Process gas supply piping system

A process gas supply system for supplying process gas to at least two process units, comprising:

• a purge gas supply piping line;
• a process gas supply piping line including at least one branching point;
• a plurality of process unit piping lines and associated process units, each said process unit piping line connected to said at least one branching point, each said process unit piping line connecting an associated said process unit to said process gas supply line;
• and
• valve means for connecting said process gas supply line, said purge gas supply piping line, and at least one of said process unit piping lines to said at least one branching point, said valve means including at least four valves, whereby each one of said process unit piping lines can be independently purged and said process gas supply system can simultaneously supply process gas through at least one of said valves to an associated process unit currently in use, and purge process unit piping lines currently not in use through at least one of said valves without stagnating either the process gas or the purge gas within the piping system in any mode of operation.