During the course of time for a project, there are few rules which are not referred anywhere and should have been deleted from the system, but due to unknown reasons those rules are not deleted and now they still exists in the application. is there any way to find out list of such rules?
I'm not aware of any mechanism to clearly identified what you are calling "orphan rules". You can check for each rule the "referencing rule" report. But this isn't always working. If the rule is called inside a Java step for example the "referencing rule" option will not find it.
You may not be able to delete "orphan" rules anyway if they are on locked rulesets. Which version of PRPC are you using ?