The Queue System has a failover recovery feature - if the farm contains multiple servers running the Project Application ...

The Queue System has a failover recovery feature - if the farm contains multiple servers running the Project Application Service, and the Queue Service fails on one server, jobs are automatically redistributed to other servers on which the Queue Service is online. A Queue Service is considered to have timed out if it has not updated its heartbeat for more than the 'Queue Timeout' interval - the heartbeat is updated by the Queue in all the Project Web App databases that it handles.

Minimum: 2
Maximum: 20
Default: 3

(Note that Queue Timeout cannot be less than 4 times the Bookkeeping Interval at any time. For example, if the Queue Timeout is 3 minutes and the Bookkeeping Interval is changed to 60000 (60 seconds), then the Queue Timeout will automatically be changed to 4 minutes.)