The main process start from a submitted Purchase Requisition (PR).
In accordance with the procurement policy, any PR are check against the corporate Delegation of Authority Matrix (DAM - http://improving-bpm-systems.blogspot.com/2012/07/practical-process-pattern-dam.html ) if the submitter are allowed to request this purchase.
Any goods/services in the valid purchase requisition can be sourced in three complementary methods. As one PR may contain several purchase items then many combinations of the sourcing methods may be initiated.
Sourcing 1 is just delivery from the corporate stock. PR becomes Purchase Order (PO).
Sourcing 2 is just ordering from an approved supplier. Of course, a contract must be prepared.
Sourcing 3 is (the most complicated case) to choose a supplier via an open bid.
The open bid process can be as the following (several variations are possible to determine "potential suppliers").
A process for annual planning of the procurement (thus replenishing the corporate stock) is trivial and not mapped.