why is pipe blocked?

Pipes and ducts do not support bidirectional setups as the a packet in the middle does not know which way to travel. In other words the packet sees both sources and destinations in both directions. Just because a source or destination is not active (in the case of the deactivated valve), does not mean that port is ignored.

In order to ensure proper flow in ONI, every section of pipe must have only sources or only destinations in one direction or the other. This has always been the case, sorry.

if that would be the case: Why is the gas passing the first valve? - it is possible to let the gas flow from the left side to the second vent... - and the gas ignores the first valve in opposite direction...

What do you mean by "first valve" ?

10 hours ago, voomdoon said:

Why is the gas passing the first valve?

No one has ever determined the rules for why packets behave certain ways in one section of “?” pipe versus another “?” section (see nice marked up image by @Sanchozz). These ambiguous sections are simply avoided completely. They do not follow any obvious rules.

 

it is possible to maintain gas flow from source S through

• valve and vent 1
• 2-way-valve A a vent 2

but is is not possible to pass two two-way-valves "A" and "B"...