank reads that pressure. There is a check valve at the manifold so gas can not flow back into the supply side. The needle valve controls the flow.

Have I been doing it wrong all these years?


iPhone. iTypo. iApologize.

If this was directed at me what I mean is each cascade tank full open one at a time. I suppose I just figured anyone reading this thread would know how to operate a cascade system. Your correct if ALL tanks were All open together they would equalize and it would no longer be a cascade it would just be a large bank tank. Although I can normally fill at least 5 al80s before having to Crack open my 2nd cascade bottle..

Sent from my galaxy S5 Active.

 

[OkByMe]

Unless I missed it, no one addressed the design of the cascade, oxygen or helium valves at all. These valves are designed to be "back seated" which seals the valve stem from leaking around the packing gland. You don't use these valves to regulate flow. There should be a flow control valve installed inline with each (or group) cylinder to control its' flow to the manifold. If using different working pressure cylinders, the highest pressure cylinders should be separated from the lower pressure cylinders by a check valve. You don't want a 6000psi cylinder open to the manifold while a valve is open to a 2250psi cylinder. Or if having different gas mixtures on the same manifold they should be isolated with check valves too.