Are these tanks legal?

[JamesCH95]

To answer OP: you need the proper documentation to prove the parts' pressure ratings to an inspector. Cut and dried IMO.

Originally Posted by trumpthero786

Hmm I can't imagine using anything BUT the plastic tanks, but you have far more experience than I do. What sorts of failures have you seen in them?

I did a hefty amount of research into this a few years ago. There were a few root causes:

-The white Clippard tanks had casting voids near their threaded fitting area, weakening a key structural area
-If the threaded fittings were over-tightened cracks could be initiated in the tank
-If tanks were subjected to rapid or extreme temperature cycles they could become prone to failure (plastic shrinks more per unit temperature than the metal fittings screwed into them, generally speaking) one team had a tank burst when pressurizing a robot
-Overly aggressive mounting solutions (like hose clamps that were tightened too much) put undue stresses in the outside of tanks, increasing the chance of failure

The black Clippard tanks have improved mold design to stop voids, molded-in push-to-connect fittings, and are a more ductile plastic. They are a vast improvement over the white Clippards that were the focus of several (many?) failures. My own un-scientific destructive testing convinced me that the black tanks are considerably more robust than the white tanks.

The penalty of failure with plastic tanks is high because (if they fail) they generally undergo brittle failure, that is to say they make shrapnel. A failure in one plastic tank might cascade to adjacent tanks. Metallic tanks (if they fail) generally undergo ductile failure; there no shrapnel, or very little shrapnel.

What are the risks of any tank failing? The plastic shrapnel is sharp and could hurt someone (wear your safety glasses near any active robot) as it gets flung around 30-100 feet. Perhaps the more traumatizing danger (IMO) is the potential for hearing damage.

Having said that, there have been few, if any, failed tanks outside of the white Clippards. There have been no failed metallic tanks that I know of.

Considering the penalty of failure when using plastic tanks our team chooses not to use them. We recognize that many teams have successfully used tanks in the past. However, having done destructive testing on both plastic and metallic tanks, we have not found the compromise in penalty of failure to be worth the weight savings.

The only argument against metallic tanks that I've heard is weight. Only using metallic tanks, we have not had an overweight robot in at least 5 years, all of which used pneumatic systems. Careful selection of metallic tanks can reduce the weight penalty associated with them, and careful pneumatic system design can reduce air usage. IIRC 558 had an aluminum tank (2 years ago?) that was lighter and easier to package than the equivalent in any plastic tank.