54 Sessions - nobody is running program

I went to Tomcat Manager to deploy an update to an intranet program.
Another intranet program caught my eye - it has 54 sessions running.
(I have two intranet programs running Vaadin)
Nobody is running the software and normally only two people use this particular program.
Looking at the sessions, One session is active almost one month - about when I last restarted that server.
In Tomcat, in the Manager, I hve sessions set to expire with idle >= 30 minutes.

So how do I actually expire sessions?

I dont know how the Tomcat Manager counts sessions, but if it does it with some heap inspection, maybe those are sessions that are kind of released already by Tomcat but still referenced e.g. some service you are using (causing a memory leak)? If you e.g. have a static set of some “active sessions” or “active UIs/views”, you should always clean those somehow (e.g. with UI detach listener).