There are two common types of fuel cell in road racing. In most SCCA classes, the cell must meet FIA Standard FT-3 or better. Additionally, in tub and production derived tube frame cars (e.g., things that aren't formula cars or sports racers), the cell must be contained in an approved metal container. finally, in all classes, there must be a complete metal bulkhead between the fuel cell compartment and the passenger compartment.
Because of space restrictions, in formula cars and sports racers where the fuel cell is under the driver's seat, one metal bulkhead is sufficient, but the cell must still be FT-3 or better. The FT-3 restriction means that inexpensive plastic cells are generally not permitted (at one point, there was a manufacturer who had gotten a plastic cell through FIA FT-3 certification, but that seems to have been a one-time event; it has not happened again so far as I know.)
The one loop hole where plastic cells are permitted are classes where fuel cells are permitted but not required; in these cases a plastic non FT-3 cell is considered a viable upgrade even if it doesn't meet the spec. Improved Touring is the class where these are likely to be seen.
How do you tell? At a superficial level, look at the filler cap area. An FT-3 cell will have a large oval metal plate with the filler neck and ports for all the various hoses and lines. The hole in the rubber bladder covered by this plate will be the only hole in the cell. By comparison, a plastic cell will have multiple holes drilled in the plastic for the various fittings.
Although the SCCA doesn't currently enforce aging of fuel cells, there are aging rules built into the FT-3 Specification. After 5 years of service, a cell bladder needs to be recertified or replaced, and recertifications are good for 2 years. I have not yet checked to see if recertification is an affordable (or even available) alternative to replacement; there's a good chance it's just not worth it.
Why would a bladder fail? That's easy enough; it's manufactured from petro-chemicals and is exposed to gasoline and whatever is in gasoline more-or-less continuously. In particular, if racers are perhaps using fuel additives that they shouldn't, one can well imagine that premature bladder failure (or disintegration of the foam inside) is a real possibility.
In the next article (Fool cell 102) I'll talk about proper mounting of a cell.