Why two serial crossfeed valves on Fokker 100?
 

Can somebody give an explanation please - why are there two crossfeed valves in SERIES installed on Fokker 100?

In B777 for example, there are two valves in parallel, but that is logical - if one fails to open, another can bypass (ETOPS requirements probably). In many other cases there is normally only one crossfeed valve.

But two in series? If one fails closed, what use of another? Crossfeeding would be impossible.

Is it about protecting the crossfeed pipeline in case of fire spreading beyond fire shutoff valve? (but then that presumes that there are problems with fire shutoff).

Thanks in advance
Mirkoni