Pubdbcall threading issue

We have configured our Oracle with minimum connection as 1, maximum connection as 3, Expiration time in ms as NULL. We are making all our database update through SP call, rather than a execSQL.The code works fine in normal situation.
The hard part begins when a load testing is being performed on our service. As 10 number of concurrent process begins, then it started throwing error in Oracle. The reason is because of the fact that “Number of session is increasing tangentialy as threads are not getting closed”.
Does the service needs an explicit pub.db:close service or call internally takes care of closing the thread as soon as it is done ?