How to Provide Access to all company departments to create PR

Our Purchasing department now handle all company departments PRs , The management requested to give access on PR creation to all departments which are around 20 department to reduce the load on purchasing department so every department should have 2 users and their manager to approve it before sending to purchasing department.

My Question is i will create around 60 new Oracle users ( 3 in every department ) and give them access to PR creation but shall i define 20 new purchasing hierarchy one for every department? as every department shouldn't see others departments PRs and will have different users.

Thanks in advance and looking for your kind assistance